Differences of Fatigability in Human Skeletal Muscles

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The relationship between force level of isometric contraction and time until three indices of muscular fatigue was studied for the shoulder abductors, elbow flexors, elbow extensors, knee flexors, knee extensors, plantar flexors and dorsiflexors in 5-7 males. Endurance tasks were performed at the contraction levels of 5, 10, 15, 20, 30, 40 and 50% of the maximal voluntary contraction (MVC). The relative force (F %MVC) versus the endurance time or the time of tiredness("darui") and pain("itai") appearance (T min) curves were fitted to the following equation; logT=a+blog(F-k), 0<=k<=30. In the endurance time the constants k, a and b in the most fitted equation were 3, 2.02 and -1.22 for the shoulder adbuctors, 0, 4.33 and -2.52 for the elbow flexors, 2, 3.05 and -1.81 for the elbow extensors, 0, 3.05 and -1.63 for the knee flexors, 8, 1.96 and -1.23 for the knee extensors, 0, 4.53 and -2.47 for the plantar flexors, and 0, 4.51 and -2.48 for the dorsiflexors, respectively. In the time of pain appearance these values were 2, 1.64 and -1.09 for the shoulder abductors, 0, 3.33 and -1.99 for the elbow flexors, 2, 2.25 and -1.43 for the elbow extensors, 0, 2.94 and -1.78 for the knee flexors, 8, 1.35 and -0.96 for the knee extensors, 0, 3.88 and -2.16 for the plantar flexors, and 0, 3.90 and -2.24 for the dorsiflexors, respectively. In the examined muscle groups, the shoulder abductors were the most fatigable. The least fatigable were the plantar flexors and dorsiflexors, followed by the elbow flexors. The knee extensors were fatigable at higher contraction levels, and relatively fatigueless at very low contraction levels. The knee flexors, which were relatively fatigueless at higher contraction levels, became fatigable at lower contraction levels, and showed similar fatigability as the elbow extensors.