Starting Strength

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 thought of as “leverage” or bending force.

    Figure 2-25. Tension, compression, and moment are the expressions of the force of gravity across the lifter/barbell system.

    When the bar is carried on the back, or overhead in the lockout position of the press, the force it applies is compression. When the bar hangs from the arms in a deadlift or a clean, the force along the arms is tension. The bones transmit compressive force, and the connective tissues and muscles transmit tension. Both the connective tissues and the bones working together transmit moment (leverage). If the bar is supported overhead and then lowered in an arc to the hang position of the deadlift, all three forces – compression at the top, moment as the arms travel through the arc to the body, and tension as the bar comes to rest on the legs – can be experienced in that order.

    Figure 2-26. Compression, moment, and tension expressed through the upper body with a loaded bar.

    A moment arm is the distance between a point of rotation and the point at which the rotational force is applied, measured at 90 degrees from the point of the force application. When you’re using a wrench, for example, the moment arm is the distance along the handle, between the point of rotation (the bolt) and the force that causes the rotation (your hand), measured at 90 degrees to the force . Moment is the force transmitted along a rigid bar to act on a pivot, or fulcrum . The moment arm (the term “lever arm” is synonymous) is essentially a way to calculate