Most abdominal exercises, such as the standard floor crunch, work your abs through an active contraction -- the muscles contract, or shorten, to perform the movement. The toe tap exercise is a basic abdominal movement that works your abs isometrically, or statically. Your abdominal muscles are not the primary movers, but they do engage during the toe tap exercise.
Lie on your back with your arms next to your sides and your palms facing the floor. Position your hips and knees at 90-degree angles -- your thighs should be perpendicular to the floor and your lower legs should be parallel to the floor. Maintain these joint positions throughout the movement. Slowly lower your left foot to the floor. Tap your toe on the floor lightly and pull your leg back up to the starting position. Repeat with the right leg. Continue alternating sides for the desired number of repetitions.
Your abdominal muscles do not actively contract, or shorten, during the toe tap movement. The iliopsoas, or hip flexor muscles, are the primary movers. These muscles connect your hip bone to your thigh bone and are responsible for pulling your thighs toward your hip. The rectus abdominis, your main ab muscle, isometrically contracts to stabilize your torso during the toe tap movement. Your ab muscles contract without a significant change in length.
Lower both feet simultaneously to increase the difficulty of the toe tap exercise. Your abdominal muscles have to work harder to stabilize your torso against the movement of both legs, versus one leg. You can also do the toe tap exercise seated on a bench. Place your hands slightly behind and lean back slightly, but keep your spine straight. Start with your hips and knees at 90-degree angles. Lower either one foot at a time or both feet together.
Isometric contractions are an important element of a fitness program. The American College of Sports Medicine recommends including isometric exercises in your program to promote strength gains and muscle growth. To thoroughly work your abdominal muscles, include exercises that move the abdominal muscles through a full range of motion. The rectus abdominis is responsible for spinal flexion, which is the main movement during the standard crunch and variations of the crunch.