ENDURANCE EXERCISE TRAINING ALTERS THE DIAPHRAGMATIC FORCE-FREQUENCY RELATIONSHIP 159

Abstract

Preliminary experiments in our laboratory have suggested that endurance training promotes a left shift in the diaphragmatic force-frequency relationship, but the results are inconclusive. In order to resolve this issue, we tested the hypothesis that endurance training will result in a leftward shift in the force-frequency curve of the diaphragm (DIA). Female Sprague-Dawley rats (120 days) were randomly assigned to either a control(CON, n=8) or endurance exercise trained (TR, n=7) group. Following 12 weeks of progressive treadmill training (≈75% VO2 max, 90min/day, 4 days/week), the animals were sacrificed, and the diaphragms were excised.In vitro contractile properties were measured on a strip obtained from the costal diaphragm. Half time to peak tension, half relaxation time and rate of force development did not differ (P>0.05) between groups. However, there was a left shift in the force-frequency relationship due to training. Specifically, tetanic forces at stimulation frequencies of 30, 40, 50 and 80 Hz were significantly greater (P<0.05) in the TR compared to CON. In contrast, maximal tetanic forces at 200 Hz did not differ (CON= 28.22 ± 2.3, TR= 31.05 ± 1.0 N/cm2). We conclude that endurance exercise training results in a left shift in the force-frequency characteristics of the costal DIA. This finding suggests that high intensity endurance training causes an alteration in excitation-contraction coupling that increases tetanic forces at submaximal stimulation frequences.