Physiology of Ski Mountaineering Racing

856 Training & Testing Physiology of Ski Mountaineering Racing Authors S. Duc1, J. Cassirame2, F. Durand1 Affiliations 1 Key words ▶ ski mountaine...
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856 Training & Testing

Physiology of Ski Mountaineering Racing

Authors

S. Duc1, J. Cassirame2, F. Durand1

Affiliations

1

Key words ▶ ski mountaineering ● ▶ heart rate ● ▶ ventilatory threshold ● ▶ performance ●

Abstract ▼

Bibliography DOI http://dx.doi.org/ 10.1055/s-0031-1279721 Published online: October 19, 2011 Int J Sports Med 2011; 32: 856–863 © Georg Thieme Verlag KG Stuttgart · New York ISSN 0172-4622 Correspondence Sébastien Duc University of Perpignan – Departement de STAPS Font Romeu Laboratory of Performance, Health & Altitude l’ermitage Font Romeu 66120 France Tel.: + 33/468/30 01 51 Fax: + 33/468/30 80 76 [email protected]

The aim of this study was to quantify and describe the exercise intensity of ski mountaineering racing, and to identify the best physiological predictors of ski mountaineering racing. Before participating in the race in which heart rate (HR) and speed were continuously recorded, 10 trained ski-mountaineers performed a field maximal test to determine the first ventilatory threshold (VT1) and the respiratory compensation threshold (RCT) in order to establish 3 exercise intensity zones (Z1: below VT1, Z2: between VT1 and RCT, and Z3: above RCT). Energy cost (EC) of each subject was estimated on the HR/V˙ O2 relationship obtained during the field maximal test. VT1 and RCT threshold were equal to 84.2 ± 3.0 and 94.5 ± 1.7 % of HRmax. Race time was significantly correlated with V˙ O2max (r = − 0.87), VT1 (r = − 0.82) and RCT (r = − 0.85) expressed for body mass unit. The mean race time and the mean HR were 101 ± 11 min and 93.4 ± 1.8 % of HRmax.

Introduction ▼ Ski-mountaineering racing is one of the most demanding winter sports. It requires a wide variety of athletic skills because a ski-mountaineering race consists of several steep ascents and downhill sections. Two trails are made during uphill so as not to penalize the leading racers. Therefore, besides climbing as fast as possible, ski mountaineers have to gain a smooth technical efficiency to glide with skis and to descend in all snow conditions, because most of the descents take place on rugged “off-piste” terrain that has not been groomed or prepared. During uphill, ski mountaineers use adhesive skins under skis and rear bindings are free whereas they remove the adhesive skins and lock the rear bindings during downhill. They are also well versed in alpine

Duc S et al. Physiology of Ski Mountaineering Racing. Int J Sports Med 2011; 32: 856–863

The % race time spent in Z1, Z2 and Z3, were 7.0 ± 4.8, 51.3 ± 4.7 and 42.0 ± 6.5 %, respectively. The mean value of EC during the two uphill of the race was 14.3 ± 2.6 J.kg − 1.m − 1. HR and speed decreased significantly during the second uphill whereas EC increased significantly by ~15 %. Data obtained in the present study represent the first qualitative description of physiology demand of ski mountaineering racing. The long period of time spent just below and above RCT suggest that ski-mountaineering can be viewed as one of the most strenuous endurance sports like cross-country skiing, running and off-road biking. In addition to high aerobic capacities, body mass seems to appear as a key factor given that performance in ski mountaineering is strongly correlated to relative common physiological variables. The changes of HR, speed and EC during the second uphill, which indicate the prevalence of fatigue, confirm the exhaustive character of ski mountaineering.

climbing techniques because in some circumstances, athletes must attach their skis to their rucksack and put cramps under shoes when they climb snow corridors or cross ridges. Ski mountaineering has 3 major kinds of races: single race, team race and vertical race. Contrary to single and team races, the vertical race consists of just one continuous uphill, without downhill sections. According to the rules for organising international ski mountaineering competitions (ISMF 2009; www.ismf-ski.org), the positive difference in height of a single race (similar to the present study) is around 1 600 m ( ± 10 %) with 2 or 3 uphill sections and at least 85 % of the track must be raced on skis. The time of a single race is generally between 1.5 to 2.5 h for the first racers, which is similar to some offroad cycling races (XO race), cross country skiing

