Tuesday, May 8, 2007

The Physiology of XC Skiing

The Physiology Of XC Skiing
By Stephan Sieler
From the MAPP http://home.hia.no/~stephens/
My approach to learning and doing a new sport starts with a lot of reading about the specific demands and physiology of the event. The basics of any endurance sport are similar, but I like to look for the details. Fortunately, there is a great deal of research on the physiology of cross country skiing. Most of it has been performed here in Europe and Scandinavia, because of the high level of participation in the sport in this region of the world. A lot of the work I have been reading is written in English, which is good. Some is in Norwegian, which is ok. And one useful dissertation I have in my hands right now is written in Danish, which is pretty darn challenging. So let's just say that studying XC has been good for my language skills!

What does an elite skier look like?
Good question. On average, top class skiers are between 27 and 29 years old when they reach their peak, but the standard of deviation is 4 years. This means that you can see Olympic medallists in their early 20s to late 30s! One important point which speaks to the need for patience and persistence is this: No junior skier has ever won an Olympic gold or World Championship. It takes years of training to achieve the highest levels of performance.

The interesting thing about XC is that there is no "perfect" body type. In sports like swimming, distance running, and rowing, observing an assembly of the elite often looks like a clone festival. In contrast, World champion male skiers have ranged in height from 5' 6 (1.68 m) to 6'6" (2.0 m). The elite skiers usually has little body fat, but not to an extreme. As a group, elite XC skiers are heavier than distance runners, but lighter than rowers. Female elite skiers tend to have a lower body mass index (mass in kg divided by height in m2) than non-athletic women of the same age.

Fiber Type Composition
What about how they look under their skin? Type I muscle fibers are predominate in the leg muscles of elite skiers, but there is considerable variability even among the elite. In the normal population the fiber composition in the vastus lateralis (a thigh muscle that is often biopsied in athletes) will approximate a 50-50 ratio between fast and slow fiber types. The fast fibers will be made up of a mixture of type II a and II b fibers. In elite skiers the percentages are more like 66% (62-75% in different studies) slow and the remainder type II a. The "pure" fast fiber, the type II b subtype, is practically non-existent in well trained skiers (and other endurance athletes). This is due to type II b to II a conversion (II a fibers are still "fast", but with much greater fatigue resistance). Now, in comparison, biopsy studies on elite distance runners suggest a slightly higher slow twitch percentage among the elite runners (78-79%). Perhaps it is adaptive for skiers to possess a higher type II a percentage, due to the varying terrain and non steady-state conditions that comprise XC racing.

Unlike running and cycling, XC is a whole body sport. Major endurance demands are also placed on the upper body musculature, including the latissimus dorsi, deltoids, and triceps groups. Surprisingly, there has been far less work done to determine upper body fiber composition in elite skiers. From what we know, the average population has more fast twitch fibers in upper body musculature compared to lower. The triceps for example is about 65-70% fast in untrained people. Consequently, the XC skier must work deligently to maximize the endurance capacity of these normally under utilized upper body muscles. Even in elite skiers, triceps fiber composition is less slow twitch dominated than the lower body, about 50-50 in one major study. Some investigators have suggested that in specific muscles like the triceps, it is advantageous to have more fast twitch fibers due to the high movement velocity of the distal arm during the "push" phase of the double poling movement.

Movement Speed on Skis
As in running, skiing velocity is a function of stride frequency and stride length. Increasing either one, without decreasing the other will result in increased movement velocity. So, which factor, distinguishes the elite skier from the "also-ran"? Elite skiers have longer strides compared to less successful skiers both in skating and in the diagonal stride. The faster skier is not faster due to greater skating or striding frequency However, when we look at the upper-body only, during double poling, then the elite skiers achieve greater velocity by using a higher rate of poling; increasing poling frequency. Finally, elite skiers are better able to change potential energy into kinetic energy than recreational skiers. This reduces the demand for changing the velocities of body segments. For example, the elite skier makes better use of the pre-stretch on the arm musculature achieved during the initial pole plant during double poling.

