Stiffness and muscle function with age and reduced muscle use.

Changes in passive muscle stiffness with age and disuse were assessed in male Fischer-344 and Brown Norway rats. Three groups of rats were studied: young (approximately 7 months old), old (approximately 33 months old), and old that had undergone 2 weeks of hindlimb unweighting, a model of reduced muscle use. Four hindlimb muscles were examined: the soleus (postural), plantaris (locomotor), extensor digitorum longus (nonpostural), and peroneus longus (nonpostural). Supramaximal stimuli elicited peak tetanic tensions throughout the available range of motion (amount of muscle elongation before the maximal attainable contractile or tetanic tension is obtained) for each muscle, permitting the creation of length-tension curves. Passive tension (amount encountered at each millimeter of change in muscle length) was also recorded throughout the available range of motion and was unchanged with aging and unchanged or reduced with hindlimb unweighting; muscle stiffness remained unchanged under both conditions. Passive tension, however, accounted for a greater proportion of total tension with age and particularly with hindlimb unweighting. A significant loss in muscle range of motion, resulting in a leftward shift in the length-tension curve, occurred with aging in only the plantaris. Hindlimb unweighting resulted in a marked loss in muscle range for all four muscles studied, suggesting that the remaining muscle force was constrained to a very small arc. Significant declines in muscle mass and peak contractile tension, associated with age and hindlimb unweighting, were observed for all four muscles.