Athletes and bodybuilders often isolate the muscle or muscle group in their workouts, emphasizing the target muscles main action. However, such a practice does not always lead to maximum development of the muscle because many major muscles also have an action at the other end.
This means you must work the muscle to move the bone to which it is attached at one end of its belly in one exercise. Then you should do a different exercise to involve the bone to which it is attached at the other end. workouts such as this are known as this is known as double ended workouts.
From physiology is well known that when you work a two jointed muscle at both ends simultaneously, the contraction is much weaker than what can be achieved with only one end in action (shortening). For example, the rectus femoris muscle of the quadriceps femoris group. This muscle is involved in knee joint extension along with the vastus muscles and is also involved in hip joint flexion.
Thus if you executed hip joint flexion and knee joint extension simultaneously, the contraction would be less than if you only did hip joint flexion or knee joint extension. When you work one end of the muscle, the other end must remain firmly in place and not be allowed to move.
This is why when you do knee extensions, your thigh is resting on a bench and not allowed to move. When you do hip joint flexion (knee drive or bent leg raises), the knee is bent so that there is no action at the knee.
The same thing can be seen on the back of the thigh with the hamstring muscles, which have an action at the hip joint (extension) and an action that the knee joint (flexion). If you normally do the conventional knee curls you’ll notice a distinctly different feeling from when you do a straight leg pulldown (paw back) with Active Cords or on a high pulley machine which involves the upper hamstrings.
Nature usually takes care this in many movements such as extending your legs in running, jumping and squatting. In addition, the leg press exercise can illustrate the intricate relationship between both ends of the muscles as well as the muscles on both sides of the thigh.
When you straighten the legs, the quads (including the rectus femoris) are involved in knee extension together with the upper hamstrings and gluteus maximus in hip extension. As these actions occur the other end of the rectus femoris and the lower end of the hamstrings are stretching under tension.
In regard to the hamstrings, it should be noted that there is one exercise available that works the hamstrings from the upper end first and then keeps the upper end under a strong isometric contraction. After this, the lower end goes into action, bringing about a double maximal contraction. This exercise is known as the glute ham gastroc raise created by Dr. Yessis.
Because of the strong movement in the knee joint the gastrocnemius is also strongly involved. Keep in mind that the gastrocnemius is also a two jointed muscle performing extension in the ankle joint and flexion in the knee joint In addition, in the glutes ham gastroc raise, the foot is held in dorsi flexion against the back plate, which puts the gastrocnemius muscle on stretch. This in turn brings about an even greater contraction at the knee joint.
The important principle to remember is that when you are working two joint muscles and you place one end of the muscle on stretch, it makes the muscle more taut. In this way, when you contract the other end, there will not be any slack to take up before the muscle begins to shorten. This produces a stronger contraction.
Or you can place one end of the muscle under a strong contraction (first concentrically, then isometrically) and then contract the other end in a double maximal contraction. This is what takes place in the glute ham gastroc raise exercise.
Other muscles are, in reality, two jointed muscles but have the same action at both joints. The biceps and triceps fall into this category. The biceps is a prime mover for elbow flexion and is also involved in shoulder joint flexion.
Thus, to effectively work the biceps it is best to have the shoulder joint in extension when doing the flexion movement at the elbow. This is why doing incline dumbbell curls is very effective. In the starting position the upper end of the biceps muscle is put on stretch so that the lower end is better prepared to shorten.
Abusing this principle, as many athletes do, results in a weak contraction – although you may think it is stronger. For example, many athletes when doing the biceps curl allow the elbow to come forward and upward. When this occurs your biceps is also working in the shoulder joint and you are bringing in other muscles to bring the elbow forward.
This repositions the elbow, changes the angle of pull of the biceps, brachialis and brachioradialis, slackens the muscle and tendon, and thus results in a weak contraction. You make up for some of this by using heavier than needed weights.
You can test this very easily by doing a strict biceps curl, i.e., by keeping your elbow alongside your body with absolutely no movement of the elbow. You will quickly find that you must use less weight when doing the exercise. By holding your elbows alongside your body you are keeping the upper end firmly in place on slight stretch which allows you to have a stronger contraction at the elbow.
The same methods for double ended workouts can be used with the triceps, which is also a two jointed muscle (but only the long head). Because of this, it is advantageous to keep the arms slightly in front, in a shoulder-flexed position. This puts the long head on stretch at which time you do a triceps extension (as in the French press, lying triceps extension) or reverse (supinated) grip triceps extension.
However, you can work the triceps very effectively when you do a two-part sequential action as in the triceps kickback. In this exercise you should completely extend the arm with the elbow alongside your body and then raise the arm after it is extended. You will feel a very strong burn in the long head since it and the posterior deltoid are the only muscles strongly contracted at this joint.
Keep in mind that working the muscle from both ends is not the same as working the muscle in more than one action simultaneously. For example, doing the incline biceps curl with pronation. In this case both actions occur at the same end.
When you work the muscles from both ends you not only create stronger muscles, but also stronger joints and make for less chance of injury to the muscles being worked. By working one end all the time, the muscle develops an imbalance in its strength at the other end, which eventually can cause injury. This may be one of the reasons athletes and many bodybuilders get an irritation of the biceps tendon in the shoulder. They work the biceps in elbow flexion but rarely in shoulder joint flexion.
The above examples should illustrate how you can create and do exercises for maximum benefit by getting true muscle isolation. Keep in mind that with proper isolation and by working the muscle from both ends, you can more effectively develop the total muscle.
By using your creativity, you will be able to come up with many more exercises based on the principles described to more maximally tax the muscles. In the process you will use less weight but develop greater strength and harmony between the muscles. And, most importantly, without undue stress on the joints.
For more information see Biomechanics and Kinesiology of Exercise