Introduction to Sports Biomechanics: Analysing Human Movement ...

INTRODUCTION TO SPORTS BIOMECHANICS
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movements, for example during growth or childbirth. Other tissues that may be present
in the joints of the body are dense, fibrous connective tissue, which includes ligaments,
and synovial membrane. The nature and biomechanical functions of these and other
structures associated with joints will not be considered here.
Joint classification
Overall, joints are classified according to the movement they allow. In fibrous joints, the
edges of the bone are joined by thin layers of the fibrous periosteum, as in the suture
joints of the skull, where movement is undesirable. With age these joints disappear as
the bones fuse. These are immovable joints. Ligamentous joints occur between two
bones. The bones can be close together, as in the interosseus talofibular ligament, which
allows only a little movement, or further apart, as in the broad and flexible interosseus
membrane of fibrous tissue between the ulna and radius, which permits free movement.
These are not true joints and are classed as slightly movable. Cartilaginous joints either
consist of hyaline cartilage, as in the joint between the sternum and first rib, or fibrocartilage,
as in the intervertebral discs. Cartilaginous joints are classed as slightly
movable.
The third classification of joint has a joint cavity surrounded by a sleeve of fibrous
tissue, the ligamentous joint capsule, which unites the bones. Friction between the
bones is minimised by smooth hyaline cartilage. Although cartilage has been traditionally
regarded as a shock absorber, this role is now considered unlikely. The functions of
the cartilage in such joints are mainly to help to reduce stresses between the contacting
surfaces, by widely distributing the joint loads, and to allow movement with minimal
friction. The inner surface of the capsule is lined with the delicate synovial membrane,
the cells of which exude the synovial fluid that lubricates the joint. This fluid converts
potentially compressive solid stresses into equally distributed hydrostatic ones, and
nourishes the bloodless hyaline cartilage. These freely moveable or synovial joints
(Figure 6.7) are the most important in human movement. The changing relationship of
the bones to each other during movement creates spaces filled by synovial folds and
fringes attached to the synovial membrane. When filled with fat cells these are called fat
pads. In certain synovial joints, such as the sternoclavicular and distal radioulnar joints,
fibrocartilaginous discs occur that wholly or partially divide the joint. Synovial joints
can be classified as follows, based mainly on how many degrees of rotational freedom
the joint allows (Figure 6.7).
Plane joints (also known as gliding or irregular joints) are joints in which only slight
gliding movements occur. These joints have an irregular shape. Examples are the
intercarpal joints of the hand (Figure 6.7(a)), the intertarsal joints of the foot, the
acromioclavicular joint and the heads and tubercles of the ribs. The joints are classed
in the literature both as non-axial, because they glide more or less on a plane surface,
and multiaxial. The latter term is presumably used because the surfaces are not plane
but have a large radius of curvature and, therefore, an effective centre of rotation