Fatigue

Lactate is a by-product of anaerobic glycolysis. Sprints in running, cycling and swimming all lead to large accumulations of lactate

However, the presence of lactate is not to blame for the feeling of fatigue

The accumulation of hydrogen ions from the dissociation of lactic acid causes ACIDOSIS. Muscle pH falls rapidly as the cells become acidified

Fatigue

Bicarbonate buffers the H+ ions so pH doesn’t drop below 6.4 - even at exhaustion

Neuromuscular Fatigue

Glycogen Depletion

Muscle pH would fall to 1.5, which would kill the cells, if bicarbonate (HCO3-) were not present in the blood

Glycogen is stored in skeletal muscle and the liver. On average, 400-600 grams of glycogen is stored in the human body (approx. 1600-2400 kcals).

Fatigue may be caused by a failure in the Nervous System. Possibilities include:

The rate of glycogen depletion is dependent on the intensity and duration of the activity

Now for the clever bit:

What do you breathe out?

Glycogen depletion is selective in terms of which muscle fibres are depleted

Depending on the activity, glycogen may deplete faster from certain muscle groups

• Decreased acetylcholine (Ach) activity
• Increased muscle fibre threshold
• Competitive inhibition with Ach
• Calcium retention in the sarcoplasmic reticulum

Fatigue is the decreased capacity or complete inability of an organism, an organ, or a part to function normally because of excessive stimulation or prolonged exertion

What does this tell us?

Glycogen Depletion

Types of Fatigue

Summary

A small amount of glucose is always in the blood but during exercise, the liver must break down its glycogen stores into glucose that can be transported to the active muscles

Energy System Fatigue

Fatigue is a concept which merits more research

Metabolic By-product Accumulation

This tends to happen more in endurance events but sometimes the glucose is used up faster than the liver can produce it

Nervous System Fatigue

PCr Depletion

Fatigue may be caused by PCr and glycogen depletion.

This causes the muscles to rely even more on their glycogen stores and accelerates fatigue

H+ ions acidify muscle and impair contraction.

PCr is used to rebuild ATP

Muscle Contractile Mechanism Failure

Central Nervous System

Nerve transmission may become fatigued.

Glycogen depletion and low blood sugar (hypoglycaemia) limit performance in activities lasting longer than 30 minutes

The CNS may be a site of fatigue although the mechanisms involved are not fully understood

Maybe it’s all in your mind?

(Harden up!)

Studies suggest that fatigue might be psychological. For example, verbal encouragement and electrical stimulation have been shown to delay fatigue

How can the athlete delay PCr fatigue?

Fatigue in shorter events more likely results from accumulation of metabolic by-products, such as lactate and H+ ions

Perhaps the CNS causes us to slow down or stop as opposed to cause ourselves injury?