The average speed of world cup races is about 6-7 meters/sec depending on the conditions. In running, there is a progressive decrease in average velocity with increasing race distance (beyond 200 meters). Top marathoners run about 19% slower during the race compared to 5000 meter runners. In contrast, the difference in average velocity during a 50k classic style ski race compared to a 10 k race is on the order of only 5-7%. The main reason for this better speed maintenance is that the longer courses are constructed with slightly less demanding climbing segments, allowing greater velocity. One other possibility is that the skier has more total glycogen available for generating energy at high intensities, due to the greater involvement of the upper-body musculature. This may allow the skier to maintain a higher average exercise intensity over the race duration without reaching performance limitation due to glycogen depletion.
For the same reasons, it is not simple to compare the racing speeds of men and women. The problem is that they often compete on different courses. However, if we use the Vasa løpet in Sweden, then both men and women go on the same course at the same time every year. In this race, physiologist Bjorn Ekblom has reported that the male winners were, on average, 16% faster than female winners. Other studies suggest differences of 14-15% in average velocity. This is a larger difference than we see in running or rowing.

VO2 max in Elite Skiers
The single physiological variable that most clearly distinguishes the champion cross-country skier from the average person, or even the highly trained but less successful skier is the maximal oxygen consumption. In the unforgiving world of XC racing, there seems to be no substitute for a BIG ENGINE!

A major question in XC skiing research has been "what is the most appropriate way to compare VO2 max values among different athletes?" One way is to just compare the absolute consumption during a maximal exercise test in liters/minute. This value is representative of the maximum capacity of the athlete to generate power through aerobic metabolism, which is what ski racing is all about. If we do that, the values are impressive (5.5 to 6.5 liters/min these days), but they don't take into account differences in body mass. The typical solution in many endurance sports is to compare values corrected for body mass. For example a 70 kg skier with a 6.0 liter VO2 max has a weight corrected VO2 max 85 ml/kg/min (yep, that's high, but typical among the world elite). Let's say another skier has an even "bigger" 6.5 liter/min max. However, he weighs 80kg, so his VO2 max is "only" 81 ml/min/kg. Consequently, our heavier skier appears to come up a little short. The problem with this very typical method of comparison is this: Skiing conditions change from minute to minute. The power needed to ski at a given speed on level terrain does not increase in proportion to bodyweight. When climbing a steep hill, added body mass is a more powerful negative factor. During a downhill it is a plus! Considering the varying conditions, physics, dimensional analysis, test data, etc., it appears that the most valid expression of maximal oxygen consumption for XC skiing is achieved by dividing VO2 max by body mass2/3. Ingjer (1991) demonstrated that the average VO2 max of world class skiers was significantly greater than that of less successful skiers only when it was divided by body mass2/3, not when it was divided by simple body mass. (In our previous example, the two skiers with maximal oxygen consumption values of 85 and 81 ml/min/kg come out to nearly identical values of 350 when expressed relative to the 2/3 power of bodyweight.) One thing is clear. The teams with the most success have skiers with the highest maximal oxygen consumption.

What is limiting Maximal Oxygen Consumption?
I have discussed the limiting factors in VO2 Max before, but some additional comments are worth mentioning here. There is strong agreement among the research community that it is the pumping performance of the heart (and therefore oxygen delivery) that limits the maximal oxygen consumption in most non-athletes and athletes. However, there now seems to be a catch. In those athletes with the really high absolute VO2 max values, driven by really high maximum cardiac output, it appears that other links in the oxygen delivery chain can become the weak link. If the flow rate of blood through the lungs becomes great enough, a point is reached where the de-oxygenated blood coming from the right ventricle of the heart is passing through the lungs before it is fully re-saturated with oxygen. At this point we say that the oxygen diffusion capacity of the lungs is limiting total oxygen delivery, and therefore, VO2 max. That may be a little more information than you want to know. The bottom line is that the single most identifying factor among the world elite will be a very high maximum stroke volume, and high max cardiac output. As a rule, you can assume that the guys winning the medals in the Olympics have maximal oxygen consumption values over 6 liters/min, maximal cardiac outputs of over 40 liters/min, and stroke volumes over 200 ml. They may look pretty ordinary on the outside, but they have a pretty extraordinary pump inside their chests. If you want to find a better heart, you probably should go out to the horse tracks and check out the thoroughbreds!

Are the skiers of today better than in the past?
Most of the increase in speed demonstrated by the XC elite in the 90s compared to say, the 60s, is due to equipment, technique and track improvements, not better trained or more talented athletes. However, the very best are still getting better physiologically, slowly but surely. Higher training volume and more total skiers competing for the top are two reasons for the progression. Here is some data from Swedish male medallists in the 60s, 70s, and 80s (from Ulf Bergh and Artur Fosberg, 1992).

