After several miles of running and sprinting in intervals, the runner's muscles ache and feel fatigues. Which is the best explanation for what has occurred?

When a muscle's ability to generate force wanes or declines, this phenomenon is termed as muscle fatigue or physical fatigue. Two main causes of this are:

1. Neural fatigue

This is where a nerve is unable to generate a sustained signal to the muscle.

2. Metabolic fatigue

This is where there is reduced ability of a muscle fiber to contract usually due to shortage of fuel in the muscle fiber or accumulation of metabolites (waste substances) within the muscle. These metabolites tend to hamper the release of calcium in the muscle or impede the ability of calcium to stimulate contraction of the muscle.

The waste substances include magnesium, chloride, potassium and lactic acid.

Lactic acid is known to form and accumulate in the muscle under conditions of high energy demand as well as during rapid fluctuations of energy requirements and insufficient oxygen and pH may decrease to 6.4 to 6.6 which may have a negative effect on the muscle activity.