The Effect of Ankle and Hallux Positioning on Hallux Force Production

The foot is not simply a rigid lever, but contributes substantially to movement energetics, primarily through muscle-tendon dynamics. However, more research is needed to understand how the small muscles of the foot generate and regulate force production. PURPOSE: We quantified the effects of ankle and hallux positioning on hallux flexion force production, isolating the contribution of the flexor hallucis longus (FHL) and brevis (FHB). We hypothesized the FHL length changes would influence hallux force. METHODS: Eighteen healthy adult volunteers participated with written consent. Participants assumed each of four testing positions on a custom-built isometric toe flexion force sensor: ankle neutral (90°) – hallux neutral (0°) (ANHN), ankle neutral (0°) – hallux extended (30°) (ANHE), ankle plantarflexed (45°) - hallux neutral (0°) (APHN), and ankle plantarflexed (45°) – hallux extended (30°) (APHE). For each position, participants maximally pressed on the device for three seconds, with a one second representative plateau extracted for analysis. Force was averaged across 3 trials and analyzed with a repeated measures ANOVA and Holm pair-wise comparisons. RESULTS: There was a significant main effect (p<0.001). The ANHE condition produced the greatest force (17.3 ± 7.2 N); pairwise, this was significantly greater than the force in all other conditions except APHE. In this condition, the FHL was in its most lengthened position. Conversely, the APHN condition produced the least force (8.7 ± 4.6 N), which was pairwise significantly less than all other conditions. Here, the FHL was in its most shortened position. ANHN and APHE resulted in similar force production (13.8 ± 6.3 N and 14.5 ± 6.4 N, respectively). In both conditions one joint was altered, resulting in roughly similar FHL lengths despite FHB being longer in APHE. CONCLUSION: In plantarflexed positions, FHL shortens, reducing its effectiveness due to less optimal positioning of cross-fibers in sarcomeres. In hallux extended positions, FHL lengthens, increasing its effectiveness with more optimal positioning of cross fibers, regardless of ankle position. It also appears likely that the FHL contributed much more to hallux force than the FHB. We hope this information will ultimately influence applications in footwear design and rehabilitation.