The beginning of a workout is often the easiest part. After slipping on your best activewear and loading up an energizing playlist, you may feel invincible when you start your exercise routine. But soon enough, something familiar happens—your muscles grow fatigued, and weights that felt like nothing a few minutes ago are suddenly impossible to lift. As you begrudgingly take a break, you may ask yourself: why do my muscles get tired so quickly? Though it may be annoying, sudden muscle fatigue is nothing to worry about. It's simply a byproduct of the way your body uses energy.

According to Livestrong, muscles are powered by a compound known as adenosine triphosphate, or ATP. During physical activity such as resistance training, your muscle fibers convert glycogen (a form of glucose), oxygen, and fatty acids into ATP. ATP gives muscles the energy they need to contract, but the body can't produce an unlimited supply of it. After several repetitions of an exercise, your ATP reserves will run low and you'll start to feel weak. This is why even the strongest bodybuilders need to take breaks between sets.

ATP is also the reason you can jog for an hour but get tired of weight training after a few minutes. Lifting weights puts more stress on your muscles than running, biking, or other types of aerobic exercise. Because of this, your stores of the compound get depleted before your body has a chance to build them back up. The exception to this rule is sprinting. This form of exercise requires intense muscle contractions, which is why it's more exhausting to sprint 30 yards than to jog five times that distance over a longer length of time.

Fatigued muscles aren't a sign that you're doing something wrong. It's just your body's way of telling you to slow down or take a break. And the more you exercise at your own pace, the easier vigorous physical activity will become. Here are some more tips for optimizing your workout from experts.

[h/t Livestrong]