Relative index of ankle muscle activations on agonistic phase between subjects with and without flat foot

Background: Although co-activation of ankle muscles has been reported, relative ankle muscle activation in subjects with flat foot has not been carefully investigated. The aim of this study was to compare the relative activation index (RAI) on the tibialis anterior (TA) and medial gastrocnemius (GTN) muscles during active ankle range of motion (ROM) between subjects with and without flat foot. Methods: There were 17 subjects with flat foot and 17 age- and gender-matched control subjects who participated in this study. The RAI based on electromyography (EMG) was measured during the agonist phase at a controlled velocity of ankle motion (10°/second). The subject was seated upright with the tested foot held firmly onto a footplate that was attached to a torque sensor. The ankle being measured was strapped to the leg support of the Intel stretch device at 60° of knee flexion. The RAI was analyzed by the summation of EMG activity from the agonistic time window divided by the total EMG activity during full active ankle ROM. Results: The RAI was significantly different on the TA muscle (t = 3.08, P = 0.004), but no difference was found on the GTN muscle (t = -1.24, P = 0.23) in subjects with flat foot. There was an interaction between group and RAI (F =7.89, P = 0.007); however, the RAI demonstrated no interaction with age (F = 2.59, P = 0.14), height (F = 3.73, P = 0.06), or weight (F = 2.96, P = 0.09). Conclusions: The RAI indicated a lack of TA muscle activation in the flat foot group. Such dissociated activation in the flat foot group might be relevant to the inefficiency of synergistic motions. The relative activation of the agonistic phase needs to be further investigated to compare co-activation of synergistic muscle activation with various functional tasks.