|
ADVANCES IN PAEDIATRIC STRENGTH ASSESSMENT: CHANGING OUR PERSPECTIVE
ON STRENGTH DEVELOPMENT
|
University of Gloucestershire, Gloucester, UK.
| Received |
|
02 March 2007 |
| Accepted |
|
18
July 2007 |
| Published |
|
01
September 2007 |
©
Journal of Sports Science and Medicine (2007) 6, 292 - 304
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| ABSTRACT |
| Our knowledge of the age and sex associated changes in strength
during childhood and adolescence is relatively limited compared to
other physiological parameters. However, those studies available on
the age and sex associated change in strength are relatively consistent,
especially for the lower limbs. Caution must be taken when transferring
this knowledge to other muscle joints as the development in strength
appears to be both muscle action and joint specific. Strength appears
to increase in both boys and girls until about the age of 14 y where
it begins to plateau in girls and a spurt is evident in boys. By 18
y there are few overlaps in strength between boys and girls. The exact
age in which sex differences become apparent appears to be both muscle
group and muscle action specific and there is a suggestion that sex
differences in upper body strength occur earlier than lower body strength.
What is less clear is the complex factors that contribute to the production
of strength during childhood and adolescence. There are few well controlled
longitudinal studies that have concurrently examined the influence
of known variables using appropriate statistical techniques. Most
studies have shown that maturation does not exert an independent effect
when other factors, such as stature and body mass are accounted for.
Also, the assumption that muscle cross-sectional area is the most
important parameter in strength production does not hold when examined
with other known variables. Consistently, stature appears to play
a key role in strength development and this may be attributed to the
strength spurt that has been linked to peak height velocity, and the
muscle moment arm. Advances in technology have provided us with more
accurate techniques to examine these explanatory variables but the
complex interaction of neural, mechanical and muscular remains to
be clearly identified from well controlled longitudinal studies.
KEY
WORDS: Strength, children, muscle size, technology.
|
| INTRODUCTION |
|
Compared to other physiological parameters our understanding of
the age and sex associated changes in strength are relatively limited,
but development of equipment, and increased understanding of growth
and maturation issues, have provided new insights into paediatric
strength development. The term 'muscular strength' refers to a measure
describing an individual's ability to exert maximum muscular force
statically or dynamically (Osternig, 1986).
Strength testing of children is performed routinely by researchers
to monitor the determinants and development of strength through
childhood but also by physiotherapists to assess the degree of muscle
disability and to diagnose the rate of recovery. Therefore, a valid
measurement method of muscle strength in children is critical. It
is important for strength test administrators to be equipped with
knowledge of the normal age- and sex-associated variations in strength
and the factors attributable to that variation. Most laboratory
and field based tests measure only the external development of force.
Additionally, there are few paediatric studies that have examined
muscle strength under electrically evoked conditions, probably associated
with the ethical constraints of such tests. Therefore, unless otherwise
stated any subsequent mention of strength shall be referred to as
the measurement of voluntarily external force. It is also inappropriate
to refer to all muscle strength movements as being a 'contraction'.
It is more relevant to refer to strength movements as a muscle 'action'
or 'moment' (the rotational effect of force). The term 'action'
is favoured throughout this text as it refers to the state of the
muscle, which is dependent upon the external force that is applied
to that muscle via the skeletal system.
| HISTORICAL
ASSESSMENT OF STRENGTH |
|
Early
studies of strength development in paediatric subjects were
primarily limited to field based tests such as number of sit
ups/press ups in 1 minute or timed flexed or straight arm
hang (Wilmore and Costill, 2004).
Essentially these tests examined muscular endurance rather
than muscle strength and may have clouded our understanding
of age and sex associated changes in strength. The best example
of this is the number of pull ups achieved by girls from 6-18
y which does not change, suggesting that strength in females
does not increase with age. The suggestion that an 18 year
old female has the same absolute strength as a 6 year old
seems absurd, and this is a good case in point to demonstrate
the importance of appropriate methods for measuring strength
in the paediatric population. Most historical studies have
used grip strength and other isometric actions to assess strength
characteristics in children. The advantages of such techniques
are well known (Wilson and Murphy, 1996),
however there are distinct mechanical and neuromuscular differences
between static and dynamic muscle actions. If the strength
of the arms and legs are to be measured then it seems logical
that they should be tested using dynamic strength tests since
their everyday functions are dynamic and not static. Although
isokinetic actions are not common everyday actions they provide
additional insight into the strength of the muscle under dynamic
conditions. Given the different neural and mechanical control
strategies of concentric and eccentric muscle actions and
the significance of these actions in everyday life, investigations
of age- and sex-associated strength development should consider
the ability of an individual to perform both types of action.
The use of cable tensiometers and other isometric dynamometers
can provide little information about the mechanical qualities
of dynamic muscle actions (Osternig, 1986).
Force, work and power are not easily measured if velocity
is not kept constant because the changing mechanical advantage
of the limb-lever system alters the force applied to the muscle
through the range of motion, i.e. the load applied to the
muscle is highest at the point of least mechanical advantage
of the muscle at the extremes of the range of motion. Isokinetic
dynamometers have become more commercially available in the
last 20 years and have contributed to an increased understanding
of the age and sex associated changes in dynamic muscle strength.
The main feature of isokinetic exercise is that the resistance
of the device precisely matches the movement speed applied
by the individual so that velocity of movement stays constant.
Isokinetic dynamometers measure torque, which is a function
of muscle force (proportional to cross-sectional area (CSA)
and the biomechanical advantage of the lever system (moment
arm). From a physiological perspective, the factors that control
the age and sex-associated variations in muscle strength are
of great interest, yet much is still unknown about the factors
that contribute to the observed age- and sex-associated differences
in isokinetic strength.
|
| METHODOLOGICAL
CONSIDERATIONS IN PAEDIATRIC STRENGTH TESTING G |
|
Paediatric
subjects provide the physiologist with added challenges relating
to varying rates of growth and maturation, and subsequently
most testing methods and equipment have been devised with
adult testing in mind. There has been an increased awareness
amongst paediatric physiologists that most commercially available
equipment needs adapting for meaningful data to be obtained.
Some of our early understanding of the age and sex associated
changes in strength may have been clouded by use of inappropriate
equipment and protocols.
Choice of testing protocols with paediatric populations may
be influenced by subjects, test equipment availability, cost
and specificity of testing. Previous authors have suggested
that the key issues relating to testing protocols should include
the muscle group to be tested, joint angle, type of muscle
action, velocity of muscle action and movement pattern (Blimkie,
1989;
De Ste Croix et al., 2003).
There are numerous generic protocol considerations when undertaking
strength testing which are beyond the scope of this chapter.
However, there are some which are specific to paediatric groups
such as adaptation of equipment, stabilization and technique,
habituation and learning effects, and safety. Advances in
technology and techniques over the years, developed to quantify
force production, have not come about without their fair share
of methodological issues. For example, modifications to isokinetic
dynamometers are required for the testing of children in order
to isolate the target muscle group. Most authors have found
the need to place a back pad behind young children to allow
their lower leg to hang freely from the edge of the seat (Henderson
et al., 1993;
Weltman et al., 1988),
or design an adjustable seat to allow for the various thigh
lengths of children (De Ste Croix et al., 1999;
2002).
