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TCurling is a game of skill and tradition and is one of the fastest
growing winter sports (Royal Caledonian Curling Club, 2008).
It is also a game that at the highest level has unique physical demands.
A typical curling game lasting 2.5 hours. At Olympic and World level it
can take up to 14 games to get to the podium, playing usually up to 2
games per day, sometimes with only a short break between. This can result
in up to 35 hours of competitive play, making curling one of the longest
of the Olympic sports. Curling has been a regular Olympic sport since
1998 and sports science is playing an increasing role in assisting in
the preparation of elite curlers. The aim of this article is to outline
how sports science can play a part in curling. It will look at the science
behind the sport and how this can inform coaching and playing strategies.
Curling is a sport in which two teams of four players (usually all male
or all female although some competitions are for mixed teams) deliver
two ~18.6kg granite stones each on an approximate 42m x 4.5m sheet of
ice towards a target or house (Figure
1). The stones are delivered from the hack and the aim is to get one
or more of your teams' stones nearest the centre of house. Curling is
the only sport where the trajectory of the projectile can be influenced
after the stone has been released. Players sweep the ice in front of the
stone to momentarily increase the temperature of the ice as the stone
passes over it and reduce the friction between the stone and the ice (Buckingham
et al., 2006).
Depending on the direction the stone is rotating (the 'handle') and the
side of the stone the player is standing to sweep, this will allow the
stone to stay straighter in its path or to curl more. The dynamics of
curling stones have been the subject of a number of studies (Denny, 1998;
Jensen and Shegelski, 2004;
Marmo and Blackford, 2004;
Penner, 2001;
Shegelski et al., 1996).
It has been shown that the motion of a stone and the amount of curl is
due to the thin liquid film between the stone and the ice. Sweeping the
ice in front of the stone can change this stone-ice interface by two possible
mechanisms in theory: 1) increasing the ice temperature momentarily; 2)
smoothing the ice by removing frost or debris. However, in frost-free
conditions, any reduction in surface roughness ('polishing') will have
a negligible effect compared to the roughness of the stone (Marmo et al.,
2006a).
Therefore, in these conditions raising the temperature of the ice by sweeping
has the greatest effect on the reduction in friction between the stone
and the ice.
Distances
are given from the Hack.
A typical curling game lasts 2.43 hours (73min allowed per team) and consists
of 10 ends. An end consists of each team playing their 8 stones. A team
of four will consist of a lead player who will always play the first 2
stones from that team, a second, a third and a skip who will always play
the last 2 stones from that team. The skip traditionally stands at the
house and controls the team strategy for the game. The two players not
playing a stone will be available to sweep the stone as directed by the
skip. The lead and second players could be asked to sweep 6 stones per
end, for 10 ends or 60 stones per game (the third player acts as vice
skip, standing in for the skip when he/she is playing their stones). A
player could theoretically sweep for up to 1.7km per game. Aggressive
sweeping is high intensity. Hard sweeping for 20s can result in the generation
of approximately 600-1600kJ of work and produce a typical average heart
rate of 170bpm. However this can result in up to 2500kJ of work and a
heart rate of almost 200bpm in some individuals [Bradley, unpublished
data collected using trials of an instrumented curling brush, (Buckingham
et al., 2006)].
This had led to the development of strategies to improve sweeping performance.
Science
of sweeping
The stone is delivered from the hack towards the house, some 38m away.
The ice surface on a curling rink consists of lots of small raised 'pebbles'
caused by water droplets being sprayed onto the ice before play. This
pebbled ice surface allows the stone to travel the full distance from
hack to house with moderate curl. A non-pebbled ice surface alters the
motion of the stone considerably, producing a greater degree of curl with
a reduced delivery distance (Jensen and Shegelski, 2004).
The stone is released with a clockwise or anti-clockwise rotation (for
a right handed curler this is termed in-turn or out-turn respectively)
which will cause it to curl to the right or left respectively. The curl
is produced due to the lower friction under the forward rotating side
of the stone (in the direction of travel) compared to the backward rotating
side. This is due to greater rotational velocity relative to the ice under
the forward rotating side of the stone causing greater heat generation
under that side of the stone. This will momentarily increase the temperature
of the ice under that side of the stone, causing a reduction in friction.
This produces an asymmetric coefficient of friction and results in the
stone curling to the right or left (e.g. a stone rotating anti-clockwise
will curl to the left).
During sweeping, the peak downwards force occurs when the brush head is
closest to the curler's feet (Figure
2). This is due to the horizontal moment arm from the curler's centre
of mass being reduced to a minimum, increasing the vertical force exerted
on the brush head (Marmo and Blackford, 2004).
This will influence the pattern of heat generation in front of the stone.
