Why it's on the list: What makes this single-joint machine move worthy of being listed among squats and deadlifts? Because too many lifters think those two exercises are "enough" for their hamstrings. They're definitely not!

In his article, "A Scientific Approach to Hamstring Training," muscle-building expert Brad Schoenfeld, Ph.D., conclusively makes the case that for balanced, strong hamstrings you need curls, too. Specifically, the lower hamstrings are activated dramatically more during seated leg curls than deadlifts—even stiff-legged deadlifts.

You wouldn't skip curls during an arm workout, would you? Don't make the same mistake for your legs.

Leg Curl Variations for Leg Development:

• Seated leg curl
• Lying machine leg curl
• Lying cable leg curl
• Standing cable leg curl
• Exercise ball leg curl

In your workout: Keep it simple: 3-4 sets of 8-12 reps, coming close to muscle failure. This is a low-risk, high-reward move, so occasionally add in some slow negatives or dropsets to make it more intense.