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.
Energy exists in a variety of forms.
Energy can be found in the form of electric, mechanical, internal energy, etc. The overall energy exchange in a working muscle is from glycolysis. In muscle function, the energy utilized is mechanical energy. Whenever an individual thinks of energy, one tends to think about the ability to move faster or enjoy greater flexibility. As such, internal energy decreases as a result of extension and increased with shortening of the muscle.