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accepted after revision April 20, 2011

University of Perpignan – Departement de STAPS Font Romeu, Laboratory of Performance, Health & Altitude, Font Romeu, France 2 Laboratory of Sport Sciences, University of Franche-Comté, UFR STAPS Besançon, France

2 120 2 100 2 080 2 060 2 040 2 020 2 000 1 980 1 960 1 940 1 920

0

200

400

600

800

1 000

1 200

Length (m) Fig. 1

Track profile of field maximal test.

Materials and Methods ▼ The study has been performed in accordance with the ethical standards of IJSM [9].

Subjects 10 competitive (national level) ski mountaineers (6 men and 4 women) volunteered to participate in this study. Their anthropometric features (mean ± SD) were: age 32 ± 7 years, body mass 61 ± 8 kg, height 169 ± 6 cm. All subjects had regularly trained for ski mountaineering competitions for at least 2 years prior to the study. Their average training was 11 ± 1 h.wk − 1 (~2 h per training session). The protocol was approved by the local ethical committee and all subjects gave written informed consent.

Methods Field maximal test During the week prior to the race each subject performed a field ski-mountaineering incremental test to exhaustion on a groomed Alpine skiing track (which was localised in “Font Romeu” Pyrénées Orientales, France) in order to determine maximal oxygen uptake (V˙ O2max), maximal heart rate (HRmax), first ventilatory threshold (VT1) and respiratory compensation threshold (RCT). The weather during the test week was sunny and the temperature between − 5 and 5 ° C. The characteristics of the track were as follow: length 1 150 m, ascent 155 m, mean ▶ Fig. 1 shows gradient 14 % (range : 9–21 %) and start 1 941 m. ● the profile of the track. After a self warm-up of ~15 min, all the subjects began the test at 3 km.h − 1 and speed was increased every minute by 0.5 km.h − 1 until subject exhaustion. To be sure that the subject sustained the fixed speed, we marked the track with flags. The distance between 2 flags matched the distance that had to be ridden at the fixed speed during 30 s. For example, at 3 km.h − 1, the distance ride during 30 s was 25 m. Subjects needed to be localised near a flag ( ± 3 m) each time they received a sound signal by walkie-talkie. The test was stopped when the subject became exhausted or when they could not catch up on their delay.

The Race Data were collected during the “Trace Catalane 2007” race (i. e., an international single ski-mountaineering race classified as a World Cup) which was held on January 7, in “Les Angles” Pyrénées Orientales, France. The race started at 7:30 (GMT + 1:00). The weather during the race was sunny and the temperature

Duc S et al. Physiology of Ski Mountaineering Racing. Int J Sports Med 2011; 32: 856–863

