PART I FOUNDATION CONCEPTS Clinical Connection 1.6
Carrying a 10-lb object away from the body (A) pro- duces a greater external torque compared with carrying the object close to the body (B). The external torque is greater at the longer distance because the moment arm of the external force is greater. The greater the external torque, the greater the force needed from
the trunk muscles to stabilize the body. This excessive force over time can produce microinjury to the spine. To carry objects using the least amount of muscle force (internal torque), the external moment arm can be shortened by carrying the object closer.
A
B
of a force couple functioning during human motion is upward and downward rotation that occurs as the scapula rotates during shoulder movement in the sagittal or frontal plane (Fig. 1.27B). As the humerus moves into fl exion or abduction, the upper and lower trapezius and the serratus anterior muscles work together to produce upward rotation of the scapula. This scapular movement preserves the alignment of the humerus with the glenoid fossa and assists with preventing shoulder dysfunction. As the humerus moves into extension or adduction against resistance, the pectoralis minor, rhomboids, and levator scapulae work as a force couple to produce scapular downward rotation.
Visit Kinesiology in Action and answer the posttest questions to evaluate your understanding of the chapter.
CHAPTER SUMMARY
The study of kinesiology and human movement requires a fundamental understanding of the planes and axes in which movement occurs and the universal language used to describe directions of movement. Motion of body seg- ments or the body as a whole can occur as linear or angular motion or as a combination of the two. Motion
