Age-related reductions in the number of serial sarcomeres contribute to shorter fascicle lengths but not elevated passive tension

We investigated age-related changes to fascicle length (FL), sarcomere length (SL), and serial sarcomere number (SSN), and how this affects passive force. Following mechanical testing to determine passive force, the medial gastrocnemius muscle of young (n=9) and old (n=8) Fisher 344BN hybrid rats was chemically fixed at the optimal muscle length for force production; individual fascicles were dissected for length measurement, and laser diffraction was used to assess SL. Old rats had [~]14% shorter FL than young, which was driven by a [~]10% reduction in SSN, with no difference in SL ([~]4%). Passive force was greater in the old compared to young rats at long muscle lengths. Shorter FL and reduced SSN in the old rats could not entirely explain increased passive forces for absolute length changes, owing to a slight reduction in SL in old, resulting in similar SL at long muscle lengths. Summary StatementThis study sought to explain the increased passive tension observed for muscles of older individuals owing to age-related changes to muscle architecture.