In a rigid, sealed container, any chemical process carried out within the vessel will do no work. The walls will not move. Where there is work, there is always heat. The reasons are beyond the scope of our studies. But this fact has important implications.
Machines produce work and energy.
The trick is to maximize the heat or the work, maximize the efficiency of conversion of fuels to the form of energy we wish to use. Maximizing heat is much easier to do. For athletes, performance will depend upon maximizing the work. And it is the maximization of work that is most difficult. We learned the foundation of thermochemistry rests on the ability to link the amount of heat released or required to the chemical equation for the specific chemical change.