Maximal Oxygen Consumption

Although I don't have data for Swedish medal winners from the 90s, I have talked to some Norwegian sports scientists who have been involved in physiological testing of Norway's national team (which has dominated Sweden in the 90s). Currently, Bjorn Daehlie sits on top of the team list with a reported maximal value of 90 ml/min/kg. He is also reining World Cup and Olympic Champion. There are one or two reports of athletes VO2with values at or above 90 ml/min/kg in the endurance sports world (in cycling and distance running). Remember though, they are very, very, rare; way out there; off the scale; WHAT PLANET IS HE FROM? type values. Indurain.......Morcelli........Daehlie.....NOT US. The air keeps getting thinner and thinner at the TOP!

The Upper Body in Skiing
Propelling the body on skies requires intense work by both the arms and the legs. When we ski hard we are "asking" the heart to deliver high blood flow in several different directions at once. Once an exercise employs a large quantity of muscle (running, rowing, cycling in experienced riders), then the oxygen consumption limitation falls back to the heart and it's ability to deliver oxygen. So, what happens in skiing when we add maximal arm exercise to maximal leg exercise? The answer is: little or nothing. Studies in the laboratory have demonstrated that adding arm exercise to maximal leg exercise during a VO2 max test increases oxygen consumption by only a tiny percentage, or not at all. The cardiovascular system works under a constant limitation related to maintaining sufficient blood pressure in the system. It is a lot like what happens in an old house when you're taking a shower and somebody turns on the faucet in the kitchen, while someone else flushes the toilet. Pretty soon, the shower becomes a drizzle. To maintain water pressure in the pipes, you can't have too many valves open at once. The same is true in our cardiovascular "pipes". When arm exercise is added to leg exercise, blood flow to the legs actually decreases due to constriction of the leg arteries. This extra blood flow is than available for the arms. The body maintains blood pressure, by controlling how how much each artery is "opened."

During skiing the contribution of the upper body to movement velocity varies from perhaps 10% during the classic diagonal stride to 100% during double poling. During skating uphill (the double dance), the upper body contributes 50% or more of the total force. The endurance capacity of the upper body has always been important to the skier. Today, with the addition of arm-intensive skating techniques, this is even more true. Consequently, there has been a lot of recent research investigating the endurance capacity of the upper body of elite skiers, and its relationship to performance.

Special ergometers have been developed for measuring oxygen consumption during either double poling, or during the alternating arm movements used during the diagonal stride. The devices range from turning a rowing machine on end to highly advanced ergometers that measure force output and movement velocities at each ski pole, while simulating the free-floating movement of the legs. One meaningful comparison to make is the "peak oxygen consumption" achieved during double poling relative to VO2 max measured during uphill treadmill running or roller-skiing. In untrained populations, upper body VO2 peak will only be about 60% of whole body max. In recreational and well trained skiers, the ratio increases to 70 to 85%. Remarkably, in the elite skiers tested in Norway and Sweden (and no doubt other word class skiers from around the globe), this ratio averages 90% and sometimes approaches 95%! I think this is a valuable point for all of us who wish to improve our skiing. One of the areas where most endurance athletes are weak is upper body endurance and power. Among elite skiers, an interesting pattern occurs during the season. Whole body maximal oxygen consumption peaks very early in the seasonal build-up. However, performance peak during the season seems to correspond to the peaking of upper-body endurance capacity, measured as upper body peak VO2.

Muscle Strength
Now we come to a common question: If I weight train, will this improve my endurance capacity? Unpublished observations by Swedish investigators (Ekblom and Berg) indicate that the maximal leg strength is only slightly greater than what is seen in the average person. However, when an endurance test is used in the same movement, such as 50 consecutive leg extensions, the skiers are clearly superior, even compared to most other endurance athletes (rowers may be the exception). What this means is that there is no relationship between maximal leg strength and leg endurance. In practice, elite skiers do little or no general weight training for the lower body. For the older (50+) skier, I would still recommend a lower body weight training program only for the purpose of maintaining muscle mass.
The upper body is a different story. Performance time for a 60 meter sprint double-poling test is strongly related to peak torque produced by the triceps group during strength testing. Faster times are produced by those with greater arm strength. Furthermore, there is preliminary evidence here in Norway that even a short term, intense upper body strength training program results in increased upper body VO2 max and endurance time in standard load testing on a special ski ergometer.

