Metabolism and Fatigue

Creatine Phosphate (PCr)

• most of the excess ATP within the muscle fibers during resting state is being used to synthesize creatine phosphate

• PCr + ADP ←→ Crea + ATP

• ←→ =CK

• formation of ATP from creatine phosphate occurs very rapidly and is the 1st source of additional energy at the beginning of muscle contraction

• initial amount of ATP + what PCr provides is enough for 15 seconds of muscle contraction 

Anaerobic Respiration

• glucose used to generate 2 molecules of ATP in absence of oxygen during heavy exercising

• glucose from either blood or glycolysis within muscle fibers

• with lack of oxygen, pyruvic acid converts to lactic acid and 80% of it will diffuse back into blood and be converted into glucose in the liver

• ATP produced anaerobically is enough for 30-40 seconds of muscle contraction

Aerobic Respiration

• rest or light to moderate exercise

• circulating blood and myoglobin in muscle gives adequate oxygen supply 

• pyruvic acid enters mitochondria where it is oxidized and produces 36 molecules of ATP, CO₂ , H₂O, and heat

• substrates used by mitochondria within muscle fibers: pyruvic acid, fatty acids, and amino acids as a last resort

• ATP produced by muscle fibers aerobically can sustain muscle contraction for minutes to hours

Oxygen Debt

• additional oxygen, over and above the resting oxygen consumption, that is taken into the body after exercise (heavy breathing and elevated blood pressure after exercising)

• extra oxygen is acquired to:

• convert lactic acid back into glycogen for storage

• Resynthesize PCr and ATP in muscle fibers

• replace the oxygen removed from myoglobin

• sustain the increase rate of chemical reactions resulting the elevated body temperature

• sustain the extra work performed by organs, such as heart and lungs, after exercise has stopped

• allow the extra energy consumption for tissue repair

Muscle Fatigue

• muscle is unable to generate or sustain force of contraction

• function of various parameters such as:

• intensity of contractile activity

• duration of contractile activity

• aerobic vs anaerobic metabolism

• composition of muscle fibers

• fitness level

• psychological effects

Central Fatigue

• usually precedes peripheral fatigue

• psychological effects and protective reflexes

• possibly triggered by the increase in acid production during maximal exertion

• could be result of neural connection failure or depletion of neurotransmitter centrally

Peripheral Fatigue

• neurotransmitter release

• receptor activation

• change in muscle membrane potential

• SR Ca leak

• Ca release

• Ca-troponin interaction

• depletion theories: PCr, ATP, glycogen

• Accumulation theories: H, Pi, lactate