Monday, March 03, 2008

To Pedal Fast or Pedal Slow?

Pedalling Cadence

This is a topic somewhat fraught with controversy, contradiction, and personal preference, not to mention habit and dare one say stubborness. On my part as much as anyone else's. Better minds than mine have studied this issue, and there are probably formal scientific studies still being written up, and started, even now. However it seems to me to be a multi-faceted problem (if it can be called a problem!) which earlier studies at least, seem to have simplified into only a few facets. Whether or not I have anything to add to the debate, I've tried to present some of the aspects of this subject here; it's somewhere between a summary and an analysis, and not the best organized thing I've written either, but hopefully you can pick out the more relevent points to your style.

Perhaps part of the two-sidedness of the debate stems from the differing objectives of different cyclists. Obviously, racers have different objectives to recreational cyclists, but rec riders too may have different objectives. For example, if you just ride for the freedom of the road and want to conserve as much energy as possible, you might want to adopt a slower cadence. If your objective is cardiovascular improvement or just general fitness, the slower cadence is less useful. Other objectives which should cause reflection might be: distances you will be covering; whether or not building stronger leg muscles is the prime consideration; or perhaps something else. Frequency of riding might be a factor: if one only rides a couple of times per week, micro-scale muscle damage caused by grinding low cadences has lots of time to repair itself between rides, whereas if one spends several hours every day on the bike muscle damage becomes cumulative when it cannot repair fully between rides.

One of the things that struck me when I started riding with the CCC was the wide range of riding styles. What I found most curious was the slow pedalling of high gears used by many of you, by preference. Having grown up with one of the single-gear coaster-brake bikes, the only way to go fast was to pedal like mad, a quality that stood me in good stead when I got into racing. Racers like to spin the pedals at a high cadence, partly because they have to at the speeds the race goes, and partly for the pedalling efficiency and better accelerations this gives them. Even racers who are reknowned for slow cadences and high gears are still working in the range of 85 rpm give or take a bit, on flats or hills.

Anyways, without making any judgements on the even slower cadence of the recreational cyclist, I tried first to find out more about the advantages or disadvantages of spinning vs stomping. Of course most of the literature on this subject is written for or by racers, but during one of the rides this past summer Gary mentioned an article in a cycling mag that had concluded that a cadence of about 60rpm provided a touring cyclist with the best efficiency that is, distance covered for nutrition requirements (ie needed to eat less). I suppose in some way that compares to the most economical ride, similar to the speed at which a motor vehicle gets the best gas mileage.
This article has the problem, for me, of not specifying the duration of the test rides. The various types of tests and analyses are interesting if you're into that sort of thing (which I am if you haven't already noticed!), but what I found more interesting and enlightening was the discussion of biomechanical issues (ie muscle and joint interactions) tucked away in the last half of the webpage, based on computer modelling which at the time may or may not have been as valid as what can be done today. Worth looking at.

It's also worth keeping in mind that on group rides the speed of the group you're in may be such that one has a choice between riding in a gear which gives a slower cadence than preferred, or the next gear down which will give a cadence higher than preferred; the temptation is of course to opt for strolling along in the higher gear/ lower cadence because settling into a cadence a bit higher than optimum is initially a bit uncomfortable.

60rpm seemed awfully low to me. (I tend to fall into a natural cadence of about 90-95 if I can find the right gear for it, but sometimes as low as the mid-80's or up around 100 feels right at the time.) But I didn't want to espouse an argument without having derived some kind of basis for it.

How to do that? (Keep in mind that I didn't do any literature research until later, and didn't have Gary's link at that point either.)

Well, I thought that with the beginning of winter and my shift from outdoor riding to my indoor trainer I might be able to prove something by comparing equal rides using the instrumentation of my trainer. I thought I would be able to see some effect on my Heart Rate caused by changing the cadence.

Note: feel free to skip the graphs if you're inclined to; they're there for anyone interested in seeing what I'm talking about but aren't necessary to
understanding it (or at least I don't think so!)

