Response to the fatigue run tended to be highly individual. Magnitudes of the active and impact peak varied between individuals, and the changes with exhaustion were inconsistent (Table 1). Increases, decreases, and no change were observed with exhaustion in different runners. Five of the six runners had consistent increases in loading rate over the run. Increased loading rate may be indicative of a stiffer leg at impact. This result compliments the observation of increased shank acceleration by Verbitsky et al. (1998). Foot contact time either increased or stayed the same. Similar to the observations of Gollhofer et al. (1987) for the upper- extremity, runners with increased foot contact time tended to have decreased peak active forces. These same runners also increased step time. These changes may be associated with the generation of vertical impulse during foot contact. With reduced force production, contact time must be increased to provide a similar vertical impulse. With exhaustion, runners exhibited increased coefficient of variation for peak active force and loading rate (Figure 1). An increase in the coefficient of variation for the loading rate might be associated with decreased control of the leg musculature resulting in variable leg stiffness on impact, although the general trend was for the impact force to increase (implying a stiffer leg). Impact peak force coefficient of variation decreased, while force magnitude generally increased, possibly indicative of the presence of an upper boundary to impact force sustainable by the body under the given conditions.
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