What Science Says About Exercise and Muscle Fatigue

As any gym rat can tell you, a thorough workout leaves muscles feeling sore and fatigued. Of course, that’s the point – muscles first must be broken down before they can be built up. Aside from trying to boost muscle strength, many people also work out to increase muscular endurance, which tends to be fairly low in those not used to exercise. But what actually causes muscles to tire from lifting weights? The possible answer to this question might surprise you.

Looking Closer at Lactic Acid

Many people, including those who work out regularly, believe that lactic acid is responsible for workout-related soreness. Lactic acid forms when the body’s oxygen levels are lower than normal. Under these circumstances, carbohydrates are transformed into energy, producing lactic acid as a sort of byproduct. In contrast, when oxygen levels are normal, these nutrients are instead broken up into carbon dioxide and water.

Severe medical conditions can deprive the body of oxygen, such as heart failure, sepsis or shock. Oxygen levels can also be depleted by a taxing workout. This explains why lactic acid regularly appears while people pump iron.

But does this mean that lactic acid can be blamed for fatigued muscles? Until recently, most health professionals would have answered “yes.” Over the last several years, however, multiple studies have reached a much different conclusion about the purpose of lactic acid. Researchers at University of California in Berkeley, for instance, reported in 2006 that this chemical is actually used by muscles as an energy source.

Australian and Danish researchers have also determined that lactic acid has been unfairly maligned. Published two years before the UC Berkeley study, this research found that muscles rely on lactic acid to endure the stress of physical activity. The reason for this relationship involves the movements of sodium and potassium ions within muscle cells (ions are atoms/molecules with differing numbers of electrons and protons).

When a muscle is physically taxed, the interior sections of its cells begin to lose potassium ions, which pile up on the cells’ outer membrane. This renders the muscle cells much less responsive to nerve signals. In response, lactic acid works to support the muscles by counteracting the effects of chloride ions. Essentially, this keeps muscles receptive to incoming signals, allowing them to hold out longer before they fail.

Another Explanation for Fatigued Muscles

So if lactic acid doesn’t cause muscles to falter, than what does? A number of recent studies have attempted to answer this very question. One possible explanation comes courtesy of researchers from the University of Utah. This group injected various substances into mouse tissue samples, including lactic acid, adenosine triphosphate (ATP) and various other acids.

When injected individually into the samples, none of the chemicals induced a reaction from the tissues’ nerve cells. A combination of these substances, in contrast, caused the cells to emit certain signals. The researchers contend that in a living mouse, such impulses would be sent directly to the brain, alerting it to growing stress in certain muscle tissues. Additionally, the nerve cells’ responses varied based on the amount of mixture added to the sample.

The next logical step was to test whether human muscles would exhibit similar reactions. Ten adults were recruited for this experiment, which involved injecting the same mix of chemicals into the subjects’ thumbs. The thumb muscles were picked for two reasons – they are relatively easy to keep still, and the researchers could access them without much trouble.

As with the mice, the participants were initially injected with just one chemical at a time. The subjects were then asked if they felt any reactions once the initial pain had subsided. None reported any lingering ill effects. The chemical mix was then administered to the subjects, in an amount similar to what is produced by moderate exercise. This time, the volunteers’ thumbs became heavy, tired, puffy and swollen a few minutes after the injections.

Two more doses of the chemical mixture were subsequently given to the subjects. The second dose was used to mimic the chemical response to strenuous exercise, while the third was comparable to what the body experiences after an especially demanding workout. With each subsequent injection, the volunteers reported increasing levels of muscular fatigue and pain.

According to the research team, the strains associated with exercise are likely caused by the substances used in the study. As we exercise, these chemicals begin to accumulate. Nerve cells in the affected muscles then start to interact with the brain, resulting in the fatigue that develops during a workout.

While these signals don’t indicate that the muscles are about to fail, they do let the body know that they will reach their physical limit at some point. Eventually, the aforementioned chemicals cluster around the tiring muscles, activating nerve cells related to pain. When muscles begin to hurt, of course, most people logically quit exercising them.

To be sure, people looking to bulk up do need to push their muscles to some degree. The trick is to do so without going too far and injuring the body. In order to get bigger and stronger, it’s generally best to exercise to the point of fatigue, but to stop before your muscles become painful.