The Moment Arm
What are you built for?
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We are not created equal. Each of us is born with different heights, limb lengths, and proportions — and those proportions determine which movements we perform with mechanical advantage and which we perform at a disadvantage.
This is most visible at the extremes. Olympic competitors are outliers in every direction, and each discipline selects for a specific body type. Weightlifters tend toward short limbs and long torsos. Swimmers are tall with disproportionately long arms. Sprinters carry their height in their legs. Marathoners are small, light, and fine-boned. These athletes are products of obsessive training, but they were born with the mechanics their discipline requires.
In 2014, sports journalist David Epstein made this point vividly by placing two world-class athletes side by side. Michael Phelps, the most decorated Olympian in history, and Hicham El Guerrouj, who has held the men's mile world record since 1999, are seven inches apart in height — Phelps at 6'4", El Guerrouj at 5'9". They have the same inseam. Phelps's extraordinary height lives entirely in his torso and wingspan. El Guerrouj's proportionally longer legs give him the stride mechanics that have made his record untouchable for a quarter century. Each man is structurally optimized for his discipline in ways that no training program could replicate in the other's body.
Most of us are not outliers — by definition — but our individual proportions will give us relative advantages in some movements and disadvantages in others. Understanding the mechanical reasons for both is more useful than grinding through movements that work against your structure.
Four concepts govern the mechanics of any exercise: Moment Arm, Direction of Resistance, Strength Curve, and Resistance Curve. Together they give you the tools to evaluate any movement, stroke, or swing.
We'll begin with the moment arm.
The Moment Arm
The moment arm is the perpendicular distance between the axis of rotation — the joint — and the line of force being applied.
It is also called the “lever arm”, or the “moment to the axis”. The terms are interchangeable. For consistency we'll use moment arm throughout.
The moment arm determines how much torque is required to move a load:
Torque = Applied Force × Moment Arm
Torque is sometimes called "moment" — which is where the term moment arm originates and where the terminology gets briefly confusing. They are the same concept.
A longer moment arm means that for a given applied force, more torque is produced. This is the “advantage” of a lever — the same force generates more rotational effect with a longer arm. Conversely, less applied force is required to produce the same amount of torque. A longer wrench makes a stubborn bolt easier to turn.
We use tools to give us an advantage to alter the world around us. Ironically, the world uses our own limbs to do the same to us.
If you hold a ten-pound weight out to your side with your arm parallel to the floor, you are not resisting ten pounds. You are resisting ten pounds multiplied by the length of your arm — because your arm is the lever and your shoulder is the axis. But your arm is not your lever in this scenario. It's gravity's lever. Gravity is using your own arm as a wrench to create torque on your shoulder.
If you bend your elbow 90 degrees, the moment arm shortens — the distance between the weight and the shoulder joint decreases — and the effective resistance drops, even though the weight in your hand is identical. Congratulations, you got some of your arm back.
Lower your arm toward your hip. Same result. The moment arm is always measured perpendicularly to the direction of resistance, which in most loaded movements is gravity — straight down. Any deviation from perpendicular shortens the moment and reduces the effective load. By surrendering to gravity you've removed any advantage it had over your arm.
The Resistance Curve
During a dumbbell bicep curl, the moment arm is the horizontal distance between the weight and the elbow joint. Horizontal, because the direction of resistance is vertical — gravity pulling straight down.
At the start of the movement, the weight hangs at your side. The horizontal distance between the weight and the elbow is zero. The moment arm is zero. The resistance on the bicep is effectively zero.
As the elbow flexes and the forearm rises, the moment arm increases. It reaches its maximum when the forearm is parallel to the floor — elbow at 90 degrees, the greatest possible horizontal distance between the weight and the joint. This is the hardest point in the movement.
Past 90 degrees, the moment arm begins to shorten again as the weight moves back toward being directly above the elbow. Resistance decreases toward the top of the curl.
The weight itself never changes, but the resistance changes continuously throughout the movement as the moment arm lengthens and shortens.
One way to think of a bicep curl is your biceps is fighting the dumbbell for control over your forearm. The longer the moment arm, the more advantage the load has over your biceps. This dynamic relationship — the changing resistance as a joint moves through its range — is called the resistance curve.
We'll examine the resistance curve in detail in a later article. For now, the moment arm is the mechanism that creates it.
