The Basics of Barbell Physics
It all starts with gravity. Gravity is the attractive force between any objects with mass, and in our case, the Earth is by far the most massive object around. The Earth’s gravitational force is constantly pulling all objects towards the Earth’s surface, giving the objects weight and accelerating them downward in a straight vertical line towards the center of the planet. When we take the bar out of the rack for a heavy squat, Earth’s gravity is applying a force to the barbell and the lifter, and trying to pull them both straight down towards the floor.
Force is really just a push or a pull that when unopposed will create a change in velocity in the object that the force is being applied to. As gravity is pulling down on the bar, the lifter must produce a force up against the bar to keep it from accelerating down towards the floor. If the lifter produces a force equal to the weight of the bar, the bar doesn’t move. If the lifter produces more force than the weight of the bar, the bar will accelerate upward.
Compression, Tension and Moment Forces
There are a several types of force at play in barbell training, and we are mainly concerned with compression, tension, and moment.
Compressive force pushes in on the ends of an object and tries to make the object shorter. A good example of compressive force is the weight of the bar resting on the lifter’s back at the top of a low bar back squat.
Tensile force pulls out on the ends of an object and tries to make the object longer. A lifter’s arms are in tension during a conventional deadlift.
A moment force is a force that has a tendency to rotate an object about an axis – just like when you pull on a wrench to tighten a bolt or the lug-nuts on your car. There are two components to the moment force: the force itself and the moment arm, which is the perpendicular distance between the direction of the force application and the axis of rotation. When a lifter is leaning over in the bottom of the squat, the heavy bar on the back acts as moment force that is trying to further rotate the hips and the knees closed.
The type and amount of force on the lifter’s joints changes as the lifter moves through the range of motion for the lift. When we’re standing up at the top of the squat the force of the bar on the lifter is basically all compressive, as the lifter should be standing up as straight as possible with the bar, the hips, and the knees all in line over the middle of the foot. As soon as the lifter begins to descend and the hips and knees move away from the imaginary line from the bar straight down through the middle of the lifter’s foot, moment arms are created, and moment forces arise at the joints.
This line from the bar thru the mid-foot is called the force vector. At the bottom of the squat, the moment arm lengths between the force vector and the hip and the force vector and the knee determine what share of the moment force must be countered by the musculature that actuates those joints. The further a joint is from the bar, the greater the moment force acting on that joint. We specifically design the way we squat so that the hips are pushed back further away from the force vector than the knees, so that the larger muscle mass of the hips carries the greater share of the moment force load.
Because we’re interested in building general strength, we want to use the most muscle mass to move the largest possible load, and pushing our hips back in the squat distributes the moment force in a way that accomplishes just that. This is the basis for the moment model that we use at Starting Strength and Starting Strength Online Coaching to select, analyze, and execute the lifts.