After a hard training session, or race, most runners experience some degree of muscle pain during, and after running. The same applies to cyclists, swimmers, and canoeists, but the post-exercise muscle soreness is relatively less.

The dictum "No pain, no gain" has some truth in that one’s maximum speed is often dependent on how you tolerate the pain- when your legs burn during a sprint, not to mention how your chest feels. It is commonly thought that "lactic acid" buildup in the muscle causes pain. However, this is not strictly true, even though it is often assumed.

Lactic acid is a by-product of anaerobic metabolism, which becomes the predominant energy pathway when there is insufficient oxygen for the preferred aerobic breakdown of fuel (carbohydrate). You will recognise these principles when thinking of sprinting. One hardly needs to take a breath for a 50m sprint on land or a 25m sprint in water.

Endurance events are almost purely aerobic in nature. An adult can store carbohydrates for about 2 hrs of running below the anaerobic threshold. Most of us burn a combination of carbo’s and fats during endurance events.

Back to why our muscles hurt during intense exercise, like sprinting. I favour the "acid buildup" theory because acid is a by-product of anaerobic metabolism is acid. To confuse you even more, the acid is associated with the lactate buildup. Human cells function within a narrow range in temperature, pH (acidity), and electrolytes. Pushing these limits result in cell dysfunction (muscles don’t work properly), and sometimes pain. At the end of a sprint, it usually takes a minute or two for the pain and nausea to settle. This coincides with the disappearance of lactic acid from the muscles.

Why do my legs ache the next day then? Delayed-onset muscle soreness is caused by actual muscle damage that occurs especially as a result of eccentric muscle contraction – for example when the quads (front of the thigh) muscles hurt after downhill running. Microscopic pictures have shown disruption of muscle fibres. Blood tests show leakage of muscle enzymes into the bloodstream. In severe instances, muscle breakdown leads to appearance of substances in the urine.

How do I prevent delayed-onset muscle soreness? Eccentric muscle training (eg. Downhill running) lessens the (quad) soreness, but will not completely eliminate it.

To put all the above in simple English:

"No train no gain. No pain no gain. No gain without train and pain!!!"