muscular sYstem 57
Muscle contractions
When a muscle cell contracts, the molecules create and release bonds between the thick
and thin filaments, which ratchet along each other and create a sliding movement that
increases their overlap and draws the two ends of the myofibril toward each other. If enough
myofibrils shorten, the whole muscle fiber slides shorter. As more and more muscle fibers
contract, they attempt to shorten the entire muscle by sliding the attachment points at the
two ends of the muscle toward each other.
Whether or not the entire muscle does actually shorten depends on outside factors,
specifically how much resistance exists. If only a few filaments are sliding together inside the
cells, they may not generate enough force to overcome the weight of whatever structure the
muscle is attached to, such as the weight of the arm or the weight of the head. The weight
of a body part is a product of the resistance created by gravity, which is a fundamental
source of resistance for everything on this planet. We negotiate this force every time we
lift an arm, stand up, roll over, or take a breath. Added resistance also comes from other
forces, such as the weight of something being carried, an opposing muscle contraction,
or even an emotional state (for example, tension, anger, or the effort not to cry will often
create resistance, while relaxation, happiness, or relief will often decrease resistance).
Muscles do not contract in an all-or-nothing way. All of the fibers do not have to con-
tract at the same time, meaning that a muscle can generate a precisely gradated amount of
force, coordinated by the dialogue between the nervous system and the muscles. Because
muscles work in this modulated way, they don’t always end up shortening, even though
the fibers might be actively contracting. A muscle may in fact be active and lengthening
when the outside force is greater than the force that the muscle is exerting.
The words concentric, eccentric, and isometric are used to describe muscle actions (figure
4.4, page 58). These terms actually describe the effects of the relationship between the
muscle and the resistance it meets.
concentric contraction The muscle fibers contract and generate more force than the
resistance that is present so that the ends of the muscle slide toward each other and the
muscle shortens.
eccentric contraction The muscle fibers contract and generate less force than the
resistance that is present so that the ends of the muscle slide apart and the muscle actually
lengthens. The muscle is active as it lengthens, so this is not the same as relaxing the muscle.
isometric contraction The muscle fibers contract and generate the same amount of
force as the resistance that is present so that the ends of the muscle neither move apart
nor move together and the length of the muscle does
not change. Isometric contractions can be distinguished
further: There is a difference in experience between
intending to hold still against the resistance of something
else trying to move you and intending to move but not
being able to overcome the resistance to movement.
There is also a difference in experience between main-
taining an isometric contraction following a concentric
contraction and maintaining an isometric contraction
following an eccentric contraction.
A relaxed muscle generally means that there is not an intentional or voluntary contraction
of the muscle fibers. If someone is conscious (even sleeping), however, there is always an
underlying level of automatic activity in the muscle fibers to maintain the resting tone of
the muscle. This resting tone keeps the muscles ready to respond and in postural muscles
automatically adjusts for slight shifts in weight and balance when we sit, stand, and walk.
Muscles don’t actually flex or
extend; these terms describe
joint actions. To be accurate,
muscles use contractions to
create all joint actions, includ-
ing flexion and extension.