How Important is the Big Toe?: Functional Anatomy of Hallux Flexion

Abstract

Introduction Bipedalism is a unique characteristic of homo sapiens allowing us the ability to stand and walk. Many muscles required for bipedalism have been studied with a focus on large superficial leg muscles. However, the functional anatomy of deeper muscles with unique foot function have not been explored. The flexor hallucis longus (FHL) of the posterior leg and flexor hallucis brevis (FHB) of the foot function to flex the big toe, are required for optimal locomotion, and have an important role in upright balance and gait. Furthermore, the FHL has been used in tendon transfers such as Achilles reconstruction. The loss of function of the FHL, including impairments in balance and walking, have been noted but not comprehensively explored. Thus, the purpose of this study was to assess the relative activation of these muscles (FHL and FHB) in toe flexion and postural movements by using indwelling electromyography (EMG) recordings. Methods In six healthy participants to date (4 males and 2 females) ages 22-25 years, indwelling fine wires were inserted into each of the following muscles of the dominant limb: FHL, FHB, soleus and tibialis anterior (TA), to record intramuscular EMG activity. Ultrasound imaging was used to ensure optimal placement of the fine wires within each muscle. After two familiarization sessions, participants were placed in a custom-built toe dynamometer to record maximal voluntary contractions (MVC) of the big toe flexors (ankle, knee, and hip were maintained at 90 degrees). Additionally, plantar flexion and dorsiflexion MVCs were recorded using a custom ankle dynamometer. Participants were then asked to perform functional tasks including standing quietly for 60 seconds, sitting and standing from a seated position, bilateral calf-raises, standing on one foot, and swaying anteriorly to their limit of stability (before requiring a step to stop falling). Data were analyzed offline using Spike2 software. EMG (root mean squared – RMS) activity from the various tasks for each muscle was normalized to the maximal EMG obtained from the MVC tasks and expressed as a percentage (%). Results Average toe flexion strength was ~174N of force. When standing on two feet the FHL was ~1% active relative to its MVC while the FHB was ~6% active. During sitting and standing from the seated position the FHL had ~3% activity while the FHB had ~10%. FHL and FHB had ~36% and 39% activity, respectively, during the body weight calf raise. When standing on one foot the FHL was ~6% active while the FHB was ~30% active. The greatest activity for these muscles within the functional tasks was during the anterior forward tilt with ~50% and 75% for the FHL and FHB, respectively. While performing toe flexion MVCs there was ~65% co-activation in the TA and ~4% in the soleus. Conclusion These results illustrate the importance of the FHL and FHB during balance (forward sway) compared with their activity during plantar flexion and other tasks, although the FHB was more active in these than the FHL. The large degree of co-activation in the TA to optimize toe flexion illustrates an important synergy required for optimal toe and foot function. In addition to providing a better understanding of the functional anatomy and biomechanics of the foot, these results also indicate consideration of the functional relevance of these big toe muscles in clinical interventions.