Force enhancement in single skeletal muscle fibres on the ascending limb of the force–length relationship

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Abstract
It has been assumed that force enhancement in single fibres of skeletal muscles only occurs on the descending, and not the ascending or plateau region, of the force-length relationship. This assumption has been based, however, on theoretical considerations or isolated experiments, in which neither stretch conditions nor fibre lengths were optimized for force enhancement. Therefore, the purpose of this study was to investigate the residual, steady-state force enhancement following active stretch in single muscle fibres of frog on the ascending limb of the force-length relationship. Twenty-nine stretch experiments on ten single fibres from the lumbrical muscle of the frog Rana pipiens were carried out on the ascending limb of the force-length relationship. Force enhancement was observed in 28 out of the 29 tests. Moreover, the force produced for stretch experiments finishing at optimal fibre length always exceeded the force obtained for an isometric contraction at optimal length. We conclude from these results that steady-state force enhancement occurs systematically on the ascending limb of the force-length relationship, and that the steady-state force in the enhanced state can easily exceed the maximal isometric force of the fibre.