During exercise, the working muscles
contract, thereby generating power and heat. In other words, physical exercise
is a form of mechanical energy. This generated energy will use up the energy reserves
within the body. During exercise, metabolites and heat are generated, which
affect the steady state of the internal environment. Depending on the form of
exercise, sooner or later, sensations of fatigue and exhaustion will settle in.
The physiological role of these sensations is to protect from the deleterious
effects of exercising, or over-exercising, and help the subject adjust his or
her exercise strategy. The exercise intensity, exercise endurance time and type
of exercise are important factors that vary among different individuals and produce
different effects within the body systems, which in turn create different types
of sensation within the subject's mind during the exercise. Physical exercise
affects the biochemical equilibrium within the exercising muscle cells: mainly,
we speak of inorganic phosphate (source of adenosine triphosphate of ATP, the
primary energy molecule), protons, lactate and free magnesium accumulate within
these cells. They directly affect the mechanical machinery of the muscle cell. The
muscle metabolites produced and the generated heat of muscle contraction are
released into the internal environment, putting stress on the steady state. The
tremendous increase in muscle metabolism triggers a large increase in muscle
blood supply. Nutrients have to be supplied to the exercising muscle by
resorting to energy reservoirs elsewhere in the body. Furthermore, the
contracting muscle fibers release cytokines (inflammation markers), which exert
many effects on other organs, including the brain. All these different
mechanisms sooner or later create sensations of fatigue and exhaustion in the
mind of the exercising subject. The final effect is a reduction or complete
cessation of the exercise.