Introduction to Sports Biomechanics: Analysing Human Movement ...

usually more flexible than non-participants owing to the use of joints through greater
ranges, avoiding adaptive shortening of muscles. Widely differing values are reported in
the scientific literature for normal joint ranges of movement. The discrepancies may be
attributable to unreliable instrumentation, lack of standard experimental protocols and
inter-individual differences.
MUSCLES – THE POWERHOUSE OF MOVEMENT
Muscles are structures that convert chemical energy into mechanical work and
heat energy. In studying sport and exercise movements biomechanically, the muscles
of interest are the skeletal muscles, used for moving and for posture. This type of
muscle has striated muscle fibres of alternating light and dark bands. Muscles are
extensible, that is they can stretch or extend, and elastic, such that they can resume
their resting length after extending. They possess excitability and contractility.
Excitability means that they respond to a chemical stimulus by generating an electrical
signal, the action potential, along the plasma membrane. Contractility refers to the
unique ability of muscle to shorten and hence produce movement. Skeletal muscles
account for approximately 40–50% of the mass of an adult of normal weight. From
a sport or exercise point of view, skeletal muscles exist as about 75 pairs. The
main skeletal muscles are shown in Figure 6.8. The proximal attachment of a muscle,
that nearer the middle of the body, is known as the origin and the distal attachment
as the insertion. The attachment points of skeletal muscles to bone and the movements
they cause are not listed here but can be found in many books dealing with
exercise physiology and anatomy (for example, Marieb, 2003; see Further Reading,
page 280) as well as on this book’s website.
Muscle structure
THE ANATOMY OF HUMAN MOVEMENT
Each muscle fibre is a highly specialised, complex, cylindrical cell. The cell is elongated
and multinucleated, 0.01–0.1 mm in diameter and seldom more than a few centimetres
long. The cytoplasm of the cell is known as the sarcoplasm. This contains large amounts
of stored glycogen and a protein, myoglobin, which is capable of binding oxygen and
is unique to muscle cells. Each fibre contains many smaller, parallel elements, called
myofibrils, which run the length of the cell and are the contractile components of
skeletal muscle cells. The sarcoplasm is surrounded by a delicate plasma membrane
called the sarcolemma. The sarcolemma is attached at its rounded ends to the endomysium,
the fibrous tissue surrounding each fibre. Units of 100–150 muscle fibres are
bound in a coarse, collagenic fibrous tissue, the perimysium, to form a fascicle. The
fascicles can be much longer than individual muscle fibres, for example around 250 mm
long for the hamstring muscles. Several fascicles are bound into larger units enclosed
in a covering of yet coarser, dense fibrous tissue, the deep fascia, or epimysium, to
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