Moderate fatigue studied at great sarcomere lengths in frog single muscle fibres

Single fibres from the anterior tibialis muscle of Rana temporaria (temperature, 2-3 degrees C) were moderately fatigued at a prestretched sarcomere length (approximately 3.6 microns) by reducing the intervals between 2 s tetani from 240 to 20 s. Changes in length of marked segments along the muscle fibre were monitored during fixed-end tetani using a photoelectric recording system. In contrast to the situation at 2.2 microns sarcomere length, the central region of the fibre was elongated during tetanus at the prestretched fibre length, whereas the segments next to the fibre-tendon junctions (end segments) shortened correspondingly. These length changes were associated with a slow climb of force (tension creep). During development of fatigue, the maximum tetanic tension was reduced to about 78% of the control value and the time course of the tetanus was markedly changed. The tension creep observed in control tetanus was greatly reduced during fatigue, the tetanic tension remaining nearly constant after an initial rising phase. The change in shape of the force myogram during fatigue was associated with a considerable reduction in the amplitude of segment movements along the fibre. The results are explainable by assuming that the end segments (having shorter sarcomeres) become more fatigued than other parts in series along the fibre. At the same time the central segments are likely to have acquired a higher ability to resist stretch by the end segments.