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Strength training exercises are classified as closed or open kinetic chain. In closed chain exercises, the end of the chain farthest from the body is fixed. A good example of a closed chain exercise would be the squat. Your feet are fixed and the rest of the leg chain moves. In contrast, when your perform open chain exercises, the end is free, such as in a leg extension machine.
Closed chain exercises promote joint compression, which will make your joints more stable. In contrast, open chain exercises create shearing forces, which, in some cases, such as the leg extension machine, can actually cause instability.
Closed chain exercises usually engage more than one muscle group. For example, the leg press or a squat will work the quadriceps, hamstrings and gluteals. In comparison, open chain exercises like the leg extension machine usually involve muscle isolation.
Consider this: The hamstring/quadriceps strength ratio should be approximately 60-75%. In other words, your hamstrings should be 60-75% as strong as your quadriceps. Many women come nowhere near this ideal strength ratio. So when you plan your strength-training workout, avoid too many exercises that isolate the quadriceps.