Depending on the handle of the stone, sweeping on the left or right of
the stone can then enhance or partially correct for the friction asymmetry
and therefore enhance or reduce the curl of a stone. (This asymmetric
generation of heat from sweeping will occur regardless of sweep length.
The full width of the running band of the stone must be swept however
as it is currently illegal to sweep only part of the running band of a
stone, so-called 'corner sweeping'). This is the basis for sweeping strategy
and controlling the stone to manoeuvre around a guard stone or draw into
the house.
To
sweep faster or harder?
Is it better to sweep faster, or to press down into the ice with more
force? Both of these strategies will affect stone-ice friction. The stone
can be delivered with a velocity of ~2 m·s-1 and be sliding
for up to 30s (Buckingham et al., 2006).
The stone will obviously be moving fastest when it is released by the
curler and moving slowest as it crosses the hog line and moves into the
house. Increasing downward pressure of the brush onto the ice will generate
more heat and a consequent reduction in friction between the stone and
the ice. Sweeping faster (greater brush head velocity) will also increase
the heat generated causing a corresponding reduction in stone-ice friction.
Using the model developed by Marmo et al. (2006a;
2006b)
it can be shown that doubling the downwards force will increase the heat
generated at the brush head by a factor of 2 and doubling the sweep velocity
will increase the heat generated by a factor of 1.55. However, sweeping
over the same piece of ice more than once has the greatest impact on heat
transferred to that part of ice and hence greatest reduction in stone-ice
friction (Marmo et al., 2006a).
The objective of sweeping is to raise ice temperature and the maximum
temperature rises occur where successive brush strokes overlap. Generally
speaking, sweeping faster to sweep the same piece of ice several times
has a greater effect on reducing stone-ice friction than applying more
pressure.
However this changes with the speed of the stone. If the stone is moving
at 2m/s, a typical 0.20m brush head used in a conventional sweeping style,
standing just in front of the stone perpendicular to the direction of
travel, will need to sweep at a rate of 10Hz (sweep 10 times per second)
for the brush to cover the same area of ice more than once (Figure
2). As it is difficult to sweep fast whilst maintaining a high level
of downwards force, sweep speed is most important at faster stone velocities
(sweeping also has less of an effect on a faster moving stone: Jensen
and Shegelski, 2004;
Marmo and Blackford, 2004).
As the stone slows down the speed of sweeping required for the brush head
to sweep over the same area of ice more than once decreases (Table
1). When the stone is moving slowest in the house, sweeping is most
effective. Here, greater downwards force will have more influence than
sweep speed as it is easy for the brush head to sweep over the same area
of ice several times at such slow speeds.
Data
collected from elite curlers during trials of an instrumented curling
brush (developed by Buckingham et al., 2006)
can be used to illustrate sweeping technique (Table
2).
Note that from these results the sweeper would struggle to effectively
sweep a stone travelling at a velocity of 1.0 m·s-1 or higher
as the maximum sweep rate is only approximately 4/s.
A typical curling stone is 0.25m in diameter and makes contact with the
ice through a circular running band of approximately 0.15m diameter. A
sweep length of 0.1071m (Table 2)
at first appears not to cover the running band but this does not take
into account the curling brush head dimensions (approximately 0.07m wide
and 0.20m long). Depending on the orientation of the brush head in front
of the stone, the entire running band can be covered. However if during
sweeping the longitudinal axis of the brush head is parallel to the direction
of stone travel (as illustrated in Figure
2) there is greatest chance that part of the brush head will sweep
the same area of ice more than once on faster moving stones (resulting
in much more effective sweeping).
As the effect of sweeping on a stone differs according to stone velocity
and sweeping style, this has implications for coaching and developing
sweep ability. The skip will have most need to sweep a slow moving stone,
being in the house area of the sheet for most of the game. The lead and
second will mostly sweep faster moving stones as they approach the house.
Skips may find the use of downwards force more effective and sweeping
speed not as vital to reducing the stone-ice friction as the other team
members sweeping when the stone is travelling faster. This has implications
for strength and conditioning. Skips and thirds should perhaps be more
inclined towards strength development and curlers who will sweep faster
stones (leads, seconds) perhaps be more inclined to speed development.
Cardiovascular fitness plays a significant role here too. As mentioned
earlier, hard sweeping produces an average heart rate of 170bpm. To be
able to repeat this throughout a game and a tournament as needed requires
considerable cardiovascular fitness. The different strategies and physical
characteristics necessary for effective sweeping of faster and slower
moving stones may also be useful when selecting the ideal team positions,
taking into account individual curlers particular strengths and weaknesses.
