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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.



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