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Posts Tagged ‘central governor’

Sports drinks don’t help for one-hour exercise (unless you’ve been fasting)

April 29th, 2010

If you’re running a marathon or cycling for several hours, you need to ingest some carbohydrates during the session to maintain your performance. If you’re sprinting for 100 metres, on the other hand, you can leave the buffet belt at home. Where things get murky is the middle ground, for sessions lasting about an hour: several decades of research have produced lots of conflicting results. A study in the April issue of the Journal of Sports Sciences, by researchers from Loughborough University in Britain, offers some new insight.

The basics: 10 trained runners did two trials in which they ran as far as they could in one hour (they used a neat treadmill that automatically speeds up if you move towards the front of the belt, and slows down if you lag towards the rear, so it was a freely paced trial). In both cases, they ate a high-carbohydrate meal three hours before the run. They drank either a sports drink containing 6.4% carbohydrates or a placebo before and during the run. The results: no difference whatsoever in performance, blood glucose, lactate, respiration, carbohydrate burning, perceived exertion, or anything else they measured.

What’s interesting is that the same group published a similar paper last year, where the only difference is that the runners were fasted before the trial instead of having a meal three hours prior. In that case, the sports drink group significantly outperformed the placebo group.

So it seems pretty clear: you only need supplemental carbs for a one-hour exercise bout if you haven’t topped up your carb supplies beforehand. There are two ways your body stores glycogen: in your muscles (which is then used exclusively by your muscles), and in your liver (which feeds glucose into your bloodstream to fuel your heart and brain and keep blood sugar levels stable). When you sleep overnight, your muscle glycogen stays relatively stable, but your liver glycogen drops by more than 50 percent (because your brain and heart are still running all night). So the researchers believe that, if you don’t have a pre-exercise meal, the sports drink is needed to make up for your depleted liver glycogen stores.

Practically speaking, this means you don’t need to worry about carbs during short exercise bouts in the afternoon or evening, since you’ll have had a meal or two. In the morning, though, you need to make sure your liver glycogen is restocked, even for a short one-hour run. You can do that by getting up early enough to eat beforehand — or, apparently, by consuming some carbs immediately before and during your session.

One other interesting note from the paper. They discuss the performance boost that comes from “rinsing and spitting” with a sports drink, which some researchers have suggested could explain why sports drinks sometimes help with short exercise sessions even when muscle glycogen stores are full. Interestingly, a couple of recent papers suggest that the same conditions also apply in this case: you get benefit from rinse-and-spit if you’ve fasted beforehand, but no benefit if you had a pre-exercise meal. That suggests that your brain is monitoring levels of carbohydrate throughout your body, and only responds positively to the carb stimulus if your body actually needs it:

Although speculative, the idea of central monitoring of whole-body carbohydrate status, which in turn influences the self-selection of exercise intensity, is worthy of further investigation.

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Perceived exertion, not muscle failure, determines “exhaustion”

April 14th, 2010

There’s an interesting preprint available online from the European Journal of Applied Physiology, by Samuele Marcora and a colleague from Bangor University in Wales. Its title is “The limit to exercise tolerance in humans: mind over muscle?,” so you might think it’s another paper supporting Tim Noakes’s “central governor” theory. It is, and it isn’t — but either way, it’s interesting.

Marcora took 10 elite rugby players and (after various preliminary testing and so on) had them do a five-second “maximum voluntary cycling power” (MVCP) test. Then they did a very intense cycling trial to exhaustion which took about 10 minutes (they offered cash prizes to the top performers and circulated the results publicly to stimulate competition and make sure the subjects went all-out), followed immediately (within one second) by another MVCP test.

Now, if you subscribe to traditional exercise physiology, you’d say that the subjects stopped the test-to-exhaustion when they were no longer physically able to generate enough power to continue. Possible reasons for their failure would include “limited oxygen delivery, metabolic and ionic changes within the active muscles, supraspinal reflex inhibition from muscle afferents sensitive to these changes, and altered cerebral blood flow and metabolism.” But that’s not what Marcora saw. The subjects had to maintain an output of (on average) 242 watts in the test to exhaustion. But as soon as they stopped, one second later, they were able to output (on average) 731 watts in a five-second burst — nearly triple the required power! Clearly the subjects didn’t stop the test because their couldn’t physically produce the needed power:

These results challenge the long-standing assumption that muscle fatigue causes exhaustion during high-intensity aerobic exercise, and suggest that exercise tolerance in highly motivated subjects is ultimately limited by perception of effort.

The interpretation of these results gets a little tangled. Marcora is an advocate of something he calls the “psychobiological model of exercise tolerance,” which seems to basically mean that we stop exercising when it gets hard. He says this is different from — and much simpler than — Noakes’s central governor theory. I’m not sure I really a see a difference that extends beyond semantics, but perhaps that’s because I haven’t given it enough thought. I downloaded a couple of Marcora’s other papers where he explains the theory in more detail, so I’ll be interested to see what he has to say. Either way, these results are certainly interesting in that they once again support the notion that, when we collapse from exhaustion, we’re generally running up against barriers imposed by our brain rather than absolute physical limits imposed by our body.

Cooling your palms enables you to bench press more weight

February 9th, 2010

Another result from the Department of Weird and Unexpected Ergogenic Aids… A forthcoming paper in Medicine & Science in Sports & Exercise, by researchers at the University of New Mexico, finds that cooling your palms between sets of bench press allows you to lift more weight.

The details: 16 subjects performed four sets of bench press at 85% of one-rep max, with three minutes rest. Between sets, they stuck their hands in a Rapid Thermal Exchanger, which heats or cools while applying a negative pressure. Their palms were either heated to 45 C (113 F), cooled to 10 C (50 F), or left at room temperature. Sure enough, they lifted more when their palms were cooled, including a remarkable 30 percent increase in the second set.

So what does this tell us? Well, for one thing, it tells us that people are going to start buying Rapid Thermal Exchangers, or at least bring ice packs into the weight room. But more interestingly — and this is becoming quite a theme on this blog — it adds new evidence in support of the “central governor” model.

If cooling our palms (which are far away from the muscles involved) allows us to lift more weight when lifting to failure, this tells us that the “failure” wasn’t due to some mechanism in the muscle fibres themselves. Instead, when the input to the central nervous system was altered by triggering cold sensors far away from the relevant muscle, the shut-down signal didn’t get sent to exhausted muscles quite as soon. It’s far from clear exactly what’s happening with this weird effect, but the researchers are quite confident that it has something to do with centrally mediated nerve signals — and thus adds support to the idea of a central governor.

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