Some dynamometers can be ordered with paediatric specifications
such as adjustable seat length to accommodate the short femurs
of children and short attachments, which may produce additional
problems (e.g. need to adapt sensitivity settings). Unfortunately,
several authors have neglected to describe changes to equipment,
which may influence subject positioning and subsequent torque
production (Gilliam et al., 1979;
Weltman et al., 1988).
Other specific methodological issues of paediatric strength
testing and reliability data have been covered in detail elsewhere
(De Ste Croix et al., 2003)
and it is beyond the scope of this review to address all of
these issues. However, specific consideration needs to be
given to familiarisation, especially if the protocol includes
eccentric actions (Kellis et al., 1999).
Children may be hesitant as the sensation of eccentric actions
is novel and unique, as are the strategies employed by the
nervous system to produce an eccentric action of maximum effort
(Enoka, 1996).
Other considerations such as warm up, the number of repetitions,
range of motion, testing velocities and gravity correction
procedures must all be based on paediatric models for meaningful
data to be obtained.
|
|
| RELIABILITY
OF STRENGTH TESTING |
|
In
order for any strength measurement to be used as an objective and
accurate measure of maximum strength it must be documented to be
a reliable measurement tool. Poor reliability may lead to erroneous
conclusions about the strength parameter being measured. Experimental
error can be minimised effectively by standardisation of test protocols
that will provide greater sensitivity to detect biological sources
of variation in a child's ability to exert maximum muscular effort.
An habituation period is critical for paediatric strength testing
as this essential period of learning facilitates a phase in which
the specific movements, neuromuscular patterns and demands of the
test become familiar to the individual. Previous studies (Deighan
et al., 2003;
Blimkie, 1989)
have reported good reliability in repeated isokinetic actions of
the knee in 6 to 8 year old (extension r = 0.95; flexion r = 0.85);
isokinetic actions of the elbow in 9/10 y (extension r = 0. 97;
flexion r = 0.87) and isometric hand grip data in 8-11y (r = 0.92).
Others have reported limits of agreement showing no systematic difference
in knee and elbow peak torque measured on two separate occasions
(Deighan et al., 2003).
A recent study on prepubertal soccer players has reported systematic
bias in concentric and eccentric knee torque, although these improvements,
3 to 7 %, were relatively small (Iga et al., 2006).
It would appear that strength testing in children, irrespective
of muscle action or muscle joint assessed, has a test-retest variation
of around 5-10 %.
It is difficult to compare results across studies as different statistical
methods, many of which are questionable, have been used to assess
reliability that is also protocol, measured parameter and dynamometer
specific. However, the available literature currently supports the
reliability of strength testing with children but suggests that
extension movements are more reliable than flexion movements and
that concentric actions are more reliable than eccentric actions.
|
| ECCENTRIC
TESTING IN PAEDIATRIC POPULATIONS |
| Eccentric
actions occur in everyday life as often as concentric actions. For
example, the knee extensors play a significant role in shock absorption
during walking, running and jumping and the knee flexors play the
role of a 'brake' as the knee extends during walking, kicking and
running. The characteristics and control mechanisms of these two actions
are very different, therefore the assessment of both types of action
is essential for a complete understanding of strength development.
It is possible that the limited information on eccentric strength
capabilities of children may be due to the concern that eccentric
testing with its potential for high muscle force production might
predispose children to higher risk of muscle injury or delayed onset
of muscle soreness. However, there is no reason to expect greater
muscle injury with eccentric actions in children compared to adults
or other forms of muscle testing, provided they are given sufficient
warm-up and familiarisation (Blimkie and Macauley, 2001).
Although still expensive, the greater choice in commercially available
isokinetic dynamometers has allowed the researcher to isolate eccentric
muscle actions during a range of joint movements. Even though eccentric
data on children is currently sparse, the technological advances in
isokinetic dynamometry has opened up avenues to explore the age and
sex associated changes in eccentric muscle actions, as well as to
examine the eccentric/concentric ratio to examine joint stability
(De Ste Croix et al., 2007). |
| ECCENTRIC/CONCENTRIC
RATIO AND KNEE STABILITY |
Conventionally,
the hamstring/quadriceps ratio is calculated by dividing the torque
of knee extensors and flexors at identical angular velocity and contraction
mode. However, previous authors have suggested that to evaluate muscular
balance of the knee the eccentric/concentric actions of the knee flexors
(KF) and knee extensors (KE) should be examined (ECCKF/CONKE
or CONKF/ECCKE ratio) and referred to as a functional
ratio rather than the conventional ratios often used (ECCKF/CONKF
or ECCKE/CONKE ratio) (Aagaard et al., 1998;
Gerodimos et al., 2003).
Examining reciprocal muscle group ratios provides information on knee
function, injury risk and most importantly, knee joint stability (Gerodimos
et al., 2003).
One previous study that used conventional ratios of the knee demonstrated
that an ECC/CON ratio of less than 60 % at 1.04 rad·s-1 represents
a 77.5 % probability of knee injury in elite soccer players (Dauty
et al., 2003).
There appear to be no available data on relative risk of injury in
non-elite performers and from functional ratios.
As dictated by the force-velocity relationship, the ECC/CON ratio
will increase as angular velocity increases (Colliander and Tesch,
1989;
Griffin et al., 1993)
but if measured at one velocity it is possible to make comparisons
of peak torque ratios between age groups and sexes. Meaningful interpretation
of the ECC/CON ratio in relation to age and sex have been problematic
due to the order of action cycling in the isokinetic protocol. For
example, during ECC/CON cycles the ECC action may theoretically potentiate
the following CON action (Hildebrand et al., 1994;
Mohtadi et al., 1990).
Gerodimos et al., 2003
reported a non-significant age effect on functional ratios between
12-17 y old trained male basketball players. It is not known whether
there is a difference between the ECC/CON ratios of children and adults
into the mid 20's where muscle mass of the upper body is still developing
in males. It cannot be assumed that the relationship between CON and
ECC actions is the same across ages during childhood and puberty because
children may have immature neuromuscular systems due to the incomplete
myelination of nerve fibres during childhood (Brookes and Fahey, 1985).
However, probably due to ethical issues surrounding invasive testing
procedures, the neuromuscular system of children is poorly understood
and requires further investigation.
Dvir, 1995
proposed that ECCKF/CONKE ratios derived from
low/medium test velocities are typically within the range of 0.95
- 2.05 for healthy adults. The few studies that have measured the
ECC/CON ratio in children have all found significantly higher ECC
compared to CON strength (Calmels et al., 1995;
Hildebrand et al., 1994;
Kellis et al., 1999;
Seger and Thorstensson, 1994)
with ratios ranging from between 1.17 at the slowest velocity to 1.47
at the highest velocity. However, in these studies the ECC/CON ratio
has been determined using ECCKE and CONKE rather
than functional ratios.
