Introduction
The performance of sport, as with all physical
exercise, is dependent upon the coordinated acti-
vation of the athlete’s skeletal muscles. The
muscles constitute the sources of the forces and
power required for skilled movement. Unfortu-
nately, the description and quantification of exer-
cise is frequently made awkward, if not difficult,
by a variety of terms, some of which are confus-
ing or inaccurate. Through the years, terms have
been regularly misused and units of measure-
ment inappropriately applied.
Exercise
The term exercise, itself, has been defined in dif-
ferent ways by different sources in the literature.
For the Encyclopaedia of Sports Medicineseries of
publications, the definition has been accepted as
‘any and all activity involving generation of force
by activated skeletal muscle’ (Komi 1992). This
would include activities of daily living, activities
of labour, activities for physical conditioning and
physical recreation, as well as participation in
sport competition. In the Encyclopaedia of Sports
Medicineseries, a sport will be considered as
any organized activity that involves exercise,
rules governing the event and the element of
competition.
To bring about movement of the body parts
and coordinate the skills of a sport, the central
nervous system activates the striated, voluntary
muscle cells which are the principal constituents
of the various structures called skeletal muscles.
The response of muscle cells to neural stimula-
tion is to produce force.
In order to develop force, skeletal muscle cells
are activated by electrochemical impulses arriv-
ing via efferent neurones, the cell bodies of which
are located in the anterior horn of the gray matter
of the spinal cord. When the threshold of excita-
tion of the muscle cells of a motor unit has been
attained, electrochemical events within each
muscle cell (fibre) result in the cylindrical fibre
generating force along its longitudinal axis in
order to draw the ends of the cylinder towards
its midsection. In this way, the activated fibres
develop force between the attachments of the
muscle in which they are contained. It has been
proposed that this process be referred to as a
muscle ‘action’ (Cavanagh 1988) rather than
‘contraction’ due to the fact that any activated
individual fibre and, indeed, an entire muscle
may: (i) shorten the distance along its longitudi-
nal axis, (ii) be held at the same length by an
opposing force, or (iii) be forcibly stretched in
length by an opposing force. The term actionhas
the advantage of being independent of a change
in length or of direction. By definition, contraction
means shortening only.
The terminology employed to identify the
three actions thus deserves discussion and expla-
nation. The interaction of muscle force develop-
ment and the external forces will result in actions
that produce static exercise (no movement about
the related joints) or in dynamic exercise (result-
ing in a change in joint angles). Static exercise of