Physics of Fitness Fridays - Working Muscles at Different Angles

There are many ways to exercise each muscle in your body. But why would you want to do that (other than to fight off boredom)? Let’s look at two different tricep extensions as an example.

Triceps are a class I lever that means that your muscle attaches next to the elbow joint OPPOSITE the weight of your arm (and whatever it’s holding). When you perform a tricep extension, your tricep is countering the torque of the weight to lift it against gravity.

A refresher on torque: torque is a force applied around a pivot. It’s calculated by taking the CROSS PRODUCT of the force applied and the torque arm (the distance from the pivot to where the force is applied). In math terms, Torque = (Force) x (Torque Arm) * sin(theta), where theta is the angle between the two. The torque – and force – your arm provides will counteract the torque created by the weight.

As the weight moves through the bottom of a tricep kickback, the force of weight is almost parallel to its torque arm. Thus, the weight feels “lighter” at this point.

At the top of the motion, however, the weight is almost perpendicular to its torque arm; thus, the weight feels a lot “heavier” here; your arm must provide a force to counteract this increased torque.

Okay, so you may be looking at the diagram here and thinking, “hold up! What about the torque arm for the tricep effort! Isn’t that almost parallel to the effort?” Yes, very true. I’m going to go out on a limb (see what I did there?) and claim that even though the torque arm and force may be very close to parallel, they will never actually BE parallel due to the insertion point of the tricep on the bone. So for this particular case, the torque will never go to zero. Also, if you were to calculate the force needed to counteract the force of the weight, you would find:

Effort = [Weight * rsin(theta_W)] / [R * sin(theta_E)]

As theta_E goes to 0, the denominator will blow up and the effort needed to make the arm perfectly straight will go to infinity!

But I digress…

The mechanics get flipped when you perform an overhead triceps extension. Here, the weight feels heaviest when the forearm is parallel to the ground and the weight is, again, perpendicular to its torque arm.

So as you can see, the orientation of your body when you perform a triceps extension allows you to exert maximum force at different arm positions; in other terms, you strengthen different lengths of the muscle. It’s good practice to work your muscles more evenly at all lengths, as this leads to better overall muscle performance. So work each muscle as many ways as you can!