Most people are slightly dominant on one side of the body and for athletes and serious exercisers, this imbalance can cause injuries and poor performance. Isolating one leg at a time keeps each leg true, making sure that each side does equal work and one side doesn’t overcompensate for the other. It also improves general balance by increasing the effort needed to maintain position on one leg. The move might feel awkward at first (practice without weights in the beginning) but it helps keep joints aligned, which prevents injuries in runners. Along with hitting the hamstrings and glutes, the move also engages core muscles to keep the spine aligned and hips from rotating.
Stand with legs shoulder-width apart, knees slightly bent and arms straight at the side holding dumbbells parallel to feet (or with a barbell held across the thighs).
Lift the left leg. Keeping the head up, back straight and right knee bent but locked, slowly lean forward at the hips. As the left leg lifts behind, maintain a straight line from head to heel. Lean forward until the back is comfortably parallel to the floor and arms are perpendicular to it.
Push down on the right heel and slowly return to the starting position. Do 10 reps on one leg before repeating the process with the opposite leg. Try to keep the lifted leg from touching the floor throughout the set.
Demonstrated here by Mike Lynch, a personal trainer with 20 years of experience and the owner of lynchfit.com