Squat Mechanics – Force Application

Moment, Moment Arm, and your Squat

You should have a good understanding of what these forces are and how they function from the previous topic, now let’s discuss how they affect your squat, and how to use them most effectively to create a more powerful squat.

Because the squat is a compound lift (multiple joints are involved), the mechanics are a bit more complicated than bicep curls, which cross only one joint, the elbow. When you squat, two joints involved in overcoming the downward force the bar exerts on your body, your hips and your knees. When considering the mechanics of the squat, you have to consider the moment force on each of these joints.

Figure 1: Moment arms during the squat

Assuming you keep the bar over your center of gravity, the moment arm is represented by the distance between the downward force of the bar at your center of gravity, and the center of the joint.

• The distance from the downward force of the bar and your hips affect how much effort your glutes and hamstrings exert during the squat. A longer hip moment increases the involvement of your glutes and hamstrings. A shorter hip moment reduces their involvement.
• The distance from the downward force of the bar and your knees affect how much effort your quads exert during the squat. A longer knee moment increases your quad involvement and a shorter moment decreases their involvement.

What does this mean for your squat technique? Here are two scenarios that affect the hip and knee moments, and may affect the effectiveness of your squat.

Knees shift forward excessively:

Figure 2: Knees shift forward

On the left Stick Man demonstrates the reference squat. The knee and hip moment arms are represented by the blue lines. On the right, while keeping the bar over his center of gravity, his knees shift forward far more than in the reference example. The moment arms for this shift are represented by the red lines. Notice that the moment arm for the knee gets longer and the moment arm for the hips gets shorter, shifting emphasis of this squat from the glutes and hamstrings from the glutes and hamstrings to the quads.

Excessive forward lean:

Figure 3: Excessive forward lean

It is natural, of course, to lean forward as you squat. The magnitude of the lean is related to your body dynamics (spine length and femur length in particular). You lean forward to keep the bar over your center of gravity as you squat. An excessive forward lean is when the bar path travels forward of the center of gravity. This creates adverse effects on your squat:

• Reduces the knee moment. As you can see from the diagram, a relatively small forward lean can have a significant effect on the knee moment, which reduces the quads’ engagement in the lift. In this example, the quads’ effectiveness in the lift is nearly eliminated.
• Shifts the bar’s weight to the hip moment. It is true, for a powerlifting squat, you do want the glutes and hamstrings more heavily engaged than your quads – the glutes are the most powerful muscle in the body. However, leaning forward of your center of gravity eliminates the quads’ ability to stabilize the lift. This brings the hips higher as you squat, making it more difficult to hit depth and also causes them to rise ahead of the bar leaving you in the good morning position to finish your squat.
• The good morning effect creates significantly more shear force in the lower back, causing more lower back fatigue during your squat session, and increasing your potential for injury.

An effective squat pattern keeps the bar directly over your center of gravity. Your knees may have a natural forward shift during the squat, but typically not forward of your toes. This pattern maintains a relative balance between the hip moment and knee moment, allowing your quads, glutes and hamstrings to work in concert to create a powerful squat.

Why does each rep get easier as you near lockout?

Figure 4: Nearing Lockout

The strongest point in other lifts (bench press and deadlift) have a strong dependency on individual body mechanics and personal strengths and weaknesses. The squat, however, is almost universally stronger as you approach lockout, with a sticking point just above parallel. This figure gives you a clear indication why.

• The downward moment force at the hip and knee is at it’s greatest at parallel, where the moment arms are their longest. Depending on your rebound point, upward momentum from the stretch often shifts the sticking point to just above parallel.
• As you move from parallel to lockout, the moment arms progressively smaller at both the hip and the knee, reducing the force required in both the quads and glutes and hamstrings to complete the lockout.

This nuance to the squat is why, in my opinion, using accommodated resistance (using bands or chains) has the greatest training effect in the squat. The band/chain resistance is at its least low in the squat where the moment force is at it’s greatest. That resistance increases as your mechanical advantage increases. For this reason you are not limited in the total weight lifted by the weakest point in the lift.

Low Bar versus High Bar Squat

Figure 5: High Bar vs Low Bar

In powerlifting, typically the low bar squat is used in competition, and in heavy strength training sessions. Why is that? Because generally speaking the low bar squat allows you to generate more explosive power than a high bar squat. Let’s look at why that is.

First, what is a low bar squat?

• High bar: Bar rests on top of your traps.
• Low bar: Bar rests between the base of your upper traps and across your delts.

The difference in position is not dramatic, but the effects are. What changes with this lower bar position, and how does it result in a more powerful squat?

• Pushes hips back: With the bar lower on your back, your upper body tilts more forward to keep the bar over your center of gravity. This increased lean pushes your hips back farther as you squat than when you squat more upright.
• Increases the hip moment: As your hips move back, the distance from your center of gravity, which is where the downward force of the bar is presented, and your hip joint increases. As we’ve discussed, this increases the moment arm between that force and your hips.
• Greater glute involvement: With a longer hip moment arm, as we’ve described above, there will be a greater load on your glutes versus your quads. The glutes are the largest, most powerful muscle in the body. Greater involvement of them in the lift equates to greater power.

By now you should have a pretty good idea what is involved in creating a strong squat. In the next few lessons we will discuss squat technique in detail.