The fiber type specific response of single muscle fiber contractile performance to endurance and resistance exercise training in healthy, sedentary adults

It is well documented that human skeletal muscle is highly responsive to exercise training. However, the impact of endurance and resistance exercise training on single muscle fiber (SMF) physiology has not been well studied in healthy, sedentary individuals. The current study documented the fiber type specific response of SMF physiology to endurance and resistance exercise training. A total of 37 participants were randomized into one of three groups: endurance exercise (EE) (n=15; 47 ± 15 y), resistance exercise (RE) (n=14; 45 ± 19 y), or no exercise (control; CON) (n=8; 45 ± 18 y). Muscle biopsies were taken from the vastus lateralis at baseline (BAS) and follow-up (FUP) after the 12-week intervention. Muscle biopsy samples were analyzed for fiber type distribution and SMF contractile performance. In the EE group, MHC IIA/IIX expression decreased 12% (p<0.05) with no other changes in fiber type observed. Limited response in SMF contractile performance was observed in the EE group aside from a 11% decrease (p<0.05) in MHC IIA normalized force. In the RE group, MHC IIA/IIX expression decreased 13% (p<0.05), MHC IIA expression increased 16% (p<0.05), with no other changes in fiber type observed. No significant changes were observed in SMF contractile performance in response to RE training. As expected, no notable changes to muscle fiber type nor SMF function were observed in the CON group. In summary, the fiber type and SMF physiology data suggest that at BAS the skeletal muscle of the sedentary individuals exhibit a high degree of heterogeneity. After training, patterns of increased homogeneity were noted, but limited responses were observed overall. This suggests that the skeletal muscles of these participants are in the early stages of remodeling and may only reflect patterns of adaptation after 12 weeks of exercise training. Further studies may seek to map the adaptive time course and the molecular mechanisms driving these responses beyond 12 weeks of endurance and resistance exercise training.