It has been suggested that sex differences in adults in the conventional
ECC/CON ratio of the knee joint are due to differences in percentage
motor unit activation (%MUA) during maximal voluntary actions, with
women having a lower %MUA than men during CON actions, (Griffin et
al., 1993; Westing and Seger, 1989) but not ECC actions, possibly because the actions have
separate neural control mechanisms (Enoka, 1996). Others have suggested that the sex difference is due
to a lower capacity for CON rather than a higher capacity for ECC
force production in females (Seger and Thorstensson, 2000). There is contrary evidence, however, in the form of
a superior ability of females compared to males in utilising stored
elastic energy in the muscle-tendon unit (Komi and Bosco, 1978). One recent study has examined the age-and-sex associated
changes in the functional ratio in prepubertal children, teenagers
and adults (De Ste Croix et al., 2007). In this study, females' functional ratio was significantly
lower than males' at both slow and fast velocities and was a product
of lower concentric torque as opposed to high eccentric torque producing
capability. This is in conflict with previous data that suggested
females have higher eccentric force producing capability compared
to males. Adults demonstrated significantly lower CONKF/ECCKE
than teenagers at 0.52 rad·s-1 and lower than the prepubertal
and teenager groups at 3.14 rad.s-1. However, for ECCKF/CONKE
at 3.14 rad·s-1 prepubertal ratios were significantly lower
than teenagers and adults. These data have highlighted for the first
time that during fast velocity movements prepubertal children have
a lower capacity for generating eccentric compared to concentric torque.
The lower CONKF/ECCKE ratio in adults appears
to be due to a greater ability to generate large eccentric torques
during slow and fast movement velocities. Longitudinal data are needed
to examine how the functional ratio changes throughout the pubertal
years. |
| STATISTICAL
ANALYSIS - THE INFLUENCE OF ADJUSTING FOR BODY SIZE ON STRENGTH DEVELOPMENT |
It
has become common in the literature to express strength in absolute
terms, with isometric data expressed in newton (N) and isokinetic
data expressed in newton metre (Nm). In the study of muscle strength
with growth and maturation, comparisons are made between individuals
of different sizes. It is therefore important that a size-free strength
variable is used for interpretive purposes. From a strength perspective
the key issue to be addressed when scaling for body size differences
are the body size variable with which to scale the performance variable
and the method to be employed. For a detailed review of adjusting
for body size on strength variables the reader is directed to a paper
by Jaric, 2002.
The most commonly used technique in the strength literature to partition
out differences in size is the ratio standard with body mass as the
most widely used denominator. However, stature and fat-free mass have
also featured as covariates. Others have used allometric scaling techniques
to examine the theory that muscle cross-sectional area and strength
are a function of second power of height. The b exponents identified
in the study of Kanehisa et al., 1995 ranged from 2.4 to 3.6, which were significantly higher
than the predicted 2.0 and the authors concluded that strength should
be scaled to stature3.0, or body mass.
Three longitudinal studies have used multilevel modelling to examine
a number of known covariates to determine their influence on the age
and sex associated changes in muscle strength (De Ste Croix et al.,
2002; Round et al., 1999; Wood et al., 2004). Most authors currently support the view that suitable
scaling factors should be derived from careful modelling of individual
data sets, and therefore be sample specific rather than adopting assumed
scaling indices. |
| AGE
AND SEX ASSOCIATED CHANGES IN STRENGTH: A HISTORICAL PERSPECTIVE |
It
is clear that many factors interact to produce the expression of strength.
Awareness of the anthropometric, neurologic, hormonal, age and sex
-associated changes in skeletal muscle strength is important from
childhood to adulthood. While there is abundant literature focussing
on determinants of strength development, few studies have used common
age ranges, muscle groups, testing protocols and muscle actions, making
comparisons difficult.
Most early strength development studies examined isometric forces
generated from handgrip data. It has been suggested that strength
measured as isometric or dynamic force reflects the same relative
strength between individuals regardless of the type of test method
(Froberg and Lammert, 1996). However, dynamic actions are far more reflective of
dynamic muscle properties, themselves a function of neuromuscular
factors and fibre type composition, more so than isometric actions.
Isokinetic assessment has primarily been recommended for strength
testing as maximal force is applied during all phases of the movement
at a constant velocity (Stocker et al., 1996). The isokinetic mode is also safe to use with children
because there is minimal risk of muscle and joint injuries, which
can result from efforts to control the load if using free weights
in 1-Repetition Max testing (Osternig, 1986). Isokinetic assessment also permits quantification of
a variety of muscle function indices such as peak and average torque,
joint angle of peak torque, work and power. Different velocities of
movements can be tested, allowing evaluation of the force-velocity
characteristics of various muscle groups. It is also possible to examine
bilateral dominance and the hamstring/quadriceps ratio has been investigated
in both adult and paediatric populations (Faro et al., 1997). The majority of previous paediatric studies have examined
concentric isokinetic knee extension and flexion torque (Alexander
and Molnar, 1973; Gilliam et al., 1979; Housh et al., 1984; 1996;
Tabin et al., 1985; Pfeiffer and Francis, 1986; Rochcongar et al., 1988) with fewer studies investigating elbow extension and
flexion (Alexander and Molnar, 1973; Deighan et al., 2002a; Wood et al., 2004, 2007)
and single studies examining hip torque (Burnett et al., 1990), plantar flexor torque (Falkel, 1978), shoulder torque (Alexander and Molnar, 1973) and trunk torque (Balageu et al., 1993). Many paediatric studies have examined elite athletes
but few studies have included female subjects or examined clinical
populations (McCubbin and Shasby, 1985; MacPhail and Kramer, 1995). |
| AGE
AND SEX RELATED CHANGES |
Most
of our early understanding of the age and sex associated development
in strength was restricted to physical performance tests. Field tests
tend to lack measurement sensitivity and therefore we are often left
with a high percentage of zero scores. As strength testing is dependent
upon motivation field tests may not be sensitive enough to detect
the more generalised gains in strength. A good example of this is
the data presented from the National Child and Youth Fitness Study
(1985) in which 60 % of girls aged 10-18 y failed to do one
pull up. As field tests require the resistance or movement of the
individual body mass it follows that children with a larger mass will
be disadvantaged. Data that have used pull-ups as the criteria for
determining sex differences in muscle strength has clouded our understanding
of strength development during growth and maturation. It is hardly
surprising therefore those correlations between strength measurements
using field tests and dynamometers are often non-significant in paediatric
populations.
It is also important to bear in mind that our understanding of the
development of strength with age will be influenced by the nuances
of the testing procedures used, such as subject positioning, degree
of practice, level of motivation, lateral dominance and level of understanding
about the purpose and nature of the test.
When examining data relating to changes in strength due to growth
and maturation it is essential to remember that the majority of data
have been derived from isometric testing. Children may not produce
maximal force during isometric actions, and this has been attributed
to inhibitory mechanisms that preclude children from giving a maximal
effort due to a feeling of discomfort caused by the rapid development
of force during isometric actions. Therefore the whole motor pool
may not be activated due to a reduction in the neural drive under
high tension loading conditions.
In his comprehensive review Blimkie, 1989 notes that while there are a number of studies examining
strength development, few studies show commonality in assessed age
ranges, muscle groups tested, methodology used, muscle action studied
and physiological condition under which muscles were tested and this
view still holds today. The abundant literature on strength during
childhood has been derived from cross-sectional studies and there
are few longitudinal studies available spanning childhood and puberty.
