48 Dance Anatomy and Kinesiology
of motion of the end of the lever and a potential
advantage in terms of speed of movement of this
distal segment. So, although a large amount of force
of the deltoid would be required to overcome the
10-pound dumbbell, a small amount of shortening of
the deltoid would result in a much greater excursion
of the hand. This ability to move the hand or foot
through a very large range of motion, and if desired,
at a very high velocity, is commonly utilized in dance
and many other human movements.Equilibrium Versus Movement
Lastly, the relationship of the net torque of the
muscle relative to the resistance has important impli-
cations for joint movement. In essence, if the torque
of the muscle is exactly equal to the torque of the
resistance, the system is in equilibrium (Fr. aequus,
equal + libra, a balance) and no net movement will
occur. If the torque of the muscle (effort) is greater
than the torque of the resistance, joint rotation will
occur in the direction of the muscle’s pull. And, if
torque of the resistance is greater than that of the
muscle, joint rotation will occur in the direction of
the resistance (table 2.3).
Applying these concepts to dance, when you hold
another dancer at shoulder height, the torque of the
muscles must be equal to the torque of the dancer
(e.g., weight of dancer × perpendicular distance to
shoulder joint axis). When you lift the dancer, muscle
torque is greater than resistance (dancer) torque;
and when you lower the dancer, muscle torque is less
than the torque produced by the dancer. Further-
more, as illustrated in figure 2.12B, if you imagine
the 120-pound weight is a dancer, moving the dancer
closer to you when you lift would dramatically reduce
the torque produced by the dancer and the amount
of muscle force needed to lift the dancer. Conversely,
moving the dancer farther away when you lift would
dramatically increase the muscle force required to
carry out the movement. Taking this concept into
consideration, it is easy to understand how functional
human movement often incorporates bending the
limbs (elbows, knees) to shorten the lever arms when
reducing resistance torque would be advantageous.
It also explains why a very long-limbed person would
have to exert significantly greater muscular effort
than one with shorter limbs to lift a dance partner
of a given weight.Angle of Muscle Attachment
While the moment arm of the resistance is key in
determining resistance torque, the effectiveness ofFIGURE 2.12 Change in resistance torque with (A)
change in shoulder joint angle and (B) change in shoul-
der and elbow joint angle.