Open chain exercises isolate one muscle group. The knee is a hinged joint that is supported by the quadriceps and hamstrings. When it comes to exercise selection for the knee, you have the option of doing either open chain or closed chain exercises. Open chain exercises allow your feet to move without contacting the floor. Close chain exercises, such as lunges and squats, involve floor contact. You can perform open chain exercises with the weight of the body and with fitness equipment.
A leg extension isolates the quadriceps on the front of the thigh, and it is performed on a leg extension machine. To begin, sit on the seat, hook your lower legs under the padded lever arm and lean back against the backrest. Reach down and grasp the support handles with both hands and lift the lever arm by bending your knees. Once your knees are just short of locking out, stop and slowly lower the arm back down. You also can do this exercise from a seated position in a chair. Simply extend your legs out and lower them back down. Strap on ankle weights to increase the resistance.
Leg curls single out the hamstrings on the back of the thighs. These are performed on a lying leg curl machine. To begin, lie face-down on the padded bench with your lower legs hooked under the padded lever arm. Grab hold of the support handles with your hands and lift the lever arm by bending your knees. Once your heels are by your buttocks, slowly lower the lever arm and repeat. You also can do this exercise on a seated hamstring curl machine.
Leg raises work the quadriceps through an isometric contraction. This means there is no lengthening or shortening of the muscle. To begin, lie flat on your back with your right leg bent and left leg straight out in front of you. Forcefully contract your left quadriceps muscle and raise your leg in the air. Try to raise your leg at least a 45 degree angle from the floor. Slowly lower it back down, repeat for a set of repetitions and then switch sides. Not only do you work your quads with a voluntary contraction, but you also work them while moving your thigh toward your body. This is called hip flexion.
Standing Hip Extension
Hip extension is the complete opposite of flexion. This takes place when you move your thigh backward. Standing hip extension works the hamstrings as well as the glutes; and even though you are standing, it's on one foot only, thus qualifying the exercise for open chain status. To begin, stand with your left side facing a table or counter and place your left hand on it. Bend forward slightly and raise your right foot off the floor. Keeping your leg straight, move it in the air behind your body in an arcing motion. Squeeze your hamstrings forcefully as you do this. Once your leg is parallel to the floor, slowly lower it back down and repeat. You can increase the resistance by wearing ankle weights.