Starting Strength, 3rd Edition - Mark Rippetoe [20]
If we develop an accurate description of each exercise based on an understanding of what each one is supposed to accomplish in terms of movement against a loaded bar, how this movement is most efficiently accomplished using muscular contractile force translated through the skeletal components that transfer the force to the load, and which physical adaptations will accompany an ability to handle increasing loads in each particular movement pattern, we will have what can be described as a model of the exercise.
This model must be grounded in an understanding of the principles that govern the motions within a physical system. And a grasp of each model makes the performance and coaching of each movement more straightforward, logical, and understandable. The science of classical mechanics studies the effects of forces on the motions of material bodies. An extensive treatment of this science is obviously outside the scope of our discussion, but a basic understanding of a few of its concepts is critical to the development of an accurate model for each exercise in this method of barbell training. These concepts are important to understand because the system of levers you will use to lift the barbell – your muscles moving your skeleton, loaded by the barbell in a gravitational framework – obeys the laws of mechanics, and you must know them before you can analyze your lifting to optimize the way you do it.
So, let’s start with the most basic concept and build on it. As noted previously, the agent that produces the weight of the loaded barbell is gravity. It is produced by the mass of the planet, and for our purposes the planet is assumed to be a uniform sphere. Every unimpeded object will fall in a direction perpendicular to the surface of this sphere. The term “level” is used to denote a surface parallel to the surface of the planet, so that if an object is dropped, it always falls perpendicular to “level,” and we describe this path as vertical. The force exerted by the weight of a loaded bar is therefore always vertical and down, and the only way to oppose the force of a freely moving barbell is with a force that is vertical and up. Horizontal force may be applied to the bar during its trip through the rep, but none of the horizontal force can contribute to the vertical motion of the bar. So, to the extent that squatting, pulling, or pressing a loaded bar works against gravity, the vertical components of the force do the work. This means that the most efficient bar path for a barbell moving in a gravitational framework is always a straight vertical line; not only is this path the shortest distance between the two points, but any force applied in any other direction is not work against the force of gravity (see Figure 2-3).
Gravity is expressed as three primary forces that affect the lifter/barbell system: tension, compression, and moment.
Tension is the force transmitted along an object that would elongate if it were deformable (not every object is deformable under normal gym circumstances). An example would be the body of a lifter hanging from the chin-up bar.
Compression is the force transmitted along an object that would get shorter if it were deformable. Compression is the opposite of tension, and an example would be the body of a lifter standing under the loaded squat bar.
Both tension and compression are said to be axial forces because they are expressed parallel to the axis of the force that generates them, gravity.
Moment is force that tends to cause a rotation about an axis. It is the force that is transmitted down a wrench handle to turn a bolt. Moment can also be