Flexor hallucis longus and tibialis anterior: A synergistic relationship

Flexor hallucis longus (FHL) is an important muscle of the foot and ankle during locomotion, contributing to hallux and plantar flexion. For optimal hallux flexion the ankle needs to be stabilized against plantar flexion which may require action of the dorsiflexors. Due to the deep location of the FHL contractile drive assessed by electromyography (EMG) has not been explored systematically. Thus, the purpose was to test the relationship between the FHL and tibialis anterior (TA), the main dorsiflexor. Using indwelling EMG during isometric maximal voluntary contractions (MVC) of hallux and ankle joint actions, 10 participants (3-females, 7-males) aged 23 ± 1.4 years were tested in custom hallux-flexion and ankle dynamometers, with bipolar wire electrodes recording from the FHL, soleus and TA muscles. During MVC, forces were 169.2 ± 28.5 N, 285.5 ± 65.4 N, and 712.3 ± 313.8 N for hallux flexion, dorsiflexion, and plantar flexion, respectively. During maximal hallux flexion, TA EMG was 53 % (±26.5) of its maximum with negligible soleus activity, 4.7 % (±3.1). No significant correlations were found between TA activity and strength, foot characteristics, sex, height, weight, or soleus activity. This higher level of relative EMG recorded from the TA during maximal hallux flexion has not been observed in prior studies during walking and indicates that the relationship between the FHL and TA is task dependent, thus highlighting the important synergistic role of the TA in allowing optimal toe flexion.