...A down and dirty reference for anyone more concerned with getting jacked than getting nerdy.
Biomechanics can be intimidating; the combination of physics, kinematics, anatomy and more comes with a ton of terminology, concepts, and even the occasional equation. Don't worry—this is not meant to be a detailed or comprehensive breakdown of biomechanics. Instead, this is intended as an overview of some of the principles and ideas with the greatest impact on everyday strength training. Even a cursory understanding of some of these fundamentals can help you choose the best squat pattern, decide how high to set a cable for a given exercise, and help you evaluate new exercises with a scientific eye.
Line of Pull
The direction in which a force—typically resistance—acts upon an object or body. For free weight or body weight exercises this is always straight down (gravity doesn’t really go in for variety), while bands and cables let us manipulate it.
Bottom Line: It’s why a pull through hits differently than a deadlift. #bootygains
Moment Arm
The distance between the axis of rotation (the moving joint) and the line of pull, measured perpendicularly to the line of pull. Movement arms are largely responsible for the difference in how a given weight “feels” at different points in an exercise. As the moment arm lengthens the application of the load is increased.
Bottom Line: Moment arms are why your sticking point in the bench and the squat is always right around parallel, and why you cheat your bicep curls right around 90º.
Force Vector
Line of pull tells you about the direction of a force, while a force vector also tells you how much force is involved, technically referred to as "magnitude". It's influenced by moment arms, line of pull, and how much weight is involved.
Bottom Line: this is ultimately what defines any exercise—how much weight you're moving and in what direction.
Center of Gravity
The point in the body around which weight is evenly distributed and all sides are balanced. As you move this shifts, and the addition of load shifts things even further. Manipulating center of gravity can influence position, and position can influence moment arms and muscle recruitment.
Bottom Line: It’s why a front squat targets your quads, and why a low bar back squat hits your hammies.
Base of Support
The area defined by the points of contact a body makes with a supporting surface—it’s the difference between a narrow stance and a wide one, or between standing on one foot or two.
Bottom Line: A bigger base of support makes it easier not to fall over. (More on that next…)
Stability
Stability—or lack thereof—is about the relationship between Center of Gravity (COG) and Base of Support (BOS). When your COG is within your BOS, you’re stable. When your COG falls outside of you BOS, you’re on your way down. While certain positions may “feel” more or less stable, stability is technically binary: you have it or you don’t. To illustrate, try this out—stand up and lean as far forwards as you can without falling over. As long as you’re still upright, you’re stable, and your Center of Gravity is somewhere within the imaginary lines that define your base of support. Go too far and start to fall and you’ve lost it.
In the weight room this often means that muscles—and let’s be clear: any muscle can act as a stabilizing muscle—are contracting isometrically to keep you from falling over.
Bottom Line: Stability is why you’d rather put weight on both sides of the bar, and why, if you didn’t, you’d want your feet set nice and wide.
Inertia
Inertia is the tendency of anything with mass to either stay put, or stay in motion. Wanna stop a freight train? You’re gonna need a lot of force. Wanna get it moving again? Same story.
Bottom Line: Inertia is why it's easier to keep a sled moving than it is to get it started.