Relationship Between the Flexion Torque of the First Metatarsophalangeal Joint and Intrinsic Foot Muscles Depends on the Ankle Joint Position.

Background: Clinicians and researchers are beginning to pay attention to the importance of the intrinsic foot muscles (IFMs). Among IFMs, the abductor hallucis (AbH) is associated with foot disorders. However, so far no method for assessing the strength of the AbH has been established. In addition, previous studies have shown increased IFM activity in the plantarflexed position of the ankle. Therefore, this study tests the hypothesis that a correlation will be found between the cross-sectional area (CSA) of the AbH and the flexion torque and that the first metatarsophalangeal (MTP) joint would be stronger in the plantarflexed (PF) position of the ankle joint than in the neutral (N) position.

Methods: Eight male and 8 female patients (16 lower limbs) were included in this study to measure the CSA of IFM and the extrinsic foot muscles of the lower leg. Furthermore, the flexion torque of the first MTP joint was measured using a handheld dynamometer at the N and PF positions of the ankle joint. Correlation analysis was performed to examine the relationship between the CSA of each muscle and the flexion torque of the first MTP joint in the N and PF positions.

Results: In the N position, a correlation was found between the flexion torque of the first MTP joint and the CSA of the AbH (r = 0.818), flexor hallucis brevis (r = 0.730), and flexor hallucis longus (r = 0.726). In the PF position, a correlation was found between the flexion torque of the first MTP joint and the CSA of the AbH (r = 0.863) and flexor hallucis brevis (r = 0.680). (P < .05).

Conclusion: Overall, this study suggested that by measuring flexion torque of the first MTP joint in the PF position, AbH strength can be estimated without using any expensive equipment.

Level of evidence: Level V, mechanism-baced reasoning.