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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.

We have recently shown that patients with chronic obstructive pulmonary disease (COPD) develop contractile fatigue of their quadriceps muscle following endurance exercise. Pulmonary rehabilitation can produce physiological adaptations in patients with COPD. We hypothesized that if pulmonary rehabilitation induces physiological adaptations in the exercising muscle, it should become more fatigue resistant. Twenty one patients with COPD, mean age 69.9 +/- 1.9 yr, FEV(1) 45 +/- 4% predicted, participated in an 8-wk outpatient, supervised pulmonary rehabilitation exercise program. Quadriceps contractile fatigue was detected by a fall in quadriceps twitch force postexercise. Twitch force was measured during magnetic stimulation of the femoral nerve. Because potentiated twitches may be more sensitive at detecting fatigue, both unpotentiated (TwQu) and potentiated (TwQp) twitches were obtained before and 10, 30, and 60 min after constant load cycle exercise. Prerehabilitation, during constant load exercise, patients exercised at 37 +/- 4 W for 11.2 +/- 1.8 min. Prerehabilitation, TwQu fell significantly postexercise down to a minimum value of 82.5 +/- 3.1% of the baseline preexercise value (p < 0.001). Similarly, prerehabilitation, TwQp fell significantly postexercise down to a minimum value of 73.9 +/- 3.9% of baseline (p < 0.001). Postrehabilitation, for the same intensity and duration of exercise, TwQu was not significantly different from baseline at any time postexercise. Postrehabilitation, TwQp fell significantly postexercise but the fall in TwQp with exercise was significantly less postrehabilitation compared with prerehabilitation (p < 0.001). In conclusion, pulmonary rehabilitation resulted in increased fatigue resistance of the quadriceps muscle in patients with COPD.

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The purpose of this study was to compare quadriceps fatigability in patients with varying severity of chronic obstructive pulmonary disease with age-matched control subjects. Ten healthy control subjects, 8 patients with severe disease (FEV1 less than 35% predicted), and 11 patients with mild to moderate disease were studied. The FEV1 was 1.75 +/- 0.13 L (SE), 50.4 +/- 2.9% of predicted in the mild to moderate group, and 0.87 +/- 0.06 L, 25.9 +/- 1.9% of predicted in the severe group. Quadriceps fatigue was quantified by the reduction in potentiated twitch force after a potentially fatiguing task. All subjects performed three sets of 10 maximum voluntary contractions of the right quadriceps muscle. Quadriceps maximum voluntary contraction force was 58.3 +/- 3.3 kg for the healthy older group, 49.0 +/- 4.2 kg in the mild to moderate group, and 44.3 +/- 4.7 kg in the severe group. The fall in potentiated twitch force after exercise was significantly greater in the patients with severe disease than in the healthy control subjects. In conclusion, the quadriceps in patients with severe chronic obstructive pulmonary disease are more fatigable than those in age- and sex-matched healthy control subjects.

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