Coach Mark Rippetoe introduces the basics of muscle action.
Understanding how a muscle works is essential to understanding how to strengthen it. This footage is taken from his Basic Barbell Certification seminar at CrossFit Brand X in Ramona, CA on February 14, 2009.
This video applies the basic concepts introduced in Part 1 to the actions in the back squat. The trunk muscles are in isometric contraction. The descent under load is driven down by gravity and there is an eccentric contraction of the muscles of the hips and knees. The ascent is a concentric contraction of those same muscles.
There is also a discussion about the difference between eccentric contractions under load and the language frequently used to describe the air squat as pulling yourself down using the hip flexors.
As simple as the concepts may be, a lot of very intelligent people still squat predominantly with their quads instead of their glutes and hamstrings. Coach Rippetoe orients his teaching of anatomy and physiology toward achieving proper mechanics in the basic barbell exercises.