Active muscle regeneration following eccentric contraction-induced injury is similar between healthy young and older adults

Abstract

Repair of skeletal muscle after injury is a key aspect of maintaining proper musculoskeletal function. Studies have suggested that regenerative processes, including myogenesis and angiogenesis, are impaired during advanced age, but evidence from humans is limited. This study aimed to compare active muscle regeneration between healthy young and older adults. We evaluated changes in clinical, biochemical, and immunohistochemical indices of muscle regeneration at precisely 2 (T2) and 7 (T3) days following acute muscle injury. Men and women, aged 18-30 and ≥70 years, matched for gender and body mass index, performed 150 unilateral, eccentric contractions of the plantar flexors at 110% of one repetition maximum. Data were analyzed using analysis of covariance, adjusted for gender, habitual physical activity, and baseline level of the outcome. A total of 30 young (n = 15; 22.5 ± 3.7 yr) and older (n = 15; 75.8 ± 5.0 yr) adults completed the study. Following muscle injury, force production declined 16% and 14% in young and older adults, respectively, by T2 and in each group, returned to 93% of baseline strength by T3. Despite modest differences in the pattern of response, postinjury changes in intramuscular concentrations of myogenic growth factors and number of myonuclear (4',6-diamidino-2-phenylindole+ and paired box 7+) cells were largely similar between groups. Likewise, postinjury changes in serum and intramuscular indices of inflammation (e.g., TNF-α and monocyte chemoattractant protein-1) and angiogenesis (e.g., VEGF and kinase insert domain receptor) did not differ significantly between groups. These findings suggest that declines in physical activity and increased co-morbidity may contribute to age-related impairments in active muscle regeneration rather than aging per se.

Keywords: aging; angiogenesis; inflammation; myogenesis; skeletal muscle.

Fig. 1.

Graphical overview of study design.

Fig. 2.

Representative blots of target proteins and Ponceau S staining used for immunblotting. BL, baseline; d, days; MRF4, myogenic regulatory factor 4; Myf5, myogenic factor 5; MyoD, myogenic differentiation antigen; HGF, hepatocyte growth factor; cMET, HGF receptor; KDR, kinase insert domain receptor.

Fig. 3.

Top: representative muscle cross-section stained for hemotoxylin and eosin. Bottom: representative section-stained laminin to identify the sarcolemmal membrane (shown in red) and 4′,6-diamidino-2-phenylindole (DAPI) to identify myonuclei (shown in blue).

Fig. 4.

Clinical markers of muscle injury and serum indices of inflammation in young and older adults before and after exercise-induced skeletal muscle injury. Dashed lines represent values for young adults; solid lines represent values for older adults. Symbols represent means adjusted for gender, physical activity (steps/day), and baseline measure of the outcome; error bars represent SE. The age × time interaction was not statistically significant for any variable (P > 0.05). Adjusted main effect for time: peak torque, P = 0.744; muscle soreness, P < 0.001; creatine kinase, P = 0.101; serum TNF-α, P = 0.929; serum monocyte chemoattractant protein-1 (MCP1), P = 0.735.

Fig. 5.

Skeletal muscle paired box 7 (Pax7)+ (satellite) cells in young and older adults before and after exercise-induced skeletal muscle injury. Dashed line represents value for young adults; solid line represents value for older adults. Symbols represent means adjusted for gender, physical activity (steps/day), and baseline measure of the outcome; error bars represent SE. A significant main effect was observed for time (P = 0.002), but the age × time interaction was not significant (P = 0.857). Right: representative muscle cross-section stained for DAPI (blue) and Pax7 (green) to identify muscle satellite cells.

Fig. 6.

Serum and intramuscular indices of angiogenesis in young and older adults before and after exercise-induced skeletal muscle injury. Dashed lines represent values for young adults; solid lines represent values for older adults. Symbols represent means adjusted for gender, physical activity (steps/day), and baseline measure of the outcome; error bars represent SE. The age × time interaction was not statistically significant for any variable (P > 0.05). Adjusted main effect for time: serum VEGF receptor 2 (R2), P = 0.039; intramuscular VEGF, P = 0.035; intramuscular KDR, P = 0.023; CD31+ cells, P = 0.040. Right: representative muscle cross-section stained for DAPI (blue) and CD31 (red) to identify muscle capillaries. A.U., arbitrary units.