Today I found out what causes muscle soreness after workouts.

Since the early 20th century, this specific type of muscle soreness, called “delayed onset muscle soreness” (DOMS), was thought to be cause by lactic acid buildup in the muscles during strenuous workouts where your body’s oxygen supply is depleted. Recent research has shown this is not the case at all and has even shown that lactic acid is actually used by your muscles for fuel when oxygen supplies are depleted. This also flies in the face of popular belief (for more on this, see the bonus factoids section).

One would have thought that it should have been obvious that lactic acid had nothing to do with DOMS, due to the fact that this type of muscle soreness doesn’t appear until around 24-72 hours or so from the time you exercised, yet the lactic acid buildup only lasts in your muscles for at most an hour or two after your workout is complete. Nevertheless, it’s only been in the last 20-30 years or so that scientists have come to realize the actual role lactic acid plays in muscles.

So if it’s not lactic acid that is causing this soreness, what is it? Delayed onset muscle soreness (DOMS) is now understood to be caused by microfractures in the muscle cells themselves. This happens when you do some activity that your muscles aren’t used to doing or do it in a much more strenuous way than they are used to.

This is also why after you exercise some specific way a few times and allow your muscles to recover, that you won’t typically get sore again from doing that activity at a similar intensity level, so long as you continue to do it on a somewhat regular bases. The muscles quickly adapt to being able to handle new activities so as to avoid further damage in the future; this is known as the “repeated-bout effect”. When this happens, the microfractures typically won’t develop unless you change your activity in some substantial way. As a general rule, as long as the change to the exercise is under 10% of what you normally do, you won’t experience DOMS as a result of the activity.

For the more technically inclined, DOMS is caused by ultrastructural disruptions of myofilaments, particularly with the z-disk and with damage to the muscle’s connective tissues. Muscle biopsies taken a day after hard exercise often show bleeding of the z-band filaments that hold muscle fibers together. The pain then is thought to be largely due to this damage to the connective tissue, which in turn increases sensitivity of the muscle’s nociceptors (pain receptors); this then causes pain with stretching and the like, basically when using the muscles.

The delayed effect is thought to be as a result of the fact that the inflammatory process that ends up making the nociceptors more sensitive takes some time to happen.

Bonus Facts:

Another common symptom of DOMS, beside the pain, is swelling in the muscles. You might notice, after workouts that cause particularly severe DOMS, that your muscles appear bigger than before. This isn’t because you’ve miraculously gained visible muscle mass in just one workout, but rather because your muscles are swelling as a response to the microscopic muscle tears.

Stretching before and after an exercise has long been considered a good way to minimize muscle soreness after a workout. However, recent research has shown that the effect stretching has on DOMS is negligible.

Methods that have been shown to minimize muscle soreness after workouts are any activities that increase blood flow to the muscles, including: massage; hot baths; low-intensity workouts; sitting in a sauna; etc.

Somewhat counter intuitively, you can also continue to do high intensity workouts to reduce muscle soreness. Experts still disagree on exactly why this appears to reduce DOMS, but it is thought to have something to do with exercise-induced analgesia. This is where your body increases pain tolerance thresholds as a response to exercise, particularly exercise structured around endurance training.

Recent research has also shown that extended warm up sessions, before exercising in some way your body isn’t used to, will also help reduce DOMS.

Alternatively, gradually increasing workout intensity, under 10% a week, should allow you to progress in your workouts while experiencing minimal or no muscle soreness.

When you first start exercising, you might notice a very quick boost in strength after only a few days of working out. This isn’t likely due to an actual significant change in your muscle composition; rather, that, as a response to exercising, your body will begin to increase the number of impulses that cause muscle contractions, which will give you a quick strength boost without your muscles actually having changed yet.

The process by which your muscles grow in length and increase in overall size is called “Muscle Hypertrophy” and is usually a response to physical exercise.

Scientists can tell how much muscle damage has occurred, which is causing DOMS, by measuring blood levels of CPK, which is a muscle enzyme. CPK is found in muscles and, when muscles are damaged, ends up being released into the bloodstream.

It used to be thought that having a nice long cool down period after exercising would reduce DOMS. This was because cool-down periods speed up the removal of lactic acid from muscles and it was thought that lactic acid was causing the muscle soreness. It has been more recently proven that cool-down periods after workouts have absolutely no affect on delayed-onset muscle soreness.

