Crosscountry Skiing

Chapter 51
Cross-country Skiing
BJORN EKBLOM AND ULF BERGH
Introduction
Recreational touring and racing skiing are
common activities in many countries today but
cross-country skiing has been practised for
several thousand years in northern countries.
Ski-racing equipment has changed considerably,
and the dimensions of the ski have changed from
being 3 m long, 10 cm wide, and weighing 2–3 kg
at the beginning of this century to 2 m, 4 cm and
about 0.5 kg in the modern era. There are specialized skis for classic and free-style ski racing. The
courses on which competition takes place have
changed as well. Now grooming machines are
used at least for the more advanced levels of
competition, which makes the tracks very hard
and durable, thus making the conditions more
equal for all competitors. It must be emphasized
that racing conditions vary due to changes in
snow and weather conditions.
Skiing competitions are classified into two
different styles: classic skiing and free style.
Three main techniques are used in classic skiing:
double pole, kick double pole, and diagonal. In
free-style events, skating techniques dominate:
these are characterized by leg movements similar
to those in ice-skating combined with various
forms of double poling.
Elite skiing competitions are performed
over distances ranging from 5 to 90 km. In the
Olympic Games and the world championships,
the distances range from 5 to 30 km in female
events and from 10 to 50 km for males. Relay
races are 4 ¥ 5 km and 4 ¥ 10 km for women and
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men, respectively. At present, individual races
last 12–90 min for women and 22–140 min for
men.
Modern rules stipulate that the courses for
international races must in length be equally
divided into uphill, downhill, and level skiing.
Since the racing speed differs greatly among
these three parts of the course, the time spent in
uphill skiing is more than half of the total racing
time, while downhill skiing time correspondingly occupies less than 10%. Even so, downhill
skiing ability is important. A fall in a downhill
part causes loss of speed and rhythm in skiing.
Compared to the winner, the time ‘lost’ is greatest in uphill and level skiing. The exact relation
between time spent in the different parts is, of
course, dependent on many factors such as level
of competition and type of terrain.
Characteristics of elite skiers
International elite competitors are often relatively old — average ages are reported to be 27
and 29 years for females and males, respectively,
indicating that it takes years of training to
achieve that level of performance. These elite
athletes do not differ very much in body size,
body weight and appearance from other nonobese persons, having relatively little body fat
but not to the extreme degree observed in some
endurance sports.
The leg muscles of elite cross-country skiers
have been found to consist of predominantly
slow-twitch fibres but the variability is consider-
cross-country skiing
able. A similar pattern has been found in the
deltoid muscle, where there is an even greater
variation. Young elite skiers have been found to
have a lower percentage of slow-twitch fibres
than older skiers (Rusko 1976), which can be an
effect either of training or further selection. The
predominance of slow-twitch fibres is logical,
since the metabolism in cross-country skiing is
predominantly aerobic and slow-twitch fibres
have a high oxidative capacity. Furthermore, the
number of capillaries is greater around a slowtwitch than a fast-twitch fibre. This enhances the
transportation of gases and nutrients between
blood and muscle cells, allowing for an effective
aerobic metabolism. All these findings are consistent with the hypothesis that physical training
for many years increases local aerobic metabolic
capacity (Saltin & Gollnick 1983).
However, the physiological variable that most
evidently distinguishes elite cross-country skiers
from the average person as well as less successful
cross-country skiers is the maximum oxygen
uptake, expressed as litres per minute as well as
in relation to body size (ml · min–1 · kg–1 body
mass). Over the decades, reports have confirmed
that elite cross-country skiers have, without
exceptions, very high values (Table 51.1). Worldclass skiers have displayed a higher maximum
oxygen uptake than less successful skiers (Bergh
1987; Ingjer 1991). Skiers of junior age display
lower values than adults (Rusko 1976; Bergh
& Forsberg 1992). These differences are also
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reflected in differences in racing speed and, thus,
racing success.
The power facilitated by the metabolism is
necessary for moving the body mass, and more
power increases speed. On the other hand, a
higher body mass demands more power at a
given speed. Thus, there is a need to compensate
for differences in body mass, otherwise it is not
possible to compare the values obtained in different skiers. Traditionally, such compensations
have been made by means of dividing maximum
oxygen uptake by body mass. However, it has
been demonstrated that the power needed to ski
at a given speed on level terrain increases less
than proportional to body mass. Thus, it is not
logical to divide oxygen uptake by body mass in
order to equate heavy and light skiers. Therefore,
dimensional analysis and empirical findings
suggest that a division by body mass raised to
the second or third power may be more valid for
cross-country skiing (Bergh 1987). This is supported by a study of Ingjer (1991), which demonstrated that world-class skiers differed
significantly from medium-class and less successful skiers if the maximum oxygen uptake
was divided by body mass raised to the second
or third power, whereas a division by body mass
did not reveal any significant difference. Thus, it
seems logical to relate maximum oxygen uptake
body mass raised to the second or third power if
the purpose is to predict the capacity for crosscounty skiing.
