Age-related fascicle–tendon interaction in repetitive hopping

Increasing age can influence the interaction of muscle fascicles and tendon during dynamic movements. The object of the present study was to examine occurrence and possible reasons for the age-specific behavior of fascicles and tendons and their interaction during hopping with different intensities. Nine young and 24 elderly subjects performed repetitive hopping with maximal effort as well as with 50, 65, 75 and 90 % intensities. During hopping joint kinematics and ground reaction, forces were measured together with recordings of ultrasound images of both the fascicle and the muscle–tendon junction part of the gastrocnemius medialis (GaM) muscle. The results showed that fascicle behavior during the braking phase of hopping was clearly age specific in nature with more fascicle shortening in the young (p < 0.001). In addition, the fascicle shortening increased in young subjects with increasing intensity (p < 0.05). At the instant of ground contact, the elderly subjects demonstrated decreased fascicle length with increasing hopping intensity (p < 0.01). Thereafter in the braking phase, the elderly showed much smaller changes in fascicle length as compared to the young. In contrast to the fascicles, the GaM outer tendon did not show major age-specific differences in stretching and shortening amplitudes during hopping although the peak tendon forces were clearly lower in the elderly (p < 0.001). These results suggest that GaM outer tendon behavior is not influenced greatly with increasing age. It is further suggested that when aging modifies the fascicle–tendon interaction, it is primarily due to the age-specific difference in the fascicle level. This notion poses a question that as compared to the young, the elderly individuals may have a different fascicle behavior for optimal SSC locomotion such as hopping.