This is an excerpt from Athlete's Clock, The by Thomas Rowland.
Cognitive Pacing Strategy
Experienced distance athletes know how they feel during a race. From having competed in similar distances in the past, they know just how intense such perceptions of effort should be in order to predict exhaustion at the finish. Breathing too hard and experiencing leg-muscle stress in the first kilometer of a 10K race cause runners to conclude that they will not make it at this pace. They consciously slow their velocity. Conversely, being too comfortable in the early phases of a race signals them to speed up.
This traditional model of pacing is rooted in the idea that athletes make cognitive, purposeful decisions regarding running velocity based on previous experience and sensory input. But which cues do they use to make these velocity adjustments?
Exercise scientists measure how athletes feel as their rating of perceived exertion (RPE). Here, athletes report subjective sensations of exercise stress, usually on a numerical scale ranging from 6 to 20. (This is called a Borg scale, with the top number indicating the very highest level of fatigue.) This, then, is an attempt to put an objective measure (a number) to a subjective feeling (“How hard does it feel?”). Using this tool, the goal of researchers has been to identify the physiological (heart and lung function, body temperature), metabolic (blood acidity, lactate levels), and neuromuscular (muscle stress) factors that are most important in signaling the athletes' level of subjective stress during a distance competition. By monitoring all this input, then, competitors figure out how to adjust race velocity to provide their best effort for a particular competition distance.
However, which particular input cues are the most critical in this strategizing are not entirely clear. A whole variety is possible, but despite a good deal of research, physiologists remain pretty much in the dark as to exactly how the athlete senses level of effort. Heart rate, oxygen uptake, and rate and depth of breathing have all been linked to RPE, as have blood-lactate level, availability of blood sugar, body temperature, and blood or muscle pH (acidity). Since all of these factors are in fact altered with increased work intensity, however, it is difficult to determine which, if any, actually cause changes in RPE. (Author's note: An informal survey of running acquaintances would seem to confirm personal experience that the agonizing discomfort of labored breathing is the most prominent signal of stress in a 5K or 10K road race. Does it help to recognize that this gasping indicates that excessive carbon dioxide is building up in your blood during the buffering of lactic acid, which is now flooding out of your muscle cells? Nope. Not at all.)
You might think, too, that external cues, such as where you are in relation to other competitors, could be important. If you, the state cross country champion, are at the back of the pack, your pace is too slow. Split times, which are provided specifically to help competitors regulate velocity, should be expected to be particularly useful in regulating effort in a distance event. But, perhaps surprisingly, this conclusion has not always been supported by research findings.
For instance, Yumna Albertus and her colleagues at the University of Cape Town shamelessly deceived trained cyclists during 20K time trials to see if providing false split times would change their pacing strategy. On the first time trial, the split times were correct, but on the second trial, the first split was actually given at .775 kilometers. This was followed by an increase of 25 meters with each subsequent kilometer until the end, when the split was given at 1.25 kilometers. On a third trial, the fraudulent times were reversed. So, did all this trickery affect the cyclists' pacing strategy? Surprisingly, not at all. There were no differences in finish times among the three trials. RPE values were the same, too. In finding that the pattern of power output was similar during the three trials, the researchers could conclude that the pacing strategy didn't change, either. Their conclusion? Pacing strategy is regulated by intrinsic clues (how an athlete feels), rather than by distance feedback.
Another investigation by Hugh Morton from New Zealand in which subjects were deceived produced different results. In this case, soccer players were asked to cycle to exhaustion (at a set resistance and cadence) on three occasions while facing a large digital clock. They didn't know that the clock was rigged to show the normal time on one occasion, to run 10% slower another time, and to run 10% faster the third. The results: When the clock ran slow, aerobic endurance time averaged 18% higher, compared to the session with normal time. There was no effect on aerobic endurance time, however, with the fast clock.
It was uncertain how to explain all this. The author himself was puzzled, concluding that “the psychological mechanisms behind the findings in this study are unclear.” But there does seem to be evidence here that external cues—in this case, the knowledge of time—are important in defining exercise tolerance. One conclusion is sure, though. If you are asked to be a subject in a pacing study, check the administrator's honesty first.
(The parallel to these deception studies in the real world of road racing obviously occurs when split times are erroneous because of inaccurately placed mile markers. This becomes evident when you find you've just passed the first-mile split, having shaved a full three minutes off the world record for this distance.)
This, then, is the customary way that coaches and distance athletes think about pacing. The competitors are in charge. They step up to the starting line, anticipating an average race speed based on previous events. Then, based on an awareness of how they feel, sensory input from the body's physiological state, plus extrinsic cues (split times), they modify their rate of muscular work (pace) during the race. These factors contribute to their optimal overall finishing time for that particular day and set of race conditions.