Table 51.1 Different measurements of maximum oxygen uptake in elite cross-country male skiers.
O2max. uptake
(l · min-1)
O2max. uptake
(ml · kg-1 · min-1)
Mean
SD
Mean
SD
Reference
5.5
5.6
5.5
6.5
6.7
0.2
0.3
0.2
0.5
0.6
80.1
82.5
75
83.8
87.0
1.4
1.5
2.7
6.4
6.9
Åstrand (1955)
Saltin & Åstrand (1967)
Hanson (1973)
Bergh (1987)
Bergh & Forsberg (1992)
Although all groups can be characterized as elite skiers, there were differences in the performance level both within
and between groups.
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sport-specific nutrition
The high values of maximum oxygen uptake
in elite cross-country skiers are the result of
a high maximum cardiac output elicited by a
large stroke volume. Maximum cardiac output
over 40 l · min–1 and stroke volumes over 200 ml
have been measured in skiers with maximum
oxygen uptake values over 6 l · min–1 (Ekblom &
Hermansen 1968). The maximum heart rates
and the arteriovenous differences were close to
values obtained in less successful athletes and
non-athletes and cannot account for the observed
differences between the different groups of
individuals. Blood volume is also high in
these athletes, while haemoglobin concentrations are within the normal ranges of nonathletes and less successful athletes (Ekblom &
Hermansen 1968).
The information on muscular strength of the
elite competitor is not very extensive. Available
data indicate that the maximum strength of the
legs is only slightly greater than that of the
average person. However, in endurance tests —
such as 50 consecutive knee joint extensions —
skiers show superior endurance values to
those of most other endurance athletes. The arm
muscle strength for poling and, thus, skiing performance, is also of the utmost importance.
Ski racing
Energy expenditure
The skier has to expend power in order to move
forward. This power is used for:
1 overcoming friction between ski and snow;
2 elevating the body mass in uphill skiing and
for each stride during level skiing;
3 accelerating the different body segments and
the centre of mass; and
4 overcoming air resistance.
The relative importance of these factors for
energy expenditure during skiing is dependent
on several factors, including body composition,
type of skiing technique, level of coordination
and technique, type of terrain, snow conditions,
and racing speed. Hence, quantitative information will only be valid under specific conditions.
It is, however, safe to state that on uphill terrain
the cost of elevating the centre of mass accounts
for the major part of the energy expenditure. In
downhill sections, the main resistances are the
friction between the ski and the snow and the air
resistance. The power to sustain this power
expenditure comes from the metabolism except
in downhill terrain. Hence, skiing speed will
depend on the power producing capacity of the
metabolism.
As stated above, the capacity and effectiveness
of the aerobic energy system is the most important factor for physical performance during ski
racing, indicating that the central circulation and
the regulation of its distribution are of primary
importance for skiing capacity. Maximal uphill
skiing produces higher oxygen uptake than
maximal running (Stromme et al. 1977). There is
no difference in maximum oxygen uptake
between maximal uphill skiing using the classic
or free-style technique (Bergh & Forsberg 1991).
Thus, the muscle mass used during maximal
skiing has a metabolic potential which exceeds
the transport capacity of oxygen of the central
circulation. Any variation in the amount of
oxygen from the heart to the peripheral muscles
will undoubtedly influence skiing performance.
During uphill skiing, the heart rate is close to or
even exceeds peak heart rate obtained during
conventional all-out maximal running on a
treadmill. During the downhill parts, the heart
rate is some 20 beats · min–1 below maximum,
mainly because the strain on the circulation is
still high. During level skiing, the heart rate is on
the average 10–15 beats · min–1 below maximum
and, during longer races, such as the 50 km, the
heart rate is on the average somewhat lower than
in the shorter races on the same parts of the track
due to the lower average speed in the longer race.
Training
Important characteristics of cross-country skiing
are as follows.
