Abstract: Objective To compare the effects of different intensities of aerobic exercise protocols on improving skeletal muscle function in aging mice. Methods Using a random number table, eighteen male C57BL/6 mice (12-15 months old) were randomly divided into three groups: the blank control group (control group), the moderate-intensity aerobic exercise group (moderate-intensity group) and the high-intensity aerobic exercise group (high-intensity group), with six mice in each group. The control group was cage-housed without exercise intervention. While mice in the moderate-intensity group and high-intensity group first received adaptive training according to the established exercise protocol, then completed 8 weeks of aerobic exercise training following different aerobic exercise protocols. After training, skeletal muscle motor function was evaluated via grip strength, exhaustive swimming, and gait analysis. Molecular changes in skeletal muscle fibers were assessed by HE staining, Polymerase Chain Reaction(PCR) and Western blot. Results The moderate-intensity group exhibited significantly increased grip strength in both forelimbs and hindlimbs, prolonged swimming endurance, increased walking and swing speeds, shortened swing duration and increased step width after intervention. After intervention, the forelimb and hindlimb grip strength of mice in the high-intensity group was improved, and the swing duration was shortened. However, there were no statistically significant differences in swimming endurance, step length, walking speed and swing speed compared with those before intervention. Compared with the high-intensity group, the forelimb and hindlimb grip strength, swimming endurance, step length and walking speed were significantly improved in the moderate-intensity group. Compared with the control group, the moderate-intensity group showed significant improvements in forelimb and hindlimb grip strength, swimming endurance, walking speed, and swing duration after intervention, and the high-intensity group exhibited improvement only in hindlimb grip strength relative to the control group. Furthermore, after aerobic intervention, the muscle fiber cross-sectional area of mice in the moderate-intensity group was significantly increased with a more uniform and compact arrangement, and the protein and mRNA expression levels of COL Ⅰ, COL Ⅱ and COL Ⅲ were significantly higher than those in the control group and the high-intensity group. Compared with the control group, the mRNA expression levels of COL Ⅰ, COL Ⅱ and COL Ⅲ, as well as the protein expression levels of COL Ⅰ and COL Ⅲ in the high-intensity group were increased (P<0.05 or P<0.01). Conclusions Moderate-intensity aerobic exercise effectively improves limb muscle strength, enhances gait function, promotes the expression of COL Ⅰ, COL Ⅱ, and COL Ⅲ, and ameliorates gastrocnemius morphology in aging mice. High-intensity aerobic exercise is less effective than moderate-intensity exercise in promoting muscle strength and gait function.

Key words: aerobic exercise, aging, sarcopenia, skeletal muscle, collagen fibers, mice