Effect of functional fatigue on vertical ground-reaction force in individuals with flat feet.

CONTEXT: Flat foot is one of the lower extremity deformities that might change kinetic variables of gait. Fatigue is one of the factors that can alter the vertical ground-reaction force (GRF). The effect of a fatiguing condition on vertical GRF has not been documented in individuals with flat feet. OBJECTIVE: To examine the fatigue effect on vertical GRF in individuals with flat feet compared with a normal group during barefoot walking. DESIGN: Repeated-measure ANOVA for the effects of fatigue on individuals with flat feet and normal feet. SETTING: Biomechanics laboratory. PARTICIPANTS: 17 subjects with flat feet and 17 normal subjects (recruited according to their arch-height ratio). MAIN OUTCOME MEASURES: Three vertical GRF measures (F1, the first peak force; F2, minimum force; and F3, the second peak force) were extracted before and after a functional fatigue protocol. RESULTS: No significant interaction between fatigue and group was observed for the 3 vertical GRF measures. For F2, fatigue and group effects were significant (P = .001 and P = .02, respectively). Furthermore, F2 was higher in the flat-feet group than in the normal group; F2 also increased after fatigue. For F3, only a significant fatigue effect was observed (P = .004). F3 decreased after fatigue in both groups. CONCLUSIONS: In the flat-feet group, a decrease in the variation of vertical GRF might be due to more flexible foot joints. After fatigue, muscles might lose their ability to control the foot joints and cause higher F2 in the flat-feet group.