Biochemical Foundations of Human Vital Activity - Volkov, N.I., Nesen, E.N. 2000

Biochemistry of Sports
Bioenergetics of Muscular Activity

As shown in Chapter 14, the direct source of energy for muscular activity is ATP. Energy is released through the Enzymatic Hydrolysis of an ATP molecule into ADP and orthophosphate:

Class="center">

During Muscle contraction, chemical energy is converted into mechanical work, while during relaxation, it powers The Active Transport of Ca²⁺ into the sarcoplasmic reticulum. A significant amount of ATP in Skeletal Muscle is consumed by Na⁺-K⁺-ATPase, which maintains the required concentration of Na⁺ and K⁺ ions within the muscle, thereby establishing an Electrochemical Potential across the sarcolemma.

The ATP content in Muscles is quite low, amounting to about 5 mmol ⋅ kg^-1 of wet tissue mass (0.25–0.40%). This level is maintained relatively constant; an increase in muscle ATP concentration inhibits Myosin ATPase—preventing cross-bridge formation between Actin and myosin filaments in myofibrils and thus inhibiting muscle contraction—while a drop below 2 mmol ⋅ kg^-1 of wet tissue mass disrupts the function of the Ca²⁺ pump in the reticulum and impairs muscle relaxation.

ATP reserves in muscle fibres can sustain intense activity for only a very brief period—0.5–1.5 s, or 3–4 maximal single twitches. Continued muscular work relies on the rapid restoration (resynthesis) of ATP from its breakdown products, utilising The energy released during this degradation:

The reaction involving The addition of a phosphate group is called phosphorylation, whereas the Transfer of phosphate from one substance to another is known as transphosphorylation.

The Energy Sources for ATP resynthesis in skeletal muscle and other Tissues include high-energy phosphate compounds present in the tissues (creatine phosphate, ADP) or generated during the Catabolism of Glycogen, Fatty acids, and other energy substrates (e.g., metabolites such as diphosphoglyceric and phosphoenolpyruvic acids), as well as the energy of the proton (H⁺) gradient across the mitochondrial membrane, produced As a result of the aerobic oxidation of various substances (see Chapter 3).

Depending on the biochemical pathway supplying the energy for ATP molecule formation, four mechanisms or pathways of ATP resynthesis are distinguished in tissues. Each mechanism possesses distinct metabolic and bioenergetic characteristics. The energy supply for muscular work utilizes different mechanisms depending on the intensity and duration of the exercise performed.