Differences in cardiorespiratory and neuromuscular responses between voluntary and stimulated contractions of the quadriceps femoris muscle

The aim of this study was to compare respiratory gas exchange variables and muscle fatigue between equal-intensity (i.e., same force output) electrostimulated and voluntary contractions of the quadriceps muscle (46 ± 10% of maximal voluntary force). Twelve healthy men served as volunteers. Oxygen consumption, ventilation and respiratory exchange ratio were recorded during the exercise bouts. Muscle fatigue was quantified as the exercise-induced reduction in maximal voluntary force. The average oxygen consumption (11 ± 3 versus 8 ± 2 mL min−1 kg−1), ventilation (23 ± 4 versus 16 ± 2 L min−1) and respiratory exchange ratio (0.96 ± 0.02 versus 0.85 ± 0.01) were significantly higher during electrostimulation compared with voluntary exercise (P < 0.05–0.001). Maximal voluntary force decreased significantly after electrostimulation (−21 ± 10%; P < 0.001), while no changes were observed following voluntary exercise. Electrostimulation-resistance exercise of the quadriceps muscle elicited greater cardiorespiratory demand and muscle fatigue compared with voluntary contractions of the same intensity. These findings probably reflect differences in the patterns of motor unit recruitment between stimulated and voluntary contractions, despite equal force productions.