Whether you've always been a faithful gym-goer or you're relatively new to getting in shape, there's one universal truth among those who exercise: No one wants to waste a workout. Yet, people do it all the time without realizing it, according to fitness instructor Amy Dixon. Especially when it comes to arm exercises.

Dixon has worked closely with clients looking to tone their triceps, and often recommends doing triceps dips for a powerful move with real impact. Sure, it seems simple, she says, but just because you're working your upper body doesn't mean you can ignore what's happening at hip-level.

"People think that as long as they've got the movement down on the top part of their body -- meaning, they're bending their elbows and straightening them -- [then] they're doing triceps dips correctly. But unless your hips are right underneath your shoulders, this exercise is ineffective," Dixon explains.

CORRECT: Hips underneath the shoulders

"When you're in the right spot, your hips will feel like dead weight," Dixon says. "That's great because it means your triceps have to work hard to lift them up and down."

INCORRECT: Hips pushed forward

"As soon as you start to push your hips forward, even a tiny bit, it's being supported by your legs," Dixon says. "You'll feel it in your hamstrings and glutes instead."

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