Compliance changes of the series elastic component of elbow flexor muscles with age in humans.

The purpose of this study was to determine the compliance of the series elastic component (SEC) of the elbow flexor muscles in young (n=13, mean age 21.5+/-2.5 years) and elderly (n=15, mean age 67.4+/-4.7 years) subjects. SEC compliance was determined using a quick-release method. Under isometric conditions, myoelectrical activity (EMG) of biceps and triceps muscles was quantified by the root mean square (RMS) value. The compliance index (CI) was defined as the slope of the regression of ln(NC) on ln(NF), where NC and NF are normalised compliance and force respectively. Maximal isometric force and neuromuscular efficiency (torque/RMS) were significantly greater in the young than in the elderly. Antagonist (triceps) co-activation was similar for both groups. The CI after quick-release movements was significantly greater in the young than in the elderly. These results suggest that the SEC compliance of the elbow flexors muscles decreases with age. This decrease in global compliance could be induced by changes both in the active and passive portions of the SEC. These findings may be of functional significance for everyday muscular activity in older people.