A comparison of EMG activity between the medial and lateral heads of the gastrocnemius muscle during isometric plantar flexion contractions at various knee angles
Affiliations: [a] University of Miami School of Medicine, Division of Physical Therapy, 5915 Ponce de Leon Boulevard, Plumer Building 5th Floor, Coral Gables, FL 33146, USA | [b] Physical Therapy Associates, P.A., 6280 Sunset Drive, Suite 606, South Miami, FL 33143, USA
Abstract: Traditionally, the two-headed gastrocnemius muscle has been considered as one muscle. The purpose of this study was to delineate if electromyographic (EMG) activity differences existed between the medial and lateral gastrocnemius heads, as well as torque output, in the normal right leg, during isometric plantar flexion contractions at three knee angles. Thirty volunteers, between the ages of 18–35, participated in this study. Isometric plantar flexion, with the ankle at 90° , was performed at three knee angles; 0°, 45°, and 90°. The EMG activity was measured in root-mean-square (RMS) microvolts on the Bioscope electromyographic feedback unit, and torque output was measured on the Biodex System II Dynamometer. Results showed decreasing EMG activity of both the medial and lateral gastrocnemius heads as the knee angle changed from 0° to 90°. EMG output decreased significantly as knee flexion increased from 0° to 45° to 90° (F = 14.29, P = 0.0007). However, the pattern of EMG activity decline for medial and lateral gastrocnemius heads was significantly different (F = 4.58, P = 0.0143). Plantar flexion torque decreased significantly across the three angles (F = 94.64, P = 0.0001). It appears that the length-tension principle played a major role in the declining EMG and torque, during isometric plantar flexion. The differences of the medial and lateral gastrocnemius pattern of decline implies that the two heads of the gastrocnemius muscle should be considered as separate entities. However, many factors not controlled for in this study, may have impacted the results. Therefore, health professionals should look at this study as a preliminary study in the investigation of the activation of the medial and lateral gastrocnemius muscles, leading to further investigation.
