The squat is probably the most important and productive exercise you can do in the weight room. In spite of its importance, it is surprising how poorly understood the squat remains within the general fitness community. Statements like “Squats don’t train the hamstrings” or yelling “Chest up!” when the lifter is about to drive his hips out of the hole are seen everywhere in this industry. It is this last misconception, that you had better squat with a vertical back angle for the safety of your back, which permeates the far reaches of the fitness world, and countless arguments can be found to back it up. Somehow, the vision of a bar on a more horizontal back triggers the alarms in the minds of personal trainers and lifters alike. It’s an instinctive reaction, and that’s fine. The problem really comes when arguments are built up from thin air to try to support a subjective perception. In this case, your lower back is about to be destroyed if kept more horizontal, rather than building the arguments from first principles to construct an objective understanding of what is really happening.

The reality is, as has been explained, not only in the book Starting Strength: Basic Barbell Training 3^rd edition, but also plenty of times in the multiple resources available at startingstrength.com, see particularly Squat Mechanics: A Clarification, that the spine is perfectly safe as long as it is kept in normal (extended) anatomical position, whatever the angle, and even if it is loaded. Moreover, it can be argued that your spine will be safe in general, be it in a flexed or extended position, if the musculature that protects it is strong. Not all implements in day-to-day life are as ergonomic as a barbell. After all, we pick up stuff off the floor with a flexed spine all the time.

The goal of this article is therefore not to argue how squatting with the spine held in extension is safe, whatever the angle of your back is. What will be shown here, from first principles and using basic physics, is that squatting with a more horizontal back, as described by the Starting Strength model, will actually significantly help your back to stay stable throughout the movement. The typical squat with a more vertical back is not only not safer, but it provides the lifter with a less stable movement pattern.

The argument is based on two premises. The first is the definition of stability in physical terms, and the second involves the understanding of the physics of the squat during hip drive, which, as I will show, is the phase in a heavy squat in which the stability of the back is especially compromised. More precisely, the second premise is the realization that there is a key mechanical difference during hip drive in a squat performed with a more horizontal back versus one performed with a more vertical back, and that a more horizontal back angle produces more stability during hip drive.

Let me succinctly present the argument: premises 1 and 2, and the corollary, and then develop it in detail, step by step.

Premise 1: Stability is the ability of the body to apply forces or moments to maintain or restore the condition of equilibrium, when it is subjected to a perturbation that tries to drive it out of this equilibrium condition. For the back in the squat, the equilibrium condition is spinal extension, and back stability is the ability to maintain spinal extension throughout the movement.

Premise 2: Inevitably, there is a perturbation that can drive the back out of its equilibrium position – spinal extension – in every heavy squat: a sudden increase in the moment of force on the back during hip drive. Crucially, as will be proven here, the magnitude of this increment in the moment of force significantly diminishes in a squat performed with a more horizontal back compared to a squat with a more vertical back.

Corollary: Therefore, a more horizontal back helps to maintain back stability throughout a heavy squat as per the definition in premise 1, since the perturbation that tries to drive the spine out of extension during hip drive diminishes compared to a squat with a more vertical back angle.

Don’t worry if this is not clear to you for now (it shouldn’t be… yet). The goal is to make it all click by the end of the article. Stay with me.

Premise 1: Stability

To understand stability in physical terms, and especially in the context of the back in the squat, we first need to have a grasp of the physics of the squat and of loaded human movement, specifically those things occurring to the back segment, as well as the concept of moment of a force (or just moment, to abbreviate).

Stability: Our musculoskeletal system is a system of levers, which are operated by muscle contractions: the muscle shortens and applies tension to the tendon, which pulls on the given bone at its attachment point, and either rotation or isometric rigidity follows around the given joint. Regarding the back segment in the squat, the bar rests on the upper back, loaded with plates, gravity applying a force on the bar vertically towards the ground. Coming out of the bottom, the hip extensors contract and pull on the hips, overcoming the weight on the back, the trunk rotates around the hips as the hips extend. The back angle goes from more horizontal at the bottom to vertical at the top.

