Biomechanical Look at Triceps

It all comes down to simple trigonometry.
(Isn’t that an oxymoron?)

Let’s start with triceps extensions typically preformed with a single, short rope.

Whether you’re using one rope or two, you have to start the exercise with your hands relatively close together to lessen the influence of the shoulder external rotators. Starting with a wider grip – ideally shoulder width apart - would require the contribution of the external rotators – not a bad thing at all, but probably not your goal in this particular exercise. (Of course, simply working unilaterally would eliminate these problems.)

From that starting position, if all you do is straighten your arms – the primary function of the triceps – your hands necessarily move apart in "diverging arcs." Any other motion would necessarily involve some other muscle.

There should be no motion at all – including internal rotation - at the shoulders since the triceps do not move the shoulder. A too short rope, too narrow bar, or anything that restricts your ability to move your hands shoulder-width apart forces you to internally rotate at the shoulder.

(To be technically correct, the long head of the triceps does have some influence at the shoulder joint, but its angle of pull on your upper arm is so small that it can cause only minimal movement at the shoulder.)

Using two of the short ropes (or one long rope) decreases the angle at clip (compare angle a to angle b ). The greater the angle, the more force against your triceps and external rotators. While this may sound good, the problem is that your triceps are weakest at the bottom position because of the combination of the Length-Tension Relationship and the poor angle of force of the triceps pulling on the humerus.

Now for the trigonometry. This involves a principle of physics and vector mathematics called resolution of forces.

Say that you have twenty pounds on the weight stack. Intuitively, you may think that you are pulling with ten (half of twenty) pounds of force with each arm throughout the range of motion, but this is not the case

In addition to the amount of force it takes to bring the ropes down toward your hips, it takes a certain amount of force to pull the ropes apart as your hands separate. This amount of force is proportionate to the angle between the ropes, or a and b in our pictures.

I will spare you the trigonometric details, but the greater the angle, the more force you have to generate to move your hands apart. Yes, obviously some of that force comes from the triceps, but remember they are in a weakened position! The rest of the force has to come from muscles you probably had not intended to work in this particular exercise.

Most handles and bars are not any better than a short rope. Unless the bar is wide enough for you to place your hands about shoulder-width apart, a bar will never allow you to move through a natural arc of motion without undue internal rotation at the shoulder, excessive stress at the elbow, or both.

Let’s take a look at another popular triceps exercise, dumbbell kickbacks. There is nothing wrong with the movement, of course. The problem is the choice of implements.

At the bottom position, due to the favorable force angle of the triceps pulling on the humerus, the triceps are at or near the strongest point in the range of motion. The resistance, however, is zero because the moment arm (the perpendicular distance between the line of action of the force of the dumbbell and the axis of rotation – in this case, the elbow) is zero.

At the top position, the opposite is true. The resistance is greatest because the moment arm is greatest at this point. Unfortunately, as pointed out previously , this is the weakest position of the triceps due to the combination of the poor leverage of the triceps on the humerus at full extension and the active insufficiency of any fully contracted muscle.

Thus, if you choose to perform kickbacks with a dumbbell, you have three poor choices:

1. You can pick a weight that is challenging in the bottom position. This means that your triceps alone cannot possibly produce enough force to allow you to complete the movement. You will only be able to go through a few degrees of motion in good form, and you will be forced to use momentum generated by other joints in order to get anywhere near full extension.
2. You can pick a weight that is challenging in the top position. As you come down from the top position, however, the muscle is getting stronger as the resistance is decreasing. This means that your triceps will not be challenged through most of the range of motion.
3. Picking a weight somewhere between the two extremes may seem to be viable option intuitively, but this compromise only results a combination of cheating as in "option" one and lack of challenge as described in "option" two.

Once you examine the position of the humerus with respect to the body – the other words, the starting length of the long head of the triceps in this particular exercise – you see that the kickback movement is the same as traditional cable triceps extensions. The salient difference is that the dumbbell version has a drastically different resistance profile from the cable version.

With a cable, the resistance increases as the muscle gets stronger. With the dumbbell, the resistance increases as the muscle gets weaker – certainly not the resistance profile that yields optimal results.

The Cable option [black line] yields a superior resistance profile. As the arm straightens and the triceps get weaker, the pull of the resistance of the cable also decreases (in accordance with the sine of the [red] angle formed by the intersection of the line of force of the cable and the forearm.)

Let’s look at another troublesome exercise, the overhead single dumbbell press sometimes known as the French Press.

A cursory examination offers a clue as to what is wrong with this exercise. We have two arms – why are we using only one dumbbell?

If all you do is straighten your arms – the main function of the triceps- your hands will naturally move apart. Holding on to a single dumbbell does not allow you to do this. When you extend your arms in this exercise, your hands are still trying to move apart although your grip on the single dumbbell will not allow them to. This force of your hands trying to move apart will be transferred to your elbow. That’s why many people suffer chronic elbow pain when performing this exercise.

The alternative is obvious. Use two dumbbells if you simply must perform this exercise bilaterally. An even safer choice would be to work unilaterally with a single dumbbell. Keep in mind also that many people have shortened lats that will not allow them to reach straight overhead without excessive arching of the low back.

CONCLUSION

The main job of the triceps working concentrically is to straighten the arms. (The long head is also a stabilizer and a very weak extensor of the gleno-humeral joint.) The biceps both flex the elbow and supinate the radio-ulnar joint - two jobs. So why are there so many triceps bars, ropes, etc.? The answer is that many of them have absolutely nothing to do with the function of the triceps or the normal movement of the elbow.

The elbow is a hinge joint and moves in exactly the same path of motion (at least at the humero-ulnar joint) whether you are concentrically working the triceps in elbow extension or concentrically training the biceps, brachialis, and brachioradialis in elbow flexion. Always keep that in mind when selecting bars and handles for either muscle group.

As Ralph Waldo Emerson wrote, "A foolish consistency is the hobgobblin of little minds." Don’t accept any exercise at face value. Examine each and every exercise that you do for safety and effectiveness as defined biomechanically.

(Thanks to my colleague Brain Brunelle for the pictures.)

References

1. Hamill, J. and Knutzen, K., Biomechanical Basis of Human Movement, Lippincott, Williams & Wilkins, 1995.
2. Nordin, M. and Frankel, V., Basic Biomechanics of the Musculoskeletal System, 2nd edition, Lea & Febiger, 1989.
3. Norkin, C. and Levangie, L., Joint Structure and Function: A Comprehensive Analysis, 2nd edition, F A Davis Co., 1992.
4. Kay, D., Trigonometry (Quick Reviews), Cliffs Notes, 1994.
5. Smith, E., et al., Brunnstrom's Clinical Kinesiology, 5th edition, F A Davis Co., 1996