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is known that impact forces increase with running velocity as well
as when stride length increases. Since stride length naturally changes
with changes in submaximal running velocity, it was not clear which
factor, running velocity or stride length, played a critical role
in determining impact characteristics. The aim of the study was to
investigate whether or not stride length influences the relationship
between running velocity and impact characteristics. Eight volunteers
(mass=72.4 ± 8.9 kg; height = 1.7 ± 0.1 m; age = 25 ± 3.4 years) completed
two running conditions: preferred stride length (PSL) and stride length
constrained at 2.5 m (SL2.5). During each condition, participants
ran at a variety of speeds with the intent that the range of speeds
would be similar between conditions. During PSL, participants were
given no instructions regarding stride length. During SL2.5, participants
were required to strike targets placed on the floor that resulted
in a stride length of 2.5 m. Ground reaction forces were recorded
(1080 Hz) as well as leg and head accelerations (uni-axial accelerometers).
Impact force and impact attenuation (calculated as the ratio of head
and leg impact accelerations) were recorded for each running trial.
Scatter plots were generated plotting each parameter against running
velocity. Lines of best fit were calculated with the slopes recorded
for analysis. The slopes were compared between conditions using paired
t-tests. Data from two subjects were dropped from analysis since the
velocity ranges were not similar between conditions resulting in the
analysis of six subjects. The slope of impact force vs. velocity relationship
was different between conditions (PSL: 0.178 ± 0.16 BW/m·s-1;
SL2.5: -0.003 ± 0.14 BW/m·s-1; p < 0.05). The slope
of the impact attenuation vs. velocity relationship was different
between conditions (PSL: 5.12 ± 2.88 %/m·s-1; SL2.5: 1.39
± 1.51 %/m·s-1; p < 0.05). Stride length was an important
factor that determined impact force magnitude. It is likely that lower
extremity posture is a determining factor influencing impact characteristics.
KEY
WORDS: Ground reaction force, impact attenuation, shock.
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