Chapter 11

At the conclusion of this chapter, the student should be able to:

• 1. Name the kinds of motion experienced by the human body, and describe the factors that cause and modify motion.
• 2. Name and properly use the terms that describe linear and rotary motion: position, displacement, distance, speed, velocity, and acceleration.
• 3. Explain the interrelationships that exist among displacement, velocity, and acceleration, and use the knowledge of these interrelationships to describe and analyze human motion.
• 4. Describe the behavior of projectiles, and explain how angle, speed, and height of projection affect that behavior.
• 5. Describe the relationship between linear and rotary movement, and explain the significance of this relationship to human motion.
• 6. Identify the critical kinematic components that would be used to fully describe the skillful performance of a selected motor task.

If we are to understand the movements of the human musculoskeletal system and the objects put into motion by this system, we need first to turn our thoughts to the concepts of motion itself. What is motion? What determines the kind of motion that will result when an object or a part of the human body is made to move? How is motion described in mechanical terms? How do these generalities about motion apply to movements of the musculoskeletal system? Indeed, how does one know that motion is occurring?

### Relative Motion

Motion is the act or process of changing place or position with respect to some reference object. Whether a body is at rest or in motion depends totally on the reference, global or local. When a person is walking down the street or riding a bicycle or serving a tennis ball, it seems obvious that movement is involved. Less obvious is the motion status of the sleeping passenger in a smoothly flying plane or of an automobile parked at a curb. If the earth is the reference point, all but the parked car are in motion relative to the earth, and even the parked car is in motion if the reference point is the sun. On the other hand, if the bicycle is the reference point, the person riding it is at rest relative to the bicycle, and the sleeping passenger is at rest with respect to anything in the plane. The relative motion of each is defined in relation to the specific reference object or point. It is possible, therefore, to be at rest and in motion at the same time relative to different reference points. The sleeping passenger is at rest relative to the plane and in motion relative to the earth. The relative motion of two bodies depends entirely on their relative velocities through space. Two joggers running at 8 km/hr in the same direction are at rest with respect to each other. However, if one jogs at 8 km/hr and the other at 10 km/hr, the slower jogger would be considered to be at ...

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