The early effects of ageing on the mechanical performance of isolated locomotory (EDL) and respiratory (diaphragm) skeletal muscle using the work loop technique

Previous isolated muscle studies examining the effects of ageing on contractility have used isometric protocols which have shown to have poor relevance to dynamic muscle performance that occurs in vivo. The present study uniquely uses the work loop technique to obtain a more realistic estimation of in vivo muscle function in order to examine changes in mammalian skeletal muscle mechanical properties with age. Measurements of maximal isometric stress, activation and relaxation time, maximal power output, sustained power output during repetitive activation and recovery are compared in locomotory EDL and core diaphragm muscle isolated from female mice 3, 10, 30 & 50 weeks old to examine the early onset of ageing. A progressive age related reduction in maximal isometric stress that was of greater magnitude than the decrease in maximal power output, occurred in both muscles. Maximal force and power developed earlier in diaphragm muscle compared to EDL, but demonstrated a greater age related decline. The present study indicates that ability to sustain skeletal muscle power output through repetitive contraction is age and muscle dependent, which may help to rationalise previously reported equivocal results examining the effect of age on muscular endurance. The age related decline in EDL muscle performance is prevalent without a significant reduction in muscle mass, and biochemical analysis of key marker enzymes suggest that although there is some evidence of a more oxidative fibre type, this is not the primary contributor to the early age related reduction in muscle contractility.