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, which 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 decrease in old age, whereas endurance during repetitive isovelocity shortening is unaffected. Midcostal DIAm strips were excised from 6-mo-old (n = 8; 4 females and 4 males) and 24-mo-old (n = 8; 4 females and 4 males) Fischer 344 rats. The DIAm was supramaximally stimulated using platinum plate electrodes to measure mechanical and endurance properties at 26°C. In 24-mo-old compared with 6-mo-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 contractions (∼30% V_max; approximating peak power output), endurance (the period during which power output was sustained) of the DIAm was not different between 6- and 24-mo-old 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 the force generation and endurance required for breathing are sustained in old age.NEW & NOTEWORTHY Aging selectively impacts the function of type IIx/IIb fibers in the diaphragm muscle. We show that sarcopenia reduces maximum specific force, maximum shortening velocity, and peak power output, whereas endurance during repetitive contractions is preserved. These findings explain why higher force airway clearance behaviors decline with age, whereas fatigue-resistant breathing behavior remains resilient.