In vivo passive mechanical behaviour of muscle fascicles and tendons in human gastrocnemius muscle–tendon units

Non‐Technical Summary Relaxed skeletal muscles behave like springs that resist joint motion. There have been few in vivo studies of the spring‐like properties of relaxed muscles. In this study, ultrasound was used to image human calf muscles while muscle length was changed by rotating the ankle of relaxed subjects. The muscles of some subjects buckled at short lengths. At short lengths most muscle fascicles (bundles of muscle cells) are slack. As the muscle is lengthened the slack is progressively taken up, first in some fascicles then in others. The increase in muscle length is due partly to increases in the length of muscle fascicles but most of the increase in muscle length occurs in the tendons.