Data from isometric actions indicate that both boys and girls strength
increase in a fairly linear fashion from early childhood up until
the onset of puberty in boys (around 13 y) and until about the end
of the pubertal period in girls (around 15 y). The marked difference
between boys and girls is caused by a strength spurt in boys throughout
the pubertal period, which is not evident in girls. Girls' strength
appears to increase during puberty at a similar rate to that seen
during the prepubertal phase and then appears to plateau post puberty.
There is some disagreement about the age at which sex differences
become evident. However, although conflicting evidence is available
it is generally conceded that before the male adolescent growth spurt
there are considerable overlaps in strength values between boys and
girls. By the age of 16/17 y very few girls out perform boys in strength
tests, with boys demonstrating 54 % more strength on average than
girls.
Throughout childhood and puberty, particularly in males, isometric
elbow flexor and knee extensor strength are highly correlated with
chronological age. Although there are some data on the age related
changes in the knee extensors and flexors for children the trends
affecting these muscle groups are limited. In line with isometric
data most cross-sectional studies of changes in dynamic strength have
demonstrated a significant increase with age. For example, increases
in males and females' absolute knee extensor (314 % and 143 %) and
flexor (285 % and 131 %) strength have been noted from the ages of
9-21 y (De Ste Croix et al., 1999).
Some studies have suggested that age exerts an independent effect
on strength development over and above maturation and stature (Maffulli
et al., 1994).
Others have recently indicated that once muscle CSA is accounted for
using a multilevel modelling procedure that age explained a significant
amount of the additional variance in isometric elbow extensor peak
torque (Wood et al., 2004). It was suggested that this positive age term may be
explained by the shared variance with maturation as maturation was
not included in the model. However, another longitudinal data set,
using multi-level modelling, has suggested that age is a non-significant
explanatory variable on isokinetic knee torque once stature and mass
are accounted for (De Ste Croix et al., 2001). This is probably attributable to differing rates of
anatomical growth and maturation, which vary independently, and thus
their effects on strength do not correlate simply with chronological
age. It would appear that although there is a strong correlation between
strength and age a large portion of this association is probably attributable
to the shared factors of biologic and morphological growth rather
than age itself.
Some authors have suggested that sex differences in muscle strength
are evident from as early as 3 years of age. As previously mentioned
there is little consensus about when sex differences in muscle strength
become apparent. Some studies have shown clear sex differences by
13-14 years of age. A recent longitudinal study using multilevel modelling
to control for known covariates suggested that there are no sex differences
in dynamic strength up until the age of 14 y. After 14 years of age
boys out perform girls in muscle strength irrespective of the muscle
action examined or with body size accounted for (De Ste Croix et al.,
2002).
Isometric data suggests that sex differences in strength are relatively
greater in muscles of the upper compared to the lower body in children.
Gilliam et al., 1979 reported no significant sex difference in 15-17 y old
knee extension peak torque but sex differences were apparent for the
elbow extensors. These data are supported by a more recent study of
9-18 y old volleyball players, which reported no significant difference
in isometric and isokinetic knee extension strength but a significant
difference in elbow flexor strength in postpubertal children (Schneider
et al., 2004). This has been attributed to the weight-bearing role
of the leg muscle. It has also been suggested that during growth and
maturation boys use their upper body more than girls through habitual
physical activities (such as climbing). This socio-cultural explanation
has recently been brought into doubt as there is no overlap in strength
between physically active girls and sedentary boys as would be expected
if this premise were true (Round et al., 1999).
For developmental physiologists understanding the complex interaction
of factors during growth and maturation that may contribute to the
age and sex associated change in strength development is challenging.
Historically, simple anthropometric characteristics (such as stature
and body mass) have been explored as possible explanatory variables
for the age and sex associated changes. As technologies have become
more advanced we have the possibility to explore muscle size and moment
arm using magnetic resonance imaging, muscle angle of pennation and
physiological cross sectional area using ultrasonography, motor unit
recruitment using electromyography, and hormonal analysis using biochemistry.
Our ability to concurrently examine possible explanatory variables,
using sophisticated techniques, may have changed our understanding
of the contributory factors of strength development during childhood
and adolescence. There are few longitudinal studies that have examined
these variables concurrently using appropriate scaling methods. The
following sections focus on the role played by the factors associated
with the development of muscle strength. |
| INFLUENCE
OF STATURE AND MASS |
The
influence of gross body size on strength development has been examined
in several studies. Stature and mass are traditionally the size variables
of choice because they can be quickly and easily measured. Early longitudinal
studies demonstrated that isometric strength per body mass varied
only slightly during childhood and through puberty in girls. In contrast,
around the time of boys' peak height velocity (PHV), i.e. age 14 y,
there was an increase in strength per body mass in boys, which was
still increasing by age 18 y.
Body mass has been found to be highly correlated with maximal voluntary
isometric strength of elbow flexors and knee extensors in males aged
9 to 18 y (Blimkie, 1989). However, age-specific correlation coefficients between
strength and body mass for males are generally low to moderate during
the mid-childhood years, tend to increase then peak during puberty
and abate in the late teens. Data on this relationship are scarce
for females but moderate positive coefficients between strength and
body mass for females during the prepubertal years and at the onset
of puberty and low correlations at the end of puberty and during puberty
have been reported (Blimkie and Macauley, 2001; De Ste Croix, 1999).
Others have found the relationship between female strength and body
mass to be high during teen years and to decline during young adulthood
(Deighan et al., 2002a, Hildebrand et al., 1994). When related to shorter periods of growth (in which
the range of the anthropometric variable in question is small), correlations
become weaker. This reliance of the correlation coefficient on the
characteristics of the sample means that comparison of correlation
coefficients between studies is made difficult. It is worth noting
here that when isokinetic knee extension and flexion torque was adjusted
for body mass using the ratio standard the rate of change in strength
between 9-21 y of age was underestimated compared to mass-adjusted
data using allometric techniques (De Ste Croix et al., 1999).
It is well recognised that peak strength velocity occurs about a year
after PHV (11.4-12.2 y in girls and 13.4-14.4y in boys). It has been
suggested that the difference in attainment of PHV and subsequent
peak strength gains account for the lack of a significant sex difference
in strength at 14 y. Girls will be in the phase of peak strength gains
at 14y whereas boys will not have experienced the strength spurt.
Three recent longitudinal studies, examining isometric and isokinetic
strength respectively, have used multilevel modelling to examine the
factors related to strength development. Round et al., 1999 reported that isometric knee extensor strength in girls
increased in proportion to the increase in stature and mass in 8-13
y old. De Ste Croix et al. (2001)
also demonstrated that stature and mass are significant explanatory
variables of isokinetic knee extension and flexion torque in 10-14
y old. This is further reinforced by Wood et al., 2004 who demonstrated a significant influence exerted by stature
on the development of isometric and isokinetic elbow flexion and extension
on 13-15 y old. Conflicting data are available and the study of Round
et al., 1999 suggested that in boys the strength of the knee extensors
was disproportionate to the increase in body size. This difference
was explained once testosterone was added to the multilevel model.
