IN-VIVO MUSCLE FORCE AND ELASTIC ENERGY-STORAGE DURING STEADY-SPEED HOPPING OF TAMMAR WALLABIES (MACROPUS-EUGENII)

In order to evaluate the role of elastic energy recovery in the hoppin g of macropodids, in vivo measurements of muscle-tendon forces using b uckle force transducers attached to the tendons of the gastrocnemius ( G), plantaris (PL) and flexor digitorum longus (FDL) of tammar wallabi es were made as the animals hopped on a treadmill at speeds ranging fr om 2.1 to 6.3 m s(-1). These muscles and tendons constitute the main s tructures that are most important in energy storage and recovery, Elec tromyographic recordings from the lateral gastrocnemius and plantaris muscles, together with highspeed films (200 frames s(-1)) and video (6 0 fields s(-1)), were also used to correlate muscle activation and kin ematic patterns of limb movement with force development, On the basis of in situ calibrations of the buckle transducers, we found that muscl e forces and elastic energy storage increased with increased hopping s peed in all three muscle-tendon units, Elastic energy recovery reached a maximum of 25% of metabolic energy expenditure at 6.3 m s(-1) and i s probably greater than this at higher speeds, Force sharing among the three muscles was consistently maintained over this range of speeds i n terms of recruitment, Although forces and stresses were generally co mparable within the gastrocnemius and plantaris muscles, maximal tendo n stresses were considerably greater in the gastrocnemius, because of its smaller cross-sectional area (peak muscle stress: 227 versus 262kP a; peak tendon stress: 36 versus 32MPa, G versus FL). As a result, ene rgy storage was greatest in the gastrocnemius tendon despite its much shorter length, which limits its volume and, hence, energy storage cap acity, compared with PL and FDL tendons, Forces and stresses (17MPa ma ximum) developed within the FDL tendon were consistently much lower th an those for the other two tendons, Peak stresses in these three tendo ns indicated safety factors of 3.0 for G, 3.3 for PL and 6.0 for FDL, The lower stresses developed within the tendons of the plantaris and, especially, the flexor digitorum longus may indicate the need to maint ain sufficient stiffness for phalangeal control of foot placement, at the expense of reduced strain energy recovery.