Influence of muscle architecture on the length-force diagram of mammalian muscle.

The functions and geometrical characteristics of a pennate and a parallel-fibered muscle in rats are quantified and compared to each other. The pennate medial gastrocnemius and the more parallel fibered semimembranosus are investigated in fourteen male Wistar rats. The length force diagram, twitch time characteristics and muscle architecture are quantified. In the parallel fibered semimembranosus the length of the muscle fibres is about 70% of the muscle optimum length (the length at which the muscle performs maximal active force), while in the pennate fibered gastrocnemius the length of the muscle fibres is 36% of the muscle optimum length. The active length force diagrams normalized with respect to the muscle optimum length show considerable differences between semimembranosus and gastrocnemius: The normalized active length force diagram runs from about 71%-129% of the muscle optimum length for the semimembranosus and from about 82-118% for the gastrocnemius. The latter muscle also has a steeper normalized passive length force curve and produces more active tetanic force per gram muscle (877 g/g for gastrocnemius versus 379 g/g for semimembranosus). No differences between the semimembranosus and gastrocnemius are found with respect to the passive tension at twitch optimum length (38 g/cm2 versus 32 g/cm2), the maximal active tension (1.27 kg/cm2 versus 1.18 kg/cm2) and the twitch contraction time at twitch optimum length (43.8 ms versus 48.4 ms). It is concluded that several functional characteristics show a linear relationship with the index of architecture, a measure for the muscle architecture, while others (especially time and tension characteristics) are independent of muscle architecture.