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Why Muscles Burn When Working Out

You Asked: What Causes the Workout Burn?

Dear Fit,
I've been on a strength training kick this month, and have even worked with a trainer to learn proper technique and new exercises. Instead of thinking about how my legs were burning while doing lunges, and how much I wanted to stand up, I embraced the burn and started wondering about what causes that feeling. So tell me Fit, what makes my muscles feel like they're about to ignite, and does it mean I'm working them too hard?
— My Legs are on Fire

"Feel the burn!" I can't tell you how many times I've heard those words from either a group fitness instructor, fitness video, or a personal trainer. That burning sensation is how many people gauge their workouts — if they don't feel the burn, then they don't feel like they got in a good workout. To find out what that burn means, what causes it, and if there are any risks, keep reading.

When doing squats or other strength training moves, as you complete more and more reps, your muscles begin to fatigue. Your body responds by producing lactic acid, which causes that tingling, heavy, or burning sensation, and lets you know when your muscles have had enough. It's definitely a good thing that you're feeling this sensation, because it's a sign that your muscles are being challenged, which makes them stronger. Just be sure to listen to your body. That burning sensation is a warning, and if you ignore it (which is hard to do because it hurts), you could be risking an injury. A good rule of thumb is to do as many reps as you can until you experience that slight burning, and then give that muscle a break.

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