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race (40–50 km), time-trial cycling race (50–60 km) or running race (marathon). Ski mountaineering racing is viewed by athletes and practisers as one of the most exhausting endurance sports, like off-road cycling, cross-country skiing and running, but none of the scientific data up to now can confirm this feeling. Only 4 physiological investigations have been realized in skimountaineering in spite of the growing practice of this winter sport in mountain regions. Bigard et al. [2, 3] showed that high protein intake during successive stage ski mountaineering minimized the exercise-induced decrease in serum branched-chain amino acid (BCAA) whereas Durand et al. [5] have focused on prevalence of exercise-induced bronchoconstriction in ski mountaineers racers. Recently, Tosi et al. [23] studied the effect of speed and ankle loading on energy cost (EC) of uphill ski mountaineering determined from the ratio between net oxygen consumption for mass unit and speed. Seven male recreational ski mountaineers with an average weight of 7 kg for the equipment (ski, boots, bindings, skins, portable gaz analyser, GPS monitor.), were asked to climb a 500 m uphill on packed snow (mean gradient of 21 %, altitude about 1 600 m), at 3 different speed levels (0.93 ± 0.15, 1.07 ± 0.17 and 1.25 ± 0.16 m.s − 1) and with 3 different additional weight bands on their ankles (0.5, 1, 2 kg, at the same preferred speed, i.e., 1.07 m.s − 1). The authors showed that a 16.8 % increase of speed enhanced a significant 5.4 % rise of EC whereas a 1.25 % increase of total weight (1 kg added to an 80 kg equipped subject) resulted in an increment of about 2–3 % in EC. They concluded that the EC of ski mountaineering depends slightly on ankle loading and speed, at least in the range considered in this study. Nevertheless, as it was suggested by the authors, total energy expenditure in ski mountaineering competitions would be probably more affected by speeds and load increments, which could justify the increasing technological effort during the last years for reducing the weight of the ski equipment as much as possible. Despite the great interest in these studies to extend our knowledge about exercise-induced bronchoconstriction prevalence, protein metabolism and energy cost in ski mountaineering, as far as we know, no studies have been interested in the physiological demand of ski-mountaineering, unlike off-road cycling [13, 22], long distance running [6], and cross-country skiing [20]. During these studies, heart rate (HR), power output and/or oxygen uptake, were used to quantify the intensity of sport-specified competition. Such data, like HR and power output, are commonly used by coaches and athletes to control the training sessions. The knowledge of time spent in intensity zones (HR exercise profile), i. e., below the first ventilatory threshold (VT1), between VT1 and the respiratory compensation threshold (RCT), and above RCT, assessed by HR recording, can be useful to match HR training zone to the predominant HR race zone, and thus increasing the effectiveness of a training program. The aim of this study was to quantify and describe the exercise intensity of a single ski mountaineering race by monitoring the heart rate (HR) and speed. We hypothesized that 1) physiological demand of a ski mountaineering race, assessed by mean % HRmax sustained across time and HR exercise profile, is similar to offroad cycling, cross-country skiing and running races; 2) physiological measures relative to mass are better predictors of ski mountaineering performance than absolute measures and 3) energy cost in uphill racing is higher compared to other locomotion sport.

Altitude (m)

Training & Testing 857

858 Training & Testing

uphill first second downhill first second

Length

Altitude

Mean

Maximal

(m)

difference (m)

gradient ( %)

gradient ( %)

3 700 3 370

660 655

17.8 19.4

35 35

3 800 3 460

− 645 − 520

16.9 15.0

36 36

between − 5 and 0 ° C. Due to the low cover of snow, the original scheduled racetrack was modified. The positive and negative altitude differences (total ascents and total descents) decreased from 1 622 to 1 342 m and from 2 020 to 1 187 m. The maximal altitude was also lower: 2 331 vs. 2 921 m. The distance was about 15 km. There were 2 uphill sections and 2 downhill sections. Characteristics of uphill and downhill profile are presented ▶ Table 1. The entire race was performed on groomed alpine in ● skiing tracks, except 2 sections in which ski-mountaineers had to cross on carrying skis. The first section was an uphill section (length: ~500 m, altitude difference: 173 m) whereas the second section was a flat section (length: ~650 m, altitude difference: < 10 m). Each ski-mountaineer had to change 7 times the configuration of their skis and boots (uphill/downhill): after the first uphill, after the first downhill, after the second uphill and before and after each section on foot. The uphill foot section was made with cramps and skis attached to the rucksack. During the race, each subject drank approximately 0.5 l of personal carbohydrate solution with a 50–70 g/l CHO concentration. None of the subjects took solid food throughout the race.

Materials Each subject performed the field maximal test and the race with his own ski equipment (skis, skins, boots, poles, bodysuit and gloves). Mean additional material weight was ~4.5 kg.

