Neural and mechanical function of flexor hallucis longus at different walking speeds and in different footwear

Abstract

Ankle plantar flexor muscles make a major contribution to body propulsion in walking. Besides the triceps surae, deep ankle plantar flexors such as flexor hallucis longus (FHL) may also contribute to this. However, FHL function has not been extensively examined in vivo. Therefore, the aim of this thesis was to examine the effects of walking speed on FHL electromyography (EMG) activity, fascicle behaviour, and forces measured under the hallux in shod walking. Agreement between surface and intramuscular EMG was also tested in shod walking at different speeds for FHL, soleus, gastrocnemii, and tibialis anterior. Furthermore, intramuscular EMG activity of FHL and triceps surae was examined in different footwear at self-selected walking speed. As expected, FHL was highly active in the push-off phase of walking, similar to other plantar flexors. Increased walking speed was associated with higher FHL EMG activity and higher forces under the hallux, indicating an increase in the relative importance of FHL at faster walking speeds. FHL muscle fascicles operated at a near-constant length throughout the stance phase of slow walking, and shortened at faster speeds. This is similar to the fascicle mechanics of medial gastrocnemius in walking, with which FHL also shares similar architectural properties. When surface and intramuscular EMG methods were compared, there was often (~60% of all cases) poor agreement between methods for FHL, likely due to the challenge of minimising cross-talk in this muscle. Walking in shoes at preferred speed required higher plantar flexor muscle activity for body propulsion than walking in flip-flops or barefoot in most individuals, however individual variability was substantial. In shod walking, peak muscle activity occurred at the same relative time in the contact phase between participants. This may be due to the fact that shoes limit individual-specific natural foot and ankle function, imposing a restrictive motion pattern. This thesis provides in vivo evidence for the important role of FHL in walking. Using intramuscular EMG and ultrasonography, future studies should examine FHL function in individuals with Achilles tendinopathy or flatfoot, which are associated with altered FHL morphology, and perhaps also altered muscle function.