Focus: strength + balance + core
Starting with no weights, place one foot on a balance disk and the other foot on the ground, legs hip-width apart. Drop down into a squat, going as low as you can without your knees jutting ahead of your toes. Keep your arms out to your sides for balance. In week one, do a set of 10, moving up and down in a continuous motion, then hold the 10th rep for 30 seconds.
To increase the difficulty, add dumbbells, starting with 5 pounds in each hand. By week four, squat with both feet on balance disks while using hand weights or a sport cord. "In a regular squat, you're using primarily quads, hamstrings, and glutes, says Keller. "When you throw balance into the mix, you're using your whole body to stay upright.