Friday, December 18, 2009

Are Cyclists Slower in Winter?

. . . the question on everyone’s mind this time of year.

If you’re worried because your average speed on cold days mirrors the dip in temperature, or that you must summon more energy to achieve your average summertime flights, fret no more. It may just be part of the natural order—you know—the laws of physics and biology. But it doesn’t mean you should stow your bike for the winter or sell it on eBay. Slower speeds in winter don’t necessarily signal a derailed training program.

Be judicious however in sharing the knowledge obtained here with your riding buddies. Unless they’ve slowed too, it may sound less like erudition and more like whining!

While it may be possible for some cyclists to go short distances as fast as they did during the regular season, the fact that it takes greater effort in cold temperatures highlights the difference between speed and power (measured in watts). In fact, power is a better determinant of workout intensity than perceived exertion, heart rate, or speed. The point is that a cyclist who churns the same wattage year-round will go slower in the cold traveling longer distances.

Others have argued persuasively, I think, that cyclists are slower in winter due to the cold. One mountain biker has attempted to estimate the effects of cold on several physical factors related to speed. Here I shall mention just a few of these factors and add a couple more of a physiological nature.

My intent is neither to quantify nor to prove that cyclists are slower in winter than summer. Instead, I shall assume that this is often the case and offer reasons why. To the extent that these variables are negligible or can be mitigated, the cyclist’s speed will be unaffected.

Increased air density due to cold: Like the wind, dense air affects every cyclist. And like the wind, the only way to confront dense air is to assume the most feasible aerodynamic position of body and equipment. Unlike a headwind, however, which can turn into a tailwind, the cold, dense air is a constant companion.

But what is air density and how does it affect speed? Air is composed of molecules such as oxygen and nitrogen. When air molecules group closer together, it makes the air more difficult for a cyclist to penetrate. Air density is related to altitude, barometric pressure, and temperature. For our discussion only temperature is relevant.

As temperature drops, air density increases.

What is the effect of increased air density in practical terms? Well, apparently, there is a rule of thumb relating the effect of air density to the speed of a cyclist.

The general rule for performance comparison is a 10°C drop in temperature equals a one minute increase in time trial time per hour.

In practical terms—time trial or no—the cyclist accustomed to traveling 16 mph when the summer temperature is 95°F (35°C) will have to settle for 15 mph at the same intensity when the temperature drops to 22°F (-5°C).

In other words, if the cyclist chooses to maintain, or is able to maintain, the same speed, there will be an increased energy cost.

The inquiring mind might wonder about density values at different temperatures. Using a readily available online air-density calculator, one might find for example that there is an 11.8 % increase in air density when the temperature drops from 90° F to 32° F. The interested reader can go a step further and determine the differences in power required to maintain a given speed at different air densities at this site.

Air density also affects tire pressure. If tires are inflated at room temperature the night before a long ride, then rolling resistance will be greater in the morning cold. Someone else may know how significant this might be in terms of affecting speed. Of course, one could inflate tires just before riding.

Bulky clothing worn in cold weather not only adds to the weight of (on) the bike but presents a less aerodynamic silhouette creating greater drag which slows the cyclist. The aforementioned on-line calculator can supposedly account for the cross sectional area of a rider and equipment (as a variable) under different conditions (including air density) as it relates to power output.

Fighting a cold headwind requires more energy than if it were a warm wind. The converse is also true. If you think of air as a loose liquid, then a strong, cold tailwind may be the closest thing to surfing a wave on a bike!
Lubricant viscosity: As mechanical devices, bicycles are propelled by moving parts that create friction on contact. Lubricants are used to reduce friction. However, the viscosity of many of the petroleum based lubricants used on drive trains, bottom brackets, and hubs increases with plummeting temperatures. To make matters worse, the inverse temperature-viscosity relationship is not linear but exponential.

A temperature-viscosity curve (like the one here), made logarithmic to obtain a straight line, indicates that the viscosity of some lubricants can double with a 10°C degree drop in temperature, say from 10°C to 0°C (50°F—32°F).

