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I know in a race, pacing is key so you can always have your power for the last sprint/climbs and ultimately at least finish the race. However, strictly speaking on a solo ride let's say 10 km, and in both of these cases you do finish. Is going all out then burning out and still forcing yourself to keep going, is slower or faster, than having steady sweet spot pace? why?

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    (But it should be noted that 10km is not a great distance. Unless it's a climb all the way many weekend bikers could run the entire distance flat out.) – Daniel R Hicks Jan 5 '18 at 0:50
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    @DanielRHicks, I don't know if that counts as going all out then. I don't think there's any distance (beyond about 200m) that I could cover entirely at my max power so there's always some restraint if I'm going to finish without hitting a wall. – Holloway Jan 5 '18 at 10:27
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    To quote Ole Ritter, former holder of the of hour record (Mexico 1968) on time trials in general: You have to start flat out, build the speed in the middle and then go even faster at the end. – Bent Jan 5 '18 at 13:24
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    @Bent That's cute, but if you analyze power and speed data for successful TT'ers and hour attempts, that's not what you see. – R. Chung Jan 5 '18 at 15:53
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    @icc97, Brad Wiggins has said that during TTs he'd constantly ask himself, "Can I maintain this effort for the remaining distance?" If the answer was "no," he'd back off. If the answer was "yes," he'd pick it up. The answer he was looking for was "...maybe." – R. Chung Jan 7 '18 at 17:29
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I don't have the time to give a full answer at the moment, but I'll upvote a full answer and delete mine. The short incomplete answer is that you're better off pacing yourself relatively evenly. The reasons are both physics and physiological. The physics answer is that drag increases nonlinearly with speed so at higher speeds you're using more of your energy overcoming drag. The physiological reason is that when you go over threshold, you get tired (that's why it's called a threshold) and the time needed for recovery is exceeded by the time you're able to spend above threshold, so the net effect is that your average power output will be lower.

These two effects, the physics effect and the physiological effect, mean that unless your ride is shorter than, say, a minute, you're better off pacing (nearly) evenly.

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    Steep hills of course make this more interesting as riding up them takes quite a lot of power even at a crawl. You have to be good for this not to be above threshold. But the conclusion holds – Chris H Jan 4 '18 at 20:34
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    Right, that's why I put "(nearly)" in parentheses. It turns out that the time-minimizing pace is constant only when the conditions (wind, terrain, road surface) are constant. When the conditions are variable, the time-minimizing strategy is slightly variable: you go a little bit harder when you're facing a headwind or climbing a hill, and you go a little easier and recover when you've got a tailwind or are descending. Pacing optimization is actually a kind of interesting problem. I've done some work with Olympic pursuit teams. – R. Chung Jan 4 '18 at 21:00
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    Perception of fatigue (i.e., psychology) may also be an important limiting factor, once you have perception of exhaustion you have essentially hit what feels like a physical limit even though your body may be capable of much more (i.e., see Martin et al 2016 and media article). This again supports pacing as a way to avoid perceived fatigue and the resulting negative effect on performance. – Rider_X Jan 6 '18 at 1:28
  • The corollary of the studies is that professional endurance athletes may superior inhibitory control and resistance to mental fatigue. – Rider_X Jan 6 '18 at 1:33
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There is an article on Runners World that asked somewhat the same question, do you start fast or do an even pace? The general conclusion is that elite runners tend to start faster than their eventual main pace, and also increase speed for the finish.

The article also cites a study done with 15 well trained cyclists on a 20km time trial. The basic methodology was they did a 20k at their own pace, then two other trials at a steady state to exhaustion. In the secondary (steady state) trials, 9 of the 15 failed to finish the 20k. (The steady state was designed to mimic the same power output as the self paced trial.)

CONCLUSION:

By adopting an uneven, parabolic distribution of work, cyclists in this study were able to achieve an average intensity during self-paced exercise in excess of their maximum sustainable power output. A subsequent matched even-paced bout resulted in cumulative metabolic stress that could not be managed by moment-to-moment changes in power output. These results challenge the notion that strict even pacing is optimal for endurance time trial events.

So at least for this one study, it appears that allowing your body to start fast, end fast and self pace in the middle, rather than adhering to a strict pace schedule has much better results. While this does not directly address going all out to exhaustion, I would suspect that you would have similar results, in that the body could not clear metabolic waste fast enough to allow completion in a time faster than a inverted U type pace.

