Active stretching prepares the body for exercise. This form of stretching warms and activates the muscles and tissues, not only stretching them but also preparing them for a specific sport or exercise. In active stretching, the muscles are stretched while they contract. During an active-static stretch, you assume the position of the stretch and hold that position with your own strength. An example is lifting your leg and holding it in the lifted position without the assistance of anything other than your muscles. The muscles being used to hold your leg in the lifted position, the actively contracting muscles, are called agonist. The muscles that are relaxed while the agonist muscles are contracting are called antagonist. An example of an active-dynamic stretch of your hamstring would be to lie on the floor with one knee bent and one knee straight. Raise and lower your extended leg. As you lift your leg, muscles contract and your hamstring is stretched. This form of stretching improves flexibility and strengthens muscles at the same time. Active stretches are usually held for ten seconds but no more than twenty seconds. Many active stretches are the same types of stretches practiced in yoga.