Fatigue and recovery practice questions.

In the first two minutes after a 200 m sprint the athlete's oxygen uptake stays well above the resting value. Which processes account for most of this fast component of recovery oxygen consumption?

• Phosphocreatine resynthesis and reloading of oxygen stores in blood and muscle
• Conversion of blood lactate into muscle glycogen inside the working leg fibres
• Replacement by the kidney of the water and electrolytes lost in sweat during the sprint
• Restoration of the liver glycogen used to maintain the athlete's blood glucose

Answer: Phosphocreatine resynthesis and reloading of oxygen stores in blood and muscle

The fast component pays for phosphocreatine resynthesis and for reloading the oxygen bound to myoglobin and haemoglobin, which is why it is largely over within about three minutes. Sweat and liver glycogen losses are restored over hours and belong to the slow component or later recovery.

A 400 m runner slows markedly over the final 80 m of the race. Which change inside the working muscle contributes most to this loss of force?

• Complete depletion of the muscle glycogen store
• Accumulation of hydrogen ions, which lowers muscle pH
• A fall in muscle temperature towards resting values
• Exhaustion of the free fatty acids in the blood plasma

Answer: Accumulation of hydrogen ions, which lowers muscle pH

Maximal 400 m running relies heavily on anaerobic glycolysis, and the hydrogen ions released as ATP is broken down lower muscle pH, inhibiting glycolytic enzymes and interfering with calcium binding. Muscle glycogen is nowhere near exhausted after one lap, and muscle temperature rises rather than falls.

One hour after an 800 m freestyle swim the athlete's blood lactate has returned to resting values. What becomes of most of the lactate that was produced?

• It is excreted unchanged in the urine and in the sweat
• It is stored in the muscle fibres until the next training session
• It is oxidised as a fuel by the heart and by slow-twitch fibres
• It is broken down in the lungs and exhaled as carbon dioxide gas

Answer: It is oxidised as a fuel by the heart and by slow-twitch fibres

Lactate is a transportable fuel, not a waste product: it is shuttled to the heart, to slow-twitch fibres and to the liver, where it is oxidised or used to make glucose. Only trace amounts leave in urine or sweat, so 'getting rid of lactic acid' is a misdescription of recovery.

During the final hour of an ultra-endurance cycling event the rider reports that the effort feels far harder although power output has fallen. Which change does the central fatigue hypothesis propose?

• A rise in brain serotonin as more free tryptophan crosses the blood-brain barrier
• A fall in the calcium ion concentration released from the sarcoplasmic reticulum store
• A reduction in the number of capillaries surrounding each muscle fibre
• An increase in the number of type IIx fibres recruited by the motor cortex

Answer: A rise in brain serotonin as more free tryptophan crosses the blood-brain barrier

As free fatty acids displace tryptophan from albumin and branched-chain amino acids are taken up by muscle, more free tryptophan crosses into the brain and serotonin synthesis rises, which is linked to a higher perception of effort and reduced drive. Reduced calcium release is real but is a peripheral change inside the fibre rather than a central one, while capillary density and fibre type do not change within a single event.

Twenty-four hours after a rugby match a player's blood creatine kinase concentration is several times his usual resting value. What does this most likely indicate?

• That his muscle glycogen stores are still only partly refilled
• That he is dehydrated and his plasma volume has not been restored
• That his blood lactate has not yet returned to the resting value
• That the membranes of muscle fibres were damaged during the match

Answer: That the membranes of muscle fibres were damaged during the match

Creatine kinase is an intramuscular enzyme, so a raised blood concentration signals that fibre membranes have been damaged and have leaked their contents, typically after eccentric or collision loading. Glycogen status, hydration and lactate are all monitored by other measures.

A researcher wants to show that the fatigue at the end of a maximal cycling test is peripheral rather than central. Which finding gives the strongest support?

• The cyclist reports a rating of perceived exertion of 20 at the end of the test
• Surface electromyography shows that motor unit firing rates fell over the final minute
• The cyclist's heart rate reaches the highest value recorded in the laboratory
• Electrical stimulation of the fatigued muscle also produces a reduced force

Answer: Electrical stimulation of the fatigued muscle also produces a reduced force

If force is still reduced when the muscle is stimulated electrically, the limitation lies beyond the nervous system's drive, inside the fibre itself. A fall in motor unit firing rates points the other way, towards a central component.