We investigated whether the capacity for repetitive submaximal muscle contraction was reduced in a group of subjects (n = 8) with a primary deficiency of myoadenylate deaminase (MAD). Quadriceps femoris muscle fatigue was evaluated using voluntary and electrically stimulated contractions during 20min of repetitive voluntary isometric contractions at 40% of maximal force-generating capacity (MFGC). After 5min of exercise, MFGC had declined significantly to 70.6±4.1% (mean±S.E.M.) and 87.2±1.6% of baseline values in MAD-deficient and sedentary control subjects (n = 8) respectively (P = 0.002 between groups). After 5min of exercise, the half-relaxation time had increased significantly to 113.4±6.1% of basline in MAD-deficient muscle, but had decreased significantly to 94.1±1.3% in control subjects (P = 0.003 between groups). All control subjects completed the 20-min exercise test. Five of the MAD-deficient subjects had to stop exercising due to early muscle fatigue; however, three of the MAD-deficient subjects were able to complete the 20-min exercise test. In conclusion, although the capacity for repetitive submaximal isometric muscle contractions for the group of MAD-deficient subjects was significantly decreased, it remains uncertain whether MAD deficiency is the sole cause of pronounced muscle fatigue.

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