Age-related changes in the passive properties of the plantarflexors: Influence of tissue size and quality.

Abstract

Limited research exists examining the influence of age on the passive mechanical properties of the muscle-tendon unit. The purpose of the present study was to 1) investigate the age-related differences in maximum range of motion (MRoM), stretch tolerance (maximal tolerable torque), and the dissipation coefficient (DC), and 2) examine the influence of plantarflexor muscle size and quality on the age-related differences in the DC. Twenty-one young and 14 older men completed a MRoM assessment to determine peak torque (PT_pass), energy stored (area under the loading curve), energy restituted (area under the unloading curve), and the DC (normalized hysteresis) from the angle-torque curve. Ultrasonography was used to determine muscle cross-sectional area (CSA) and subcutaneous fat corrected echo intensity (EI) of the gastrocnemii. The PT_pass, MRoM, and energy restituted were greater (P ≤ 0.046) in the young men, whereas the DC and EI were greater (P ≤ 0.024) in the older men. When accounting for CSA, differences between groups for the DC remained (P = 0.028), while there were no longer differences between groups when accounting for EI (P = 0.120). PT_pass was also strongly related to MRoM (r = 0.755, P < 0.001). Older men exhibited lower dorsiflexion MRoM and a greater DC, which may be explained by an altered stretch tolerance and qualitative changes (i.e., non-contractile tissue infiltration) in aged skeletal muscle leading to greater energy dissipated, respectively.