Inertia Principle

Newton's first law of motion, or the Law of Inertia, describes the resistance of all objects to a change in their state of linear motion. In linear motion, the measure of inertia is an object's mass. Application of Newton's first law in biomechanics is termed the Inertia Principle. This section will discuss how teachers, coaches, and therapists adjust movement inertia to accommodate the task. Our focus will be on the linear inertia (mass) of movement, so the inertial resistance to rotation will be summarized in chapter 7.

The first example of application of the inertia principle is to reduce mass in order to increase the ability to rapidly accelerate. Obvious examples of this principle in track are the racing flats/shoes used in competition versus the heavier shoes used in training. The heavier shoes used in training provide protection for the foot and a small in-ertial overload. When race day arrives, the smaller mass of the shoes makes the athlete's feet feel light and quick. We will see in chapter 7 that this very small change in mass, because of its position, makes a much larger difference in resistance to rotation (angular inertia). Let's add a little psychology and conditioning to the application of lowering inertia. Warm-up for many sports involves a gradual increase in intensity of movements, often with larger inertia. In baseball or golf, warm-up swings are often taken with extra weights, which when taken off make the "stick" feel very light and fast (Figure 6.6).

In movements where stability is desired over mobility, the Inertia Principle suggests that mass should be increased. Linemen in football and centers in basketball have tasks that benefit more from increasing muscle mass to increase inertia, than from decreasing inertia to benefit quickness. Adding mass to a golf club or tennis racket will make for faster and longer shots if the implement can be swung with the same velocity at impact. If an exercise machine tends to slide around in the weight room, a short-term solution might be to store some extra weights on the base or legs of the machine. If these new weights are not a safety risk (in terms of height or potential for tripping people), the increased inertia of the station would likely make the machine safer.

Another advantage of increased inertia is that the added mass can be used to mod

Figure 6.6. Mass added to sporting implements in warm-up swings makes the inertia of the regular implement (when the mass is removed) feel very light and quick. Do you think this common sporting ritual of manipulating inertia is beneficial? If so, is the effect more biomechanical or psychological?

ify the motion of another body segment. The preparatory leg drives and weight shifts in many sporting activities have several benefits for performance, one being putting more body mass in motion toward a particular target. The forward motion of a good percentage of body mass can be transferred to the smaller body segments just prior to impact or release. We will be looking at this transfer of energy later on in this chapter when we consider the Segmental Interaction Principle. The defensive moves of martial artists are often designed to take advantage of the inertia of an attacker. An opponent striking from the left has inertia that can be directed by a block to throw to the right.

An area where modifications in inertia are very important is strength and conditioning. Selecting masses and weights for training and rehabilitation is a complicated issue. Biomechanically, it is very important because the inertia of an external object has a major influence on amount of muscular force and how those forces can be applied (Zatsiorsky & Kraemer, 2006). Baseball pitchers often train by throwing heavier or lighter than regulation baseballs (see, e.g., Escamilla, Speer, Fleisig, Barrentine, & Andrews, 2000). Think about the amount of force that can be applied in a bench press exercise versus a basketball chest pass. The very low inertia of the basketball allows it to accelerate quickly, so the peak force that can be applied to the basketball is much lower than what can be applied to a barbell. The most appropriate load, movement, and movement speed in conditioning for a particular human movement is often difficult to define. The principle of specificity says the movement, speed, and load should be similar to the actual activity; therefore, the overload should only come from moderate changes in these variables so as to not adversely affect skill.

Suppose a high school track coach has shot put athletes in the weight room throwing medicine balls. As you discuss the program with the coach you find that they are using loads (inertia) substantially lower than the shot in order to enhance the speed of upper extremity extension. How might you apply the principle of inertia in this situation? Are the athletes fully using their lower extremities in a similar motion to shot putting? Can the athletes build up large enough forces before acceleration of the medicine ball, or will the force-velocity relationship limit muscle forces? How much lower is the mass of the medicine ball than that of the shot? All these questions, as well as technique, athlete reaction, and actual performance, can help you decide if training is appropriate. The biomechanical research on power output in multi-segment movements suggests that training loads should be higher than the 30 to 40% of 1RM seen in individual muscles and muscle groups (see the following section on muscle power; Cronin et al., 2001a,b; and Funato, Matsuo, & Fukunaga, 1996). Selecting the inertia for weight training has come a long way from "do three sets of 10 reps at 80% of your maximum."

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