Endurance athletes are accustomed to thinking of metabolic stress
(the high rate of energy use associated with endurance exercise) and
exercise fatigue (the involuntary loss of work capacity) as
inextricably linked. In other words, we tend to think of energy
depletion as the cause of declining performance during exhaustive
exercise. However, it is possible to fatigue the muscles without
significant metabolic stress, for example by repeatedly jumping to the
ground off a platform. So, how would local muscle fatigue induced
without metabolic stress affect endurance exercise performance? If
metabolic stress were truly THE cause of exercise fatigue, then it
would have little or no effect.
Muscle fatigue during endurance exercise is typically associated
with an elevated cardiovascular response. The heart rate increases at
any given level of sustained work output. It is commonly assumed that
metabolic stress is the link between muscle fatigue and the elevated
cardiovascular response associated with it. But in light of the fact
that it is possible to induce muscle fatigue without significant
metabolic stress, researchers from Bangor University in Wales recently
investigated the effects of muscle fatigue induced by drop jumps on
cycling performance and heart and breathing rate.
In this study, subjects rode stationary bikes to exhaustion at a
high intensity level. The ride was later repeated after the subjects
completed 100 drop jumps from a height of 18 inches. This resulted in
an average 18% decline in the maximum amount of force they were able to
generate with their quadriceps. Despite the fact that the drop jumps
had cost the subjects little energy, their time to exhaustion following
them was significantly reduced compared to the control trial and their
heart rate and breathing rate were significantly elevated.
So, what caused these effects if the subjects' muscles still had
plenty of energy available even after the drop jumps? The study's
authors concluded, "These effects seem to be mediated by the increased
central motor command and perception of effort required to exercise
with weaker locomotor muscles." In other words, the muscle fatigue
induced by the drop jumps impaired the brain's capacity to drive the
same level of cycling work output compared to the rested state.
So, what does this mean for you? Aside from the obvious
lesson--don't do 100 drop jumps before an endurance workout if you want
to perform at your best in the latter--there's nothing practical to be
gleaned from this particular study. I just find it interesting as
further evidence that the brain is truly the governing organ in
relation to exercise performance.