Differences in End-Point Force Trajectories Elicited by Electrical Stimulation of Individual Human Calf Muscles

Abstract

The purpose of this study was to investigate the end-point force trajectories of the fibularis longus (FIB), lateral gastrocnemius (LG) and medial gastrocnemius (MG) muscles. Most information about individual muscle function has come from studies which use models based on electromyographic (EMG) recordings. In this study (N=20 subjects) we used electrical stimulation (20Hz) to elicit activity in individual muscles, recorded the end-point forces at the foot and verified the selectivity of stimulation by using magnetic resonance imaging. Unexpectedly, no significant differences were found between LG and MG force directions. Stimulation of LG and MG resulted in downward and medial or lateral forces depending on the subject. We found FIB end-point forces to be significantly different than those of LG and MG. In all subjects, stimulation of FIB resulted in downward and lateral forces. Based on our results, we suggest that there are multiple factors determining when and whether LG or MG will produce a medial or lateral force and FIB consistently plays a significant role in eversion/abduction and plantarflexion. We suggest that the inter-subject variability we found is not simply an artifact of experimental or technical error but is functionally relevant and should be addressed in future studies and models.

How much does the human medial gastrocnemius muscle contribute to ankle torques outside the sagittal plane?

“…While specific studies reported frontal and horizontal torques for the cat gastrocnemius (Carrasco et al, 1999; Lawrence et al, 1993), in humans, similar quantifications were recently studied during electrically elicited contractions at constant stimulation amplitudes (Giordano et al, 2009). Perhaps because of the predominant ankle movement in the sagittal plane, active contribution of individual calf muscles to ankle torques outside the sagittal plane is often disregarded.…”

“…With a markedly small stimulation electrode, we aimed to stimulate the nerve branch serving exclusively the MG muscle (Wolf & Kim, 1997) and thus to quantify the electrically elicited twitches in different planes. Previously, Giordano et al (2009) quantified the amplitude of plantar flexion and inversion moments produced by electrically elicited twitches in both gastrocnemius heads and in the fibularis longus muscle. By using different current intensities, in the current manuscript we further evaluate the relative contribution of different proportions of the whole population of MG motor units to frontal and sagittal torque components.…”

Scientific Reforms, Feminist Interventions, and the Politics of Knowing: An Auto‐ethnography of a Feminist Neuroscientist

“…The design of the experiment was not intended to measure variability but to look for the “normal” movements produced by activating individual muscles; however, it was not possible to average the data for individual muscles. I explained the significance of this in my dissertation and pushed for more recognition and research into variability in a subsequent peer‐reviewed publication (Giordano, Segal, and Abelew ). Access to feminist theories allowed me to view my data in new ways and to produce a different scientific result.…”