Mechanics and control of the flat versus normal foot during the stance phase of walking

Objective. To compare walking stance phase rearfoot and forefoot motion, ankle joint moments and extrinsic foot muscle EMG profiles between normal and pes planus feet. Design. A cross-sectional comparative study. Background. Musculoskeletal conditions are often attributed to pes planus, in which motion is assumed to be excessive and the muscle control inadequate. However, many of the speculated differences in mechanics and control between the normal and the pes planus foot have not been quantified. Methods. Kinematic and kinetic data were obtained from video recordings of skin surface markers and a force plate, and EMG was recorded with surface electrodes. Analysis of variance was carried out to compare the group profiles. Results. In the pes planus group: the forefoot was less adducted (P<.05) at toe-off, and total transverse plane range of motion, at 8° versus 10°, was less (P<.01); the peak plantarflexor ankle moment at push-off was greater (P<.05); the invertor moment was greater at foot flat (P<.05); for the EMG profiles, activity early in stance, relative to the mean stance phase activity was higher (P<.01) in tibialis anterior and lower (P<.05) in the peronei, soleus and medial and lateral gastrocnemius. Conclusions. Despite reaching statistical significance, the group differences were small for the task of laboratory walking at a natural pace. The main implications of the differences were for restraint of motion. The expectations of excessive motion and muscle effort in the pes planus group were therefore not substantiated.