Lunges with Tubing

Stand with your feet hip width apart. Take your left leg and step back approximately 2 feet with the ball of your foot on the floor. Place the tube under your front foot and hold the ends in your hands.
 
While keeping your back straight, lower your body by bending your right knee until your thigh is parallel to floor. 
 
Reverse the motion to return to the starting position.
 
Do all repetitions with one leg, then switch and do the same number of repetitions with the other leg.