Correct answer: micro-tears, where weak muscle fibers are eliminated
Long-held beliefs die hard. But it’s time, once and for all, to do away with the century-old hypothesis that muscle soreness is caused by a build-up of lactic acid after prolonged exercise. In reality, your body eliminates lactic acid – which accumulates when glycogen depletes – within an hour after you exercise and well before any soreness appears. The pain is actually due to micro-lesions in your muscle fibers. When you engage in a new or highly strenuous physical activity, you subject your muscles to an unaccustomed effort, causing the muscle fibers to stretch and tear. Those fibers are then replaced by other – stronger – fibers that are able to withstand the strain.
Muscle soreness, the painful reward for your hard work, sets in around 24 hours after you exercise and can last for several days. The micro-traumas cause calcium to escape from the muscle fibers. This process spreads slowly in the cells and triggers an inflammatory response – information that is sent to your brain through your nervous system.
The pain serves to protect the muscles. Do you really feel like doing the same exercise the next day when it hurts so much to sit down, climb stairs or move your arms? This gives your muscles time to get better. It is nevertheless recommended to engage in some light exercise such as walking or stretching in order to get your blood flowing and help the recovery process.
But when the pain goes away, that doesn’t mean your muscles have totally healed. Using MRI exams, doctors have discovered that the soreness ends before the muscles have fully repaired themselves.
To strengthen their muscles, athletes follow a training regimen that includes eccentric (versus concentric) movements, which force the muscles to lengthen and contract at the same time. For example, bicep curling 5kg weights is a concentric movement on the way up; slowly lowering the weights back down is an eccentric exercise. One of the best illustrations of this phenomenon is running up and down hills – and the intense soreness that follows. Running downhill is particularly hard on your thighs: your quadriceps are forced to contract while lengthening, which makes for some serious resistance work.
The good news is that the next time you run hills, you’ll experience little or no soreness. The key, of course, is to train regularly.
Thanks to Dr. Laurent Koglin at the sports medicine department of the Hôpital de La Tour for his input.