IV.1. Muscle energetics

• The average adult oxygen consumption is 0.25 l/m which means 5 kJ of energy use. If the muscle activity increases the oxygen demand and consumption of the muscle grows as well (up to 20-50 fold).
• At the beginning of the increased muscle activity a certain level of oxygen deficiency occurs. Muscle vasodilation forms along with an increase in cardiac output and the breathing becomes faster.
• In work physiology the difficulty of physical work is measured by the rate of energy consumption.

In terms of energy consumption the following categories can be mentioned:

1. Light physical work occurs when <627 kJ/hr, the oxygen consumption is <0.5 l/m.
2. Moderate physical work occurs when 627-1254 kJ/hr, the oxygen consumption is 0.5 <1 l/m.
3. Hard physical work occurs when 2058 kJ/hr, the oxygen consumption 1<2 l/m.  In case of a well-trained athlete this value can go up to 4 l/m.

• The immediate source of energy for muscle contraction is called adenosine triphosphate (ATP). Creatine phosphate serves as a quick replacement for ATP. In the tired muscle the level of creatine phosphate decreases but the ATP level is constant.
• ATP is formed during aerobic circumstances by terminal oxidation and under anaerobic conditions by the released energy caused by the fermentation of lactic acid.
• The created ATP fills in the creatine phosphate reservoir.
• During aerobic conditions the power source of the muscle contraction is created by the oxidation of glycogen.
• During anaerobic conditions the energy production is accomplished by glycogen-lactic acid transformation. Tired muscles and fatigue are caused by the depletion of O2, the accumulation of acidic metabolites, hypothermia, and hypoglycemia (low blood sugar) which lead to the exhaustion of the contractile apparatus.