Effect of a task’s postural demands on medial longitudinal arch deformation and activation of foot intrinsic and extrinsic musculatur

It is not well established how motion and muscle activation of the medial longitudinal arch (MLA) of the foot vary under different loading conditions. Intrinsic and extrinsic foot muscles may play a role in postural control, which may be investigated by comparing loading tasks with differing postural demands. The objective of this study was to investigate the interaction of MLA flexibility and loading task on muscle activation. Methods: Twenty healthy adults completed two instrumented single-foot loading tasks: controlled external load of 50% body weight while sitting and bilateral standing. Fine-wire intramuscular and surface electromyography collected flexor hallucis brevis, abductor hallucis, tibialis posterior, flexor hallucis longus, tibialis anterior, and peroneus longus activation. MLA deformation was measured as a percent change in navicular height with loading. Results: During seated external loading, greater MLA deformation was associated with greater muscle activation for all instrumented muscles (R2 = 0.224–0.303, p < 0.05) except for tibialis anterior. During bilateral stance, there were no correlations between MLA deformation and muscle activation. Activation of all extrinsic muscles except for tibialis anterior were greater during bilateral standing than during external loading ( p = 0.002–0.013), indicating activation of these muscles was caused by postural demands of the standing task, not simply load. Conclusions: MLA deformation and muscle activation are strongly task-dependent.