Stephen M. Roth

 This passage is adapted from Stephen M. Roth, “Why Does 

Lactic Acid Build Up in Muscles? And Why Does It Cause 

Soreness?” ©2006 by Scientific American, a division of 

Nature America, Inc.

As our bodies perform strenuous exercise, we 

begin to breathe faster as we attempt to shuttle

more oxygen to our working muscles. The body 

prefers to generate most of its energy using aerobic 

methods, meaning with oxygen. Some circumstances, 

however, require energy production faster than our 

bodies can adequately deliver oxygen. In those cases, 

the working muscles generate energy anaerobically. 

This energy comes from glucose through a process 

called glycolysis, in which glucose is broken down

or metabolized into a substance called pyruvate 

through a series of steps. When the body has plenty 

of oxygen, pyruvate is shuttled to an aerobic pathway 

to be further broken down for more energy. But 

when oxygen is limited, the body temporarily 

converts pyruvate into a substance called lactate, 

which allows glucose breakdown—and thus energy 

production—to continue. The working muscle

cells can continue this type of anaerobic energy 

production at high rates for one to three minutes, 

during which time lactate can accumulate to high 

levels.

A side effect of high lactate levels is an increase in 

the acidity of the muscle cells, along with disruptions 

of other metabolites. The same metabolic pathways 

that permit the breakdown of glucose to energy 

perform poorly in this acidic environment. On the 

surface, it seems counterproductive that a working 

muscle would produce something that would slow its 

capacity for more work. In reality, this is a natural 

defense mechanism for the body; it prevents 

permanent damage during extreme exertion by 

slowing the key systems needed to maintain muscle 

contraction.

Contrary to popular opinion, lactate or, as it is 

often called, lactic acid buildup is not responsible for 

the muscle soreness felt in the days following 

strenuous exercise. Rather, the production of lactate 

and other metabolites during extreme exertion 

results in the burning sensation often felt in active 

muscles, though which exact metabolites are

involved remains unclear. This often painful 

sensation also gets us to stop overworking the body, 

thus forcing a recovery period in which the body 

clears the lactate and other metabolites.

Researchers who have examined lactate levels 

right after exercise found little correlation with the 

level of muscle soreness felt a few days later. This 

delayed-onset muscle soreness, or DOMS as it is 

called by exercise physiologists, is characterized by 

sometimes severe muscle tenderness as well as loss of 

strength and range of motion, usually reaching a 

peak 24 to 72 hours after the extreme exercise event.

Though the precise cause of DOMS is still 

unknown, most research points to actual muscle cell 

damage and an elevated release of various 

metabolites into the tissue surrounding the muscle 

cells. These responses to extreme exercise result in an 

inflammatory-repair response, leading to swelling 

and soreness that peaks a day or two after the event 

and resolves a few days later, depending on the 

severity of the damage. In fact, the type of muscle 

contraction appears to be a key factor in the 

development of DOMS. When a muscle lengthens 

against a load—imagine your flexed arms attempting 

to catch a thousand pound weight—the muscle 

contraction is said to be eccentric. In other words,

the muscle is actively contracting, attempting to 

shorten its length, but it is failing. These eccentric 

contractions have been shown to result in more 

muscle cell damage than is seen with typical 

concentric contractions, in which a muscle 

successfully shortens during contraction against a 

load.

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