Although simple body dimensions appear to be important in the development
of strength with age only 40-70 % of the variance in strength scores
of 5 to 17 y-old children could be accounted for by age, sex, stature
and body mass leaving a large portion of the variance unexplained. |
| MATURATION
AND HORMONAL INFLUENCES ON STRENGTH DEVELOPMENT |
Early
studies indicated that maturation, determined using Tanner's (1962) indices of pubic hair development, is a better predictor
of 1-Rep max isotonic maximal knee extension and flexion than simple
chronological age. A recent longitudinal study of 10-14 y-old indicated
that maturation was a non-significant explanatory variable in the
development of isokinetic knee extension and flexion, once stature
and mass were accounted for, using multilevel modelling procedures
(De Ste Croix et al., 2002). However, the authors do acknowledge that their sample
consisted of a narrow range of maturational stages. Supporting data
are available with previous studies indicating that maturation does
not exert an independent effect upon isometric strength development
in 10-18 y old athletes (Maffulli et al., 1994) and 12-14 y old football players (Hansen et al., 1997).
An important consideration regarding the development of muscle function
is the effect of endocrine adaptations typical of sexual maturation
such as increased levels of testosterone ([T]) and growth hormone
(GH). There is both direct and indirect evidence to demonstrate the
association between [T] and strength development during puberty.
[T] levels accelerate from a modest 4 fold increase during the early
stages of puberty to a rapid 20 fold increase in mid-late puberty
in boys (around Tanner stage 3). It is not surprising that [T] levels
appear to coincide with the divergence of strength between boys and
girls as circulating [T] begins to rise one year before PHV, increasing
steadily and reaching adult levels about 3 years after PHV. Testosterone
has been shown to stimulate anabolic processes in skeletal muscle
and appears to be the principal hormone responsible for the development
of strength. As previously stated, Round et al. (1999) suggested that [T] accounts for the sex difference that
exists in isometric strength even after making allowances for body
size. Full analysis showed that there was an increase of 0.7 % in
isometric knee extension strength for every nmol.L-1 of
circulating [T]. The analysis showed that the young men in the sample
were 15 to 20 % stronger as a result of the [T] than might be expected
from their overall body stature. In contrast, the same analysis for
biceps showed that sex differences could not be fully accounted for
by the effects of [T] in teenage boys. These authors speculated that
the linear measure inserted into the model for biceps should be humerus
length as opposed to stature. Their plausible suggestion was based
on the well-known increase in the upper limb girdle dimensions in
boys during puberty that provides an additional stimulus for muscle
growth with the direct action of [T] in the muscle. Jones and Round,
2000 indicated that increasing levels of oestrogen in the girls
causes inhibition of muscle growth as a result of a speedier skeletal
maturation, which removes the lengthening stimulus for muscle growth.
Ramos et al. (1998) also reported that body mass did not eliminate the age
effect in isokinetic peak torque in boys and that [T] increased with
age in boys but not in girls. This increase in [T] preceded the gains
in muscle strength but more importantly there was a moderate positive
correlation (r = 0.64) between serum [T] and isokinetic angle specific
torque. |
| MEASURING
MUSCLE SIZE - MAGNETIC RESONANCE IMAGING |
It
has always been assumed that the size of the muscle, in particular
physiological muscle cross-sectional area (pCSA) is the most important
parameter in the development of force in adults. The role that muscle
CSA (mCSA) plays in the production of force in the growing child has
also been extensively examined, based on the relationship between
force production and strength in adults. The difficulty with paediatric
subjects is the influence of other explanatory variables that relate
to growth and maturation and subsequently the production of force
in the growing child may not simply be prescribed to the size of the
muscle.
When examining studies that have explored the role that the mCSA has
on strength development during growth and maturation the technique
used to determine muscle size must be examined. When measuring mCSA
in children for research purposes the technique used should be non-invasive
with no potential side effects. Many studies with children have used
anthropometric techniques to estimate mCSA because they are low cost,
equipment is easily accessible and often easily portable, the measurement
protocols take little time and few personnel, which is important if
the number of subjects is large. Every effort should be made to ensure
accuracy by standardising the technique with measurements always made
by the same trained observers. This is particularly important if measurements
are to be taken over time, in order to safeguard the validity and
usefulness of the data.
At the simplest level, coaches have been known to take circumference
measurements alone to estimate mCSA but circumference measurements
ignore the obvious fact that limb circumference is influenced by fat
and bone cross-sections as well as muscle, such that a larger circumference
need not mean a larger muscle. Efforts have been made to take into
account the contribution of fatness to the circumference measurement
by incorporating skinfold thickness into the equation (Jones and Pearson,
1969).
The techniques described by Jones and Pearson, 1969 are the most widely used anthropometric technique for
estimating thigh muscle volume plus bone in children although more
recent equations by Housh et al., 1995 for determining total mCSA plus bone have also become
popular. The main problem with both of these techniques is that the
regression equations have been derived from adult data and therefore
cannot be confidently applied to children. Work exploring the reliability
of the Jones and Pearson technique in children, comparing the anthropometric
technique to muscle volumes determined using Magnetic Resonance Imaging
(MRI), found that the anthropometric technique underestimates lean
thigh volume by 31 % (range 14-46 %). Limits of agreement further
support this conclusion by identifying a consistent bias towards an
underestimation in total thigh volume. Therefore while anthropometric
estimates may be valid for a 'snapshot' of mCSA plus bone or for characterising
various populations, they are not acceptable for monitoring changes
over time (Housh et al., 1995). Anthropometric techniques may be a useful way of
approximating mCSA plus bone on a one-time basis but not used in paediatric
studies examining changes during growth and maturation.
Radiography is a technique, which can potentially provide estimates
of mCSA, but due to the radiation exposure required to produce well-defined
radiographs, ethical considerations mean this technique is unsuitable
for use with children. In any case, conventional radiographs depict
a three-dimensional object as a two dimensional image so that overlying
and underlying tissues are superimposed on the image which makes determination
of mCSA difficult.
Computerised Tomography (CT) overcomes this problem by scanning thin
slices of the body with a narrow x-ray beam, which rotates around
the body, producing an image of each slice as a cross-section of the
body and showing each of the tissues in a thin slice. Unlike conventional
radiography, CT can distinguish well between muscle, bone and fat.
Children are particularly sensitive to radiation therefore this technique
is contraindicated in children.
Ikai and Fukunaga, 1968 made the first measurement of strength per mCSA using
ultrasonography with children. The technique has also been applied
in studies concerned with fibre pennation of the quadriceps muscle
after training, mCSA of the calf of the dominant leg of junior soccer
players and the effect of strength training on upper arm mCSA of children.
One of the major issues of ultrasonography is the difficulty in distinguishing
tissue boundaries and the difficulty in determining individual muscles/muscle
groups.
MRI is a technique that in recent years has offered exciting opportunities
for the study of gross structure and metabolism of healthy and diseased
muscle. With MRI ethical constraints are avoided unlike CT and radiography.
