"My legs and shoulders feel so heavy I have a hard time lifting them. I'm going to take the day off."
Common words after a intense interval training session on the mountain bike or a hard session in the weight room.
Why? What makes muscles sore and stiff? How could your body let you down like this? You, who works out in the gym several hours per week and had plenty of miles in your legs, should not feel any pain. You, who up until this point, always believed your body could take any punishment and your muscle tone was excellent.
Whatever you did, you did too much, too soon. Now it's the day after and you have athletic hangover. You are experiencing muscle soreness.
Causes of your post-exercise soreness vary from overuse to minor strains to your individual muscle fibers. Despite what your high school coach may have told you, the culprit is not lactic acid. Lactic acid build-up has been blamed for prolonged muscle fatigue and discomfort; however, this concept is not widely accepted today.
Lactic acid is produced during intense levels of exercise when the oxygen demands of the muscle fibers increase beyond what the blood is capable of delivering. To produce the energy needed, the body begins another process, which works in the absence of oxygen. Lactic acid—a byproduct of this process—locks up your muscles, and because it is an acid it causes your muscles to experience that burning sensation.
But lactic acid is completely washed out of the muscles within 30 to 60 minutes after you finish riding. Since muscle soreness does not show up until 24 to 36 hours later, scientists have been exercising their brains to come up with another explanation.
Currently, the most popular theory is that when you overdue your cycling, skiing or weight work, you cause "microtrauma" to the muscle fibers—localized damage to the muscle fiber membranes and contractile elements.
Over the 24 hours, the damaged muscle becomes swollen and sore. Chemical irritants are released from the damaged muscles and can irritate pain receptors. In addition to the injured fibers, there is increased blood flow from increased activity to the muscle, causing a swelling of the muscle tissues, which causes enough pressure to stimulate pain receptors. Instead of having free-moving muscle fibers the next morning, you have fibers that are fatigued, have microscopic tears and are swollen.