The Kettlebell Swing
This move is the perfect prescription to treat lagging muscles of the posterior kinetic chain: the calves, hamstrings, glutes, low back and rear delts. Since the kettlebell is moved by ballistic force (think of a ball hit by a tennis racquet) it exerts less stress on the spine than exercises such as the deadlift where the muscles are moving the load through the whole movement.
Setup: Grasp a kettlebell with both palms in an overhand grip and let it hang in front of your body. Stand with your feet slightly wider than shoulder-width with your head in neutral alignment with your spine. This can also be performed with a dumbbell.
Move: Drop into a quarter-squat and let the kettlebell hand between your legs. Explosively extend your knees and snap your hips forward, propelling the kettlebell in an upward arc, keeping your arms straight. The kettlebell should be moved by the force generated from opening your hips. Your arms are only used to guide the weight, not lift it. When the weight reaches eye level, let it drop down following the same arc. Seamlessly descend into a quarter-squat and snap your hips and send it back up.