Muscles involved: hip, knee and spinal extensors
Form: Stand with feet hip width apart, braced core, barbell at feet. Squat to pick up the barbell, with an alternate grip. In a standing position, slowly lower the barbell close to the front of the thighs, elongating the hamstrings, keeping knees slightly bent. Do not lower past the point of comfort.

Cautions:
Make sure core is braced. Use DBs for this exercise if barbell is too much weight, or have a beginner do movement without weight. Make sure knees stay slightly flexed.
Trainer techniques:
cue breathing, 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.