During the whole movement, the spinal erectors have to contract and fight against the weight on the back in order to keep the spine in extension. Their origin is in the sacral region, and their insertion is left and right along the vertebrae all the way up to the base of the skull, such that under contraction they produce spinal extension. The action of the spinal erectors in maintaining extension is key for the back to be safe and stable during a squat, and to be able to efficiently transmit the force applied by the hip extensors to the bar.

More precisely, the physical quantity against which the hip extensors and spinal erectors have to fight during a squat is a moment of force. The moment of a force can be defined as the turning effect of a force, applied to a rotational system along a segment, at a certain distance from the axis of rotation. The moment is equal to the magnitude of the component of the force perpendicular to the segment, multiplied by the distance between the axis of rotation and the point of force application. During a squat (see Figure 1), gravity is acting on the bar on your back, resulting in a force on the bar directed vertically towards the ground.

 The moment of this force on your back is the distance from the hips (axis of rotation) to the bar (point of force application) multiplied by the component of the force perpendicular to the back. The moment can be increased therefore in three ways: by loading more weight on the bar, by placing the bar higher on the back at a certain back angle, or by setting the back in a more horizontal position, so that the force of gravity on the bar is “more perpendicular” to the back segment.

The equation for the moment of a force, which we will represent by the letter τ, is

τ = L F sin(θ),

where L is the distance between the hips and the bar, F is the force of gravity on the bar (perpendicular to the floor), and θ is the angle of the back measured from the vertical – sin(θ) is the sine function, which value is 0 for an angle θ of 0º and goes monotonically up to 1 for an angle of 90º. The equation means that the moment τ is the multiplication of L times F times sin(θ), and the quantity Fperp = F sin(θ) is basically the magnitude of the component of the force perpendicular to the back, depicted in Figure 1. Therefore, for a fixed weight on the bar (fixed F), and for a fixed bar position (fixed L), the moment (nearly) vanishes when standing up at the top of the squat, since the angle is (nearly) 0º and the sine function vanishes. The moment on the other hand is maximum when the back is in its most horizontal position at the bottom of the squat, which we will take to be, for a general lifter, approximately at an angle of θ0 = 45º in a correctly performed low bar squat.

Stability: the car illustration. If you are already familiar with the physics behind the definition of stability, or just feel in a rush, you can skip this aside. However, I think the car example is enlightening, and since most certainly all of you reading this article have driven a car, or at least have been a passenger, and have felt the forces going on during a ride, it’s the perfect example to understand the concept of stability.

Let me recall our physical definition:

Stability is the ability of the body to apply forces or moments to maintain or restore the condition of equilibrium, when it is subjected to a perturbation that tries to drive it out of this equilibrium condition.

Let’s take the car as the “body,” and let’s analyze the forces going on during a turn. The equilibrium condition is the car staying in contact with the road, ideally on all four tires. Let’s make the driver perform a sharp sudden turn. There is a perturbation that tries to drive the car out of its equilibrium condition: the centrifugal force during the turn pushes you laterally, directed to the outside of the turn. On the other hand, the force that tries and maintains the equilibrium condition is the force of gravity, pushing the car downwards against the road.

The key physical quantity that determines whether the car remains with its four wheels on the ground is the moment of force on the car. There is a moment due to the centrifugal force, trying to topple the car over towards the outside of the turn, as well as a moment due to the force of gravity, trying to maintain the car glued to the ground (see Figure 2). The axis of rotation is the line connecting the tires located on the outside of the turn, the point of application of both forces is right at the center of mass of the car (that point around which all the distribution of mass is even), and the segment is the distance between the axis defined by the outside tires and the center of mass. The forces are applied at a certain angle from this segment. Please have a look at Figure 2, a drawing helps massively to visualize it all

Now, if the moment due to the centrifugal force exceeds that due to the force of gravity, the condition of equilibrium would be lost, and the car would be on two wheels. So what aids to the stability of the car? A wider car: the segment length between the center of mass and the tires would be longer, such that the force of gravity would be “more perpendicular” to this segment, and the centrifugal force “more parallel.” This would improve stability, since the moment due to gravity would be increased relative to that due to the centrifugal force, and therefore would be more effective in maintaining the equilibrium condition: all four wheels in contact with the ground. A similar effect would be achieved by keeping the width fixed but lowering the center of mass.