Field maximal test During the maximal test, a K4b2 breath-by-breath telemetric and portable gas analyser (Cosmed, Rome, Italy) was used to collect the metabolic data: oxygen uptake (V˙ O2, l · min − 1), carbon dioxide output (V˙ CO2, l · min − 1), respiratory exchange ratio (RER =V˙ CO2/V˙ O2), minute ventilation (V˙ E, l · min − 1), tidal volume (V T, l), ventilatory equivalents for oxygen and carbon dioxide (V˙ E/V˙ O2 and V˙ E/V˙ CO2, respectively) and breathing frequency (BF, breaths · min − 1). The pneumotachograph and analysers of the Cosmed K4b2 system were calibrated before every test session according to the manufacturer’s specifications, using respectively a 3-l syringe and a gas bottle of known O2 and CO2 concentrations (16 and 5 %, respectively).

The race During the race, and according to the safety rules of the ISMF, each subject had to wear a helmet and a rucksack which contained a cramps set, a probe, a snow shovel, an avalanche transceiver, sweatpants, and snow jacket. The weight of the rucksack was about 2.5 kg. Each subject was also equipped during the race and also during the field maximal test with a chest belt (Polar Electro, Kempele, Finland) and a GPS device (B100, FRWD Technologies, Finland) to collect heart rate (HR, beats.min − 1) and position data.

Data analysis Field maximal test V˙ O2max determination The highest mean V˙ O2 and HR values obtained during the increment test for 15 s were defined respectively as the V˙ O2max and the HRmax. Criterions used to verify that the test was performed at maximal subject’s capacity were the exhaustion of subjects, a RER superior to 1.1, a plateau of V˙ O2 and the incapacity of the subject to maintain the speed desired. All subjects achieved these criterions.

VT1 and RCT determination The first ventilatory threshold (VT1) was determined by visual analysis of the breakpoints of ventilatory equivalents for oxygen (V˙ E/V˙ O2) and carbon dioxide (V˙ E/V˙ CO2) and minute ventilation (V˙ E) changes over time with an increase in V˙ E/V˙ O2 without increase in V˙ E/V˙ CO2. The respiratory compensation threshold (RCT) was determined by visual analysis of the breakpoints of ventilatory equivalents for oxygen (V˙ E/V˙ O2) and carbon dioxide (V˙ E/V˙ CO2) and minute ventilation (V˙ E) changes over time with an increase in both V˙ E/V˙ O2 and V˙ E/V˙ CO2 [18, 24]. Two blinded and experienced investigators proposed individually the heart rate and oxygen uptake corresponding to the VT1 and RCT estimated with ventilation measurement. In the case of a difference below 10 bpm between the 2 observers, the mean value between the 2 observers was considered. When the difference exceeded 10 bpm, a third observer was requested and the mean of the 2 closed observations was considered. Intensity zone determination From these data, 3 intensity zones were established to describe the exercise intensity profile of ski-mountaineering race: easy zone (Z1) for intensity below HR corresponding to VT1, moderate zone (Z2) for intensity between HRs corresponding to VT1 and RCT, and hard zone (Z3) for intensity above HR corresponding to RCT [6, 13].

The race Speed and HR data Linear speed (km.h − 1) and heart rate (HR) data were recorded every second throughout the race and were downloaded on a portable PC using the specific software and subsequently analysed. All data were averaged every 5 s. Climbing speed (km.h − 1) was also calculated every 5 s. Mean and maximal linear speed, climbing speed and HR were determined for each uphill and downhill section. The amount of race time (in % total race time) spent below VT1 (Z1), between VT1 and RCT (Z2) and above RCT (Z3) was computed for each subject using HR data. Energy cost estimation Energy cost (EC) of uphill ski-mountaineering during the race was determined from the HR-V˙ O2 relationship obtained during the field maximal test [1]. Mean HR and ventilatory equivalent of 5 kcal.lO2 − 1 were used to evaluate the individual level of energy consumption for each subject. EC (J.kg − 1.m − 1) was calculated by the average value of energy consumption (J) for mass unit (body mass plus race equipment (~4.5 kg) divided by the uphill average linear speed (m.s − 1).

