The Muscular System 57suggest and as illustrated in figure 2.16, a single-joint
(uniarticulate) muscle crosses only one joint, while
a two-joint (biarticulate) muscle crosses two joints
and a multijoint (multiarticulate) muscle crosses two
or more joints. A single-joint muscle can produce
motion only at the one joint it crosses. For example,
the gluteus minimus shown in figure 2.16A crosses
only the hip joint and so can produce movement
only at the hip joint. In contrast, a multijoint muscle
can produce motion at all the joints that it crosses.
With a two-joint muscle, both of its tendons are pulled
nonselectively toward the belly of the muscle, result-
ing in the tendency to cause movement at both of its
joints. This can be advantageous from a perspective
of efficiency when both actions are simultaneously
desired, such as when both functions (hip flexion
and knee extension) of the two-joint rectus femoris
(figure 2.16B) are used when kicking.
Using one function of the muscle concentrically
and the other eccentrically at the same time can
also offer an advantage for two-joint muscles. For
example, in locomotion, when the leg is initially swung
forward (concentric hip flexion), the knee is often in
a flexed position (knee extensors working eccentri-
cally). This combination stretches the rectus femoris
over the knee while shortening is occurring across
the hip. In essence, this enhances the efficiency of
the rectus femoris by keeping the whole muscle at a
length where it can generate more muscle tension,
as well as allowing for greater force due to the use
of the stretch-shortening cycle. Other examples
of muscles that cross two or more joints—that is,
multijoint muscles—are the sartorius, tensor fascia
latae (figure 2.16B), hamstrings, gastrocnemius,
biceps brachii, the long head of the triceps brachii,
and many muscles of the hands and feet, such as the
flexor digitorum longus shown in figure 2.16C.Active and Passive Insufficiency
However, despite this advantage in terms of efficiency,
multijoint muscles hold a disadvantage in terms of
allowing either fully active or fully passive range of
motion at two or more joints simultaneously. Regard-
ing the former, active insufficiency occurs when
active contraction of the muscle is unable to produce
as much range of motion as could be produced if
an external force (e.g., gravity, momentum, another
body part, or another person) was responsible for
the movement. This limitation is due to the fact that
the average muscle fiber can shorten only about halfActive Insufficiency
- Establishing active insufficiency.
Create active insufficiency with the ham-
string muscles by performing both of its joint
actions (knee flexion and hip extension)
together. Stand on your left foot and lift the
right foot off the ground and flex the right
knee as fully as possible. Then, with the
knee fully flexed, try to lift the leg as high
as possible to the back (hip hyperextension)
as if doing a parallel back attitude. - Angle of knee flexion.
- You will find that either the ham-
string starts to cramp or the knee
starts to decrease its degree
of flexion as the thigh is raised
higher toward the ceiling. Why? - Holding the end position, use
your hand to bring your right heel
toward your buttocks. Why can you bend the knee further when you use your hand?