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The purpose of this study was to analyze the end-point forces at the foot created when selected muscles of the human calf are electrically stimulated. We hypothesized that the lateral (LG) and medial (MG) gastrocnemius muscles and the peroneus longus (Per) muscle would produce varying degrees of an abduction/ plantar flexion force. Surface stimulating electrodes placed on a subject’s calf and an electrical stimulator were used to isometrically activate the LG, MG, and Per of the four subjects, whose force outputs were recorded by a force/torque transducer. As expected, the Per produced an abduction/plantar flexion force while the MG and LG produced adduction/ plantar flexion forces in most cases. Though preliminary, these data offer some insight into the role muscles may play during movement.
Locomotion requires muscle activation in the sagittal plane but torques in 3D are needed for a full range of lower limb motion (Inman et al., 1981). When isometrically, electrically stimulated, cat hindlimb muscles create distinct, 3D torques (Lawrence et al., 1993). When individual muscle activation in human cadavers is simulated, different muscles also produce different torques (Arndt et al. 1999). Little is known about the force trajectories generated by individual human muscles in vivo. This information may influence movement control theories.
Subjects
n = 4 (2 female, 2 male, mean age = 37.5 yr.)
Measurement/Instrumentation
Muscle activation: Thera-touch 4.7 (Richmar)
Force-recording transducer: Multi-axis Force/Torque Sensor System (ATI)
Range: 330 N in Fx, 660 N in Fz
Resolution: .25 N in Fx, .5 N in Fz
Procedure
Stimulating electrodes were placed on skin over subject’s muscles (LG, MG, Per). Subjects were seated with their right foot and leg strapped down to isolate foot forces. Calf muscles were electrically stimulated for 10 seconds at 20 Hz with rest periods of 120 seconds between repetitive stimulations. Two stimulation levels were used: maximum stimulation that a subject could tolerate, and a level that produced half the force output of the maximum stimulation trial. The forces created by the foot were recorded using the transducer.
Data Analysis
Recorded data were processed by a custom Labview program that plotted the force data in its three components on a time-force graph. A custom Matlab program generated numerical values from the time-force graphs. Vector and direction cosine plots, organized by subject or by muscle, were created. The sample size was too small to run a meaningful statistical analysis.
1. The LG, MG, and Per generate different force trajectories.
2. The force trajectories for one muscle during a single testing session do not change as the magnitude of the force output varies.
The laterally directed x-component of the Per suggests an abduction or eversion force component and is consistent with previous data reported in cat and cadaver studies. Data from cat and cadaver studies suggested that the MG and LG are foot abductors. Our preliminary data suggest they are adductors. Medial force components reported in this study, however, may represent adduction, inversion, or a combination of both. The variability in LG may have been caused by subtle variations between stimulations and/or by the compartmentalization of the muscle. Discrepancies in all the muscles from trial to trial may have been caused by small differences in electrode placement, affecting which muscle/muscle compartment was stimulated.
We would like to thank William Goolsby for assistance with this study. Supported by NICHD, NCMRR under Grant No. 32571, the Howard Hughes Medical Institute under Grant No. 52003727, and by the Graduate Division of Biological and Biomedical Sciences, Emory University.
Although scientists know a great deal about the human body, there is a lot about muscles and their connection with the central nervous system that is still unknown. For example, the lateral and medial heads of the gastrocnemius muscle in the calf were once thought to be used for the same function. However, recent studies show that there are differences between the two branches of the gastrocnemius including the torques the two heads create when electrically stimulated in the cat and in the activation of the two heads during activities such as turning as one walks. If these muscles are proven to be different, then perhaps the idea that they have the same function is incorrect. This may help explain which and why certain muscles are used for certain tasks, which may lead to improved rehabilitation techniques.
One of the first steps to solving this puzzle is to examine the forces at the foot that are created when the calf muscle is stimulated. Consequently, this study involved using surface stimulating electrodes to activate selected muscles of a person's calf and then recording the forces that were produced at the foot. We found that different muscles generate different force trajectories and that these trajectories were different than those found in cat studies. Although preliminary, the data on the lateral and medial heads of the gastrocnemius mean that there is a detectable difference between the two heads but whether this difference is relevant to explaining if and how the central nervous system can distinguish between the two for specific tasks remains to be seen.
Muscle stimulation using surface electrodes
lateral and medial gastrocnemius muscle, peroneus longus muscle, electrical stimulation, end-point force, abduction/adduction
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