Free Starting Strength by Mark Rippetoe Page B

semimembranosus, and the semitendinosus, all three of which attach to the ischial tuberosity of the pelvis. They all insert at various points on the tibia, behind the knee on the lower leg. This configuration means that the hamstring group crosses two joints, the hip and the knee, and therefore technically has two functions: the proximal function (hip extension), and the distal function (knee flexion). The hamstrings can also act isometrically against both attachments to control the back angle.

    Figure 2-35. (A) The relationship of the bones of the lumbar spine, pelvis, femur, and upper tibia and the actions of the muscles that move them, in profile. The squat has the reputation of being a quadriceps exercise, but the hamstrings are also strongly developed during the full squat. (B) The spinal erectors attach to the pelvis, ribs, and vertebrae, and extend the spine when in contraction. This “arching” action is accomplished in conjunction with the underlying multifidis, rotators, interspinales, and intertransversarii muscles. When contracted, these muscles move the spine into the position shown by the arrows.

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