Meeting Abstract
In pennate muscles, shortening of muscle fibers is accompanied by a change in their pennation angle. This fiber rotation can increase whole muscle shortening velocity for a given fiber shortening velocity, a relationship characterized by a muscle’s architectural gear ratio (AGR). AGR can change as muscles undergo different shape changes at different levels of force. As a result muscles can shift gears dynamically during different types of contraction. Muscle shape changes are largely determined by the interaction between muscle force and the mechanical constraints imposed by connective tissues. We used an ageing model system to examine how changes in contractile and connective tissue properties alter the variable gearing mechanism. Muscle ergometry and sonomicrometry were used to determine muscle and fiber length changes in young (6mo, n=12) and old (33mo, n=13) rat m. gastrocnemius muscles during isotonic contractions at a range of relative forces. Muscles shortened faster in young rats (p<0.001), particularly at lower relative forces. However, there was no effect of age on fiber shortening velocity (p=0.66). Muscle power output was significantly lower in old rats (p<0.05) due to a decrease in force (35%), and muscle shortening velocity (23%). AGR decreased with increasing relative force in young rats (p<0.001) but there was no relationship between AGR and relative force in old rats (p=0.72). These findings show the loss of variable gearing in old rats and suggest that an increase in intramuscular connective tissue and a reduction in force capacity constrain muscle shape changes. The loss of a variable gearing mechanism suggests that the muscles of old rats achieve lower velocity during low force contractions and lower forces during forceful contractions. Therefore, changes in the mechanical properties of muscles contribute significantly to the age related decline in muscle performance. NIH AR055295 & NSF 1436476.