MRI can accurately measure anatomical mCSA, distinct muscle groups
can be differentiated and it appears to be more suitable than other
imaging techniques used for the examination of mCSA. With unparalleled
picture clarity it is possible to differentiate individual muscle/muscle
groups and identify both intramuscular fat and blood vessels. Despite
the financial limitation numerous studies have recently used MRI with
paediatric populations to determine muscle volume and mCSA (Deighan
et al., 2006; De Ste Croix et al., 2002; Wood et al., 2004, 2007). |
| SITE
OF MCSA MEASUREMENT |
| A
methodological problem with many previous studies of force or torque
per mCSA with growth and maturation is that the optimal site for the
measurement of maximum mCSA within and between subjects has not been
clearly identified. Instead, an arbitrary location on the limb has
been used for mCSA determination of mid femur in the case of thigh
muscles and mid humerus in the elbow flexors and extensors. Adult
data suggests that for the knee extensors 2/3 upper femur height and
1/3 lower femur height for the knee flexors should be used as the
site of maximal mCSA. De Ste Croix et al., 2002 measured the maximal mCSA of each individual subject
using MRI and found maximal thigh mCSA to occur between 51 and 69
% of ascending femur length in 10-14 y old. A recent cross-sectional
study, using MRI determined thigh and arm mCSA in 9 y old, 16 yold
and adults, demonstrated that there are age differences in the location
of maximal mCSA of the elbow extensors (Deighan et al., 2006). In order for age, sex and muscle group comparisons to
be made, an optimal site of mCSA needs to be individually determined
for the paediatric population. Therefore the site chosen to determine
mCSA should be taken into account when interpreting the age and sex
associated development in mCSA. |
| AGE
AND SEX ASSOCIATED DEVELOPMENT IN MCSA |
CSA
of muscle fibres reach their maximal adult size by 10 y in girls and
14 y in boys. Although muscle fibres appear to reach their maximal
CSA early in childhood this does not mean that muscle has reached
its maximal length as muscle will continue to grow in length simultaneously
with growth in limb length segments.
The Harpenden Growth study examined age and sex differences in radiographically
determined upper arm and calf widths of British children from infancy
to age 18 y. Boys' muscle widths appeared greater than those of girls
during childhood but the difference was small. MRI studies have also
found no significant sex difference in knee and elbow mCSA up until
13 / 14 y. A large cross-sectional study using MRI demonstrated a
significant age effect in elbow mCSA up until 24 y (Deighan et al.,
2002a). These data indicate that from 9 - 24 y elbow extensor
and flexor mCSA increases 207 and 210 % in males and 65 and 78 % in
females respectively. By adulthood CT determined muscle size found
that the mCSA of the arm and thigh of adult females is around 57 %
and 72.5 % than that of adult males.
According to Blimkie, 1989 "it is likely that quantitative differences in muscle
width account for a large proportion of the observed age and sex differences
in strength development during childhood and adolescence" (p127).
It is important at this point to reconsider that there have been variations
in the methods used to measure both strength and muscle size. The
relationship between muscle size and strength during growth has been
examined by measuring muscle widths, muscle volume and mCSA. It is
also important to note that most studies have reported anatomical
mCSA due to the difficulties in determining physiological mCSA.
There are considerable data that support the contention that differences
in muscle size account for differences in muscle strength during growth.
One of the earliest studies examined the relationship between isometric
elbow flexion strength and mCSA determined by ultrasonography in 12
to 29 y-old (Ikai and Fukunaga, 1968). Although correlation coefficients were not given, the
authors indicated that strength 'is fairly proportional' to elbow
flexor mCSA regardless of age or training status. The relationship
appeared weaker for girls than boys. Others (Deighan et al., 2002a, 2002b; Round et al., 1999) have reported a strong positive correlation between muscle
size, and isometric knee strength (r = 0.87), isokinetic knee strength
(r = 0.73), isokinetic elbow strength (r = 0.82) and isokinetic triceps
surae strength (r = 0.91). In addition, based on grip strength data
on children, the sex related growth curve patterns for body muscle
are virtually identical to those for strength suggesting a strong
association between muscle growth and gains in strength. Numerous
longitudinal studies (Wood et al., 2004;
De Ste Croix et al., 2002) have shown that mCSA is a significant explanatory variable
in the age associated development in strength when examined as an
independent covariate. However, it would appear that when additional
variables are examined concurrently alongside mCSA its influence is
reduced or disappears. |
| AGE
DIFFERENCES IN STRENGTH PER MCSA |
| There
is still some debate about whether strength per mCSA increases with
age. Early studies demonstrated increasing strength per mCSA from
age 7 to 13 y. Also, Kanehisa et al. (1995) suggested that isokinetic strength per mCSA, measured
using ultrasonography, was greater in older age groups (18 y) than
younger age groups (7 y) in every muscle group measured. It was hypothesised
that children in the early stages of puberty may not develop strength
in proportion to their muscle anatomical CSA. It is likely that the
deficiency in strength per mCSA in the younger age groups might be
the result of a lack of ability to mobilise the muscle voluntarily.
The same group of authors found that the isometric strength of the
ankle dorsi flexors and plantar flexors per mCSA measured by ultrasonography
in boys and girls aged 7 to 18 y was significantly greater only for
plantar flexion in 16-18 y old boys compared to the other groups.
In a comprehensive cross-sectional study others have reported a significant
increase in isokinetic knee and elbow torque per MRI determined mCSA
from 9-16 y but no significant difference from 16-24 y (Deighan et
al., 2002a, 2002b). Further investigation is required to establish whether
these differences in torque per mCSA are due to biomechanical or neuromuscular
factors. What these data do suggest is that torque per mCSA of the
elbow and knee extensors and flexors are at adult levels by 16 y of
age. Conflicting data are available and Tolfrey et al., 2003 recently reported that despite smaller MRI determined
triceps surae mCSA early pubertal boys torque scaled to muscle size
is not different from adult males. These conflicting data emphasise
the need to measure the strength per mCSA ratio in a variety of muscles
as the strength development characteristics of one muscle or group
of muscles may not be the same as another, even within the same joint. |
| SEX
DIFFERENCES IN STRENGTH PER MCSA |
Debate
surrounds whether sex differences exist in strength per mCSA. Early
work reported that absolute isometric strength differences between
sexes disappeared when data were normalised to anthropometric muscle
(plus bone) CSA in 9-12 y old. Sunnegardh et al., 1988 showed that boys had significantly greater torque per
CSA than girls at 13 y. Deighan et al., 2002a recently reported significant sex differences in isokinetic
elbow flexion per mCSA in 9/10 y old and 16/17 y old. These studies
are in contrast to others that have demonstrated similar strength
to mCSA ratios between sexes (Deighan et al., 2002b; Ikai and Fukunaga, 1968; Wood et al., 2004). Deighan et al., 2002b reported no significant sex differences in isokinetic
torque per mCSA of the knee extensors and flexors and elbow flexors
in 9/10 y-olds, 16/17 y-olds and adults. Using multilevel modelling
procedures on longitudinal data Wood et al., 2004 also reported that gender effects for isokinetic elbow
extensors and flexors became non-significant once mCSA was controlled
for. The majority of recent studies would lead us therefore to the
conclusion that there is no significant sex difference in strength
per mCSA irrespective of the muscle joint or action examined. It would
appear that factors in addition to mCSA may account for the age and
sex associated development in strength.
