Load-Specific Distribution of Muscle Activity in Human Triceps Surae Muscles

Abstract

Motor units are arranged topographically within some muscles, and are activated in the order of their size, which enables individual regions to be activated in contraction. The triceps surae muscles are composed of the gastrocnemius and soleus (Sol) muscles. The gastrocnemius contains a higher percentage of type II fibers than that in the Sol. We hypothesized that these two muscles show different distribution characteristics of muscle activity. PURPOSE: This hypothesis was tested by comparing three-dimensional (3-D) distribution of muscle activity in the medial gastrocinemius (MG) and Sol during plantar flexion exercise at two different loads with use of muscle functional magnetic resonance imaging. METHODS: Six male subjects performed seven sets involved 10 repetitions of plantar flexion exercises with load that was 25 or 75% of their 12 repetitions maximum (RM). Transverse relaxation time (T2)-weighted images (repetition time 2500 ms; echo time 25, 80 ms; slice thickness 10 mm; gap 0 mm) were acquired from calf before and after each exercise load. To threshold images, considering as “active muscle” only those pixels showing T2 greater than the mean + 1SD of regions of interest (ROIs) in the pre-exercise images and a T2 lower than the mean + 1 SD of ROIs in the post-exercise images. Thresholded images were 3-D reconstructed, from which area of active muscle was determined along the x-axis (transversely), y-axis (longitudinally), and z-axis (vertically), respectively. RESULTS: The percentage area of activated muscle, which is relative to the anatomical cross-sectional area, was significantly larger after exercise with heavy load (75% of 12RM) than with light load (25% of 12RM) at each axis in the MG. On average, percentage area of activated muscle was 53.5 ± 6.4% in heavy load and 35.5 ± 3.3% in light load at x-axis (P< 0.05), 53.7 ± 6.9% in heavy load and 36.2 ± 3.0% in light load at y- axis (P <0.05), and 54.2 ± 4.8% in heavy load and 37.0 ± 3.7% in light load at z-axis (P <0.05). However, Sol did not show such differences. In the MG, distribution of muscle activity showed similar pattern between heavy and light loads along the x-axis and y-axis, but it varied across along the z-axis (P <0.05); it was higher at the proximal portion with greater load. No interactive effect was found at any axes in the Sol. CONCLUSION: Increase in load of repetitive plantar flexion exercises was accompanied by increased activations in the MG, especially at around the region of proximal portion and at throughout in a transversal and longitudinal axes. These findings suggest that load-specific distribution characteristics of muscle activity would be different between MG and Sol.