No abstract

Three muscles from the proximal equine forelimb were dissected in order to investigate their potential to contribute to proximal limb mechanics. Muscle mass, fibre length, tendon mass and tendon length were measured from biceps brachii, triceps brachii, supraspinatus and lacertus fibrosus (biceps lateral head mass 171-343.4 g and fibre length 0.5-0.8 cm; biceps medial head mass 283 -500 g and fibre length 2.2-4 cm; biceps tendon mass 121.8-260 g and tendon length 35 -44 cm; triceps long head mass 3200-6663 g and fibre length 19-26.3 cm; triceps lateral head mass 513.8-1240 g and fibre length 17.5 -24 cm; triceps medial head mass 85.2-270.6 g and fibre length 9-16.8 cm;supraspinatus mass 793 -1546 g and fibre length 4.7-12.4 cm; lacertus fibrosus mass 4.6-12.4 g and length 10-16 cm). Physiological cross-sectional area (PCSA) and maximum isometric force were estimated for each muscle, and moment arm measurements were taken at the shoulder and elbow joints. Biceps has a greater isometric forcegenerating capacity than supraspinatus. It also appears to have a larger shoulder moment arm, so could therefore have the potential to make a greater contribution to the shoulder moment than supraspinatus. Supraspinatus is likely to function primarily as a shoulder stabilizer rather than a shoulder extensor. Biceps also functions as an elbow flexor and data here indicate that it has a greater PCSA and isometric force-generating capacity than its antagonist triceps brachii. Calculation of tendon forces showed that the biceps tendon can withstand much greater forces than lacertus fibrosus. This study will enable further investigation into the interaction between energy recycling in elastic tissues and the generation and absorption of mechanical work by adjacent muscle groups in the equine forelimb.

SUMMARYThe equine biceps brachii (biceps) initiates rapid limb protraction through a catapult mechanism. Elastic strain energy is slowly stored in an internal tendon and is then rapidly released to protract the forelimb. The muscle fibres are short, have little scope for length change and can therefore only shorten slowly compared with the speed at which the whole muscle must shorten, which makes them poor candidates for driving rapid limb protraction. We suggest that the muscle fibres in the biceps act to modulate the elastic energy output of the muscle-tendon unit (MTU) to meet the demands of locomotion under different conditions. We hypothesise that more elastic strain energy is stored and released from the biceps MTU during higher speed locomotion to accommodate the increase in energy required to protract the limb and that this can be achieved by varying the length change and activation conditions of the muscle. We examined the work performed by the biceps during trot and canter using an inverse dynamics analysis (IDA). We then used excised biceps muscles to determine how much work could be performed by the muscle in active and passive stretch-shorten cycles. A muscle model was developed to investigate the influence of changes in activation parameters on energy storage and energy return from the biceps MTU. Increased biceps MTU length change and increased work performed by the biceps MTU were found at canter compared with at trot. More work was performed by the ex vivo biceps MTU following activation of the muscle and by increasing muscle length change. However, the ratio of active to passive work diminished with increasing length change. The muscle model demonstrated that duration and timing of activation during stretch-shorten cycles could modulate the elastic energy storage and return from the biceps. We conclude that the equine biceps MTU acts as a tuneable spring and the contractile component functions to modulate the energy required for rapid forelimb protraction at different speeds. Supplementary material available online at