For example, the peak gain in muscle strength in boys occurs more
often after peak stature and mCSA velocity but there is no such trend
for girls. Therefore, particularly in boys there may be factors other
than mCSA that affect strength expression during puberty. Also, it
has been shown that the sex differences that occur in strength of
boys and girls of the same stature cannot be accounted for by muscle
size alone. A longitudinal study of upper arm area and elbow flexor
strength have shown that boys have muscles ~5 % greater in area but
which produce ~12 % more strength. Others have indicated that mCSA
is a non-significant explanatory variable once stature and mass are
accounted for (De Ste Croix et al., 2001).
Peak muscle mass velocity has also been shown to occur at an average
of 14.3 y, whereas peak strength velocity appeared at age 14.7 y.
This supports the view that muscle tissue increases first in mass,
then in functional strength. Consequently, this would seem to suggest
a qualitative change in muscle tissue as puberty progresses and perhaps
a neuromuscular maturation affecting the volitional demonstration
of strength. |
| BIOMECHANICAL
FACTORS - THE MUSCLE MOMENT ARM |
The
mechanical advantage of the musculoskeletal system is variable across
different muscle groups and is considered unfavourable because the
measured force or torque is somewhat smaller than the corresponding
tension developed in the muscle tendon. Another unfavourable biomechanical
influence on the measured force lies in the internal muscle architecture,
i.e. the greater the angle of pennation to the long axis of the muscle,
the smaller proportion of force in the muscle fibres that is transmitted
to the muscle tendon. The age-associated relationships between these
factors have not yet been extensively investigated in children.
It is probable that small differences between subjects in the location
of the centre of rotation of the joint or in the length of the lower
limb could contribute to the observed variability in the ratio of
muscle strength to mCSA. It is difficult to account for biomechanical
factors but some authors have divided strength values by the product
of mCSA and stature (Nm·cm-3), i.e. the product of mCSA and possible
differences in moment arm length or mechanical advantage which they
assumed to be proportional to stature. There are few published data
on the relationship between strength per mCSA and mechanical advantage
covering different age groups, both sexes and different muscle groups
but it seems sensible to correct strength for possible differences
in mechanical advantage, especially if comparing children of different
sizes by normalising to mCSA*limb length (LL) (Blimkie and Macauley,
2001). One of the major assumptions with using this method
is that the relationship between the muscle moment arm and limb length
are proportional.
Numerous authors have demonstrated a moderately strong, positive correlations
between stature and isometric torque per mCSA for the elbow flexors
(r = 0.67) and knee extensors (r = 0.57); isokinetic knee extensors
(r = 0.85) and flexors (r = 0.84); and isokinetic elbow extensors
(r = 0.79) and flexors (r = 0.80) (Deighan et al., 2002a). Kanehisa et al., 1994 found that isokinetic torque was significantly correlated
to mCSA*thigh length (r = 0.72 to 0.83). These data suggest that at
least part of the age associated variability in voluntary strength
may be attributed to differences in mechanical advantage that occur
with growth.
Blimkie, 1989 reported that age effects were the same whether dividing
torque by the product of mCSA and stature or just mCSA. Young adults
have been found to have significantly higher ratios of isokinetic
knee extension torque per unit of mCSA*thigh length than children
with the difference becoming greater with increasing velocity of movement.
Deighan et al., 2002a suggests that the influence of mechanical advantage on
the development of isokinetic strength may be muscle group specific.
Data showed a non-significant age effect for the elbow extensors and
flexors but a significant difference between 9/10 y-olds and 16/17y-olds
in knee extension and flexion torque per mCSA*LL. The knee data suggest
that mCSA*LL alone cannot account for the age differences in strength.
It is difficult to attribute physiological reasons to the muscle group
differences but it is possible that part of the explanation may lie
in the differing function of the arms and legs. For example, there
is some evidence to suggest that the extent of motor unit activation
of the arm muscles remains essentially unchanged with growth but increases
in the muscles of the thigh.
Early work indicated that sex differences in absolute torque remain
statistically significant, although diminished, when expressed per
unit mCSA*thigh length. Kanehisa et al., 1994 reported no significant sex differences in young children
but that sex differences become apparent in adulthood when expressing
torque per mCSA*LL. A recent study by Deighan et al., 2002a reported non-significant sex differences for the knee
and elbow extensors and flexors in torque per mCSA*LL in 9/10 y old,
16/17 y old and adults. Recent data therefore shows that sex differences,
at least for dynamic strength, can be accounted for by the product
of mCSA and limb length. Only 1 study appears to have longitudinally
examined leverage in children from isometric actions and using MRI
to determine mCSA and muscle moment arm using a geometric model (Wood
et al., 2007). Wood et al., 2007 reported no significant age or gender effect on muscle
moment arm length in the elbow from 13-15 y. However, using multi-level
modelling mechanical advantage significantly contributed to the explanation
of torque variance at 10° of flexion. The authors note, however, that
both muscle length and architecture were not determined in this study
and further investigation of physiological mCSA is needed to enhance
our understanding of strength development in children.
There has been speculation that the angle of muscle pennation plays
a role in the group differences in strength per mCSA (Blimkie, 1989). Conventional scanning techniques all measure mCSA at
right angles to the limb, i.e. anatomical mCSA. However, the maximum
force a whole muscle or muscle group can produce is a function of
the tension generated by each individual fibre in the direction of
the muscle's line of pull. Most muscle groups that are tested in humans
are pennate (with the exception of the biceps brachii). By design,
the total capacity for tension development is enhanced in pennate
muscle by having more sarcomeres arranged in parallel and fewer in
series within a given volume of muscle. In other words fibres are
not orientated in true parallel to the long axis of the muscle. This
can affect measurements of in vivo strength per anatomical mCSA in
two opposing ways. Firstly, the shortening force transferred to the
tendon at the muscle's insertion is less than that generated along
the axis of the muscle fibres. Secondly, individual fibres do not
span the whole length of the muscle, therefore anatomical mCSA will
not include all fibres contributing to the force. Therefore, physiological
mCSA is thought to be a better predictor of force producing capacity
than anatomical mCSA. However, true physiological mCSA cannot easily
be determined in vivo and to date there are no paediatric studies
that have examined physiological mCSA. |
| NEUROMUSCULAR
FACTORS AND STRENGTH DEVELOPMENT |
Investigation
into the 'quality' of children's muscle is sparse due to the methodological
issues of determining neuromuscular function. Measured voluntary strength
depends highly on the degree of percentage motor unit activation (%MUA).
Both the level of voluntary neural drive or motor unit recruitment
and the level of activation or frequency of stimulation govern %MUA.
The ideal way to measure the contractile capacity of a muscle is to
record the force developed during supramaximal electrical stimulation
of the nerve innervating the muscle. When an electrical stimulus is
applied to a motor nerve near the muscle, the resultant muscle force
is free of any inhibitory influence from above the point of stimulation.
