THE DYNAMIC SUPPORT STRUCTURE AND THE FOOT STABILITY RELATED TO RUNNING SPEED
AbstractIntroduction - The alterations of the dynamic support structure in running, related to the horizontal speed increase, has been studied by several authors; Cavanagh (1989)studies the displacement of the support center and the pressure distributions; Nigg et all (1981) analyses the active and passive loader Bates (1983) study the critical variables defining the support dynamic. Simultaneously, the study of the rearfoot mobility around the horizontal axes of the subtalar joint during ground contact has been gaining in importance (Nigg et all, 1978). The above mentioned subjects are integrated in the present study for a more complete knowledge of the foot contact. Relations between the increase of running speed and respectively the variables defining the rearfoot mobility and the critical variable6 of the ground reaction forces are establish. Methods - Six sprinter of Portuguese national level carried out each of them 6 performances, barefoot, at three different speeds (Vl=3.3 7 me-I+/-0.56 ; V2=4. 37me"+/-0.37 e V,55 -44 me-I+/-0.31). The kinetic data were obtained on a force platform (Kistler 9281B, sampling frequency 1000 Hz) connected t6 the computer via A/D card (DT 2801A Data Translation). The runners speed was controlled by means of photoelectric censors that synchronized the collection of the kinetic and video images. The kinematic data was processed according to the methodology proposed by Nigg et all (1978). Results and Discussion - The obtained kinetic results are similar to those contained in the corresponding bibliographic references , in form patterns as well as in quantitative results (Cavanagh,1989 ; Nigg et a11,1981; Batee et all, 1983). Regarding the rearfoot mobility the displacement/time curves are similar to the ones presented by Nigg et all (1978) as far as barefoot runners are concerned. Significant statistical correlations (correlation coefficient, Bravais-Pearson) were ascertained showing that the running speed increase ie related to : (1) decrease of ground contact time (r=-0.96, P-ZO.01) , (2) increase of anteriorposterior speed variation (aVy) (r50.77, P<0.01) , (3) decrease of the braking phase of the anterior-poeterior speed variation (A Vy(-) ) (r=-0.70, P<0.01), (4) increase of the propulsion phase of the anterior-poeterior speed variation (A Vy(+))(r=0.66, P
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