Are Deadlifts for Back Day or Leg Day?Strength Training 6 min Read
I’ve heard this topic come up a lot over the years, and I’ve most recently seen it on Instagram….
When it comes to pulling your next deadlift PR off the floor, why does physics matter?
Because matter has mass, and a massive PR is what we are trying to achieve.
Sir Isaac Newton, one of the greatest scientists of all time, bestowed on us his three laws of motion. Just like any other large-scale matter, the bar on the floor behaves in a predictable way.
Let’s see how his first two laws of motion apply to the deadlift.
Newton’s first law is all about inertia. An object at rest stays at rest and an object in motion stays in motion unless acted on by a force.
That means the bar isn’t going to lift itself. The weight is perfectly content sitting on the floor until you come along and disrupt its inertia by applying a force.
That is fairly obvious, but how much force is actually needed to get the weight moving? That brings us to Newton’s second law.
Newton’s second law gives us a better understanding of force. Force can be described as an interaction with a mass that causes a change in motion. One of the fundamental forces, gravity, acts on all the mass on Earth to pull objects to the ground. Gravity is your enemy in the weight room. The mathematical representation of Newton’s second law is given by:
This means that the sum of forces on an object is equal to the mass of the object multiplied by its acceleration. Without going too far down the physics rabbit hole, acceleration is the rate of change of velocity (speed with direction) of an object.
In order to get the weight to accelerate from its resting state to a velocity that will allow you to reach lockout before your muscles fatigue, the net force must be positive.
You have to apply MORE force to the bar than the force of gravity pulling it down!
When the bar is sitting on the floor gravity pulls down with a force equal to the bar’s mass multiplied by the acceleration caused by gravity. This force is countered by the force of the floor pushing up on the weight (equal and opposite remember?) so that your deadlift doesn’t drop straight to the pits of hell.
Assume you are going for a 315-pound deadlift PR. This means that gravity is exerting 1,400 N(acceleration of gravity on the mass of the bar) on the bar towards the ground, and the ground is pushing up with the same force. As you begin to pull on the bar the downward force of gravity remains the same, but you begin to take over the upward force that was being applied by the floor. Just before the bar breaks off the floor, you are pulling on the bar with 1,400 N as gravity matches you.
The takeaway here is that in order to get that bar off the floor you need to pull with more force than gravity is pulling it back down!
Once the bar breaks off the floor if you maintain that greater force, the bar will continue to accelerate and will begin to move faster. If your force begins to decrease back down to 1,400 N we can look back at Newton’s first two laws.
The net force is 0 but the object is in motion. You can finish the lift at a constant velocity pulling with less force than it took to get the bar off the floor. At no point in your lift do you need to apply more effort than right off the floor!
Visit the Serious Guide to the Conventional Deadlift to learn how to put Newton’s laws into motion. It’s an article covering technique, accessory work, and programming for the convention deadlift. Everything you need to know to learn the lift and set big PRs.