ELASTIC AND LENGTH-FORCE CHARACTERISTICS OF THE GASTROCNEMIUS OF THE HOPPING MOUSE (NOTOMYS ALEXIS) AND THE RAT (RATTUS-NORVEGICUS)

The aim of this study was to compare the contractile and series elasti c properties of terrestrial mammals that use bipedal versus quadrupeda l gaits. The gastrocnemius muscle of the hopping mouse (body mass 30.2 +/-2.4 g, mean +/- S.D.) and the rat (313+/-10.7 g) were compared with data from the literature for the wallaby and the kangaroo rat to dist inguish scaling effects and locomotion-related effects on muscle prope rties, Contractile length-force properties and series elastic stiffnes s were measured in situ during maximal tetanic contractions. The rat h ad a larger muscle-fibre-to-tendon-length ratio. The rat and hopping m ouse showed similar normalised length-force characteristics of the gas trocnemius. Normalised stiffness in the hopping mouse was higher. The hopping mouse showed a higher capacity to store elastic energy per uni t of contractile work capacity, as well as per unit of body mass. Acco unting for body size differences, the rat had a smaller relative muscl e mass and thus smaller work capacity than the three hopping animals c onsidered. This is in agreement with a quadrupedal versus bipedal loco motion style. The differences in contractile and elastic properties of the gastrocnemius of the rat and hopping mouse seem to be closely rel ated to locomotion patterns, Small animals seem to be able to utilise the storage and release of elastic energy to a far lesser extent than larger animals, However, even in animals as small as hopping mice, the storage and utilisation of elastic energy during locomotion is of fun ctional significance and probably depends on locomotor behaviour.