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!
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 medalsists 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 enteresting 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 heigh in m2)
than non-athletic women of the same age.
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 compostiion in the vastus lateralis
(a thigh muscle that is often biopsied in athetes) 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.
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 maintainance 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.
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 chnage
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.
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 outputs, 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!
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
trainied 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 medalists in the 60s, 70s, and 80s (from Ulf Bergh
and Artur Fosberg, 1992).
|
|
Maximal Oxygen Consumption |
||
|
Body mass (kg) |
liters/min |
ml/min/kg |
ml/min/kg2/3 |
1960s (n=4) |
68 |
5.56 |
82 |
335 |
1970s (n=4) |
72 |
6.14 |
84.9 |
353 |
1980s (n=4) |
73 |
6.33 |
87.2 |
363 |
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
reigning 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!
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. Remember, 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 sufficent 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 buildup.
However, performance peak during the season seems to correspond to the peaking
of upper-body endurance capacity, measured as upper body peak VO2.
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, elete 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.
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."
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.