On the other hand, force or torque measured during a voluntary action
is the result of neuromuscular influences from the brain and inhibitory
reflex influences from the spinal cord in addition to the maximum
force producing capacity of the muscle. The results of tetanic electrical
stimulation may not be comparable to voluntary muscle actions, since
in the former method synergistic muscles may not be excited and the
procedure is very painful leading to methodological, compliance and
ethical issues in children.
Due to these problems with tetanic stimuli of children's muscles,
most studies that have investigated maximum force producing capacity
in children have used twitch stimuli because various properties of
an electrically evoked twitch reveal information about intrinsic muscle
properties and %MUA. Assuming that %MUA stays constant with age, then
the ratio of evoked twitch force to voluntary force should stay constant
with age. Based on this assumption, Davies, 1985 measured both evoked twitch force and maximum voluntary
force in groups of 9, 11, 14 and 21 y old males and females. The twitch
torque/voluntary torque ratio of the triceps surae was similar in
boys and girls aged 9 y but it gradually decreased with age in the
males. However, no conclusion of a greater %MUA with increasing age
in boys could be made because there was also a change in the twitch
to evoked tetanus ratio with increasing age. On examination of the
tetanic/voluntary ratio it appeared that %MUA may vary with age but
not sex. The possibility that an inability to fully recruit the available
motor unit pool may be reflected in smaller strength per mCSA scores
in children than in adults has not been extensively investigated.
The interpolated twitch technique (ITT) has been used to provide an
answer to the painful tetanic stimuli method and to allow %MUA to
be calculated more directly. Blimkie, 1989 used the ITT on maximum voluntary isometric actions of
the elbow extensors and knee flexors. He found that %MUA of the knee
extensors increased with age in boys from 77.7 % at 10 y to 95.3 %
at 16 y, an increase in %MUA of 17.6 %. A different pattern was found
for the elbow flexors whose respective values were 89.4 % and 89.9
%, indicating no change in the %MUA of elbow flexors. No studies have
investigated this phenomenon in females. However, it appears that
boys at least are unable to fully activate the available motor units
during maximum voluntary muscle actions of the knee extensors but
not the elbow flexors. In support of this others (Davies, 1985) have reported that %MUA in prepubertal boys was 78 %
of the intrinsic force producing capacity during maximum voluntary
knee extension. Also, the maximum rate of force production, being
largely dependent on the amount and rate of neural activation has
been found to be lower in children aged 8 to 11 y compared to college-age
men and women.
In adults a sex difference has been demonstrated in the rate of force
development which is an important quality for dynamic muscle actions
in which there is limited time to generate force. Recent data examining
isokinetic time to reach peak torque suggests that there are non-significant
sex differences in the knee and elbow extensor and flexor muscles
(Barber-Westin et al., 2005; De Ste Croix et al., 2004). In the De Ste Croix et al., 2004 study age related changes in time to peak torque were
muscle group and muscle action specific leading the authors to the
conclusion that care must be taken when making assumptions on differing
muscle groups and muscle actions.
Time to peak twitch torque and twitch relaxation indices can be used
as measure of rate of energy turnover and fibre type composition.
Backman and Henriksson, 1988 found that twitch relaxation times were similar in boys
and girls and were not influenced by age. Also, it has been found
that time to peak twitch force and relaxation times were the same
regardless of age during childhood (Davies et al., 1983). Likewise, similar time to peak twitch tension was demonstrated
in 3 y-olds as 25 y old adults. These data suggest that muscle fibre
composition and muscle activation speed is similar between these age
groups and that there is no difference in the fibre type distribution
from the age of around 7 y. Previous authors have suggested that the
neuromuscular system is still maturing with respect to the myelination
of the nerves in younger children. Also muscle fibre conduction velocity
has been seen to increase with age in children. Therefore, the influence
that neuromuscular factors has on the development of muscle strength,
concurrently with other know variables, remains to be established. |
| CONCLUSION |
There
is still a clear need for further longitudinal investigation into
the static and dynamic development of muscle strength through childhood
and adolescence into adulthood. Our major difficulty in describing
the age and sex associated development in strength is that much of
the current data reveal muscle group and muscle action specific differences
in the relationships described. For example, the factors responsible
for the development of isokinetic eccentric elbow flexion may be different
from isometric knee extension. Despite this, the age-associated development
of strength is reasonably consistent, irrespective of the muscle group
or action examined. There is slight disagreement about when sex differences
occur. Importantly, many of the factors discussed in this chapter
play a role in strength development when examined as independent variables.
It would appear that for dynamic muscle actions in particular that
mechanical factors may play a large role in the development of muscle
torque and accurate investigation of the muscle moment arm, employing
MRI techniques, would provide us with a clearer picture of the age
and sex associated development. Our greatest challenge is to elucidate
the factors that contribute to the age and sex associated development
in strength concurrently with other known explanatory variables. |
| FUTURE
DIRECTIONS IN PAEDIATRIC STRENGTH ASSESSMENT |
Despite
the growing number of longitudinal design research papers on the development
of strength, and the use of differing methods to control for differences
in body size, there are still unexplained factors that may contribute
to the age and sex associated development in strength. We know relatively
little about muscle fibre types in children, probably due to the invasive
nature of muscle biopsies and the associated ethics that preclude
the use of such techniques with paediatric subjects. There has been
some tentative exploration into the use of MRI to determine fibre
type (Houmard et al., 1995) but this technique has not been validated for use with
children and require further investigation. Researchers are turning
to new technologies to advance our understanding of the mechanisms
that contribute to the development of force. For example, studies
with adults have identified, using ultrasonography, muscle pennation
angle (Maganaris et al., 2001). Others have begun assessing muscle tendon stiffness
using ultrasonography in adult subjects (Kubo et al., 2006) but as yet there appear
to be no longitudinal studies that have examined the age and sex associated
changes in muscle tendon stiffness.
There is no doubt that development in techniques to measure muscle
forces (eg isokinetic dynamometers), muscle size (MRI) and newer techniques
for controlling for differences in body size (allometric scaling and
multilevel modelling) have contributed to our greater understanding
of the age and sex associated development in strength. However, there
is still much we do not know and continuing advances and access to
sophisticated technologies e.g. DEXA, MRI, ultrasonography, may elucidate
new thoughts in this area over the coming decade. |
| KEY
POINTS |
- The
age associated development in strength is attributable to changes
in growth and maturation. Sex differences appear at around 14y
and very few girls out perform boys in strength tests at 18y.
- Stature
and mass appear to be important explanatory variables in the development
of muscle strength. PHV is a particularly important time for maximal
gains in strength during childhood.
- The
muscle moment arm is possibly the most important factor in the
development of muscle strength with age but further longitudinal
studies using MRI are needed.
|
| AUTHOR
BIOGRAPHY |
Mark
De Ste Croix
Employment: Faculty Research Director for Sport, Health
and Social Care and Principal Lecturer in Sport and Exercise
Physiology, at the University of Gloucestershire, UK.
Degree: BA (Ed) (Hons), PhD.
Research interests: Paediatric exercise physiology, muscle
physiology, isokinetic strength, electromyography and fatigue,
injury prevention, talent identification.
E-mail: mdestecroix@glos.ac.uk |
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