Introduction
Physical, and perhaps mental, training, along
with adequate nutrition, is generally thought to
decrease fatigue and optimize physical per-
formance. However, the specific mechanisms of
such strategies are not fully understood because
not much is known about the specific causes of
fatigue. The problem is complex because fatigue
can be caused by peripheral muscle weakness
(peripheral fatigue) or by a failure to initiate or
sustain voluntary drive to the muscle by the
central nervous system (CNS fatigue). It may
also vary with the type, duration and intensity of
the work, the individual level of fitness and
numerous environmental factors (Davis & Fitts
1998). Even the specific definition of fatigue is
often debated. For the purpose of this review,
fatigue is defined as the loss of force or power
output in response to voluntary effort that leads
to reduced performance of a given task. CNS
fatigueis the progressive reduction in voluntary
drive to motor neurones during exercise,
whereas peripheral fatigueis the loss of force and
power that occurs independent of neural drive.
Peripheral mechanisms of fatigue could in-
clude impaired electrical transmission via the
sarcolemma and T-tubule, disruption of calcium
release and uptake within the sarcoplasmic retic-
ulum, substrate depletion and other metabolic
events that impair energy provision and muscle
contraction (Davis & Fitts 1998). Much less is
known about CNS mechanisms, even though it is
well known that ‘mental factors’ can affect physi-
cal performance. In fact, inadequate CNS drive to
the working muscles is the most likely explana-
tion of fatigue in most people during normal
activities. Most people stop exercising because
the exercise starts to feel too hard (i.e. there is
increased perceived effort) which almost always
precedes an inability of the muscle to produce
force. Therefore, CNS fatigue may include neuro-
biological mechanisms of altered subjective
effort, motivation, mood and pain tolerance, as
well as those that directly inhibit central motor
drive in the upper most regions of the brain
(Gandevia 1998).
Evidence for specific inhibition of motor drive
within the brain during fatiguing exercise has
only recently appeared in the scientific literature.
The best evidence comes from recent studies in
humans using a new technique called transcra-
nial magnetic stimulation (TMS). This technique
has been used to assess the magnitude of the
motor responses elicited in the muscle by mag-
netic stimulation of neurones in the motor cortex.
Recent reports show that the electrical stimulus
reaching the muscle following magnetic stimula-
tion of the motor cortex (motor-evoked potential)
is suppressed following fatiguing exercise
(Brasil-Netoet al.1993; Samii et al.1996). Gande-
via and colleagues (Gandevia et al.1996; Taylor et
al.1996) also showed that fatigue was accompa-
nied by a prolonged silent period in response to
TMS that likely results from inadequate neural
drive by the motor cortex. These data suggest
strongly that specific mechanisms within the
brain are involved in fatigue during exercise.