Effect of endurance training on muscle activation and force sensation.
One of the consequences of endurance training is a reduction in force sensation in trained muscles at any exercise intensity. To study the central and peripheral contributions to this adaptation, we trained six male subjects with single-leg cycling at 60% VO2 peak (30 min/day x 3 days/week x 8 weeks); six others were matched controls. Measurements were made during separate 20-min, single-leg rides at 70% pre-training VO2 peak, with trained (TR), untrained (UT), and control (CT) legs, before and after training. No pre-post differences were observed in the control group. VO2 peak increased 18% (p < 0.05) in the TR leg and 6% (p < 0.05) in the UT leg of the trained subjects. Force sensation was significantly less in both the TR (70%; p < 0.05) and UT (50%; p < 0.05) legs during 20 min of single-leg cycling after training. Vastus lateralis EMG, plasma lactate, and heart rate were all significantly (p < 0.05) lower when cycling with either the TR or UT leg, which were both lower than when cycling with the CT leg, at the end of each 20-min ride. These data reflect an intramuscular environment that is better adapted to endurance performance by virtue of both central and peripheral mechanisms. Thus, there is less need to recruit additional motor units to maintain the same power output, and this reduced motor outflow leads to a decline in force sensation.