Test "protocol": first attempt. My first attempt was to try several cadences for short segments all in one ride (following a warmup period, of course). The objective was to hold a steady speed of 25kph no matter what, the cadence determined by the gear selected. Each segment consisted of 4 sections: first about 1 minute at a slope of -1 during which I switched to the new gear and settled into the new cadence. Then about 2 minutes at a slope of +1, about a minute at a slope of +3 (to apply a bit of stress), and then back to slope +1 for another 2 minutes. I chose 25kph speed and the slope value of +1 rather than zero to make sure that I would be working hard enough to create a difference. The gearing on my bike gave me the following cadences, tested in the order listed: 54rpm, (101rpm*), 57 rpm, 92rpm, 62rpm, 84rpm, 66rpm, 76rpm, 53rpm, and 92rpm. To cover for the case of being insufficiently warmed up I repeated the lowest and second highest segments at the end. I didn't repeat the highest segment* because I couldn't maintain the 25kph test speed in the lowest test gear.

[Graph of heart rate, power, cadance and slope during the first run; speed 25kph throughout most. In following runs the cadence and speed were held constant and the slope program was the same as here.]

There were no obvious trends in the data. When I graphed analysis,
the general shape of the resulting curves could be interpretted as showing that something optimal happens between cadences of 70-90rpm. Alternatively one could say that it looked like the lower the cadence the better things looked. Can that be right? Nah- more testing required.

Test "protocol": second attempt. It seemed obvious that I was going to have to ride at each cadence for an extended period. I decided to use the same "ride profile" as for the initial test, for each of the repeat tests. That is, for each cadence I would ride a warmup and then 10 segments of -1, +1, +3, +1 slopes. I ended up doing the tests in pairs, on day 1 I would do a lower cadence test, day 2 a higher cadence test, then a few days of rest or other riding to keep my interest alive-- this grinding out 25kph at a constant cadence for over an hour was awfully boring despite the slope changes, especially at lower cadences!

Here's an example of raw data, for runs at 52 and 84rpm:

And here, zoomed into the last two slope cycles.

What I was hoping to see was a difference in the change of HR going into and out of the +3 slope segment, or in the smoothness of the power output at the slope changes. Nothing significant that I could see. Darn!

Results were mixed. That is, what the numbers were showing me was not, for the most part, what my body was feeling like. It was a great strain to do the runs at the lower cadences whereas the higher cadence runs were much easier, but yet my average HR was lower at the lower cadences, which should indicate a lower effort. The minimum HR's were at a cadence of 62 rpm, which correlated with what Gary had read.

At first brush, the "Energy expended" (Kcal) per run was steady across all runs. That makes sense, because the Energy data (blue line in the next graph) I was looking at was recorded by my I-magic trainer Catalyst software, which measures the actual work done by the back wheel to rotate a spindle and small flywheel against an electromagnetic brake. So this pick-up sees only the speed of the spindle and the resistance of the brake (set by the Slope function, here). However, most HR monitors for athletes on the move (as opposed to on stationary equipment) incorporate a calculation of "Energy expended" (E) which is calculated from the HR data, as a reasonable approximation. I also had these numbers available ("Polar" curve, pink in the graph below), and when I looked at them of course the 62rpm run gave the lowest E, while the more comfortable higher cadences gave the highest E. However I was also feeling physically somewhat better on the day of the 62rpm run so maybe that minimum is a bit more minimum than it should normally be; still, the trend is there in the 57 and 67 rpm runs.

So that should be the end of discussion, right?

Uh, no.

First there is the immensely subjective, qualitative issue of how the various cadences felt to my legs and body (always bearing in mind that I am used to the higher cadences by choice). Notes made at the time of each run:

  • 52rpm: Thighs tight and tightening; Knees and lower back bothered by it, also neck and shoulders from deathgrip on handlebars for much of the ex. A bit of a struggle after 7th group (of 10). No muscle fatigue or stiffness the next day though.
  • 57rpm: (no notes made but essentially felt the same as at 52)
  • 62rpm: HR's low the last 2 days, I am better rested somehow. Kind of messes up the project?
  • 67rpm: A minor struggle to keep the pedals turning.
  • 75rpm: Thighs, Knees and Lwr back good. Got a bit harder on the quads after about the hour mark, otherwise a comfortable slow cadence.
  • 83rpm: (no notes; this cadence is well within my comfort zone so all felt normal and easy)
  • 92rpm: a good roll but tired from prev day and today activities (off the bike). So HR higher than I think it should be. Quads painless.

