The hamstrings (made up of the biceps femoris, semimembranosus, and semitendinosus), move the hips, flex the knees, and help stabilize joints. Yet research suggests that they're often 50 to 80 percent weaker relative to the quads, limiting the muscle group’s power and its ability to support the hips and knees. Athletes tend to train the front sides of their bodies, so it's not surprising that this back-of-the-leg muscle falls behind. Prone leg curls are the gold standard of hamstring strengtheners, but two machine-free moves work just as well: the kettlebell swing and single-leg deadlift. Add one (or both) to your routine to stop playing favorites with your thighs.
Stand with feet slightly wider than shoulder-width apart, kettlebell between legs and a little in front. Reach down and lift the kettlebell, then let it hang softly in front of you, maintaining soft knees, a strong core, and keeping shoulders back and tall. With knees bent, hinge forward slightly from hips, lowering kettlebell between legs and behind you. Driving from your hips, simultaneously extend legs and swing kettlebell straight up to shoulder-height in front of you. Use momentum to lower back into hinge position; repeat immediately.
Place a moderate weight kettlebell or dumbbell on the floor to the outside of your left heel with your foot directly behind the kettlebell. (Your weight is on your right foot.) Maintain a straight line from the top of your head to your left heel. Push the left foot straight back off the floor and hinge forward over your right leg, grasping the handle of the kettlebell with your left hand. At this point, your spine and leg would ideally be parallel to the floor. Drive the right heel into the floor to stand all the way up with a long spine. Repeat to bring the bell back down to the floor, then stand again with the bell.