Holding the medicine ball at your chest, take a big step forward with your right foot and lower into a lunge. Make sure your front knee doesn't go too far beyond your ankle and your back (left) knee bends straight down to the ground. Press off your front foot to step back to the starting position. Repeat, this time taking a big step forward with your left foot. Keep your chest lifted during the entire movement. Continue alternating legs until you've done 10 lunges on each side.
