Jamie Carruthers noted in a recent post that education is a two-way
process,
and encouraged those of us who have ‘sat back’ to share some of their
findings
with the rest of us.
With this in mind, below is a post regarding central nervous system
fatigue.
After putting together notes from Dr. Hornby, I wrote this insight,
then
simply forgot to send it! Hope this gives a new spin on an old thread.
The issue of CNS fatigue has been investigated in numerous labs for the
past
twenty years, yet it still defies a simple answer. When the issue came
up in
a few posts, I decided to contact Dr. George Hornby at Northwestern
University
here in Chicago for his perspective.
Dr. Hornby mentioned Doug Stuart, one of the top researchers in the
field who
organized a symposium back in 1994 on the issues of central vs.
peripheral
muscle fatigue. This symposium resulted in a book which came out in
1995:
Fatigue: Neural and Muscular Mechanism, by S.C. Gandevia, et. al.
Although ten years old, this book is still one of the best resources to
date
on muscle fatigue
Regarding the central issue of CNS fatigue, there are indeed central
mechanisms which account for fatigue, and ample evidence of this in the
literature.
What we know is that the electromyographic signal from a muscle during
a
sustained maximal contraction decreases somewhat proportionally to the
force during
the contraction, so there is something that is decreasing the drive to
the
motor neuron or motor pool. Dr. Hornby mentioned that there are three
central
physiological mechanisms that may account for this decreased drive:
- chemical and mechanical stimuli from a contracting muscle can be
sensed by
different sensory fibers, and this in turn might inhibit the discharge
of
motor units. This appears to have a protective effect on the muscle. - When contraction occurs, the drive to motor the neurons which
innervate
sensory stretch receptors (gamma motoneurons) decreases, and this
decreases the
sensory input to the motor pool. Dr. Hornby referred to this as
disfacilitation. - If you stimulate a motor neuron or its axons directly, the discharge
rate
of the fibers will decrease, without any input from the muscle (i.e.,
there is
an intrinsic adaptation of the motor unit discharge). The decrease in
discharge rate is greater in the fast vs. slow motor units.
Sounds good…and technical… but what about recovery, which is at the
heart of
all our discussions. According to Dr. Hornby, this depends on the task
performed, and it’s difficult to separate out the effect of peripheral
mechanisms
vs. central mechanisms, even more so if the stimuli from a contracting
muscle
can indeed be sensed by different sensory fibers.
In addition, there are supraspinal effects, which means that brain
pathways
that activate the spinal motor pools can adapt their rate of firing as
well.
Evidence of this can be found in Gandevia’s book.
As Dr. Hornby notes, " There are also motivational factors, arousal,
sense of
effects, and pharmacological agents that can alter fatigue"
Kindest regards,
Ken Jakalski