Increased cardiorespiratory fitness and skeletal muscle size following single-leg knee extension exercise training

BACKGROUND: It is unclear to what extent an improved skeletal muscle phenotype influences maximal aerobic capacity (VO_2max) when changes in cardiopulmonary function are limited. Thus, to primarily target skeletal muscle adaptations, we employed an alternating single-leg knee extension exercise training protocol and evaluated the impact on skeletal muscle morphology and whole-body exercise capacity. METHODS: Eight sedentary volunteers (20±1 years; VO_2max: 38.4±2.6 mL/kg/min) completed 12 weeks of progressive alternating single-leg training. Subjects performed a bilateral maximal graded exercise test on a cycle ergometer, a single-leg peak workload test on a knee extensor ergometer, knee extension muscle function testing, and a dual energy X-ray absorptiometry scan before and after training. Skeletal muscle biopsies were obtained before and after the training protocol to assess muscle fiber type, by determining myosin heavy chain isoform composition using gel electrophoresis, and fiber size using immunofluorescent staining of muscle cross sections. RESULTS: Following training there were increases (P<0.05) in VO_2max (2.53±0.25 vs. 2.76±0.20 L/min; 9±3%), the VO2 at ventilatory threshold (1.88±0.19 vs. 2.01±0.18 L/min; 8±2%), peak two-leg cycling workload (183±17 vs. 206±19 W; 13±2%), single-leg peak workload (40.0±4.4 vs. 55.3±5.1 W; 43±8%), skeletal muscle lean mass (3±1%), citrate synthase protein content (44±20%), as well as slow and fast myofiber size (13±4% and 14±4%, respectively) while there was a decrease (P<0.05) in hybrid fiber content (-33±8%). CONCLUSIONS: These data suggest that inducing a more oxidative phenotype in skeletal muscle with isolated aerobic exercise training improves cardiopulmonary responses to maximal exercise. Additionally, the low cardiovascular burden associated with isolated single-leg exercise may be a useful training approach for clinical populations with skeletal muscle dysfunction or cardiovascular limitations to exercise.