But what about the movement pattern? Doing a turn picking a more open line, with longer turn radius, versus just doing a sharp sudden turn, would help to maintain the car stable, since we would be minimizing the perturbation that tries to drive the car out of its equilibrium condition: the moment due to the centrifugal force.

Stability: the back in the squat. Now we are better equipped to understand what is back stability during the squat. There are two aspects of back position in the squat and the pulling exercises: back angle, meaning more horizontal or vertical relative to the floor, and spinal position, meaning flexion or extension of the vertebral segments. Note that back angle and spinal position are separate quantities: the back angle can become more vertical or horizontal whether it is in extension or not, and is controlled by the hip extensors. And the spine can move in three different planes – it can flex or extend, rotate, and laterally flex, whether it is more vertical or horizontal; its position is controlled by the muscle mass that surrounds the spine – the spinal erectors and abs. The stable spinal position of the back during a squat is normal anatomical extension, and zero rotation and lateral flexion, and this is independent of back angle.

This extended spinal position is what you want to maintain during the whole movement, even as the back angle changes over the whole range of motion of the lift. The major factor that tries to break this condition of spinal position equilibrium is the moment on the back due to the force of gravity acting on the load on the bar, and the moment force the lifter must generate to overcome the load and complete the squat. If spinal position is not controlled isometrically by the spinal erectors and abs – the circle of muscles around your spine, the spine will come out of extension. This is what I will be concerned with, so for our purposes I will use a more specific definition of stability in the context of the back in the squat:

Back stability in the squat is the ability of the lifter to maintain spinal extension throughout the movement, against perturbations in the moment of force on the back that try to drive it out of extension.

Now, the moment of force that tries to maintain spinal extension, fighting against the moment due to the force of gravity, is that produced by the contraction of the spinal erectors. They have to win this battle for you to perform a squat with a stable, extended spine. Sure, you want the spinal erectors to work hard – that’s the way you strengthen them. However, any perturbation in the form of a sudden increase in the moment on the back, if strong enough, can potentially exceed their ability to maintain the condition of equilibrium. Remember that for a fixed bar position and a given weight, it is back angle that dictates the magnitude of the moment on the back. It is therefore the perturbations resulting in a sudden increase in back angle horizontality that can specially compromise back stability, coming from bad technique like “good morning-ing” your squats, or, as I will show in a moment, just as an intrinsic part of the lift during the hip drive phase out of the hole.

Then, knowing all this, what adds to the stability of the back? Strong spinal erectors and abs will be able to produce more force and will be able to maintain spinal extension better. And what about the movement pattern? Well, if you are able to squat in a way that minimizes any sudden changes in the moment on the back, you will be helping your back to stay stable throughout the movement. Assuming you are balanced and have decent technique, this means squatting in a way that minimizes the sudden increase in the moment during hip drive. I will proceed next to explain what hip drive is and to analyze in detail its effect on back stability.

Premise 2: Mechanics of Hip Drive

I will present here the argument for the second premise: how a more horizontal back minimizes the sudden increase in the moment of force on the back during hip drive. But first, what is hip drive and why does it happen?

Hip drive: what and why? We can think of hip drive as that subtle movement of the hips leading out of the hole, resulting in a slight change in back angle to more horizontal (see Figure 3). It is going to occur during a successful squat whether you want it to or not, and irrespective of you trying to keep the most vertical back possible while you are being yelled at “Chest up!!!” if the weight is heavy. In fact, in general, the more vertical the back is at the bottom, the more pronounced the change in back angle during this hip drive phase will be. This has been explained in previous articles (see Back Angle in the Squat, Part 1: Why it Matters, and is continuously demonstrated in detail by Rip in a lecture in the seminars, by analyzing videos of lifters lifting heavy weights.

The inevitable occurrence of hip drive is due to the fact that the muscle mass involved in hip extension – which in a correctly performed squat consists of the glutes, adductors, and hamstrings – is much bigger and stronger than the muscle mass involved in knee extension, which is basically the quads. Therefore, heavy weights can only be lifted if the moment of force is skewed towards the hips, as your musculoskeletal system arranges itself during a squat so that there is more moment on the hips and less on the knees. And it does so whether you want it to or not, because it is the only way you can successfully squat heavy. Maximal involvement of the hip extensors is necessary for lifting the heaviest weights, which is achieved by a more horizontal back angle and a more open knee angle. This is also why, if your back is too vertical at the bottom, the change in back angle out of the hole will be more pronounced than if you had properly bent over in the first place.