1 Metabolism is mainly aerobic.
2 Oxygen uptake can be taxed maximally.
3 Certain techniques cannot elicit maximum
cross-country skiing
oxygen uptake and, in these cases, the attainable
level can vary considerably between individuals.
4 The technique has to be learned.
5 The duration of races may be such that the
glycogen stores become emptied.
6 The training necessary to obtain the performance level of elite competitors is such that only
an extremely well-trained individual can endure
this.
As a consequence of these characteristics, the
training should contain practices that: (i) challenge the cardiovascular system considerably, (ii)
activate all of the muscles used during competetive skiing, (iii) improve the technical skill, and
(iv) last for periods of up to several hours.
Moreover, the amount of training should be
increased gradually during the course of each
year and from one year to the next. Otherwise,
there is a considerable risk of overtraining and
overuse injuries.
Running and roller-skiing can elicit approximately the same oxygen uptake as that achieved
in all-out skiing (Bergh 1982). Ski-walking
(walking up a steep hill using poles to imitate
skiing) and cross-country skiing have been
found to produce slightly higher levels of oxygen
uptake than running in individuals trained for
cross-country skiing (Hermansen 1973; Stromme
et al. 1977). Roller-skiing has an advantage over
running in regard to training of the upper body:
the activity patterns of the muscles are similar in
skiing on snow and roller-skiing. This is advantageous for the development of the local aerobic
power. This is important because the muscles
involved in poling must have endurance since
this activity contributes significantly to performance in skiing. Moreover, it has been demonstrated that individuals who can attain a
relatively high oxygen uptake during upperbody exercise benefit in regard to the maximum
oxygen uptake that can be attained during combined arm and leg exercise (Bergh et al. 1976).
Elite skiers rarely use barbells or other resistance devices in order to improve maximum
muscle strength. However, repeated doublepoling on roller-skis at maximal speed for 10–30 s
is used as strength training. This exercise is a
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minor part of the training (3–5% of the time
during summer and early fall).
Skiing technique should be learned by skiing,
because other exercises, e.g. roller-skiing and
ski-walking, do not display the same muscular
activity patterns as in skiing, judging from
electromyographic recordings. In general, it is
preferable to concentrate on technique with
youngsters because they learn more easily than
adults. In total, elite male and female skiers train
about 650–750 and 500–700 h · year–1, respectively. In addition, they normally compete in
about 35–45 ski races · year–1.
Metabolic energy yield
In order to evaluate the average metabolic
energy yield during a ski race, heart-rate recordings during the race and blood lactate and core
temperature measurements after the race have
been used. Using these measures, it can be estimated that the average energy expenditure
during ski racing between 5 and 30 km is in the
range of 90–95% of maximum oxygen uptake.
During the longer ski races, it is some 5–10%
lower. There are no reasons to believe that
genders differ in this respect.
Combining this information on fractional
utilization of oxygen with data on maximum
oxygen uptake of elite skiers, racing metabolic
cost can be estimated. Such calculations indicate
that the metabolic rate of an average male elite
skier is about 1.5–2 kW during the shorter races.
During longer races (50 km and longer), the
metabolic rate is about 10% lower. The total
energy yield for a normal 15-km race is about
4–5 MJ (950–1200 kcal) and for a 50-km race about
13–15 MJ (3100–3600 kcal). Corresponding calculations for females indicate that they use about
30% less energy than males for a given distance,
which is due to the lower maximum aerobic
power and body mass in the females.
Heat balance
Since cross-country skiing is often performed in
cold climates, problems related to cold injuries,
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sport-specific nutrition
breathing problems, and hypothermia might be
expected. For the body as a whole, the metabolic
heat production during ski racing is usually
greater than the heat loss due to convection, conduction and radiation. Therefore, the skier must
sweat to maintain heat balance. During ski
racing, the weight loss, mainly due to water loss,
might be some 2–3% of body mass during 15- to
30-km races. This will undoubtedly impair performance. Therefore, fluid replacement during
races longer than 15–20 km is needed.
Although heat production is high, cold injuries
to peripheral parts of the body, such as fingers,
toes, nosetip and ears, are not uncommon during
cold weather, since the wind velocity is high,
especially during long downhill segments of
a course. Furthermore, pulmonary ventilation
may be at least 100–150 l · min–1 and, in many
cases or parts of a track, over 200 l · min–1. This
puts large demands on the airways, since cold air
is very dry and must be heated and saturated
with water before it reaches the alveolae. Many
skiers experience coughing problems during
races and after exercise. Therefore, many top elite
skiers use antiasthmatic medications. To avoid
local cold injuries and breathing problems, competitions and hard training sessions should be
avoided at temperatures below –20°C.
