muscular sYstem 59
If, however, stretch implies a particular quality of sensation in the muscle, then it is not
interchangeable with lengthen. It is possible to lengthen a muscle without a stretching
sensation—in fact, most of us do it all the time. Actions such as walking, talking, or pick-
ing up a cup all involve lengthening and shortening muscles, often without any particular
muscular sensation at all.
(^1) The word agonist comes from a Greek word meaning contender or contestant. Antagonist comes from the Greek word for
opponent.
Origin and insertiOn Fallacy
The places where muscles attach to the bones are often classified as being the origin
and the insertion. The origin is the attachment that is closer to the torso or the center
of the body, and the insertion is the attachment that is farther from the center, closer
to the fingers, toes, skull, or coccyx. The underlying implication is that the origin is
the fixed point and the insertion is the point that moves; however, this is only true
for some of our movements. Any time we move the torso through space, we reverse
the so-called origin and insertion points.
This classification of attachment points also implies that muscles develop from
one point to another and they somehow grow from the origin toward the insertion.
Embryologically, however, they do not do this. Instead, clusters of future muscle cells
migrate to the area of their future home and organize themselves once they get there.
It is not a linear point-to-point process at all.
Muscle relationships
No muscle works in isolation; all muscles in the intricate web of the muscular system con-
stantly engage with each other to balance, reinforce, modify, and modulate one another
through the matrix of the connective tissue.
The relationships between muscles can be organized in a variety of ways. We can focus
on how muscles balance each other around a single joint, how the layers of muscle have
different effects as they shift from deep to superficial, or how kinetic chains of muscle and
connective tissue integrate the limbs and torso.
agonist–antagonist pairs
One of the common paradigms for organizing muscles is into agonist–antagonist muscle
pairs. This perspective is oriented around specific joint actions and the muscles that create
and modulate those joint actions.
The starting place is a specific joint, the focal joint, and a specific joint action. For every
joint action there are muscles that create the movement and muscles that oppose the move-
ment. The muscles that create the joint action are called the agonists, or prime movers,
and the muscles that create the opposite joint action are called the antagonists.^1 These
pairs of agonist–antagonist muscles can have direct relationships in the nervous system at
the level of the spinal cord. When one muscle of the pair acts, the other muscle receives
a message to respond and modulate. This relationship is called reciprocal innervation or
reciprocal inhibition. Not all agonist–antagonist muscle pairs have a relationship at the level
of the spinal cord; some are paired together through repeated movement patterns that are
recorded at higher levels in the brain rather than the spinal cord.