activated muscle is traditionally referred to as
isometric. Force is developed but, as there is no
movement, no work is performed. All other
muscle actions involve movement and are
termeddynamic. The term concentricis tradition-
ally used to identify a shortening action and the
termeccentricis used to identify a lengthening
action, although the origin of these terms is
obscure (Table 1.1) (Knuttgen & Kraemer 1987).
The International System
Some years ago, the world of science adopted an
International System of Measurement (Bureau
International des Poids et Mésures 1977; Le
Système International, abbreviated as SI) to quan-
tify all physical entities and processes. The unit
of force in the SI is the newton (N). One newton is
quantified as the force which imparts to a mass of
1 kilogram an acceleration of 1 metre per second
per second. To develop force, a muscle cell
requires energy, and the SI unit for energy is the
joule (J).
When force is expressed through a displace-
ment (i.e. movement of body parts is occurring),
work is measured as force (N) multiplied by the
distance (m) of the displacement, and work can
be calculated as force¥distance: 1 N¥1m=1J.
During movement, the performance of work
involves conversion of one form of energy (J) to
another. The SI unit for energy is the same unit
used to quantify work. One joule is the energy of
1 newton acting through a distance of 1 metre.
Any energy used by the muscle for force devel-
opment that does not result in work becomes
heat, the SI unit for heat also being the joule.
Obviously, direct relationships exist among
energy, work and heat and they are quantified
4 nutrition and exercise
with the same unit, the joule. Throughout this
publication, the term energywill most often refer
to metabolic energy.
Whentime[SI unit, the second (s)] becomes a
factor in quantifying energy release, the perfor-
mance of work or the generation of heat, then the
rate of energy release, work performance or heat
generation is presented as power, the SI unit for
which is the watt (W) (1 J¥1s–1=1 W). In exercise
in which 150 W of external power is produced at
a metabolic cost of 750 W, then the rate of heat
production is 600 W.
Attention should be called at this point to the
fact that, when describing exercise and sport,
physiologists and nutritionists can be interested
in the available energy content that can be
metabolized from the food ingested (J), the total
stored energy available for the muscle cells (J),
the total energy utilized during a conditioning
session or sports performance (J), or the rate at
which muscle cells are called upon to produce
power (W).The joule and the calorie
As described above, the joule is the SI unit used
to quantify energy, work and heat. This provides
a simple and efficient basis for describing the
relationship among nutrition, exercise perfor-
mance, body heat generation and heat dissipa-
tion, both in terms of total amounts (in joules) or
as power (in watts). Unfortunately, the calorie
and its multiple, the kilocalorie (kcal), have been
utilized for so long in nutritional circles that a
change to the description of the energy content of
foods in joules is being implemented very slowly.
Instead of utilizing the convenient relationships
among newtons, joules, seconds and watts,
conversion factors need to be employed. For
example, 1 cal=4.186 J and 1 kcal=4.186 kJ.
When a mechanically braked cycle ergometer
is used for an exercise bout, one method of
obtaining the desired power production would
be to have the subject cycle at a pace that would
produce a ‘velocity’ of the flywheel rim of 5 m · s–1
and provide an opposing force (sometimes
termed ‘resistance’) of 60 N. The simplest way ofTable 1.1Classification of exercise and muscle action
types.
Exercise Muscle action Muscle length
Dynamic Concentric Decreases
Dynamic Eccentric Increases
Static Isometric No change