This is called the stretch-shortening cycle. Gaseous work is evaluated in terms of volume change and the same holds true for skeletal muscles. In the stretch-shortening cycle, an eccentric contraction produces a decrease in volume, while a concentric contraction returns the muscle to its relaxed state and thus increases the volume. The value of efficiency increases when first negative, then positive, work is done. Taken together, both types of work are directly related to another thermodynamic variable, heat.
Thermal changes occur in the muscle as it goes through the stretch-shortening cycle.
The first category involves heat produced before the muscle action and while it is in progress. In the heat of activation, initial heat is actually at a maximum before any movement is detectable. There is no gradual buildup. In the heat of shortening, this initial heat value is proportional to the change in length. As a whole, heat and work considered together lead to the third variable, energy.