No abstract

Recent data suggest that the potentiated twitch is a more sensitive index of contractile fatigue than is the unpotentiated twitch. We hypothesized that after a potentially fatiguing load, the fall in twitch amplitude of the potentiated twitch would be significantly greater than that of an unpotentiated twitch. We compared the response of the potentiated and unpotentiated twitches to a series of potentially fatiguing loads using magnetic stimulation of the femoral nerve in 10 healthy subjects. The baseline unpotentiated quadriceps twitch force (TwQu), potentiated quadriceps twitch force (TwQp), and maximal voluntary contraction (MVC) were 129 plus minus 6 N, 198 plus minus 6 N, and 622 plus minus 25 N, respectively. During a fatigue protocol that was designed to induce a spectrum of fatigue from mild to marked, the percent fall in quadriceps twitch force was significantly greater for the potentiated method than for the unpotentiated method at all levels of fatigue (P <.005). The within-subject within-day coefficient of variation was 7.5 plus minus 0.5% for TwQu and 5.6 plus minus 0.9% for TwQp. Thus, TwQp is reproducible and is superior to TwQu for detecting early muscle fatigue.

The purpose of this study was to determine whether diaphragmatic fatigue occurs after high-intensity constant-load whole-body exercise to volitional exhaustion. Ten sedentary subjects with a maximal oxygen uptake of 2.52 +/- 0.47 L/min were studied. Subjects exercised on a bicycle ergometer at 80% of their maximal working capacity until volitional exhaustion. Minute ventilation during the last minute of exercise was 89.9 +/- 13.6 L/min, which represented 50 +/- 6% of the subjects' 12-s maximal voluntary ventilation. During the last minute of exercise, mean inspiratory esophageal pressure was 18.1 +/- 5.3 cm H2O, which represented only 15 +/- 4% of the subjects' maximal static inspiratory pressure. Bilateral transcutaneous supramaximal phrenic nerve stimulation was performed before and 10, 30, 45 and 60 min after exercise. Twitch diaphragmatic pressure (twitch Pdi) was significantly decreased after exercise in seven of the 10 subjects. For the group as a whole, twitch Pdi fell from 28.9 +/- 3.7 cm H2O during control to 23.9 +/- 5.1 cm H2O at 10 min after exercise (p< 0.005). The fall in twitch Pdi was due to a significant decrease in twitch esophageal pressure from 19.6 +/- 4.3 cm H2O during control to 15.5 +/- 4.9 cm H2O (p < 0.001). Twitch gastric pressure was not significantly different: 8.7 +/- 4.0 cm H2O, compared with 9.2 +/- 3.8 cm H2O during control. Twitch Pdi recovered to 93 +/- 7% of control values at 60 min after exercise. The fall in twitch Pdi after exercise indicates that diaphragmatic fatigue can occur following heavy endurance exercise in sedentary healthy persons.