The Basic Movement
At its most basic, an exerciser will stand with a dumbbell in hand, feet in a wide stance -- slightly wider than your hips, bent at the waist. The dumbbell should be extended toward the opposite foot, the other hand on the waist. Keeping the back flat and core supported -- stomach muscles engaged -- the arm holding the dumbbell pulls up toward the chest, drawing a diagonal line up the body. The exerciser will stand up slightly, moving from a 90-degree angle to a 45-degree angle, and the working arm will finish the movement with the elbow extended directly out from the working shoulder, the dumbbell held just below the chest, close to the body. The easiest way to envision this motion is to imagine leaning down to grip a pull cord on a lawnmower and making the subsequent motion required to pull the cord to start the mower.
Modification for Triceps
To build triceps and increase the focus on the back of the shoulder, the movement can be modified to include a triceps kickback at the end of the motion. To perform a kickback, keep the upper arm tight by the side of the body. Extend the lower arm, keeping the wrist holding the dumbbell strong and straight, until the elbow is almost straight. Squeeze at the end of the movement -- you should feel this in your upper arm -- and then return to a bent position at your side.
Engaging the Obliques
To increase engagement of the core muscles, the exercise can be done beginning in a lunge position, facing the front foot. As the exerciser pulls the arm back, he should also twist the torso and pivot weight back onto the back foot, ending the motion standing upright with weight evenly on both feet. During any core exercise, keep the abdominal muscles tight and pulled in.
The lawnmower exercise can be performed with a dumbbell or resistance tubing. If using tubing, exercisers should stand with one foot on the tubing, holding the handle in the working arm. The shorter the length of tubing between the foot and hand, the more resistance supplied. As with all exercises, if there is any doubt about proper form, exercisers should consult a trainer.