and
triceps of the upper arm. Contraction of the biceps flexes the forearm
at the elbow; the contrary movement, extending the forearm, results from
the contraction of the triceps. This principle of opposition applies to
the entire muscular system. One set of muscles raises the ribs in
inspiration, another set lowers them in expiration; one group flexes the
fingers and clenches the fist, an opposed set extends the fingers and
opens the hand. Muscular opposition does not imply that the entire
structure is made up of parallel pairs of muscles, like the biceps and
triceps, located on opposite sides of the same bone. It means only that
the opposed sets pull in contrary directions.

Each opposed set consists of muscles of about equal strength. Under
normal conditions of relaxation the entire muscular system exerts a
slight degree of contraction. To this normal state of oppositional
contraction the name "muscular tonicity" is given. The present purpose
does not call for a discussion of the subject of muscular tonicity. This
form of contraction has no direct bearing on the performance of
voluntary movements.

What effect has the voluntary contraction of all the muscles of any
member, each opposed set exerting the same degree of strength? No motion
of the member results, but the member is brought on tension and
stiffened. This is well illustrated in the case of the arm. Extend the
arm and clench the fist; then contract all the muscles of the arm, about
as the athlete does to display his muscular development. You will notice
that the arm becomes stiff and tense.

This state of tension is commonly called "muscular stiffness," but the
term is open to objection. It is really the joints which are stiffened,
not the muscles. We are, however, so accustomed to speak of muscular
stiffness, and particularly of throat stiffness, that little is to be
gained by substituting a more accurate expression.

A condition of muscular stiffness results from the contraction of all
the muscles of a member, whether this contraction be voluntary or
involuntary. This condition does not prevent the normal movements of the
member; it only renders the movements more difficult and fatiguing and
less effective. It is readily seen why this is the case. More than the
necessary strength is exerted by the muscles. Suppose the biceps and
triceps, for example, each to be contracted with five units of strength;
then let some work be performed by the flexing of th