Mean sarcomere length-force relationship of rat muscle fibre bundles

To study how sarcomere length inhomogeneities and the duration of activation affect sarcomere length-force characteristics of muscle, the mean sarcomere length-force relationship was determined for twitches and at 100 and 300 ms during tetanic activation for rat extensor digitorum longus and gastrocnemius medialis muscle fibre bundles. Mean sarcomere length is the mean length of all sarcomeres within the fibre, calculated by dividing fibre length by the number of sarcomeres in series in the fibre. The twitch mean sarcomere length-force relationship is shifted to larger sarcomere lengths (optimum mean sarcomere length = 2.69 microns) compared to the relationships determined at 100 or 300 ms of tetanic activation (optimum mean sarcomere length = 2.38 microns), which were the same. It is shown that the normalized Gordon et al. rationale results in a large overestimate of force (at most 68% of force at a sarcomere length of 1.60 microns) for mean sarcomere lengths between 1.4 and 2.0 microns, and in an underestimate of force between 2.3 and 3.0 microns. It is concluded that modelling skeletal mammalian muscle length-force relationships can be improved by using mean sarcomere length-force relations of mammalian fibres instead of the normalized rationale of Gordon et al. derived from a selected homogeneous part of frog fibre.