You do 10 minutes of vigorous exercise with your arms, rest for a few minutes, then hop on your bicycle and ride as hard as you can for 10 minutes. Will the arm exercise affect your performance on the bike?
The answer (which is "yes") may seem obvious, but it's far from clear why tired arms should affect your legs – in fact, scientists vigorously disagree about the explanation. A pair of recent studies offer new hints about the different forms of fatigue that affect either individual muscles or your whole body, and how understanding them can help you recover more quickly during interval workouts.
The arms-then-cycling study, from exercise physiologist Dr. Michael Johnson and his colleagues at Nottingham Trent University in Britain, was designed to separate the role of metabolites that accumulate in the blood during intense exercise from the effects of fatigue of the muscles themselves.
When you exercise above a certain "critical power" – a threshold below which you feel you could sustain your effort almost indefinitely – your muscles produce and accumulate chemical byproducts such as lactate, hydrogen ions, phosphate and potassium.
These metabolites also begin to accumulate in the bloodstream, where they circulate throughout the body and thus can potentially affect other muscles.
Johnson used "arm cranking" – an obscure form of exercise that basically amounts to pedalling a bike with your arms – to manipulate the level of metabolites circulating in the blood of his volunteers before they performed intense cycling tests (this time with their legs). The results, published last month in the journal Medicine & Science in Sports & Exercise, showed that the amount of work above critical power that the volunteers could perform with their legs depended on the level of metabolites in their blood when they started cycling.
That confirms that exercise in one muscle affects performance in other muscles – but exactly how this happens is where scientists begin to differ. One possible explanation is that these metabolites directly interfere with muscle function. For example, there is evidence that elevated potassium levels and changes in blood acidity (due to accumulation of hydrogen ions) may disrupt muscle-fibre contraction.
Another possibility, Johnson notes, is that the brain keeps tabs on metabolite levels and automatically reduces its signals to muscles throughout the body when levels get too high. This "central fatigue" hypothesis remains highly controversial; the anonymous peer reviewers who approved the study for publication asked that references to the idea be removed from the manuscript's discussion section.
Or it could be something else entirely: The apparent link between metabolites and fatigue could be a mere correlation, in the same way that exhaust from your car's tailpipe doesn't "cause" it to run out of gas. Dr. Samuele Marcora, an exercise physiologist at the University of Kent in Britain who studies the role of the brain in endurance, suggests mental fatigue and tired breathing muscles as alternate mechanisms that could explain the link between arm and leg exercise.
Still, the correlation suggests that if you can figure out how to get rid of the metabolites in your blood, you may also be able to speed your recovery from exercise. Most metabolites return to normal levels somewhere between a few minutes and an hour, but you can accelerate this process with light exercise such as jogging or cycling to keep blood circulating rapidly, as another recent study, from St. Mary's University College in Britain, demonstrated.
The researchers compared performance in two bouts of intense cycling separated by either "active recovery" (light cycling) or "passive recovery" (doing nothing). When the recovery period was just 45 seconds, there wasn't enough time to get rid of the metabolites in the blood, so the cyclists performed better when they just rested.
In contrast, when the recovery period lasted longer than about two minutes, metabolite levels in the active recovery group began to drop and they outperformed the resting group in the subsequent sprint.
This finding is most relevant if you're doing high-intensity interval training, an increasingly popular form of workout that involves alternating intense bursts of exercise with brief recoveries. While the temptation is to spend your rest period bent over with your hands on your knees, a light jog or spin may actually help you recover more quickly.
Practical tips aside, this research reminds us of how much we still don't understand about even seemingly basic questions like why exercise makes us tired. Fortunately, if you're doing high-intensity interval training, you can stick with a definition of fatigue borrowed from former U.S. justice Potter Stewart's definition of pornography: You'll know it when you see it.
Alex Hutchinson blogs about exercise research at sweatscience.runnersworld.com.