Age-related changes in isolated mouse skeletal muscle function are dependent on sex, muscle, and contractility mode

Aim: The present study aimed to simultaneously examine the age-related, muscle-specific, sex-specific and contractile-mode-specific changes in isolated mouse skeletal muscle function and morphology across multiple ages. Methods: Measurements of mammalian muscle morphology, isometric force and stress (force/cross-sectional area), absolute and normalised (power/muscle mass) work loop power across a range of contractile velocities, fatigue resistance, and myosin heavy chain (MHC) isoform concentration were measured in 232 isolated mouse (CD-1) soleus, extensor digitorum longus (EDL) and diaphragm from male and female individuals aged 3-, 10-, 30-, 52-, and 78-wks. Results: Ageing resulted in increased body mass, and soleus and EDL muscle mass, with atrophy only present for female EDL by 78-wks despite no change in MHC isoform concentration. Absolute force and power output increased up to 52-wks and to a higher level for males. A 23%-36% loss of isometric stress exceeded the 14%-27% loss of power normalised to muscle mass between 10-wks and 52-wks, though the loss of normalised power between 52-78-wks continued without further changes in stress (P>0.23). Males had lower power normalised to muscle mass than females by 78-wks, with the greatest decline observed for male soleus. Ageing did not cause a shift towards slower contractile characteristics, with reduced fatigue resistance observed in male EDL and female diaphragm. Conclusion: Our findings show that the loss of muscle quality precedes the loss of absolute performance as CD-1 mice age, with the greatest effect seen in male soleus, and in most instances without muscle atrophy or an alteration in MHC isoforms.