Hip drive: a sudden increase in the moment. At this point, we already know why the moment of force on the back suddenly increases during hip drive: there is a sudden increase in back angle, which means that the force of gravity is applied “more perpendicular” to the back segment. See Figure 3 and the comparison between the components of the force perpendicular to the back, before and after hip drive. The magnitude of the perpendicular component clearly increases after hip drive, due to the change in back angle to more horizontal.

Now I want you to have a look at Figure 4. There you can see how the moment increases monotonically with the angle of application of the force. In our case, this angle is basically the angle of the back as represented in Figure 1. 

But I want to direct your attention to something else, which in fact is the center of the argument for premise 2: two equal increments in back angle, Δθ, produce two very different increments in the moment on the back, Δτ, depending on what angle they occur. The bigger the angle around which Δθ occurs, the smaller the resulting Δτ. For a given increment in back angle during hip drive, the resulting increment in the moment on the back will be smaller if the back is more horizontal (bigger θ), than if it is more vertical (smaller θ).

Let me make some numbers to show this. In a correctly performed low-bar back squat, a reasonable estimate for the small change in angle of the back during hip drive could be around 4º. The increase in the moment of force, Δτ, on the back is just the moment of force right after hip drive, when the back is 4º more horizontal, minus the moment of force before, when the back is at a given angle θ0 at the bottom of the squat. We write this as

Δτ = L F sin(θ0 + 4º) - L F sin(θ0).

I am going to compare Δτ in a correctly performed squat, with back angle around θ0 = 45º, to the Δτ that would happen if the squat was performed with a more vertical back angle, something resembling the typical high-bar squat you can see in gyms all around the world. I will assume a back angle for this more “upright” squat in between 25º and 30º. Let’s be generous, and take θ0 = 28º. Even more generously, as I will argue in a moment, I will also assume that the change in back angle during hip drive is going to be the same in both squats, that is, 4º. Finally, I will take the same bar placement and weight in both cases, so L and F remain the same. I get

Δτhorizontal /Δτvertical = 0.8,

where the subscripts horizontal and vertical refer to the change in the moment of force in the squat performed with a more horizontal and a more vertical back angle, respectively. For more clarity, let me write the above equation in a different way

Δτhorizontal = 80% of Δτvertical.

This means that the sudden increase in the moment on the back during hip drive in a squat performed with a more horizontal back, as dictated by the starting strength model, is 80% of that occurring in a squat performed with the typical more vertical back angle. A more horizontal back means a reduction that can be around 20% in the perturbation that tries to take your spine out of extension during the movement, a perturbation that will happen if the weight is heavy, whether you want it to or not.

Furthermore, the numbers given here are actually somewhat generous towards the more vertical back case. The reality is that when the squat is performed with a too-vertical back, the amount of change in back angle out of the hole increases considerably. It may well be closer to 10º than the 4º assumed here. That means that Δτvertical will generally be even bigger, so the 20% reduction in Δτ by squatting with a more horizontal back calculated here, if anything, falls a bit short.

Corollary

Back stability in the squat is the ability of the lifter to maintain spinal extension throughout the movement, against perturbations in the moment of force on the back that try to drive it out of extension.

At this point we know that, in a squat with decent form, the major perturbation that tries to drive the spine out of extension is the sudden increase in the moment on the back during hip drive, which occurs in every heavy squat, whether you want it to or not.

We also know that a squat performed with a more horizontal back, as described by the Starting Strength model, minimizes such perturbation by at least a 20% compared with the typical squat with a more vertical back.

Therefore, as per the definition of back stability above, squatting with a more horizontal back significantly helps to maintain back stability throughout the movement, since it minimizes the perturbation that tries to drive the spine out of extension.

So, listen to the numbers and bend over in your squats. Not only will you use more muscle mass, lift more weight, and get stronger this way, but you will also do a favor to your back by providing it with a more stable movement pattern. Numbers matter… sometimes.

My thanks to John Petrizzo DPT for his help with this essay.

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