Introduction to Sports Biomechanics: Analysing Human Movement ...

INTRODUCTION TO SPORTS BIOMECHANICS
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cover two slightly different types of joint. One of these has a spheroidal surface
that articulates with a spheroidal depression, as in the metacarpophalangeal or
‘knuckle’ joints (Figure 6.7(d)) – ‘condyloid’ means ‘knuckle-like’. These joints are
potentially triaxial but lack the musculature to perform rotation about a vertical
axis. The other type, which is sometimes classified separately as ellipsoidal joints, is
similar in most respects except that the articulating surfaces are ellipsoidal rather
than spheroidal, as in the wrist joint.
Saddle joints consist of two articulating saddle-shaped surfaces, as in the thumb
carpometacarpal joint, shown in Figure 6.7(e). These are biaxial joints, with the
same movements as other biaxial joints but with greater range.
Ball and socket joints, also known as spheroidal joints, have the spheroidal head of
one bone fitting into the cup-like cavity of the other, as in the hip joint and the
shoulder (or glenohumeral) joint. The latter is shown in Figure 6.7(f). These are
triaxial joints, permitting movements in all three planes.
Joint stability and mobility
The stability, or immobility, of a joint is the joint’s resistance to displacement. It
depends on the following factors:
The shape of the bony structure, including the type of joint and the shape of
the bones. This is a major stability factor in some joints, such as the elbow
and hip, but of far less importance in others, for example the knee and shoulder
joints.
The ligamentous arrangement, including the joint capsule, which is crucial in, for
example, the knee joint.
The arrangement of fascia, tendons and aponeuroses.
Position – joints are more stable in the close-packed position, with maximal contact
between the articular surfaces and with the ligaments taut, than in a loose-packed
position.
Atmospheric pressure, providing it exceeds the pressure within the joint, as in the
hip joint.
Muscular contraction – depending on the relative positions of the bones at a joint,
muscles may have a force component capable of pulling the bone into the joint (see
Figure 6.17); this is particularly important when the bony structure is not inherently
stable, as in the shoulder joint.
Joint mobility or flexibility is widely held to be desirable for sportsmen and sportswomen.
It is usually claimed to reduce injury. Although this is probably true, excessive
mobility can sacrifice important stability and predispose to injury. It is also sometimes
claimed that improved mobility enhances performance; although this is impeccably
logical it is not well substantiated. Mobility is highly joint-specific and is affected by
body build, heredity, age, sex, fitness and exercise. Participants in sport and exercise are