The two ways of working a muscle

If we train the quadriceps muscle, we can sit on the bench of the gym and lift the weight that remains before our foot. But we can also do a squat. The two exercises train the quadriceps muscle but in different ways. 

In the first exercise, thigh is static and the leg moves. In the second, the feet are in contact with the ground and the moving part are the thighs. In the two exercises the knee flexes but in first does so with open chain and the second with closed chain. These names refer to the body part that is in movement: the free extremity (for bench gym) or body (in the case of the squat).

Ways to do this depending on the body part to move

The first exercise is more analytical and serves to evaluate the strength of a particular muscle or train in a specific way. The second, however, is more functional and is used for performance. Trains the muscle in combination with other nearby muscles and also intervenes balance and not in isolation from other muscles.

Except in times of rehabilitation or general work, you should work the muscles in closed chain exercise to look more like the sports movement. For example, squats and pul ups are two ideal exercises to incorporate into your workout routine instead of the quadriceps bench and dumbbells for biceps.