Total energy yield
The energy cost of cross-country skiing is high, as
mentioned earlier. During the preparation or
main training part of the year, which often
includes two training sessions per day, the
estimated total energy turnover is some 20–
25 MJ · day–1 (4800–6000 kcal · day–1). During
training camps, it can be 4–8 MJ (950–1900 kcal)
higher. The total energy turnover for a 15- and
50-km race is about 4–5 MJ (950–1200 kcal) and
13–15 MJ (3100–3600 kcal), respectively.
One of the main nutritional problems is to
cover this high energy demand. In many cases,
this problem is solved by having three main
meals — breakfast, lunch and dinner — and,
added to that, small meals after each training
session. A carbohydrate-rich meal just before
bedtime facilitates
glycogen.
restoring
the
muscle
Quality of the meal
The glycogen concentration in activated arm and
leg muscles is low or almost empty in many
muscle cells at the end of a race or a long training
session. Thus, a meal rich in carbohydrates is an
essential part of an elite cross-country skier’s
diet. The post-training meal is especially important, since the rate of glycogen resynthesis and
accumulation in the muscles seems to be faster
when a high-carbohydrate meal is consumed just
after the exercise. It is a general experience that
most skiers are not hungry immediately after a
race, but failure to eat at this time may delay
recovery and limit the training load. Of interest
for the hard training cross-country skier, therefore, is the observation that postexercise muscle
glycogen concentration can be enhanced above
normal with a carbohydrate–protein supplement
as a result of the interaction of carbohydrate and
protein on insulin secretion.
During the racing season, there may be specific
nutritional problems at hand. Racing and prolonged hard training sessions may damage the
muscle cells as indicated by a leakage of protein
and other molecules from slow-twitch fibres. If
this occurs, the rate of glycogen resynthesis may
be reduced after exhaustive exercise. Therefore,
it might not always be possible to fully replenish
glycogen stores within 24–48 h after hard races
and training sessions.
Rehydration during skiing
During races, skiers may sweat a lot even in a
cold climate. The body mass loss for a 15- and 50km race may be in the range of 2–4% of initial
value of body mass. It is a well-known fact that
this can impair physical performance. Rehydration is therefore of great importance for counteracting the negative influence of the dehydration.
However, not only rehydration is of importance.
During prolonged exercise, as in cross-country
skiing, a carbohydrate intake during prolonged
cross-country skiing
exercise will also enhance performance. The
mechanism is not clear but it is most likely
that the glucose uptake from the bloodstream
may contribute considerably to the aerobic
metabolism.
Therefore, most skiers consume some 100–
200 ml of a 5–10% carbohydrate drink about each
10–15 min in races with a duration longer than 1
h. The intake is of the same order of magnitude as
the minimum of 40–60 g · h–1 suggested by Coyle
(1991). However, some skiers also take in a solution with up to 25–30% of carbohydrates. The
reason for this is that the net uptake of glucose is
higher in such a solution than with a traditional
5–8% concentration, although the water uptake
is, of course, less.
During training camps at altitude, the water
turnover is increased because of the increased
urine output and the high respiratory water
losses. Some elite skiers drink up to 8–10 l · day–1
in order to compensate for the increased rate of
dehydration during altitude training.
Vitamins and minerals
Vitamins and minerals are essential nutrients for
optimal performance. It is well known that deficiencies impair general health and human functions but, in present-day society, obvious vitamin
deficiencies are rare. Cross-country skiers have a
high energy intake. Since the amount of nutrient
intake mainly follows energy intake when the
athlete consumes ‘normal’ food (Blixt 1965),
there is a general agreement that the risk of an
inadequate nutrient intake is low in athletes with
high total energy intakes.
Conclusion
Cross-country skiing is dynamic exercise involving a large muscle mass. There are many different
skiing techniques. The energy yield is mainly
aerobic and the cardiovascular system can be
taxed maximally during skiing. Therefore, crosscountry skiing is effective in regard to endurance
training. The elite skier is characterized by an
extremely high maximum oxygen uptake and
661
the skeletal muscles contain predominantly
slow-twitch fibres. Body size is not very different
from the average person of corresponding
gender and age. Training is mainly performed by
skiing, roller-skiing and running. The energy
demand is very high, and in longer races, the
glycogen stores may be emptied. Proper rehydration procedures during races are of greatest
importance.
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