Strength and size relationships of toe flexor muscles in three different functional force production tasks.

Purpose: Toe flexor strength (TFS) has been determined to evaluate the toe flexor muscle function. However, it is unclear how strength and size relationships of toe flexor muscles vary depending on the toes intended for force production. We aimed to clarify this by examining the relationship between TFS and toe flexor muscle size, and hypothesized TFS produced by all toes (TFS-All), the great toe (TFS-Great) and lesser toes (TFS-Lesser) would be specifically associated with the size of the muscles specialized in each corresponding toe flexion. Methods: The maximal anatomical cross-sectional area (ACSA_max) of each toe flexor muscle was measured by magnetic resonance imaging in twenty healthy young men. The three types of TFS were measured using a custom-made toe push dynamometer. Results: TFS-All was significantly associated with ACSA_max of the adductor hallucis transverse head (ADDH-TH) (r = 0.58, P = 0.01) and flexor hallucis brevis (FHB) (r = 0.56, P = 0.01). TFS-Great and TFS-Lesser were not significantly correlated with ACSA_max of any analyzed muscles, except for a significant correlation between TFS-Lesser and dorsal/plantar interosseous muscle (r = 0.48, P = 0.03). Conclusions: The size of two plantar intrinsic foot muscles, FHB, anatomically specialized for the great toe flexion, and ADDH-TH, supplementary flex the great toe, may be the determinant for TFS-All. However, TFS-Great and TFS-Lesser are not associated with the size of the muscles anatomically specialized in each corresponding toe flexion, perhaps due to difficulty in maximally and separately activating individual muscles (i.e., neural/anatomical reasons) during the TFS-Great and TFS-Lesser production.