Starting Strength, 3rd Edition - Mark Rippetoe [27]
When you squat, ultimately it is hip extension – straightening out the hip joint, their proximal function – that you produce with the hamstrings, along with the glutes and adductors. (In reality, the hamstrings can control hip extension, knee flexion, and back angle while functioning eccentrically, concentrically, and isometrically; the definitions of these functions are blurry, and are really significant only when we isolate joints on exercise machines. The complexities of normal movement do not lend themselves to such constructed distinctions.)
Squatting power is generated by the hips and legs and is transmitted up the rigid trunk segment to the load resting on the shoulders. The spinal column is held rigid in its normal anatomical position by the muscles of the back, sides, rib cage, and abs so that the force can be safely transmitted through the trunk to the load. Before you lift anything heavy, you squeeze your abs (really, you squeeze everything in the vicinity of your abs) into contraction. This squeezing transforms your trunk into what is essentially a rigid cylinder that surrounds and supports the spine. The effect is that of a hydrostatic column – an uncompressible column of fluid that is therefore capable of transmitting compressive force – between the contracting abdominal wall and the spine. The force of contraction transmitted through this fluid medium braces the spine into the position set by the back muscles until the load overcomes your ability to stay in position. These muscles contract isometrically – that is, they stay in contraction but cause no movement to occur – and in doing so, they permit no movement to occur.
The pelvis articulates with the spine in the L5/S1 area of the lower back, the area above the tailbone. The muscles of the lower back – the erector spinae group, or “spinal erectors” – insert on the pelvis and at numerous points along the spinal column. When these muscles are contracted, the pelvis remains in a constant position relative to the lumbar vertebrae. The spinal erectors and associated lower back ligaments serve to lock the pelvis and spine into a rigid structure – to protect the vertebral column from movement under load and to hold all these joints in their normal anatomical positions when you’re lifting heavy loads – so that the intervertebral discs are not damaged. This area needs to stay arched to stay safe when you’re lifting. And this is why the pelvis must tilt forward at the same angle as the lower back as you lean forward.
Figure 2-36. Proper spinal alignment ensures the anatomically correct distribution of forces across the intervertebral discs during loading. Improper vertebral position under load can result in either anterior or posterior squeezing of the discs and the injuries that accompany this bad position.
However, as the squat approaches the bottom position, the necessary forward lean of the trunk can have a tendency to make the lower back assume a flexed, rounded position. This tendency is caused by the hamstring anatomy and the position of the thighs. As squat depth increases and the torso assumes a more forward tilt, the bottom of the pelvis (the origin point of the hamstrings) comes under tension from the direction of the proximal tibias (the insertion point of the hamstrings just below the knees). As the hamstrings reach the limit of their ability to stretch, they become tighter and begin to exert more pull on both the knees and the muscles’ pelvic attachments. If your knees are not far enough apart, your thighs will also crowd your torso as you approach the bottom.
There are two problems.