Squat Mechanics – Force and Leverage

As I discussed in the introduction to this lesson, your body’s mechanics plays a significant role in how much weight you lift. We’re going to discuss in this topic the concepts that you have some control over, so you can optimize your lifting patterns for maximum power.

Important Terms

• Force: Mathematically, force is the product of mass times acceleration. For example, when you squat, the force applied is the bar weight (let’s say 600lbs) times the acceleration created by gravity (9.8 meters/second squared). If we get past the physics mumbo jumbo, the thing to remember that force is linear, the resistance it applies during a lift is constant. That squat is always 600lbs x m/second squared. Or, most simply, 600lbs is always 600lbs. Now this may seem pretty damn obvious, but I bring it up to differentiate from the concept of the moment.
• Moment: In contrast for Force, which is linear, and 600lbs is always 600lbs, moment force applied rotationally. 600lbs is not necessarily 600lbs (ok, if you’re a physics student or mathematical purist this is not entirely true…or true at all, but let’s suspend mathematical absolutes for the purpose of this topic). When discussing the rotational moment force, as you increase the distance from the axis, the greater force applied grows. Confusing? I’ll dig into it more as this lesson progresses.
• Moment Arm: The moment arm is simply the distance from the rotational force and the axis it is applied to. Have you ever changed a tire? Consider the lug nut the axis, and the lug wrench is the moment arm.

Moments

When you squat, the 600lb squat is always 600lbs and applies the force linearly against your back. However, the force it applies on your knee and your hip is not linear. As described above, the farther you get from the axis of a rotational force, the greater the applied force becomes. Here’s an example.

Let’s say you’re on your way to the gym and you get a flat tire. You try to quickly change the tire and just grab a wrench from your tool kit to loosen the lug nuts. They don’t budge. The only result is bloody knuckles and a lot of cursing.

Figure 1: Using a short lever

Now instead of the wrench, you remember what your father taught you and you grab the lug wrench from your trunk. With a little effort and a lot less cursing the nuts quickly pop free. The longer handle of the lug wrench applied a much greater force on the lug nuts allowing you to loosen them, and soon be on your way to the gym.

Figure 2: Using a long lever

The rotational force you apply on the lug nut is the moment. The length of the wrench is considered the moment arm. If you’ve ever changed a tire, you know that the longer the moment arm (wrench), the more force you can apply against the lug nut.

Let’s apply this concept to your lifting.

Figure 3: Leverage during a bicep curl

Have you ever noticed that when you curl a weight, the movement is relatively easy as you start each rep, gets more difficult through the middle of the range of motion, then gets easier again as you near the top? Why is that, magic? Broscience? Have you noticed how easily Stick Man curls 155lbs?

Figure 4: Distance to axis is short, moment force is small

Figure 5: Distance to axis is long, moment force is large

In this example the external force gravity applies on the weight is the moment, the axis is your elbow, and the moment arm is, well, your arm – your lower arm to be specific. As you start each rep, the distance from the weight to your elbow (horizontally) is relatively small. As your arm reaches parallel to the floor, the horizontal distance from the weight to your elbow is at its maximum. The distance then closes as you raise the bar to the top.

Now let’s move on and apply these concepts to some real work – your squat.