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JOURNAL
OF
SPORTS SCIENCE &
MEDICINE
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Research
article
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NUMBER OF SUCCESSIVE CYCLES NECESSARY TO ACHIEVE STABILITY OF SELECTED GROUND REACTION FORCE VARIABLES DURING CONTINUOUS JUMPING |
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Vitamor Racic  ,
Aleksandar Pavic and, Jasmes M.W. Brownjohn |
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Department of Civil & Structural Engineering, University of Sheffield, Sheffield, UK. |
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© Journal of Sports Science and Medicine (2009) 8, 639 - 647 |
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| ABSTRACT | |||
| Because of inherent variability in all human cyclical movements,
such as walking, running and jumping, data collected across a single cycle
might be atypical and potentially unable to represent an individual's generalized
performance. The study described here was designed to determine the number
of successive cycles due to continuous, repetitive countermovement jumping
which a test subject should perform in a single experimental session to
achieve stability of the mean of the corresponding continuously measured
ground reaction force (GRF) variables. Seven vertical GRF variables (period
of jumping cycle, duration of contact phase, peak force amplitude and its
timing, average rate of force development, average rate of force relaxation
and impulse) were extracted on the cycle-by-cycle basis from vertical jumping
force time histories generated by twelve participants who were jumping in
response to regular electronic metronome beats in the range 2-2.8 Hz. Stability
of the selected GRF variables across successive jumping cycles was examined
for three jumping rates (2, 2.4 and 2.8 Hz) using two statistical methods:
intra-class correlation (ICC) analysis and segmental averaging technique
(SAT). Results of the ICC analysis indicated that an average of four successive
cycles (mean 4.5 ± 2.7 for 2 Hz; 3.9 ± 2.6 for 2.4 Hz; 3.3 ± 2.7 for 2.8
Hz) were necessary to achieve maximum ICC values. Except for jumping period,
maximum ICC values took values from 0.592 to 0.991 and all were significantly
(p < 0.05) different from zero. Results of the SAT revealed that an average
of ten successive cycles (mean 10.5 ± 3.5 for 2 Hz; 9.2 ± 3.8 for 2.4 Hz;
9.0 ± 3.9 for 2.8 Hz) were necessary to achieve stability of the selected
parameters using criteria previously reported in the literature. Using 10
reference trials, the SAT required standard deviation criterion values of
0.49, 0.41 and 0.55 for 2 Hz, 2.4 Hz and 2.8 Hz jumping rates, respectively,
in order to approximate the ICC results. The results of the study suggest
that the ICC might be a less conservative but more objective method to evaluate
stability of the data. Based on these considerations, it can be recommended
that a force time history due to continuous, repetitive countermovement
jumping should include minimum of four (the average from the ICC analysis)
and possibly as many as nine successive jumping cycles (the upper limit
of the ICC analysis) to establish stable mean values of the selected GRF
data. This information is important for both experimental measurements and
analytical studies of GRF signals due to continuous, repetitive countermovement
jumping. Key words: Reliability, stability, variability, jumping, ground reaction forces. |
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