In food products, lactic acid is primarily found in sour milk products such as yogurt, cottage cheese, leban, koumiss, etc. Lactic acid is also commonly found in modern detergents, as it makes a good soap scum remover; is an anti-bacterial agent; and is environmentally friendly.

The idea that lactic acid was the cause of muscle soreness and fatigue dates all the way back to the early 20th century from research done by Nobel laureate, Otto Meyerhof. The specific research was where he cut a frog in two; placed the bottom half in a jar; then applied electric shocks to the muscles in the legs. After a few shocks, the muscles no longer would twitch. Dr. Myerhoff then examined the muscles and discovered they were saturated with lactic acid. Thus, lack of oxygen must lead to lactic acid, which then leads to fatigue. Because of this incorrect theory, athletes throughout the 20th century were taught to exercise only aerobically, where glycogen is used as fuel. Once they got to the anaerobic zone, lactic acid would buildup and damage their muscles and force them to stop working out for the day. It turns out, though, that these theories were incorrect, as shown by Dr. George A. Brooks who is an integrative biology professor at Berkley. He showed all the way back in the 1970s that muscles use lactic acid as fuel. It took many years and numerous other research projects, but eventually, his theory that lactic acid is actually used as fuel by muscles has been shown to be correct.

How muscles use lactic acid as fuel is as follows: muscle cells convert glycogen to lactic acid when there is not enough oxygen present to convert it normally to adenosine triphosphate (ATP); the lactic acid then can be used as fuel by mitochondria, which are the energy factories in muscle cells. The mitochondria have a special transporter protein that help move the lactic acid into themselves.

Lactic acid is extremely important because it allows the body to convert glycogen to energy without the need for the presence of oxygen, as with normal aerobic glycolysis (the process by which the body uses glycogen for energy). By converting to lactic acid instead of ATP, when there isn’t much oxygen available, it allows for the glycolysis process to last for several minutes instead of only a few seconds. Once your body has enough oxygen present, it can then go back to converting the glycogen to ATP and the lactic acid can be converted back to glucose by the liver and other tissues to be used later. This makes for much more efficient usage of glycogen when your body is low on oxygen.

Intense, endurance training can more than double the mitochondrial mass in your muscles cells, which can aid in your ability to use lactic acid as fuel. This allows your muscles to work harder and for longer periods in extended low oxygen situations such as when you are endurance training or the like. So one of the reasons trained athletes can perform at the level they do for as long as they do is because their intense training actually allows their muscles cells to absorb lactic acid faster and more efficiently due to larger mitochondrial mass.

Also contrary to popular belief, the buildup of lactic acid does not directly cause acidosis (increase in acidity in the blood that, among other things, is associated with causing a type of fatigue).

The system used by your body to give muscles their energy from the conversion of glycogen to ATP is known as an aerobic energy producing system. The system used by your body to use glycogen converted to lactic acid, when there is very little oxygen available, is called an anaerobic energy producing system.

One of the single best measures of cardiovascular fitness level, or more aptly, maximum aerobic potential, is called “VO2 Max”. This is a measure of the maximum capacity of a body to transport and utilize oxygen during exercise. This is often measured in liters of oxygen per minute (l/min) or milliliters of oxygen per kilogram of body weight per minute ((ml/kg)/min). The name comes from “Maximum Volume per time unit of O2”

VO2 max levels in untrained people are typically 40-60% higher in men than in women with the average VO2 max of an untrained man being around 3.5 l/min and the average VO2 max of an untrained woman being around 2.0 l/min. Interestingly, conditioning can more than double the VO2 max in some people and in others has little affect at all.

For reference, five time Tour de France winner Miguel Indurain at his peak had a VO2 max of 88 ml/kg/min. Seven time Tour de France winner Lance Armstrong at his peak was at 85 ml/kg/min. Cross country skier Bjørn Dæhlie had them both beat with a VO2 max of 96 ml/kg/min. Astonishingly, he achieved that in the off season. It is thought that his peak is likely over 100 ml/kg/min during the season, when he is in optimal physical shape.

For further perspective, typical thoroughbred horses have a VO2 max of around 180 ml/kg/min. Siberian dogs trained for the Iditarod have VO2 maxes as high as 240 ml/kg/min.

Expand for References: