50 Dance Anatomy and Kinesiology
the muscle force in producing desired rotation is
greatly influenced by the angle of the muscle’s attach-
ment relative to the bone. The muscle force or effort
has a direction determined by its angle of attach-
ment and a magnitude determined by how hard the
muscle is contracting, and hence is a vector. A basic
property of vectors is that they can be resolved into
vertical or perpendicular and horizontal or parallel
components. In the human body, the perpendicular
component of the muscle effort will tend to produce
rotation of the joint and hence is termed the rotary
component, as shown in figure 2.13. At all angles of
pull other than 90°, the parallel component (running
parallel to the distal bone and through the axis of the
joint) of the muscle effort will tend to pull the distal
bone toward the joint (stabilizing) or away from the
joint (dislocating), depending on the joint angle. So,
at angles less than 90°, only part of the muscle effort
will produce rotation (rotary component), while part
contributes to joint stability (parallel component),
as seen in figure 2.13A. This offers a disadvantage
for movement but an advantage for joint stability
that can be particularly useful when the limbs are
weightbearing. When a muscle’s angle of pull is per-
pendicular (90°) to the bone on which it is pulling,
virtually all of the muscle’s effort will contribute to
joint rotation (figure 2.13B), an optimal situation
in favor of movement. Lastly, when the muscle’s
angle of pull is greater than 90°, again only part of
the muscle effort will produce joint rotation (rotary
component), while part acts to dislocate the joint
(parallel component), as seen in figure 2.13C. Forthis reason, a joint that is loaded in extreme flexion,
such as during dance floorwork, is at heightened
risk for injury. However, careful simultaneous use
of muscles that tend to dislocate the joint in the
opposite direction can be used to help protect and
stabilize the joint.Types of Muscle Contraction (Tension)
The relationship of net torque of the muscle relative
to the net resistance previously discussed can also
be associated with types of muscle contraction or
tension. Before looking at possible types of muscle
contraction, it is important to remember the sliding-
filament theory and to recall that this muscle tension
can only pull the ends of the muscles toward each
other (termed the law of approximation) and not
push them away. This gives rise to the commonly
used statement that “muscles can only pull and not
push.” However, although the same internal pro-
cess of cross-bridge formation is occurring within
a muscle cell, the muscle as a whole may shorten,
lengthen, or stay the same length, depending on
whether the torque resulting from the contraction
of the muscle is more than, less than, or the same
as the resistance torque. These differences have
traditionally been described as types of muscle
contractions. However, because contraction implies
“shortening,” some authors prefer the terminology
“types of muscle tension.”TABLE 2.3 Relationship of Net Torque, Joint Movement, and Type of Muscle Contraction