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    My theory is that at the start of a race you still have high oxygen, ATP etc. and low lactic acid levels so you can go faster until you reach balanced levels. At the end of the race you can enter a short-term debt because you have time to rest after the finish line. – Michael Jan 5 '18 at 6:41
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    Note that the time taken to complete a certain distance isn't just a function of average power. For a given average speed, it's optimal in terms of energy expenditure to ride at constant power (in ideal conditions, on a flat course, that is). Variations in speed (and therefore power) will increase the average power required to keep the average speed fixed, as R. Chung describes in his answer. – Will Vousden Jan 5 '18 at 12:08
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    The only point I'd disagree with is "much better results". Much is a very inexact word. I certainly don't think people will be doubling their speed. Nor is it a magic sauce as a lot of people will tend to do this anyway. – icc97 Jan 6 '18 at 6:41
  • @icc97 semantics, but I would consider a pacing protocol that produces 60% better results to be substantially better. :) – JohnP Jan 6 '18 at 14:09
  • Are you sure of that 60%? I don't see that mentioned in the study. That would mean that a good cyclist who can do 40k in an 60 mins (40 km/h), if they change their technique you expect that they can do 40k in ~38 mins (64 km/h)? (56 km is the world record for 1 hour). Given the effort it takes to even change your time by 1 min, this seems unbelievable. – icc97 Jan 7 '18 at 15:35
8

One of the things we sometimes have to accept in science is that we have observed facts that we can't fully explain. Fatigue is not well understood physiologically.

For middle and long distances, the human body has enough muscle glycogen and liver glycogen to fuel vigorous exertion for about 2 hours. It's not a coincidence that a world-record marathon pace is a little over 2 hours, and that amateur athletes also tend to bonk after about 2 hours.

Since you have enough glycogen to go for a couple of hours, the question is why you can't maintain the same pace for two hours that you could for 15 minutes. Nobody really knows. Anaerobic metablism is relevant, but only on very short time scales. It used to be believed that fatigue was caused by the accumulation of waste products such as lactic acid, and changes in pH in muscle tissues. Recent work does not support that idea. The kind of model that currently has the most experimental support is that fatigue is something the central nervous system does in order to maintain homeostasis.

Maintaining homeostasis requires that the body protect itself from damage, keep itself from overheating, and avoid running out of fuel. Factors such as lactic acid and pH may be inputs that the CNS uses to make these decisions, but they are probably not physically limiting factors. This hypothesis is supported, for example, by the observation that when the weather is hot, performance decreases before core body temperature goes up. This suggests that the CNS is anticipating that it will overheat. Similarly, the CNS may anticipate that it will run out of fuel in the future.

People have built mathematical models of this sort of thing, e.g., Reardon 2012. Reardon succeeds in reproducing data at middle distances showing that people tend to slow down later in a race, which he interprets as meaning that there is some optimal pacing strategy that involves deceleration. It is unclear how or whether a model like this corresponds to any fundamental physiological limitation or gives any insight into the underlying mechanisms. I'm not an expert in this kind of thing, but a recent book that seems to do a decent job of outlining the state of the art from the perspective of an elite athlete is Magness 2014.

As an amateur athlete, I don't find much useful guidance in the scientific data, except in the negative sense that it encourages me not to worry to much about what the experts say, because the experts don't really seem to know what is going on.

Magness, The science of running, 2014

Reardon, Optimal Pacing for Running 400 m and 800 m Track Races, 2012, http://arxiv.org/abs/1204.0313

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    The thermal issues are interesting. I've seen a discussion about performance on indoor stationary trainers vs outdoors cooled by the wind the rider creates. That also hinted that some measure of surface heat loss rather than core temperature rise is the issue. This makes sense as the thermal mass of a human (80kg of water to a good approximation) is large: at typical power levels and efficiency, with no heat loss to the outside world the effort will take several minutes to raise the core temperature by 1°. Of course we do lose heat but this gives an order of magnitude for time constants – Chris H Jan 5 '18 at 9:32
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    All out anaerobic sprinting uses available ATP and creatine phosphate (CP) to replenish, sustainable for up to 8ish seconds. After that, the aerobic mechanisms take over, and they are slower to replenish ATP from glycogen than the CP mechanism. (As an aside, that is why creatine works. It puts more CP in the cells to fuel work a little longer before alternative sources). – JohnP Jan 5 '18 at 14:47
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    It's not a coincidence that a world-record marathon pace is a little over 2 hours Sure it is -- because the distance of a marathon is arbitrary. If the battle of Marathon had been fought forty miles away from Athens instead of 26, you wouldn't see two-hour times. – John Gordon Jan 5 '18 at 19:09
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    @JohnGordon we could also say "it's not a coincidence that the longest mainstream running event is the marathon" There are plenty of shorter races, and there are certainly ultramarathons, but they have very niche appeal, while the marathon is both a relatable goal for many runners and an elite sport. What's interesting is that the equivalent challenge for many cyclists could be regarded as the century, for which records are well over 3 hours. – Chris H Jan 5 '18 at 22:25
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    @JohnGordon Or maybe it's not arbitrary. If the battle of Marathon had been fought forty miles away from Athens, perhaps the outcome there would have been a lot different and our running contests would have been named after a battle at a different location where the run was close to two hours. – Michael Jan 6 '18 at 17:16

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