Immediate Effects of a Wearable Thermoplastic Polyurethane Arch Pad Sock on Lower-Extremity Muscle Activity in Adults with Flexible Flatfoot

Flatfoot (pes planus) is a common foot deformity associated with altered lower-extremity muscle activity during gait. Conventional foot orthoses and insoles can provide arch support but have limitations related to comfort, adherence, and everyday usability. This study evaluated the immediate effects of a thermoplastic polyurethane (TPU) arch pad sock on lower-extremity muscle activity in adults with flexible flatfoot. Twenty adults with flexible flatfoot participated. Surface electromyography signals were recorded bilaterally from the tibialis anterior (TA), peroneus longus (PL), medial gastrocnemius (MG), and lateral gastrocnemius (LG), which yielded eight recording channels. Muscle activity was measured before and immediately after the TPU arch pad sock application and normalized to percentage of maximum voluntary isometric contraction. Muscle activity was significantly reduced in five of the eight channels following the TPU arch pad sock application (all p < 0.05), including the right TA, right PL, right MG, left TA, and left PL. The largest effect was observed in the right MG (Cohen’s d = 1.157). Although these reductions were statistically significant, all five channels remained below the minimal detectable change (MDC95), indicating that the immediate effects did not reach the threshold for clinically meaningful change; longitudinal studies evaluating cumulative use are needed to establish clinical significance. Wearing the TPU arch pad sock was immediately associated with altered lower-extremity muscle activity in the subjects, although the single-arm design precludes establishing a definitive causal attribution. These findings suggest that the device may affect neuromuscular responses during walking by providing medial longitudinal arch support.