Muscles involved: hip and knee extensors

Form: stand with feet shoulder width apart, holding DBs if desired. Brace core, and inhaling, lunge one leg forward, dropping back knee downward, aiming for a 90 ̊ angle at the hip, and keeping the knee from extending over the toes. Press through the front heel, exhale, and come back up. Perform with the other leg.

Cautions: Keep the spine aligned properly, no spinal flexion or arching. Keep knee from extending over toes.
Trainer techniques: Cue breathing, contractions in gluteals and quads, watch for proper position of the spine and pelvis, and observe the motion at the hip, knee and ankles. Spot with your hands under the rib cage.