Contributions of flexor hallucis longus and brevis muscles to isometric toe flexor force production

Purpose Morphological differences between the two primary great toe flexors – flexor hallucis longus (FHL) and flexor hallucis brevis (FHB) – likely drive differences in how these muscles contribute to functional toe flexion torque production. The aim of the study was to investigate FHL and FHB activation in two isometric toe flexion tasks – one called a “toe-pushing” task with the metatarsophalangeal (MTP) joints extended and the interphalangeal (IP) joints in neutral and another called a “toe-gripping” task with the MTP joints in neutral and flexed IP joints. Methods Twenty participants’ FHL and FHB muscles were instrumented with intramuscular electromyography electrodes. Muscle activation was normalized to a maximum voluntary contraction and compared between the two isometric toe flexor force production tasks. Results Overall, participants utilized these two toe flexors completely differently in the two tasks. In the toe-gripping task, the FHL was activated to a much greater extent than the FHB. In fact, 18 our of 20 participants activated FHL at more than seventy percent maximum voluntary contraction and half of participants activated FHB at less than ten percent. In contrast, muscle activation during the toe-pushing task appeared more reliant on the FHB for most participants. Conclusions Different contributions from the FHL and FHB to toe flexor force production in these two tasks are potentially driven by differences in muscle functional length among other factors. These findings help to inform the selection of rehabilitation and training exercises meant to preferentially target intrinsic or extrinsic foot musculature.