We have investigated the appropriate joint angle for detecting frequent alternating activity in synergistic muscles and the relationship between muscle activation patterns and endurance during static low-level contractions. Eleven healthy men performed prolonged static plantar flexion of the ankle at 10% of the maximal voluntary contraction, with the ankle flexed at 100°, 110°, or 120°, while seated with the right leg in full extension. The onset of muscle activation and/or inactivation was detected using quantitative analysis, and alternate activity among muscles was detected using a threshold criterion of ×e or ×1/e multiplied by the levels of mean electromyograms (EMG) calculated at 1-min intervals. Surface EMG revealed frequent alternations of activity among the lateral and medial gastrocnemius and soleus muscles at an ankle flexion of 110°. The first alternation occurred after approximately 15 min of exercise. The number of alternations per hour was four- to sevenfold higher at 110° than at 100° or 120°. Endurance was longest and shortest at 110° and 120°, respectively. These findings suggest that synergistic motor pools activated at a specific joint angle (110°) affect muscle endurance during static low-level fatiguing tasks.