What Causes Muscle Soreness After Exercise
Muscle soreness after hard exercise is common, especially weight lifting and resistance training exercises. The muscle soreness may not happen immediately after the activity. It may occur the next morning after you wake up, or even the day after that. It sometimes takes 8 hours later or 24 or 48 hours later. This is known as delayed onset muscle soreness, or DOMS.
The soreness usually is more noticeable right after waking up. But once you get up and get moving a bit, the body temperature increases and there is more blood flow to the muscles bringing it fresh oxygen and nutrients which helps alleviate the soreness.
Muscle Soreness Caused by Microscopic Tears
The soreness occurs when you push your muscle beyond what they are normally accustomed to. Many fitness experts believe that pushing your muscles to this limit is needed in order to build muscle strength.
At this limit, there will be microscopic tears in the muscle tissues. It is these tiny tears coupled with localized inflammation that causes the muscle soreness. When given the chance to recover, the body will build back newer and stronger muscle fibers.
As muscle fibers slide over each other during heavy resistance training, there are some bleeding damage of the z-disk filaments as evidenced by a muscle biopsy taken on the day of a hard exercise. There is also release of creatine phosphokinase (CPK) muscle enzyme into bloodstream as muscles are damaged. The more CPK found in the bloodstream, the more sore the person feels. These microscopic damages initiates an inflammation response and white blood cell counts have been found to go up.
Minor Muscle Soreness is Normal
Having soreness and microscopic tears is normal. Normal minor soreness should go away on it own. If soreness doesn't go away, or if there is excessive swelling, discoloration, or severe pain, then you should see a doctor as it may indicate a serious injury.
The important point is to allow the body rest until the soreness goes away before re-stressing them. This allows time for the body to build up the new muscles. Although working the muscles stimulate growth, it is during muscle rest periods that is actually when muscle building takes place.
That is why some athletes cross-train so that they do weight training one day and do aerobics the next day. The soreness usually occur from weight training the muscles. So the next day, they do aerobics to work the heart and give the muscles a chance to recover from the soreness. Alternatively, the work different muscle groups on alternate days.
Muscle Soreness in Eccentric Resistance Training
Concentric contraction is when the muscle fibers shortens as the muscle is contracted. When most people think of muscle contraction, they are referring to the concentric contraction.
However, it can sometimes be the case that the muscle fibers lengthen during a muscle contraction. This can occur if the force generated by the muscle is not sufficient to move the load in the direction intended. Letting down a heavy load slowly is an example. This is know as eccentric resistance.
Wikipedia provides a good definition ...
"In concentric contraction, the force generated is sufficient to overcome the resistance, and the muscle shortens as it contracts. This is what most people think of as a muscle contraction.
In eccentric contraction, the force generated is insufficient to overcome the external load on the muscle and the muscle fibers lengthen as they contract. An eccentric contraction is used as a means of decelerating a body part or object, or lowering a load gently rather than letting it drop."
In particular, the soreness is most readily occurs after strenuous eccentric resistance training (as opposed to concentric resistance training). Eccentric resistance training can cause more of the microscopic muscle damage than concentric training.
Another example of eccentric resistance is when running downhill, which is why you get more sore running downhill than running on flat ground.
Weight trainers sometimes call eccentric resistance training "negatives". And incorporation of both eccentric and concentric workout in a routine can result in greater strength gain than concentric resistance training alone.
Muscle Soreness Not Due to Lactic Acid
One previous theory on muscle soreness was thought to be caused by lactic acid buildup. Although it is true that there is an accumulation of lactic acid in the bloodstream during strenuous workouts, this level returns to normal within one-hour after the exercise. So it is not likely to be the cause of soreness 24 hours later.
Lactic acid is responsible for the "burning" feeling in the muscles during the workout, not the soreness of the muscles a day or two later.
Muscles require oxygen in combination with glucose to do its work. Under heavy load, it may require more oxygen that you can supply it. That is why you pant and breath harder. At some point, even that is not enough to supply the muscle with the needed oxygen fast enough. Now the muscles have to work without oxygen, a condition known as anaerobic generation of energy. This is when lactic acid is produced which allows muscle to continue to work in this anaerobic state for a little bit more time. Hence instead of producing fatigue, lactate acid is an alternative fuel for muscles enabling them to work a bit longer.
Although lactic acid may increase the acidity of the muscle cells, minor lactic acid build up is not particularly dangerous. When you slow down and allow oxygen supply to be restored, the lactic acid will dissipate within hours.
Is There A Way To Reduce Muscle Soreness Faster?
Not really. You just have to let time for the body to rebuild the muscles.
Some people suggest that one should do "cool down" exercise at a slower pace after a vigorous bout. This is not a bad idea, since these cool down exercises does increase the rate of removal of lactic acid. But since lactic acid itself is not responsible for the soreness, you still will have the soreness.
Some people also suggest anti-inflammatory drugs. But these are not recommended. Article in Scientific American says ...
"Although anti-inflammatory drugs do appear to reduce the muscle soreness--a good thing--they may slow the ability of the muscle to repair the damage, which may have negative consequences for muscle function in the weeks following the strenuous event."
-  Sore Muscles After Exercise? Causes, Treatments, and When to Take a Break
-  Delayed onset muscle soreness - Wikipedia
-  Skeletal muscle damage and repair - Google Books
-  Why does lactic acid build up in muscles? And why does it cause soreness?: Scientific American
-  LACTIC ACID - in the human body
Lactic acid is naturally present in humans, as well as in animals. There are many misconceptions about lactate and lactic acid in the body. Lactate was long considered one of the causes of both fatigue during exercise and the stiffness felt after.