In Part 1 of this series we looked at the symptoms of DOMS. Matt (aka The Fitness Nerd ) addresses the causes of DOMS in part II of this series.
Causes of Delayed Onset Muscle Soreness
The causes of delayed onset muscle soreness aren't entirely understood by doctors and scientists. But there are a couple of theories:
Sore Muscles = Lactic Acid Buildup?
For years, the leading hypotheses was that DOMS was caused by a build up of lactic acid in the muscle during intense, prolonged exercise.
L-lactate is a chemical continuously produced from pyruvate during metabolism and exercise. Under normal circumstances, L-lactate is removed by the body at a rate that exceeds production of it, but intense exertion (like weight training or running) can result in more L-lactate being produced than the body can immediately remove.
This is actually a good thing - since it allows (through a related process that I won't get into here) energy production to be maintained and your body to continue the exercise.
However, until this was widely understood, many people believe that L-lactate caused acidosis in the muscle tissue - in other words, that "burning" sensation you feel during a particularly long or grueling set of squats, sprints or bicep curls. Scientists now believe that muscle burn during exercise is not a product of L-lactate (or the acid form of L-lactate - lactic acid), but rather a result of tissue PH falling when the body switches from aerobic metabolism to anaerobic during activities like weight training.
The other reason scientists have eliminated L-lactate as a cause of DOMS is that L-lactate is cleared fairly quickly from the body once the activity causing the concentrated build-up has ceased (often within 30 - 60 minutes). It's just not present in the tissue long enough to explain muscle soreness that develops 24 - 48 hours after experience.
So, scratch that hypothesis.
Cellular Damage and Microscopic Tears?
The newest theory around what causes DOMS has nothing to do with L-lactate or Lactic Acid. In fact, it's a much simpler explanation: muscle tears.
Now, before you start getting freaked out, understand that what we are really talking about here is tiny, microscopic tears in muscle fibers as a result of performing high-intensity work. We're not talking about a ripped bicep or pulled groin.
One of the things exercise physiologists observed when trying to find an explanation for DOMS was that certain types of exercises caused more server cases of DOMS - specifically exercises or activities that involved eccentric contraction of muscles - activities like downhill running, plyometrics, or lowering a weight very slowly under tension or resistance. This suggested that maybe the type of muscle contraction was important to understanding DOMS.
For some time now, exercise physiologists have known that eccentric muscle contractions (when the muscle "lengthens" during movement) can cause greater damage to muscle tissue than concentric motions (when the muscle "shortens" - for example, during the first part of a bicep curl). And, since DOMS seems to be more severe after exercise that involves a higher-degree of eccentric contractions, they put two-and-two together.
A New Hypotheses for What Causes DOMS
So, here's the new hypothesis for what causes DOMS:
Whenever you put the muscle under more stress than it's use to, tiny, microscopic tears develop in the muscle tissue. In other words, you've literally caused structural "damage" to the muscle and underlying cells. This isn't necessarily a bad thing - since the process of repairing the muscle will typically make it stronger or larger.
There is also cell membrane damage that occurs in the injured tissue, which causes a nasty sounding thing called "necrosis". Necrosis describes the death of cells, in this case due to muscle trauma. When this happens, the body sends in macrophages, which are immuno-cells that clear away the dead cellular material and flush it from the body.
However, this process also sets off an inflammatory process that may create soreness, swelling and tenderness in the muscle tissue. As the body clears the wasted material, inflammation decreases and symptoms subside.
There is also some evidence that fast-twitch muscle fibers are more susceptible to eccentric-contraction muscle damage than slow twitch muscle fibers. The length of the actual muscle during exercise may also determine severity of DOMS. This may explain why the calves are particularly prone to developing DOMS.
Could Gender Play a Role in DOMS?
Interesting enough, there is some evidence that men may be more prone to DOMS than women.
Researchers believe the mechanism for this may be estrogen, which could have a protective effect on muscle and cells, reducing damage. This effect has been observed in both rats given estrogen and then exposed to muscle stress, as well as in humans using oral contraceptives. However, these studies did not look at reported muscle soreness, but rather Creatine Kinase (CK) activity, which may not be a reliable measure of muscle damage.