This is due to the fact that the muscle moves in the same directions as the joint angle change. The type of contraction responsible for what we think of as work is concentric, or an acceleration. However, eccentric contractions of skeletal muscles, or deceleration, produce the greatest force, many times more than concentric contractions. Furthermore, these two contractions occur together. 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.