A large force acting on a body during a very brief period of time may instantaneously alter the velocity of the body. Such forces often occur when two bodies collide. A runner crushes down upon his or her heel with a briefly sustained but intense force that often reaches many times the body weight. Each heel strike sends shock waves through the body, causing accelerations as high as 15 g. Running is not the only mode of motion during which impulsive forces act on humans. Accidental falls are the leading cause of death from injury among persons aged 65 years and older. Nearly 300,000 hip fractures occur in the United States in a year. Finally, it must be noted that car-crash injury is still the number one killer for adults under the age of 35 years. To that end, the mechanics of objects impacting on padded surfaces are of great interest in biomechanics. The risk of injury from striking an automobile dashboard is of obvious importance in everyday life. In this chapter, we present an introduction to impact mechanics. We illustrate how the impact forces affect the movement and motion of the human body. The analysis is based on the mathematical relationship between impulse and momentum.
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The use of dumbbells gives you a much more comprehensive strengthening effect because the workout engages your stabilizer muscles, in addition to the muscle you may be pin-pointing. Without all of the belts and artificial stabilizers of a machine, you also engage your core muscles, which are your body's natural stabilizers.