Response to Endurance Training Using Critical Speed: Influence of Genetic Background and Exercise Intensity.

Exercise is used as a therapeutic option to improve an individual's quality of life or as a method to attenuate the severity of cardiovascular, respiratory, or metabolic-related diseases. However, changes in cardiorespiratory fitness in response to endurance training are often heterogenous with individuals being more or less responsive to a similar exercise protocol. Heterogeneity in responses to exercise training is observed in humans and rodents. Low responses to training might be mitigated by modifying the standard exercise training protocol by altering the frequency, intensity, or duration of training. Threshold-based exercise training prescription, e.g., critical speed, has also been proposed as a means to reduce the variability of training responses. The aim of this study was to identify the effect of different exercise training intensities on changes in exercise capacity in two inbred strains of mice, NZW/LacJ (NZW) and FVB/NJ (FVB). NZW mice were selected because they respond poorly to moderate intensity endurance exercise whereas, FVB mice respond well to endurance training. We hypothesized that increasing the intensity of the training would reduce the number of low or non-responding NZW mice. To test this hypothesis, female mice from each strain were assigned to one of three treadmill training groups based on critical speed (CS): 80% of their critical speed (CS80%), 90% of their critical speed (CS90%), or sedentary controls (SED). Exercise groups within each strain were volume matched and thus ran the same distance over the 6-week training period. Pre-training exercise capacity differed between NZW (30.5 ± 3.5 min) and FVB (36.1 ± 2.8 min) mice (P < 0.001). Critical speeds also differed between strains (NZW: 25.1 ± 2.4 m/min; FVB: 32.9 ± 1.6 m/min, P < 0.001). After training, both FVB exercise groups, CS80% (10.0 ± 4.9 min, P = 0.004) and CS90% (14.0 ± 7.7 min, P = 0.007), improved their endurance capacity. In contrast, NZW CS80% did not significantly improve after training (0.4 ± 4.6 min), but NZW CS90% improved significantly (3.6 ± 2.7 min, P = 0.02). Collectively, these data indicate that the high-responder FVB strain can adapt to moderate-intensity and high-intensity endurance training. The low-responder NZW strain only showed significant improvement after sufficient homeostatic stress was induced through high-intensity endurance training. These data suggest that increasing exercise intensity can improve responses to exercise training in low- or non-responding mice.