Why Do Your Muscles Get Tired When You Exercise?

During exercise your muscles work hard to lift a load. Whether it be resistance training or aerobic exercise, your muscles are under siege. Your body's neural response to this load is to recruit muscle fibers to complete the task, but muscle fibers become tired rather quickly. This is due to muscle tissue's main source of energy, called adenosine triphosphate, or ATP. As your muscles use up this energy source, they become tired and fatigued.

A man is doing crunches outside. (Image: m-imagephotography/iStock/Getty Images)

Resistance vs. Aerobic

Your muscles tire more quickly during a heavy weight training session compared to jogging because more ATP is used up when the stress of heavy weight training takes a toll on ATP stores in your muscle tissue. ATP is used up very quickly in your muscles, which is why you can't do an unlimited number of bicep curls, for instance. At some point, your muscles tire and you can't do another rep. Aerobic exercise, on the other hand, uses your body's aerobic system to generate new ATP to fuel your muscles. Oxygen is the key ingredient that helps create new ATP to replenish the burned up ATP in your muscles. While efficient, your aerobic energy system is a slow process that kicks in several minutes after your workout begins.

Between Sets

Resting for a minute or two helps restore some, but not all, of the ATP stores in your muscle. This physiological process is referred to as your body's phosphagen system. What happens here is your muscle tissue converts stored creatine into new ATP to power your muscles. While your muscles may still feel tired between sets, some muscle energy will get restored. This is why you are able to lift a load on your next set, although it may be a lower resistance.

Muscles on Fire

As muscle fatigue sets in during exercise and your ATP stores are fast diminishing, your muscles begin to produce an acidic environment known as acidosis. This is why your muscles burn during an intense bout of exercise. The release of lactate helps neutralize this burning effect and as your body naturally removes this lactate can actually help regenerate energy. By increasing your training volume and intensity, you can make this entire process in your body more efficient over time. Your muscles become more efficient at using lactate to reduce acidosis and converting lactate waste to energy.

More Muscle Means Less Fatigue

Your muscles will fatigue less often and for shorter durations if you increase your lean muscle mass. More muscle fibers means your body can handle bigger loads and for a longer duration. The optimal recipe for muscle growth includes exercising a muscle group with 12 to 20 total sets per workout, six to 12 reps per set, train to failure, eat 12 to 15 percent of your calories from protein, and sleep about eight hours per night. Don't exercise the same muscle group two days in a row; this could lead to overtraining which won't help your goal.

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