The Skeletal System and Its Movements 29Joint Stability and Mobility
Joint stability can be defined as the ability of a
joint to withstand forces and avoid being separated
(disarticulated) without injury. This is a very impor-
tant property of joints, from a perspective of both
defining movement capacity and promoting safety.
When forces exceed the stability of a joint, injury to
the various tissues can occur. Two common types of
injury are injury to the ligament, termed a sprain,
and injury to the muscle, termed a strain.
In contrast to joint stability, joint mobility can
be defined as the range of motion allowed prior to
tissue restraints. The functional capacity of joints to
move through a full range of motion, also termed
flexibility, is an important aspect of physical fitness
and essential for allowing the achievement of dance
aesthetics. Key factors that influence relative joint
stability versus mobility include the joint architec-
ture (type of joint, shape and depth of articulation);
arrangement of the ligaments and capsule; vacuum
created in the joint due to the negative atmospheric
pressure; extensibility of the muscles, tendons, and
fascia crossing the joint; neural factors that influence
the resistance to movement; effects of gravity; and,
in some cases, occlusion of adjacent soft body parts
or impingement of bone against bone. These factors
are discussed in more detail and as they relate to spe-
cific key joints in subsequent chapters. However, it is
important to note that although dancers often imag-
ine that continuing to increase joint mobility should
be their goal, excessive mobility can be associated
with decreased stability and increased injury risk.
Instead, the goal should be a balance of joint stability
and mobility, which will allow for protection of joints
while still meeting desired dance aesthetics.
Close-Packed and
Loose-Packed Positions of JointsJoint stability is also influenced by the specific posi-
tion of a given joint, and articulating surfaces of
synovial joints have a position that offers the great-
est mechanical stability. This position is termed
the close-packed position and often occurs at the
extreme in the range of motion, such as full exten-
sion of the elbow, knee, wrist, or fingers (interpha-
langeal joints); flexion of the metacarpophalangeal
joints; and dorsiflexion of the ankle. In this close-
packed position the following conditions generally
occur: (1) The joint surfaces have the greatest con-
tact area and fit (congruency); (2) a majority of key
ligaments are under tension; (3) the capsule is taut;
and (4) the joint is under compression and difficult
to separate (Smith, Weiss, and Lehmkuhl, 1996).
In close-packed positions stability is facilitated, and
often little or no muscular contraction is required
to maintain the position during weight bearing. This
offers advantages in terms of energy expenditure
that can be very valuable in positions such as upright
standing. In all other positions, the joint surfaces
have less contact and are termed loose-packed. In
loose-packed positions key capsular and ligamental
structures are slack, and motion, rather than joint
stability, is facilitated.Closed and Open
Kinematic Chain MovementsIn the human body many joints occur in series and
often their movements occur together, rather than
in isolation. This has led to the use of the engineer-
ing term “closed kinematic chain,” in which a seriesClose- and Loose-Packed Joint Positions
- Loose-packed position of the fingers. While sitting, bend your elbows to 90° with your palms
facing upward and the fingers in line with the metacarpals. Spread the fingers and then bring them
back in. Note the degree of abduction and adduction of the fingers that can occur in this loose-packed
position of the metacarpophalangeal joint. - Close-packed position of the fingers. Bring the fingers up to face the ceiling while keeping the
rest of the hand in place (90° of metacarpophalangeal joint flexion). Now, try to spread the fingers
and notice how limited abduction and adduction of the fingers are in this close-packed position of
the metacarpophalangeal joints. What advantages could the change in mobility present in different
positions offer functions performed by the hand?