You probably assume that the more you sweat during a workout, the harder you’re exercising. Pretty simple, right? Unfortunately, the reality is a lot more complicated, and you need to be careful of how much (or how little) you sweat to maximize your workout. We asked Walt Thompson, president of the American College of Sports Medicine, for his best pieces of advice.

Your hydration status can sabotage your workout if you’re not careful.

“A person’s core temperature rises as a result of exercise, but if you’re not sufficiently hydrated before a workout you will not be able to sweat as much. That means your core temperature will rise much more rapidly,” Thompson explains. Result: You’ll experience muscle fatigue, loss of coordination, decreased energy, and decreased athletic performance.

It’s important to drink up after your workout as well.

If you don’t, your “core temperature can remain elevated for a more extended period of time. The same loss of muscular strength and endurance can be seen post-exercise as during exercise when people don’t hydrate enough, particularly in hot and humid environments,” Thompson says

Sweat doesn’t have superpowers.

Perspiration may be your built-in defense system against overheating, but it doesn’t protect you entirely from heat-related illnesses, especially if you are exercising outside on a hot day. “If your sweat isn’t enough to decrease the core temperature, it can continue to rise, leading to heat illness or life-threatening heat stroke,” Thompson says. Get hot enough and you may stop sweating altogether. “Your body starts to realize the sweat defense mechanism against overheating has failed, and other systems may start to shut down,” explains Thompson.

Smelly sweat? Blame your stress.

Your body cools itself with sweat from the eccrine glands and is mostly odorless. But when you’re stressed or anxious, your sweat comes from the apocrine glands, and it also contains traces of protein and fat. When this type of sweat mixes with bacteria on the skin, it can start to stink.