Some of those notes might lead one to believe that a few runs should be redone. Well, the protocol is only loosely repeatable (certainly not to scientific standards!) because I didn't follow a consistent activity plan or diet for the duration of the test period (17Nov to 3Dec). Nor did I drink at the same times in each run (I always have a bump in HR for about 30 sec after drinking on the bike!). However, the significance of the Energy output minimum at about 62rpm deserves a better look, in view of the noted aberation.

From the feeling of my thighs during the lower cadence runs, I think that if I had continued for an even longer effort, maybe 2 hours, they would have been sufficiently fatigued to cause a change in HR, whereas at 83rpm I know that the numbers would have been about the same after 2 hours. Maybe next winter...

One of the numbers the Polar instrumentation on my bike (separate from the Imagic gadget the bike sits on) gives me is a thing called Pedalling Index (PI). Basically, the feet do not apply equal force to the pedals during the entire 360degrees of the revolution; PI gives an indication of how distributed the force is. A higher PI means the force is applied more evenly and occurs during a larger arc of the pedal stroke; a smaller PI correlates to a jerky pedal motion with large force of shorter duration. When I went back and looked at PI data for the various runs, I found the following:
[In the graph, the averages are the more useful numbers than the maximums. "slope3 avg" is the average PI of a single representative 1-minute interval at Slope of +3 (I used the same time interval from each run)].

What I find interesting in the graph is how the overall average (entire 70-minute run) is about as good at 75rpm as 83 and 92, but the slope3 avg for 75rpm is as bad as for the slower cadences. This seems to tell me that to optimally handle the little dips in the road and gusts of wind, a cadence of 83 or slightly lower is best (for me at least; the question remains just how "average" am I in this regard?). There is also a pretty large difference in the average run PI between the lower cadences and the higher ones.

Let's examine this a bit more closely. Although it seems like the bike maintains a steady speed while being pedalled at a cruising rate, in fact it is constantly accelerating at each pedal stroke and decelerating between them. During each test I was going the same speed and the power outputs were always the same for each slope setting regardless of cadence. That means that where the PI was low I was exerting a relatively greater force on the pedals for a small arc and then not much force for the rest of it, whereas at a higher PI the peak force in each revolution was smaller but it was spread over a larger arc. In effect, when the PI is low the larger force is required to accelerate the bike back up to speed, but the legs cannot maintain that level of force for very long and so ease off from that maximum very quickly, although they never relax to the same degree as they do at higher cadences. So the bike is continually speeding up and then slowing down as it goes along the road. At higher PI less effort is needed to accelerate back up to speed, partly because of the lower gearing and partly because the bike has not had as long to slow down (faster pedal rpm's and less of each revolution without much power).

On the trainer, I am working with the momentum of my back wheel plus a small flywheel whereas on the road there would be the entire momentum of bike plus rider. The trainer actually slows down faster than I would from the same speed on the road, but it speeds up much more quickly. So on the road the alternating slowing and accelerating of the bike would have the result that the force required at lower cadences would be larger, the rider's legs would tire sooner.

If you are ever riding on the road with someone who has knobby tires on their bike, you will be able to hear this (the speeding and slowing, not the tiring of the legs! but maybe that too) in the sound the tires make, the sound increasing in pitch with the accelerating part of the pedal stroke and decreasing in pitch when decelerating. If you can get the rider to try different gears while staying at the same speed you will hear the different amounts of deceleration.

In practice this means that if I were to re-do all these tests on the road, I would expect to see higher energy output per test on the road compared to on the trainer, but significantly higher at the lower cadences while only marginally higher at the higher ones. I think that this would shift the cadence at which the minimum effort occured to something higher than the 62rpm.