In North Carolina, for example, it is not uncommon for seasonal riding temperatures to range from 30°C to O°C (86°F to 32°F) which could cause a quadrupling of viscosities of some lubricants. For those who might wish to compare viscosity values against those on this particular chart, one source notes that lubricant viscosities ranging from ISO VG10 to ISO VG32 are commonly used on mountain-bike chains. There are many different kinds of lubricants, not all of which are petroleum based.

Just how significant is lubricant viscosity in terms of affecting a cyclist’s speed over the temperature range of a year? Perhaps a materials engineer might be able to offer an informed opinion.

Physiology: Cold can trigger a series of physiological events that can halt any ride. However, because there is a difference between being outside in the cold and being cold, care can be taken to prevent becoming too cold and triggering the physiological responses that can adversely affect athletic performance.

Before I address the extreme physiological effects of cold which are triggered if we get too chilled, I shall say something about normal physiology.

Although there is a difference between the season of winter and the cold of winter, a great number of people are affected by the change of season beginning in fall that is unrelated to the cold. Those affected may experience slightly reduced energy levels, increased appetites, and weight gain. Weight gain reduces VO2max independently of any other considerations. The winter season may also affect motivation in some people, the very last topic of discussion.

What about warming up just before an athletic event? If warming up on a spin cycle in July prior to a time trial affects the performance of the elite cyclist, what do you think about the activities of the ungloved randonneur standing outside on the street in sub-freezing temperatures prepping a bike for a Winter Solstice 200km Permanent? Granted, randonneurs seldom if ever hop a stationary bike to warm up for a 200km event, although some do ride to such events! But the point is that warm muscles, joints, and connective tissues perform better than cold ones even when cold does not mean extreme cold as in the case of our elite time trialist!

Now let’s turn to what happens physiologically when an athlete gets too cold. Again, be aware that this response need not be triggered even when cyclists ride all day in the cold. But, as a sports scientist explains:

When unprepared for the cold, skin and core temperature receptors stimulate a response called peripheral vasoconstriction in cutaneous and skeletal muscle circulation.

Essentially what happens when a cyclist gets too cold is that in an attempt to preserve core temperature, blood vessels reduce the flow of blood to the skin and muscles located near the body’s surface. Not only does this reduce the supply of energy to the working muscles in those areas but the extensive energy source located in the subcutaneous adipose tissue is now made unavailable. It should be noted that fat is the primary fuel source for aerobic respiration. A reduced blood supply to this area also means that muscle waste products are trapped. The net effect is that performance suffers.

When blood flow is shunted from the surface to the core the periphery cools.

Sports scientist Amy Mason notes the effects of cooling on muscles and nerves:

Muscle temperature decreases in the cold reducing maximal strength, power, and endurance. The superficial nerves cool and it takes more time to develop force and reduced nerve conduction results in recruitment of fewer muscle fibers, especially those closest to the muscle surface. Chemical reactions slow down and ATP utilization decreases at low muscle temperatures.

The authors of Physiology of Sport and Exercise, Wilmore, Costill, and Kenney concur, while noting the eventual fate of the athlete if the cooling process is not reversed.

When muscle is cooled, it is less able to produce force, and fatigue can occur more rapidly.

Working at a higher rate results in an earlier onset of fatigue, as there is an increased reliance on anaerobic energy production.

In other words, when the energy becomes unavailable in subcutaneous fat tissues, stored glycogen is burned at a faster rate.

During prolonged exercise in the cold, as energy supplies diminish and exercise intensity declines, metabolic heat production decreases and people may become increasingly susceptible to hypothermia.

Eventually, the cyclist’s intensity will slow as glycogen stores are depleted and core temperature drops. As the core cools, the heart rate slows. Because of the reduced cardiac output concomitant with a slower heart rate, VO2max declines.

In summary, according to Wilmore, Costill, and Kenney, the cyclist who attempts to maintain pace faces daunting odds: reduced blood supply to exercising muscle, slower heart rate, and reduced energy stores.

Again, these physiological responses are not triggered when the athlete is exercising in the cold, only if the body becomes sufficiently cold.

