Cyclists who participate in 40-K races and time trials often wonder if there is a simple test which can indicate how well they will perform over the 40-K distance. Such a test could provide confidence that an upcoming 40-K race will go well - and might even predict performance time in the 40-K event. Of course, determination of cycling velocity (or power output) at lactate threshold would serve both functions, telling a cyclist whether fitness had improved and also foretelling a specific time in the 40K. Some research has indicated, for example, that if power output at lactate threshold is assumed to be the power output which produces a blood-lactate concentration of 4 mmol/liter, then lactate-threshold intensity is nearly the exact intensity which will be harnessed during a 40-K race. EXPLOSIVE

Unfortunately for most cyclists, determination of lactate threshold is not so easy. For precise determination of threshold, a visit to an exercise-physiology laboratory is mandated, and the test requires time and a fair amount of money (including the cost of traveling to the lab). But - no worries: There is a much-simpler way to assess 40-K fitness. All a cyclist has to do is cover 5K as fast as possible. That's right - just 5K. Recent research indicates that for well-trained cyclists the speed sustained in a 40-K race will be 92 to 94 percent of the average velocity attained in a 5-K test. Furthermore, the power output displayed in a 40-K race can be predicted quite nicely with the following equation: Power Output(40-K Race) = .58(power output in 5-K trial) + 89.4 The power output in both cases (40K and 5K) is expressed in Watts.

Thus, a simple field test - the performance of an all-out 5-K trial - will not only provide an adequate estimation of current fitness; it will also predict rather neatly how well one will perform in a 40-K race. EXPLOSIVE

The glory days of glycogen are here. The long-chain compound, formed within your muscles when glucose molecules are linked end to end, has traditionally been viewed as a rather - sleepy carbohydrate which simply lies around passively until it is "burned" for energy during exercise. The true story of glycogen, however, is quite different. Yes, glycogen does contain energy which is crucial for muscle contraction. However, the concentration of glycogen in your muscles also produces a "signal" which your brain uses to set the speeds of your cycling efforts. In addition, glycogen determines the rates at which fat and carbohydrate are metabolized during exercise. Amazingly, glycogen can also influence the degree to which you improve physiologically in response to the training you conduct. Glycogen: Not Just A Carbohydrate Any More

Let's start with the link between glycogen and fatigue first. It is true that when your muscle glycogen stores reach a certain low level as you bike along at a high intensity, you are going to become too tired to continue - there is simply no way to get around it (1). Oddly enough, however, this sunken glycogen level is not zero: when your sinews reach the point at which they are unable to work, there is still a fair amount of glycogen fuel lying around.

This seems strange, of course. It doesn't seem right that not much exercise-boosting glycogen can be stored in your body in the first place - maybe just 2000 calories worth or so - and yet that not all of the socked-away stuff can actually be utilized to sustain exercise. You can store a seemingly infinite amount of fat in various regions of your frame, but you get to stockpile a rather meager amount of glycogen, and you don't get to use the entire, modest amount that you put away - even when you are trying desperately to win a race or set a PR.

Those of us who are accustomed to operating a motor vehicle certainly find this situation to be unusual, too: it's like having a car stop running completely when there are still a couple of gallons of gas left in the tank. The shutdown makes sense, however, when you realize that it is probably a protective mechanism. If your muscles ever reached rock-bottom glycogen quantities during a workout or race, their overall metabolism would be impaired, the possibility of injury would probably increase, and it would be very difficult - after exercise ended - for the muscles to sustain themselves and successfully begin the post exercise recovery process. Glycogen: Not Juct A Carbohydrate Ny More

Since the exhaustion-inducing glycogen level is not zero, exercise scientists have searched for the glycogen concentration which actually does stop cyclists in their tracks. Interestingly enough, they have found that this level can vary considerably from athlete, with some bikers becoming exhausted when glycogen stockpiles are 70-percent gone - and others requiring a significantly greater wipe-out before exhaustion is reached (2).

As mentioned, glycogen concentration is linked with several other things, in addition to the onset of fatigue. Notably, your muscle-glycogen content regulates your rates of carbohydrate and fat breakdown at the beginning of a competition or a good workout. If your muscle-glycogen depots are well-stocked, for example, your glycogen "burning" rate will be very lofty as you begin your session or race; if they are medium to low, glycogen breakdown will proceed at a significantly lower rate, and you will be relying more heavily on fat to keep you going (3).

This is true even if you gulp down large quantities of sports drink at the last minute, just before you start cycling, in hopes of giving your glycogen-poor muscles a carbo-boost. A couple of studies have shown that athletes with low muscle-glycogen concentrations at the beginning of a bout of exercise have elevated rates of fat oxidation, even when hyperglycemia (a high level of blood glucose) is induced (4).

Such findings have suggested to some researchers that some factor, originating outside muscle, must determine the rate at which muscles breakdown fat in response to shifting glycogen levels during exercise (this "factor" could be either neural or hormonal in origin). If this is true, muscles must have some way to communicate with the nervous and/or endocrine systems, and many investigators have hypothesized that muscles are able to send out a "signal" as their glycogen concentrations fall (5). In theory, this signal leads to an increase in circulating noradrenaline levels and a decrease in circulating insulin. Noradrenaline boosts fat breakdown, and insulin tends to shoot carbohydrate into muscles, so overall result would be a stimulation of fat oxidation and a potential decrease in carbohydrate utilization. Glycogen: Not Just A Carbohydrate Any More

Of what practical use is this information to you? The answer is that if muscles can truly send out a signal (or signals) in response to changing glycogen levels, then the quality of your intense workouts and competitions could vary - right from the beginning of exercise - on the extent of your muscle glycogen storage.

This would be rather startling. After all, we already know that when muscle-glycogen levels get pretty low, exhaustion is reached. But it has not been clear that moderate (non-exhaustion-inducing) depletions of glycogen might slow down efforts right from the beginning of a workout or race. It has been assumed that muscles operate quite well until they reach a critical (low) point of glycogen concentration, after which performance falls. If this is not true, then maxing-out glycogen would become an even-more important issue than if it has appeared to be in the past. Even in relatively short bike races (lasting less than 60 minutes or so), glycogen pile-ups could be quite important.

Glycogen concentrations can even determine how effectively muscle cells respond to rigorous training.

To learn more about Glycogen: Not Just A Carbohydrate Any More (the full article can be read by purchasing Vol.2 Issue 2 of Cycling Research News) located in the back issuessection of our site, and many more cycling related topics.