Physiological effects at lower cadences: we've seen, unexpectedly, that the HR during low cadence runs was less than at higher cadences even though the power measured at the trainer roller was the same in both cases. At the same time, my qualitative notes are telling me that my legs were more tired and less likely to last for hours at the lower cadences. So what's going on here? In a (hopefully) educated guess, I think we're looking at reduced blood circulation due to the leg muscles being tighter from the stronger forces they are producing, essentially like a weight-lifter's muscles get "pumped up". Less blood flow, less oxygen, therefore more premature lactic acid production in the muscles. But the reduced blood flow masks the extent of lactic build-up because less waste products are getting into the blood stream (to signal that more oxygen is required). So the body goes merrily on its way with an aerobic-zone HR, even though anaerobic activity is going in in the muscles. The cardiovascular system lacks correct feedback and produces a false effort (HR). Possibly there is a blood pressure issue involved as well. In contrast, at higher pedal rpm the lesser contractions but greater relaxations of the leg muscles probably enhance the blood flow through the muscles, not only perhaps assisting the heart in actual pumping but also keeping the blood vessels more open. So the cardiovascular system may be able to respond with great efficiency at detecting muscle combustion by-products. At ridiculously high cadences and low gearing there is so much blood rushing around that the HR gets much higher than the power output warrants.

Since doing those tests on the bike, I've noticed that (on the trainer) whenever I shift gears there is an almost instant small but transient shift in my HR (only a few beats per minute, but there nonetheless, for a short while. When shifting to a higher gear (slower cadence) my HR drops, and when shifting to a lower gear (higher cadence) my HR increases. The increases generally last longer than the decrease but I haven't timed them. The fact that the effect is almost instantly seen indicates to me that it probably has more to do with a circulation issue (blood pressure or some such) than to heart rate increase due to a change in work rate.

I noticed some hints about the above effects in some training exercises prescribed for competitive cyclists. (Chris Carmichael: "The Ultimate Ride"). For basic aerobic system development workouts, he directs the rider to use a cadence of 85-95. Interspersed therein come sub-exercises of various sorts (don't use the following as basis for training; there are detailed instructions on use of them for best results which should be read first):

  • Fast Pedal: short intervals at 105 to 130+ rpm, with equal recovery periods between them. "heart rate will climb during intervals, but should not be used as an indicator of intensity"
  • "MuscleTension"(TM) Intervals: hard pushing at low cadence, 50 to 55rpm. Maximum duration of 12 minutes (max 4 repeats with recovery between) and at least 48 hours between sessions. "This is a muscular workout, so heartrate is not a good indicator of intensity and should remain low".

There are also a number of other workout types of which 2 seem to offer some insight here:

  • Recovery rides: Cadence 75-85 (with low gearing and low HR).
  • "Tempo"(TM) riding: this is a bit of a complicated and hard workout, targeting a HR close to anaerobic. But the salient points as far as this discussion are concerned are: uses a cadence of 70-75rpm; trains the body for "better fuel utilization during long races or rides"; strengthens leg muscles; duration up to 2 hours, and at least 36 hours between sessions.

There are essentially two movement types of getting power to the pedals: using a circular foot motion, or linearly using the legs as pile-drivers with essentially up-down motion. How can that work, you ask, since the pedals go in a circle. In the circular case, power is applied tangentially to the crankshaft, first by straightening the knee deliberately when the crank is vertical upwards, then progressively adjusting the angle of attack as the crank progresses around, pulling back through the bottom of the stroke and finally lifting its own weight on the up-stroke; this gives a high Pedalling Index. (Easier said than done, requiring concentrated practice.) At higher cadences the momentum of the foot performing a circularly-driven pedal stroke adds to the muscular effort to enhance the efficiency. In the pile-driver case, the rider is just concerned with a big downstroke and some energy is dissipated in fighting the fore-and-aft vector of pedal motion; low Pedalling Index. The momentum of the circular foot motion is lost. This motion can also be hard on the hamstrings which function in decelerating the leg, because this pedalling motion requires hard decelerations rather than the softer ones of the circular motion. I find that a more circular pedalling motion (but certainly not a perfect one by a long shot) comes naturally to me at higher cadences, and that at lower cadences I really have to think about what I'm doing to avoid being a pile-driver. Circular pedalling also stabilizes the pelvis to some degree (or vice versa?), whereas linear pedalling often results in the body rocking along from the hips and even the shoulders (fighting with the bike). The cyclist is most efficient when the upper body can relax, and any tension and motion of the upper body takes energy away from the power available for pedalling. Ironically, body-rocking is more likely when the rider is fatigued, a time at which efficiency is most desired but hardest to achieve. Having said all that, if one watches the Tour de France on tv one can see some pros doing quite nicely with what looks like very inefficient style. So in the end, it boils down to "whatever works best for you".

