The recruitment of different compartments within a muscle depends on the mechanics of the movement

Abstract

Muscles are commonly assumed to have uniform activations across their bellies. Yet animal studies have shown that different regions across a muscle can vary in their architecture, fibre type, levels of activation and the transfer of forces to the bones and tendons. The purpose of this study was to test whether regional variations in activity occurred across the soleus, medial gastrocnemius and lateral gastrocnemius in man, and whether these regional variations changed with altered mechanical demands on the limb. Arrays of surface electrodes were placed over these muscles and the electromyograms (EMG) measured for a range of cycling tasks where the resistance and pedal velocity was independently altered. Significant variations in the magnitude and timing of the EMG occurred across these muscles, which were most pronounced in the gastrocnemii and occurred in both the proximodistal and mediolateral directions. The patterns of variation across the muscles changed in response to the altered mechanical demands during the cycling. It is likely that the muscle fascicles in the gastrocnemii contribute varied mechanical functions to the contractions that depend on both their location within the muscle belly and on the mechanical requirements of the movement.

Figure 1

Regional changes in EMG intensity for the (a) LG, (b) MG and (c) Sol when cycling at increasing crank torques. Data show the mean (thick lines) ±s.e.m. (thin lines) averaged over 504 pedal revolutions. Lateral and medial recording sites are denoted by blue and red colours, respectively. Proximal and distal sites are denoted by light and dark colours, respectively. (i) 140g, 6.5 N m; (ii) 100g, 6.5 N m; (iii) 60g, 6.5 N m; (iv) 60g, 25.1 N m; (v) 60g, 39.9 N m.

Figure 2

Differences in the patterns of activity from the muscle regions for the varied mechanical demands. (a) PC loading score plots for the patterns of activity are shown for each muscle. Each point shows the mean (n=504) loading score, and the coloured ellipses show the 95% confidence limits for these means. Lateral and medial recording sites are denoted by blue and red colours, respectively. Proximal and distal sites are denoted by light and dark colours, respectively. Data are grouped together and shown for the three extreme mechanical regimes that were performed. (b) PC I loading score is on the vertical and represents levels of activity, with greater scores being associated with greater muscle activity. (c) PC II loading scores are on the horizontal and represent a shift in timing, with more positive scores representing later timing of muscle activity. The percentage of the signals explained by each PC is shown (b,c).