What is going on here?
I have told you repeatedly that whole body maximal oxygen consumption is limited by the heart (along with the endurance capacity of the muscles), not how much muscle or strength you have. So how can strength training improve upper body endurance and peak oxygen consumption? Here is the difference. The total muscle mass of the upper body is not great enough to maximally stress the heart during high intensity work. For example, peak heart rate achieved during a double poling test may be 10-20 beats lower than observed during maximal treadmill running. What this means is that in the unique condition of upper-body only endurance exercise, the heart is no longer the limiting factor, the muscle is. Consquently, dedicated specific training designed to increase skiing specific strength AND endurance can result in more total muscle available during double poling, or other arm-intensive skiing techniques. In the summer training of the elite, it is common to see arm-intensive work like kayaking added to the program in order to help close the endurance gap between the upper and lower body. This is a useful lesson many masters skiers can take away from observing the "big-boys."

Race Day
So far, I have not mentioned the two other major endurance qualities, the lactate threshold, and movement economy. Both are important in skiing, just as in other endurance venues, but the conditions in skiing are pretty special in two ways. First, XC race courses are laid out on terrain that is constantly changing. Uphills, downhills, flat areas, curves etc. Consequently, the athlete is almost never in a condition that could be considerd a steady-state. This makes the lactate threshold somewhat less powerful as a predictor of performance. Second, unlike rowing, running or cycling, the techniques used in skiing vary from moment to moment during the race. This makes a simple investigation of economy impossible. I will discuss these issues futher in the context of data collected under race conditions.

A good cross country race course will have equal proportions of flat, uphill, and downhill segments. It is possible to estimate the energy expenditure during a race by analyzing the heart rate responses during a race plus body core temperatures and lactate levels after the race. The average workload during 5-30k races by both elite men and women is between 90and 90% of VO2 max. This is similar to what we would see in running or cycling time trials. However, unlike these events, during a ski race the climbing portions of the course present tremendous physiological demands. Heart rates of elite skiers reach maximum levels during every significant climbing portion of a course. In fact, some skiers will reach slightly higher heart rates during a race climb than during a mximal treadmill running test. What this tells us is that the top skiers are working at 100% of VO2 max many times during a race. When a down hill segment comes, the heart rate drops, but not as much as you might think. Even though oxygen demand for downhill skiing is much lower, the skier doesn't get much of a break. That heavy oxygen deficit accumulated during the climb is being repaid during the fast downhill, so heart rate may drop only 20 beats. Then we are on a flat. Now heart rate climbs again, to 10-15 beats below max. Analysis of world cup races reveals that the winners make their biggest time gains during the climbs. This is why having the biggest engine is so important for the skier. They guys with the biggest engines climb the fastest, then descend at about the same speed. Bjorn Daehlie does his damage on the hills.

Measurements of lactate threshold using standard laboratory tests reveal what we would expect in the elite. Lactate accumulation during a progressive load test doesn't occur before about 85% of max. The problem is "lactate threshold" seems to have little to do with XC ski racing. Dr. Erik Mygind in Denmark did extensive testing of Swedish and Danish elite skiers under both laboratory and racing conditions. In order to ensure ideal conditions and conditioning, the testing was performed during the racing season, so the athletes were "race ready." For just this reason the Swedish senior elite declined to participate. So the Swedish skiers were national and world-class juniors (19 yrs old). What he discovered was that blood lactate concentration reaches very high values within minutes of the start of a competition and then stays reasonably stable throughout a 40-50 minute race. The lactate levels averaged about 10 mM at the end of the race. In one skier, lactate levels were 14 mM after the first 2.5 km and finished at 18mM 10km later! These findings are supported by previous investigations from other labs in the 60s and 80s.

One could argue that the lactate levels were really rising and falling from minute to minute during the race, and only high at the point in the race were the measurements were made. This is unlikely, because blood lactate levels do not recover on such a short time scale, even using ideal active-rest recovery methods. Even 7 minutes after the race was over, lactate levels were nearly unchanged in all the skiers.

What all this tells us is that "velocity at lactate threshold", or other lactate based measurements have little predictive value in a short to medium length XC ski race. This doesn't mean that increases in lactate threshold percentage aren't an important training adaptation for skiers. It just means that unlike a marathon in running, the LT doesn't set the speed limit for the athlete. Both the winners and the losers are tolerating very high lactate levels throughout the race. The capacity to race at such high average lactate levels is probably also a training adaptation. One study in skiers who were untrained for racing measured lactate levels after a 10km race and found values of only 5-7 mM Blood samples were not taken during the race in this study.

Economy and Skiing Technique(s)
Now we come to another unique aspect of XC skiing. There are Many different ways to go from Point A to Point B, even on the flats: Diagonal stride, kick double-pole, double pole, marathon skate, V1 skate, double skate with-out poling, to name just the flat ground techniques. There is no simple answer to questions about skiing economy differences among competitors.

