Force Length Relationship

The amount of force that a sarcomere and consequently a muscle can deliver is strongly dependant on the length of the sarcomere at which it is delivering force. This is caused by the amount of overlap between the myosine and actin filaments at a given length. The greater the number of myosin head that can bind to the actin filaments, the higher the force that can be produced. However, when the sarcomere is maximally shortened, it can occur that the oposite actin filaments colide with one another and slide past each other. This results in less cross bridges being formed between actin and myosin filaments and extra resistance during contraction and therefore a reduction in produced force. In the figure below can be seen that a sarcomere builds up strength during shortening until it peaks and decreases due to a decrease in crossbridge forming surface on the filaments. Force Length relationship

References:
-Gordon, A.M., Huxley, A.F., and Julian, F.J. (1966). The variation in isometric tension with sarcomere length in vertebrate muscle fibers. J. Physiol., 184, 170-192.

-Wilmore, J.H., Costill, D.L., Kenney, W.L. (2008). Structure and function of exercising muscle. Physiology of Sport and Exercise Fourth Edition. USA Human Kinetics