Muscle shortening, heat and stretch
Muscle shortens during active contraction. For example we effectively instruct our biceps muscle to shorten in order to lift a weight held in the palm of our hand. The weight rises, gaining potential energy. It is also gaining momentum which depends on the speed of the movement and on the mass of the weight. The work carried out by the muscle is the product of velocity and mass. The energy for the work is derived from the hydrolysis of ATP. The conversion of energy into work is not 100% efficient. The efficiency depends on the speed of shortening and on the type of muscle fibres used for the movement. We shall discuss fibre types later on. Energy that is not converted into work is converted into heat. Heat production is an important role of muscle and contributes to thermal regulation of the body. Shivering is an example of small scale contractions designed to maximize heat production. Excess heat during exercise is dissipated through the blood supply, skin, perspiration and exhaled air and water. Insufficient heat dissipation may limit muscle performance in some circumstances. Heat measurements during exercise can be used to calculate the efficiency of contraction and of ATP hydrolysis (See the excellent review by Woledge, Curtin and Homsher, 1985).
We have seen that muscle is stretched by antagonistic muscles to restore the initial length. It is also stretched during ex-centric exercise where muscle resists stretch. This occurs also when we use our legs to absorb the shock of landing after a jump, or when we lose at arm wrestling. This is an important function which muscles share with the tendons. During stretch, the muscle is not performing any work, but work is performed on the muscle. Muscle acts like a brake. This requires special properties: it must become stiff very rapidly, and it must be able to absorb stretch gradually, over relatively long distances, and without damage. We are interested in stretch as this reveals important properties of the molecular interactions between myosin and actin.