Don’t forget hydration. Mason describes an interesting fact about renal function in the cold as it could relate to dehydration:

Cold temperatures increase urine formation by decreasing tubular re-absorption of water in the kidneys.

This sets up the potential for dehydration because of a peculiarity of our sensory physiology, particularly the thirst center of the hypothalamus. In the cold, an exercising athlete’s thirst response may be dampened. Moreover, because the water carried on the bike is cold, it is more likely to quench thirst prematurely than warm water.

We also need to remember that coffee, tea, and chocolate which help us stay warm contain caffeine, a diuretic. Used properly, caffeine can be a cyclist’s ally.

Alluded to above, the question arises as to whether the winter season can affect mood and energy levels and therefore affect performance. For a small percentage of the population, the season does affect mood, although it is not clear whether exercise performance itself is affected. The converse is true, however: exercise enhances mood in many people.

Neither athleticism nor motivation hibernates in winter. In fact, some of the highest VO2max recordings belong to Nordic cross-country skiers. Moreover, there is that thing every four years they call the Winter Olympics!

Research suggests that cycling performance increases as cycling season nears. In other words, many cyclists are simply not geared toward speed in winter. Their off-season training is aimed at other goals such as building core strength and preventing mental stagnation by cross training. Many cyclists in fact choose to ride slower during the off-season to build what is known as the “base,” the training of fat-burning muscle fibers. And some elite cyclists ride less in winter.

While there may be a few good reasons cyclists slow during the winter, slowing itself may be no cause for concern. In fact, it may provide just the right path for attaining regular-season goals!

I’m grateful to riding buddies, John, Maria, Mike D, and Ron for their insights on some of the ideas expressed here. Any and all mistakes, misrepresentations, and/or mal entendu are of course my own fault. But if any factualness, truth, or clarity should have inadvertently infected this essay, it should be laid at their wool-sock, cycling-shoe, toe-warmer covered feet.

Any observations that you the reader might have in any of these areas please feel free to share under comments!

Let’s ride!


Anonymous said...

Hey Dean:

You forgot got :It's F'n cold. Let's ride slow."


dean furbish said...

Hi, Rico!

I thought you were going to say that based on my summer speed it was hard to tell that I had slowed any in winter! :)

Stay warm!

Anonymous said...

Great post!

Now I'll have all sorts of excuses for my poor showing :)

dean furbish said...

Thanks, Anonymous!

It seems I didn't think this one through too clearly . . . What if my speed doesn't just jump up this summer?

Can I draft you?

Doctor on a bike said...

Very interesting post Dean. I also wondered why I had such intense carb cravings on a recent 200k and this helps me understand.

dean furbish said...

Now that you mention it, Keith, in retrospect, on my last two rides, sipping my powdered food formula mixed in my drink bottles has not been sufficient. I found myself almost guzzling it on occasion and wondering why. Thanks for you observation!

Anonymous said...

Dean, it's either all that or my brake is rubbing. Cool article!

Mike / Raleigh

Anonymous said...

Great've inspired me to make a spreadsheet to see what holds true.

I'm gonna track my TH200k power output using kcal and time in HR zones from my heart rate monitor, avg. speed from my cyclometer.(Note this is not the same as power output measured using a cycling power (HRM measurement) should be greater since all of the energy doesn't go into moving the bike.)

Oct. 302 W chilly
Nov. 377 W cold
Dec. 421 W 4 wool jersey day

Gettin' my geek on. Oh, yeah.


Charles Lathe said...

Wow! It took me so long to read that article, I think I'm going to throw a log in the stove so I can read the next one faster.


dean furbish said...

I love your subtle humor, Chuck! It's always hard to keep up with you, but especially in the cold!

Elizat said...

I have been keeping a record and have noticed this. i thought it was just me, but it is good to read that it is not imagination or I'm losing fitness. It is real. Having said that and despite being slower I love riding in the cold (in appropriate warm clothing) and enjoying the fog, frosts and clear mornings.

dean furbish said...

Elizat, it's great to hear that you still enjoy riding when the temperature dips. Until a few years ago, I didn't know the thrill of winter riding, although our winters are temperate as compared to other parts of the country.