Lichtwark Glen A, Watson Johanna C, Mavrommatis Sophia, Wilson Alan M
Structure and Motion Laboratory, The Royal Veterinary College, Hawkshead Lane, North Mymms, Hatfield, Hertfordshire AL9 7TA, UK.
J Exp Biol. 2009 Aug;212(Pt 15):2454-63. doi: 10.1242/jeb.027995.
The 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.
马的肱二头肌通过弹射机制启动肢体的快速前伸。弹性应变能缓慢地储存在一条内部肌腱中，然后迅速释放以伸展前肢。肌纤维较短，长度变化的范围很小，因此与整个肌肉必须缩短的速度相比，它们只能缓慢缩短，这使得它们不太适合驱动肢体的快速前伸。我们认为，肱二头肌中的肌纤维起到调节肌肉-肌腱单元（MTU）弹性能量输出的作用，以满足不同条件下运动的需求。我们假设，在更高速度的运动过程中，肱二头肌MTU会储存和释放更多的弹性应变能，以适应伸展肢体所需能量的增加，这可以通过改变肌肉的长度变化和激活条件来实现。我们使用逆动力学分析（IDA）研究了肱二头肌在小跑和慢跑过程中所做的功。然后，我们使用切除的肱二头肌来确定该肌肉在主动和被动拉伸-缩短循环中能够做多少功。我们开发了一个肌肉模型，以研究激活参数的变化对肱二头肌MTU能量储存和能量回返的影响。与小跑相比，发现慢跑时肱二头肌MTU的长度变化增加，所做的功也增加。肌肉激活后以及增加肌肉长度变化时，离体肱二头肌MTU所做的功更多。然而，主动功与被做功的比值随着长度变化的增加而减小。肌肉模型表明，拉伸-缩短循环中激活的持续时间和时机可以调节肱二头肌的弹性能量储存和回返。我们得出结论，马的肱二头肌MTU起到一个可调节弹簧的作用，收缩成分的功能是调节不同速度下前肢快速前伸所需的能量。
