Fatigue & Recovery

This resource explores fatigue and recovery looking at: lactic acid accumulation, glycogen restoration and oxygen debt.

Lactic acid accumulation

  • When is lactic acid produced (intensity)?

There is always some lactic acid being produced by your body. However, we don’t notice it due to the aerobic system breaking it down. Lactic acid accumulation occurs as a result of the body’s inability to deliver sufficient oxygen. The result is that the body then has to rely onanaerobic glycolysis.

  • What effect does it have on performance?

Lactic acid inhibits the action of enzymes responsible for the breakdown of glucose. Therefore, glucose available or stored cannot be used. Lactic acid also inhibits the action of calcium ions on the actin filament (see sliding filament theory). This makes muscular contraction impossible.

The more efficient the aerobic system, the higher the intensity you can operate without accumulating lactate. The aerobic system also plays an important role in breaking lactic acid down via the Krebs cycle. A more efficient aerobic system will help perform this task more effectively.

Other causes of fatigue

Creatine phosphate and glycogen depletion

  • The effect of exercise intensity on PC and glycogen stores

PC stores – if intensity of work is maximal, PC stores (which are stored in the muscle) are depleted in approximately 10 seconds.

Glycogen – is stored in the liver and muscles. Once depleted, performance deteriorates as the body switches to fats. As a result, performance is reduced.

  • The effects of using fats as a fuel and its effects on performance

Fats release less ATP per molecule/gram than glycogen. Rate and amount of depletion is dependent on exercise intensity, diet and initial levels of the performer.


  • The factors that lead to dehydration and its effects on performance

Dehydration is largely a result of increased body temperature. The body’s most effective method of cooling is through evaporation of water. The increase in body temperature can be dangerous (40 degrees can lead to unconsciousness). Losses of water greater than 2% result in a significant decrease in performance.

Dehydration is detrimental to performance as it restricts blood flow, which is responsible for carrying oxygen and removing waste from muscles. The result is slower restoration of PC stores and more rapid accumulation of lactic acid, both leading to fatigue.

Muscle fibre types

  • Fibre type recruitment vs. exercise intensity and link to characteristics of fibre type.

Fast twitch fibres are suited to anaerobic activities. Explosive efforts of short duration only.

Slow twitch fibres are suited to aerobic conditions. Slower contractions, slower to fatigue.

When you finish exercising, energy demands decrease. However, the amount of oxygen required by the body remains above resting levels. This is known as oxygen debt.

There are two components:

Alactacid debt – refers to the restoration of PC stores (complete in only 3 minutes).

Lactacid debt – refers to the removal and breakdown of lactic acid from the muscles and blood. This takes much longer – approximately 2 hours.

PC replenishment

What is the time taken for PC replenishment (what is the role of oxygen system here)?

70% – 30 seconds

90% – 90 seconds

100% – 3 minutes

Glycogen restoration (muscle)

Recovery meals and rehydration

Full glycogen restoration takes approximately 2 days. It depends on the duration of the exercise performed and the carbohydrate intake during the recovery.

Glycogen replenishment is fastest during the first 2 hours, and is complete in 2 days if a high carbohydratediet is followed. This can be extended to 5 days if a high carbohydrate is not used.

Hydration is also important during this process. For every gram of carbohydrate stored, the body needs 3 grams of water.

What are the effects of interval vs. continuous training on glycogen depletion/restoration?

Glycogen depletion is less rapid if the training is intermittent. It also results in more rapid replenishment of glycogen stores. As a result, a person can do more training – if done in the form of interval training and combined with a high carbohydrate diet – without risking severe depletion.

Glycogen can only be resynthesised if lactic acid, pyruvic acid and glucose are available to the muscles and liver. The levels of these are low in continuous exercise; however, they are higher in intermittent exercise, thus resynthesis can occur sooner.

Glycogen resynthesis occurs more quickly in FT fibres, as more of these are recruited during intermittent work.

  • What is the relevance of varying carbohydrates diets on recovery rates?

A high carbohydrate diet will ensure a more rapid recovery of glycogen. This can be further enhanced if carbohydrates are ingested in the first 30 minutes-2 hours.

Lactic acid breakdown and removal

  • What is the role of the oxygen system in this process?

The aerobic system is responsible for the breakdown of lactic acid. Therefore, light exercise, where the aerobic system is stimulated, will speed up the removal and breakdown of lactic acid.

Warm down

Be able to discuss the benefits of active vs. passive recovery with regard to the following:

  • muscular contractions (stimulate faster circulation of the blood)
  • oxygen levels (assist in the breakdown of lactic acid)
  • venous pooling and effects on recovery (inhibits recovery)