The descending limb of the force‐sarcomere length relation of the frog revisited.

Abstract

SUMMARY1. We studied the descending limb of the force-sarcomere length relation in single frog muscle fibres using sarcomere isometric contractions.2. Sarcomere length was measured simultaneously with two independent methods: a laser diffraction method and a segment length method that detects the distance between two markers attached to the surface of the fibre, about 800 ,um apart. Both methods were used to keep sarcomeres at constant length during contraction.3. Fibres were selected for low resting tension since it was known from previous experiments that for such fibres the force developed by fixed-end tetani is much higher than that predicted by the degree of filament overlap.4. With fixed-end tetani, force decline with increase of sarcomere length was small between 2-0 and 3'0 gim. At a sarcomere length of 3 0 gim, force was about 90 % of maximal.5. With sarcomere isometric tetani, force was considerably lower than with fixedend tetani. Force was maximal at about 2-1 ,um and decreased to zero at about 3-6 ,tm. At intermediate lengths the descending limb was within 80 nm of the values predicted from filament overlap.6. We investigated why force of fixed-end contractions was much higher than that generated by sarcomere isometric contractions. 7. During the force plateau of fixed-end tetani at sarcomere lengths longer than about 2'0-2-2 /im, sarcomeres in the fibres mid-region were not isometric, but instead stretched slowly. By measuring the force-velocity relation it was shown that this slow stretch elevates active force well beyond sarcomere isometric force. 8. Stretch of the central region was also observed during the tetanic force rise. This was shown to result in an increase of passive force that grew larger at longer sarcomere lengths. At about 3-6 ,m the increase of passive force was similar to the total force generated by fixed-end contractions at this length.9. Laser diffraction and segment length methods gave the same results, diminishing the chance that any systematic artifact underlies our findings.

Microfluidic perfusion shows intersarcomere dynamics within single skeletal muscle myofibrils

“…Our microfluidic perfusion system allowed the control of a long known phenomenon of muscle contraction, the nonuniform lengths of sarcomeres during activation (26,27). From the proposal of the sliding filament theory (1, 2), nonuniform behavior of sarcomeres and its effects in force production during contraction have puzzled scientists.…”

Controlled intermittent shortening contractions of a muscle–tendon complex: muscle fibre damage and effects on force transmission from a single head of rat EDL

“…However, in the present study, the muscles were allowed to recover until isometric muscle force remained constant and the shape of the time-force trace resembled that of an isometric tetanic contraction of a muscle in the unfatigued state. (b) The distribution of lengths of sarcomeres arranged in series within muscle fibres (Huxley and Peachey, 1961;Granzier and Pollack, 1990;Pollack et al, 1993), and (c) the distribution of fibre mean sarcomere length (Ettema and Huijing, 1994;Willems and Huijing, 1994). Finite element modelling has shown that such distributions are affected as a result of more compliant connections between muscle fibre and extracellular matrix (Yucesoy et al, 2002).…”

The mechanisms of the residual force enhancement after stretch of skeletal muscle: non-uniformity in half-sarcomeres and stiffness of titin

“…Non-uniformities are associated with several experimental observations: the force creep in contractions produced at long sarcomere lengths [31,35], differences in force obtained when fibres are activated with sarcomere length control or fibre length control [13, 31,36], and the kinetics of force development and relaxation [34,37,38]. Historically, sarcomere length nonuniformity has been incorporated into a hypothesis to explain the residual force enhancement [39,40].…”