Dance Anatomy & Kinesiology

46 Dance Anatomy and Kinesiology

from the line of action of the force to the axis of

rotation or potential rotation. This latter quantity

(perpendicular distance from the line of force to the

axis of rotation) is termed the force arm or moment

arm. The key point here is that it is not just the

magnitude of the force but also how far away from

the axis (e.g., the joint) that this force is acting that

is vital in determining its effect. So, when analyzing

joint movement, the force resulting from muscle

contraction can be termed the effort (E); the line

of this force would be the line of pull of the muscle,

and the point of application of this force would

be the muscle attachment to the bone. So, as can

be seen in figure 2.11, torque of the muscle (T) =

effort (E) × effort arm (EA). However, this torque

related to contraction of the muscles trying to effect

a given motion is countered by the torque related

to the resistance to this motion. When the torque of

the resistance is due to external forces such as the

effect of gravity on a dumbbell or a body segment,

then the torque of the resistance = resistance (R) ×

resistance arm (RA).

Mechanical Advantage

The relation of the moment arm of the effort to the

moment arm of the resistance has important conse-

quences for the potential to overcome resistance and

FIGURE 2.10 Three classes of lever systems used in human movement.