After completing this chapter, you
will be able to:
- Define torque, quantify resultanttorques, and identify
the factors that affect resultant joint torques.
- Identify the mechanical advantages associated with the different
classes of levers, and explain the concept of leverage within the
- Solve basic quantitative problems using the equations of static
- Define center of gravity, and explain the significance of
center of gravity location in the human body.
- Explain how mechanical factors affect a body’s stability.
Why do long jumpers and high jumpers lower their centers of gravity
before takeoff? What mechanical factors enable a wheelchair to remain
stationary on a graded ramp or a Sumo wrestler to resist the attack
of his opponent? A body’s mechanical stability is based
on its resistance to both linear and angular motion. This chapter
introduces the kinetics of angular motion, along with the factors
that affect mechanical stability.
Many athletic skills require
As discussed in Chapter 3, the rotary effect created
by an applied force is known as torque,
or moment of force. Torque, which may
be thought of as rotary force, is the
angular equivalent of linear force. Algebraically, torque is the
product of force and the force’s moment
arm, or the perpendicular distance from the force’s
line of action to the axis of rotation:
Thus, both the magnitude of a force and the length of its moment
arm equally affect the amount of torque generated (Figure 13-1).
Moment arm is also sometimes referred to as force
arm or lever arm.
Which position of force application is best for opening
the swinging door? Experience should verify that position C is best.
As may be observed in Figure 13-2, the moment arm is
the shortest distance between the force’s line of action
and the axis of rotation. A force directed through an axis of rotation
produces no torque, because the force’s moment arm is zero.
The moment arm of a force is the perpendicular distance
from the force’s line of action to the axis of rotation
(the door hinge).
Within the human body, the moment arm for a muscle with respect
to a joint center is the perpendicular distance between the muscle’s
line of action and the joint center (Figure 13-3). As a
joint moves through a range of motion, there are changes in the
moment arms of the muscles crossing the joint. For any given muscle,
the moment arm is largest when the angle of pull on the bone is
closest to 90˚. ...