Skating vs Classic
The reason we now have "freestyle" races and "classic" races is that without this distinction, everyone would be skating every race, and classic skiing would eventually disappear into the ranks of the wilderness trail blazers. Skating is faster, plain and simple. Depending on temperature and snow conditions, skating races are 5-15% faster over the same distance. In very wet snow or extreme cold conditions, the difference in speed between skating and classic decreases. As a rule of thumb, we can say that skating is 10% faster for a given group of athletes. Why?
There have been several hypotheses presented and tested:
1. Skating allows the athlete to achieve a higher aerobic capacity compared to classic. In other words, perhaps skating creates a bigger work capacity.
2. Skating allows more of the work output of skiing to be delivered to the skis and forward progress.
3. Skating results in decreased frictional resistance.
Here is what the studies have shown so far. First regarding possibility #1. This is not correct. Several studies have indicated no difference in VO2 max when measured in the same athlete performing either skating or classic techniques. Of course, this could be a different story if the skier is technically weak in one or the other technique. However, at the top levels, this is rarely the case. Even as early as 1986, a study of junior world class skiers demonstrated that the race placement in classic and skating races was very similar. Watching the world cup season also indicates the same. The same skiers are dominating the top 10 places in both skating and classic races.

Possiblity #2 seems to play a role. On flat terrain at a constant speed, skating (V1) has been shown to require 10% less oxygen compared to the same speed via the diagonal stride. Heart rate, perceived exertion and lactate accumulation are all lower at similar intensities while skating compared to diagonal striding. One explanation for this seems to be that the velocity changes by the limbs are much smaller in the skating technique. Skating results in a longer force development period for the limbs. Minimizing repetitive acceleration and deceleration of the limbs increases movement economy.

Finally, regarding #3, the elimination of the grip wax during skating results in a small but significant decrease in friction, and increased speed for the same effort. Since skating results in a slightly lower body position, air resistance may also be a little lower during skating, but data to support this contention is lacking.

There ARE exceptions to the general trend of skating based techniques being more economical than classic techniques. The classic double pole technique is even more economical than skating on level ground. (Double poling on ski-skates is the MOST economical technique). However, since double poling involves a smaller muscle mass to generate the work, the strain on the muscles is higher and so is the perceived exertion. If double poling is the most economical, why not use this style all the time? Double poling does not allow the athlete to use his/her maximal work capacity. Being efficient is not effective if too little power is generated! So when push comes to shove, and you are climbing a hill, the prize still goes to the guy with the biggest engine and economy goes out the window!

The LEAST economical style is the classic diagonal stride. A study by Hoffman and Cliffard (1990) measured several physiological variables during skiing at a constant speed while using different techniques on level ground. The oxygen cost was 33% higher during the diagonal stride compared to double poling on classic skis. This isn't hard to believe when you consider how much limb movement is going on for a given amount of forward progress. Consequently, this technique is most frequently seen during hill climbing (in Classic races), when distributing the high workload over the largest muscle mass possible is important. The V1 technique required about 15% more energy than double poling but 15% less than diagonal striding.

Can Technique Decide a Race?
Well, sure it can. And, there are significant differences in skiing efficiency between the elite and the recreational skier at a given speed. The elite are technically superior. But, who cares about that comparison. The world class guys and gals could ski with no poles and kick our butts (I once saw Thomas Alsgaard finish a major relay with one pole and a broken hand. He was still skiing pretty darned fast!). What I am really getting at is "how big are the technique differences among the best skiers?" Again, this is a tough question. Part of racing efficiency probably involves optimally timing technique selection at different stages along the course. You can't measure that in a lab test. Several studies suggest that you can find national class skiers who are no better technically than good recreational skiers. There is pretty good variation at this level. However, if you just look at international class skiers, the variation gets much smaller (7% in one study). At this level, efficiency is not a strong predictor of performance placing. Inefficient skiers never make it to the international level. Again, we go back to who has the biggest performance engine. A good example of this was 8-yime Olympic gold medalist Bjorn Daehlie. Among those with discerning eyes for this stuff, they would tell you that he was not necessarily the smoothest skier on the list. His double poling stood out as frenetic. And, he hated to get into a sprint situation, because that was a major weakness for him. However, he rarely NEEDED to sprint at the end, and he WON and WON. Why? A 90 ml/min/kg VO2 max, a love of training, and an unquenchable competitive spirit. When it's all said and done, that’s ALL you need to win in World Class Cross country skiing!

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