Aging Slows Maximum Shortening Velocity and Reduces Peak Power Output of Rat Diaphragm Muscle

Sarcopenia is the age-related atrophy and weakening (decrease in force per cross-sectional area) of muscle fibers. In rat diaphragm muscle (DIAm), sarcopenia primarily affects type IIx/IIb fibers that generate greater specific force, have the fastest maximum shortening velocities (V max ), but display greater fatigue with repeated activation. We hypothesized that because of the selective effect of sarcopenia on type IIx/IIb DIAm fibers, V max and peak power output of the DIAm decreases in old age, while endurance during repetitive isovelocity shortening is unaffected. Mid-costal DIAm strips were excised from 6-month (n=8; 4 female and 4 male) and 24-month (n=8; 4 female and 4 male) Fischer 344 rats. The DIAm was supramaximally stimulated using platinum plate electrodes to measure mechanical and endurance properties at 26oC. In 24- compared to 6-month old rats, maximum specific force of the DIAm decreased by ~35%, V max slowed by ~20%, and peak power output was reduced by ~35%. During repetitive isovelocity (~30% V max ; approximating peak power output) contractions, endurance (the period during which power output was sustained) of the DIAm was not different between 6- and 24-month rats. The changes in DIAm mechanical properties corresponded to an age-related decline in the cross-sectional area of type IIx/IIb muscle fibers (~35%). We conclude that age-related atrophy of type IIx/IIb DIAm fibers underlies the changes in mechanical properties of the DIAm, which would only impact more forceful expulsive airway clearance and voiding behaviors of the DIAm. The resilience of type I and IIa DIAm fibers with aging ensures that force generation and endurance required for breathing is sustained in old age.