Central fatigue of the diaphragm and quadriceps during incremental loading.

Anecdotal observations suggest that low frequency fatigue, as judged by a fall in twitch tension, is more difficult to achieve in the diaphragm than nonrespiratory muscle but this hypothesis has not previously been directly tested. We studied 7 subjects by performing incremental repetitive contraction loading protocols of the diaphragm and quadriceps. We measured twitch transdiaphragmatic pressure (Tw Pdi) and twitch quadriceps tension (Tw Q) during both muscle contraction and relaxation phases during the run. Unpotentiated and potentiated Tw Pdi and Tw Q were measured before and at 20, 40, and 60 minutes after the run. During the run, greater activation of the quadriceps was achieved; for example, at 70% of maximal voluntary effort the interpolated Tw Q was 12.5% of the relaxation phase Tw Q (implying activation of 87.5%) compared with 29.4% (i.e., 70.6% activation) for the diaphragm (p = 0.05). A significantly greater fall in Tw Q than Tw Pdi was observed (unpotentiated Tw Pdi at 20 minutes 94% baseline versus Tw Q 59% baseline, p = 0.007). Low frequency fatigue in humans is more difficult to generate in the diaphragm than in the quadriceps muscle due in part to reduced central activation.