Abstract Body : Introduction: Many muscles required for bipedalism have been studied with a focus on the larger 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. The loss of function of the FHL, as with tendon transfers, 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 during big toe flexion and postural movements using indwelling electromyography (EMG) recordings.
Methods: In five healthy participants to date (3 males and 2 females) ages 22-25 years, indwelling fine wires were inserted with a small hypodermic needle separately into the FHL and FHB, on the dominant limb, 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 isometric toe dynamometer to record maximal voluntary contractions (MVC) of the big toe flexors (ankle, knee, and hip were maintained at 90 degrees). Participants also performed functional tasks including swaying anteriorly when standing to their limit of stability (before requiring a step to stop falling), bilateral calf-raises (plantar flexions), and standing from a seated position. Data were analyzed offline using Spike2 software. EMG (root mean squared – RMS) activity and rate of EMG rise (RER) from the various tasks for each muscle was normalized to the maximal EMG obtained from the MVC tasks and expressed as a percentage (%) or percentage per unit time, respectively.
Results: Average toe flexion strength was ~160N. The FHL was activated ~150ms before the FHB during the varying functional tasks, including toe flexion MVCs. However, the FHB relatively was as much as 3 times more active than FHL. Additionally, the rate of EMG rise was up to 20 times faster in the FHB compared to the FHL – RER was 1.6%/s and ~7%/s for the FHL and FHB, respectively, when testing the limit of stability; 1.8%/s and ~6%/s during the bilateral calf-raises; and 0.1%/s and ~2%/s when standing from a seated position. During the toe flexion MVCs the RER was 1.5 times faster in the FHL compared to the FHB.
Conclusion: The results illustrate the importance of the FHL in initiating big toe flexion regardless of task, and its essential role when producing larger forces. Also, these results highlight the crucial role of the FHB in possibly maintaining foot stability with its higher rate of EMG rise and higher overall level of activity relative to its maximum in the various functional tasks. In addition to providing a better understanding of the functional anatomy of the foot, these functional results may be useful for clinical considerations.