I did run across one medical perspective in my reading (Bernard Hineault: "Road Racing Training and Tactics): "When spinning is not smooth and regular, the cyclist puts excessive demands on the lumbo-sacral muscles which in turn subject the lumbar-sacral joint to extreme and abnormal stress." This can result in back pain.

As always, there is certainly a time and a place for grinding along at a slow cadence to build leg strength, but I think it is better done towards the end of a group ride rather than earlier, or when solo.

Time for an anecdote, maybe. At about the time that I got back on my bike after 13 years off it, I had a high-school kid working for me in the nursery who rode some BMX. Some of the work involved going a few km down the road to check on plants at a "remote" location (that commute is what got me back on the bike, rather than waste gas going such a short run in my van). Several times I took him along on a mountain bike, and over the next couple of years we did a few road and off-road rides. Anyways, to get to the point, he was initially very much a "power" rider, high gear/ low cadence and very set in his ways, but after watching me a bit and being surprised by the out-of-shape old guy beating him up the hills while he gasped and wheezed, I noticed that eventually he started using gears and cadences much like mine with better results, and did not go back to his previous style, at least while riding with me.

So what's the bottom line for the touring cyclist in all this cadence stuff? First, everyone has their own comfort zone in cadence. But your comfort zone may be one that you've fallen into by default, without trying anything else, and there may be a more comfortable and more efficient or useful cadence zone for you if you look around for it. Second, any attempts to change any aspect of your riding style should only be done gradually. So if you want to try to increase your cadence work into it a bit at a time. Third, definitely try a higher cadence, with lower gearing, when climbing hills or fighting a headwind. Apart from rationing leg-power, I personally find a psychological benefit from pedalling lightly with rapid leg motion into adverse conditions (ie strong winds or up hills); the speed of the legs makes it seem like there is more progress even if the ground is just creeping past. When leg muscles start to burn even a little bit or have become heavy, slow down and gear down and open up your cadence and let your heart rate back into the gossip zone so that the rest of the ride remains pleasant. And I think it is helpful to change cadence from time to time during a ride anyways, just to keep your legs interested, in much the same way as you change the position of your hands on the bars from time to time to keep your upper body more comfortable.

I find it is useful to change to a faster cadence for a few minutes every now and again, say about every 10-15 minutes. I may "lose ground" during the little spin, because I do slow down a bit, but maybe in the long run I might make it up again. And a high cadence the last few minutes before the end of the ride also helps start your legs into recovery by getting the blood moving.

When cresting a hill and starting down, instead of immediately changing clunk-clunk-clunk into a high gear and grinding down, spin out each gear before changing to the next lower (that is stay in a gear and just keep pedaling faster until you reach a fairly high cadence, say about 100rpm or when you start bouncing on the saddle, whichever comes first). You should find the down-shifting much easier on the legs because they won't be jarred by such a sudden increase in resistance. And actually you can accelerate faster that way too, if that matters.

But of course it's all about having fun, at the end of the day.

And here's a related curiosity. Suppose you are pedalling first up a hill in a small gear at 80rpm cadence at a work rate of 150watts, and later you are going down a hill in a big gear also at 80rpm and 150watts, obviously at a higher speed, will the pedalling feel the same to your legs? Logically, same rpm same work output, should feel the same but in fact No!! For me, the high gear segment will feel harder on the legs, and different, than the low gear segment. And apparently it's not just me. Don't know why, haven't seen an explanation for it, but it just is. If anyone out there feels the opposite, I'd be curious to know of it.

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Anonymous Ella said...

Thanks for writing this.

Tue Nov 11, 01:58:00 AM  

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