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 that use models based on electromyographic (EMG) recordings. In this study (N = 20 subjects) we used electrical stimulation (20 Hz) 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 from 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 plantar flexion. We suggest that the intersubject 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.

Figure 1

Overview of Experimental Protocol. Subjects participated in two sessions. The order of testing is demonstrated in this figure. E-stim is electrical stimulation. LG, MG, FIB were randomly assigned to M1, M2, M3 for each subject. MR is magnetic resonance.

Figure 2

Experimental setup with foot, ankle and leg constrained (left). Foot resting on force plate with Fx, Fy, Fz reference system superimposed (right). Fx represents a lateral or outward force. Fz represents a downward force. Fz direction is downward through the page.

Figure 3

End-point force trajectories by subject. Each plotted point represents the average ratio for an individual subject during session 1. Error bars represent standard deviation. A) Fibularis longus. Electrical stimulation caused a lateral and downward force for all subjects. B) Lateral gastrocnemius. Electrical stimulation caused a downward force for all subjects but a lateral or medial force depending on the subject. In most subjects medial forces were elicited. C) Medial gastrocnemius. Electrical stimulation caused a downward force for all subjects but a lateral or medial force depending on the subject. There were an almost equal number of subjects with lateral and medial forces elicited by electrical stimulation.

Figure 4

An example of one subject’s MR images before and after stimulation of FIB. The brighter areas indicate higher T2 times. A) Pre-stimulation images; B) Post-stimulation images. In the post-stimulation images, the increases in T2 time are located in the area of FIB.

Figure 5

Correlation of average Fx/Fz data between Day 1 and Day 2 by subject. Points which lie on the line are ones in which the Fx/Fz ratio was the same on Day 1 and Day 2 for a given subject. Average Fx/Fz values were significantly correlated for LG and MG. We did not find a significant correlation for FIB data points. However, note that there are only four points of comparison for FIB and one point is clearly farther away from the equality line than the others.