A comparison of iEMG activity between the medial and lateral heads of the gastrocnemius muscle during partial weight bearing plantarflexion contractions at varying loads

The purpose of this study was to determine the differences between iEMG activity of the medial head (MH) and lateral head (LH) of the gastrocnemius of normal right legs during partial weight bearing isometric plantarflexion contractions under varying percentages of body weight (BW) and maximal contractions. Fifty-three healthy volunteers ranging in age from 18 to 35, with no history of right lower extremity injury or disease volunteered for this study. Each subject's age, height, weight, and medical history were recorded. A chronaximeter was used to identify the motor points of the MH and LH. After performing a 5-minute warm-up, surface electrodes were placed just distal to the motor point, and subjects were positioned on the sliding board of the Total Gym Therapy System. The percentage of BW used as resistance was determined by the angle of inclination of the slide board. The subjects were then directed to plantarflex to 20 degrees, with 8 second iEMG recordings, with a proprioceptive cue, under four conditions: 30%, 50% and 70% of BW (3 trials each) and maximal isometric (1 trial). Paired t-tests concluded the MH consistently demonstrated a larger percent of total iEMG activity than the LH in the 30, 50, and 70% bodyweight trials (p < 0.0001). The percent of total difference between iEMG activity of the MH and LH during the maximum isometric trial was not statistically significant (p = 0.2958). A repeated measures ANOVA and post-hoc analysis demonstrated the percent of total iEMG differences to be greater at the lighter loads (D30 > D50 > D70; p < 0.0001). These findings strongly suggest separate but complementary functions of the two heads of the gastrocnemius muscle.