Starting Position: Stand with your feet together. Pull your shoulders blades down and back without arching your low back. Engage your abdominal/core muscles to stabilize your spine. Hold a light-to-moderate weight medicine ball in front of your chest, firmly holding the ball with both hands.
Step forward: Slowly lift the right foot off the floor, finding your balance on the standing leg. Keeping the abdominals engaged will help to avoid any sideway tilting or swaying in your upper body. Press the standing foot firmly into the ground to reduce any unwanted movement and maintain stability. Hold this position briefly before stepping forward. The right foot should land on the floor heel first. With the right (forward) leg placed firmly on the floor, shift your body weight forward. Focus on lowering yourself downwards toward the floor rather than forward. As you load your bodyweight into the right leg, avoid any sideways tilting or swaying in your upper body and try not to move the left (back) foot. As you lower yourself into your right leg and hip, push the medicine ball straight overhead. Keep the abdominals braced.
While still in the bottom (downward) phase of the lunge, pull the medicine ball back down to your chest. Continue to maintain a strong core and drive your right foot into the ground to push back, straightening the leg and returning to the start position. Repeat for a specific number of repetitions or a certain period of time.
Exercise Variation: Begin with a light-weight medicine ball in order to learn how to maintain stability through the entire movement. To increase the level of difficulty and to improve core and shoulder strength, once you can easily perform 8-12 repetitions on each leg, increase the weight of the medicine ball. Another option is to hold one dumbbell in each hand. Start with the elbows bent at ninety degrees and tucked in to the side; at the bottom of the lunge, bend the elbows to bring the weight up to the shoulders and press it directly overhead.