The back serves as a fundamental part of our body to protect our spine, keep our posture upright, and being involved in any movement we perform.
Our back is built to bear and carry loads of all types. Its anatomy is characterised by a set of muscles which are orientated in a way that lock the spine into rigidity and thus provide it with greater stability.
In the squat, the back serves as the interface between the bar and lower extremities. Upon the descend, the weight of the bar is transferred from the upper back into the legs and floor - which results in ground reaction forces coming back up into the legs, helping us to "stand up" to an upright position.
When our back is set in rigidity, there is an efficient force transfer between upper and lower body - making our squat stronger and more efficient.
For simplicity of understanding, there are two parts of our back we need to set correctly in order to achieve as much rigidity as possible: the upper and lower back.
To set the upper back correctly, we need to first pull the sternum towards the chin. This elevates the chest and moves the shoulders back and down - leading to a tight contraction in the muscles of the upper back (i.e. rhomboids, mid-deltoids, posterior deltoids, lats).
Imagine keeping the elbows "down" and "in" when positioning yourself under the bar. This stops the elbows from lifting too high and thus prevents a less than favourable anterior tilt (i.e. forward movement) of the shoulder joint.
To set the lower back correctly, we need to perform an anterior pelvic tilt. In other words: we need stick our "bum out" to form an arch in our back. This creates a contraction in the lower compartment of the spinal erectors which lock the pelvis and lower back into rigidity.
Now that the upper and lower back are set in position - like two ends moving towards each other - the back efficiently performs its duty of stabilising and transmitting force.
As an additional cue, we need to think of our back being like a block of concrete in the squat: immovable, unbendable, unshakeable.
With this in mind, our back can serve the purpose of bearing the load of the bar so our legs can do their job of absorbing force and powering the body out of the hole in the squat.
A rigid back is a strong back.
Strong backs are healthier and hurt less than weak backs.