The idea that exercise makes you tired is so deeply ingrained that it’s often the first thing people think of when the topic of “exercise” comes up. The explanation is a little more complicated than that.
Your muscles burn energy while they’re contracting. As you exercise, you increase the rate that your muscles use energy, which causes them to burn more. But your muscles don’t burn energy just because they’re moving. They also burn energy while they rest between contractions. There are several reasons your muscles might be sore after working out, and each reason may explain a different part of your soreness. Muscle soreness doesn’t make you tired because it takes a long time for your muscles to recover from a workout.
When you work out, you release lactic acid. Lactic acid is a small, acidic molecule that carries your energy from one muscle to another. Your body makes more lactic acid during intense exercise, which may explain why you feel tired after working out. When you work out, your muscles recover by contracting and relaxing.
The time between contractions is called the “resting period.” A longer rest period causes your muscles to recover more slowly. And your muscle’s ability to adapt to that longer rest period is why your muscles feel sore after a workout. Muscle soreness also doesn’t feel like fatigue because your muscles feel sore even when you haven’t worked out.
Muscle soreness is a normal response to physical activity, and appears in everyone, from elite athletes to couch potatoes. The muscles in your upper body may ache, while the muscles in your lower body may feel sore. And your soreness may not go away for several days or longer. Muscle soreness is a normal part of building strength and endurance. But you may feel uncomfortable and embarrassed if you’re sore for more than a few days.