Fatigue
The sweep rate and force generated are the principle factors that control
the heat generated during sweeping. Sweep rate has been shown to decline
during a 20-25s period of hard sweeping. However, the sweep rate recovers
quickly and remains relatively consistent in successive bouts separated
by a recovery period (Buckingham et al., 2006).
In an analysis of sweeping action in 17 elite curlers performing three
20s bouts of hard sweeping separated by 1min recovery (using an instrumented
curling brush developed by Buckingham et al., 2006)
we looked in more detail at sweep length and vertical force. Sweep length
was very similar between male and female curlers and remained remarkably
consistent (Figure 3). Vertical force
generated by male curlers however was nearly double that generated by
female curlers (Table 2). This led
to the vertical force in successive 20s bouts of hard sweeping falling
significantly in male curlers but being more consistent in female curlers
(Figure 4). This is despite the work
done remaining relatively constant (Table
2). This is due to work done being calculated from the product of
horizontal force and sweep length, both remaining consistent in successive
bouts of sweeping. Fatigue may be more evident in male curlers during
repeated bouts of hard sweeping but the fatigue profile of both male and
female curlers within one bout of 20-25s of sweeping will still result
in a decline in sweeping rate. This can have considerable impact on the
heat generated from sweeping but could be minimised by the development
of sweeping strategies.
Sweeping
strategies
Sweeping is the most physical aspect of curling. As mentioned above, depending
on the rotation of the stone the side the curler stands when sweeping
the stone can have very different effects on the stone trajectory. The
preferred sweeping side may even change during the course of a throw if
a stone needs to stay straight to get past a guard stone then curl get
into a scoring position for example. The geometry of a stone is such that
the running band of the stone (the area in contact with the ice) is 0.05m
from the outside of the stone (Marmo et al., 2006a).
Sweeping closest to the stone will have the greatest impact
on the stone-ice friction, allowing the shortest time for the ice to re-cool
before the stone travels over the swept area. The ice temperature re-cools
very quickly following sweeping. Sweeping 0.05m in front of a stone travelling
at 0.5 m·s-1 means 0.2s will elapse before the running band
passes over the swept ice. In this time the ice temperature that had been
increased due to sweeping can fall from approximately -3.7ºC to -4.7 ºC
(on well prepared ice with a temperature of approximately -5.0 ºC; Marmo
et al., 2006a).
Sweeping further than 0.05m in front of the stone allows a longer period
to elapse before the stone reaches the swept ice, further reducing any
reduction in stone-ice friction due to sweeping. This can be useful in
situations when a skip does not want a stone to be swept (for example
if it is moving too fast). Sweeping 1m or more in front of a stone will
clear any debris (to avoid the stone picking anything up that may alter
its intended trajectory) but the ice will have re-cooled completely when
the stone reaches the swept area.
Sweeping in pairs as a team can be highly effective here. One role of
the sweeper may be to clear frost and debris from the path of the stone.
With two curlers sweeping in tandem, the sweeper next to the stone will
have the greatest impact on stone-ice friction and be sweeping vigorously.
The second curler can be sweeping in front of the first curler, clearing
frost and other debris from the path of the stone, requiring sweeping
of much less intensity. As discussed earlier, the fatigue profile within
a period of 20-25s of sweeping and in repeated bouts of hard sweeping
can show a considerable decline. To overcome this, highly practised pairs
of sweepers can change who is sweeping with high intensity next to the
stone and who is clearing debris mid-way through a delivery, to sustain
vertical force and sweeping rate and maintain a greater effect on reducing
stone-ice friction. If the two curlers are on opposite sides of the stone
(as is the convention), changing who is sweeping next to the stone mid-way
through the stone trajectory will impart some ability to 'steer' the stone
on the ice. Changing sweeping sides will allow the stone to stay straighter
or curl more depending on the stone rotation.
The high intensity nature of effective sweeping and the long duration
nature of curling games and tournaments place considerable emphasis on
musculoskeletal conditioning, particularly of the upper body and trunk
muscles. Knee, back and shoulder injuries are the most common injuries
reported from curlers (Reeser and Berg, 2004).
Sweeping the stone carries the greatest risk of provoking an injury, followed
by the action of stone delivery (Reeser and Berg, 2004).
Focussing on these areas in strength and conditioning will produce more
injury-resistant athletes. The ability to sweep on either side of a stone
is also a considerable advantage to tactics and strategy of a game and
better conditioned curlers will be more able to achieve this skill more
effectively.
Recently conditioning programmes for curling have been developed, containing
resistance training, cardiovascular training, balance training, core training
and flexibility training (Behm, 2007).
Adding to this training programme the specific demands of sweeping outlined
above and the injury profiles of curlers (Reeser and Berg, 2004)
adds support to the development of a specific conditioning programme to
support the unique demands of curling.
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