Statistical analysis Descriptive statistics (mean, SD: standard deviation, CV: coefficient of variation) for data were calculated for the entire race

Duc S et al. Physiology of Ski Mountaineering Racing. Int J Sports Med 2011; 32: 856–863

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Table 1 Characteristics of uphill and downhill sections of the “Trace Catalane” 2007.

Training & Testing 859

V˙O2max

V˙O2max

HRmax

V˙O2 VT1

HRVT1

V˙O2RCT

HRRCT

(bpm)

(l.min − 1)

(bpm)

( % HRmax)

(l.min − 1)

V˙O2RCT ( % V˙O2max)

HRRCT

(ml.kg − 1.min − 1)

V˙O2VT1 ( % V˙O2max)

HRVT1

(l.min − 1)

(bpm)

( % HRmax)

182 ± 7

3.12 ± 0.53

75.5 ± 4.1

153 ± 7

84.2 ± 3.0

3.85 ± 0.61

93.4 ± 2.2

172 ± 6

94.5 ± 1.7

4.12 ± 0.63 67 ± 8

Table 3 Race time, rank, average and maximal HR for each subject during the race. Subjects

Race Time

Overall rank

HRmean

(gender)

(h:min:s)

(female rank)

(bpm)

( % HRmax)

MS (M) MC (M) LA (M) EL (M) MR (M) SD (F) AT (F) VL (F) OM (M) GA (F)

01:26:03 01:30:03 01:32:02 01:34:25 01:35:07 01:41:57 01:51:09 01:52:19 01:53:34 01:55:08

21 32 33 45 51 60 (4) 72 (11) 73 (12) 75 (13) 77

175 173 161 179 166 176 170 170 165 164

95 93 91 91 94 96 92 95 92 94

HRmean

Table 4 Time, mean and maximal HR, and energy cost of all subjects during each uphill and downhill section of the race.

uphill first second downhill first second

Race time Mean HR Mean HR

Maximal

Energy cost

(min)

(bpm)

( % HRmax)

HR (bpm)

(J.kg − 1.m − 1)

40 ± 4 44 ± 5

173 ± 6 170 ± 6#

95.5 ± 1.8 93.4 ± 2.1#

178 ± 6 176 ± 6#

13.3 ± 2.2 15.2 ± 2.9

5±1 6±1

153 ± 8 159 ± 9

83.8 ± 3.9 86.8 ± 4.3

– –

– –

Significant difference compared to the first uphill (* : p < 0.05, ** : p < 0.01, *** : p < 0.001)

and for each uphill and downhill section. Because the number of subjects was small, non-parametric statistical analysis was used (SigmaStat version 2.03). Wilcoxon signed rank test was used to compare the mean and maximal HR, linear speed, climbing speed and EC between the 2 uphill sections. Spearman regression was used to estimate the degree of correlation between performance (race time) and V˙ O2 variables of field test (V˙ O2max, VT1, RCT). For all tests, the level of statistical significance was set at p < 0.05.

Results ▼ Field maximal test ▶ Table 2 shows the physiological characteristics of the subjects. ● The CV of HRmax, VT1, RCT, and V˙ O2max were 4, 15, 16 and 17 %, respectively. Field maximal test duration (mean ± SD) was 489 ± 128 s. Maximal speed and maximal climbing speed were 7.92 ± 1.22 km.h − 1 and 1.258 ± 0.194 km.h − 1, respectively.

Race data Heart rate data ▶ Table 3 shows race time, rank and mean HR of each subject. 86 ●

2007” and the best time was 87 min for the first man, and 95 min for the first woman. Mean race time of studied subjects was 101 ± 11 min. Time spent in uphill and downhill sections was 84.0 ± 1.3 % and 9.7 ± 1.2 % race time, respectively. Time spent for changing ski configuration between uphill/downhill was estimated about 3.1 ± 0.8 % of race time. Mean HR of all subjects during the race was 170 ± 6 bpm ▶ Fig. 2 shows a representative curve of (93.4 ± 1.8 % of HRmax). ● one ski-mountaineer HR (subject SD) over the entire race. HR was very close to 180 bpm during the 2 uphills. The HR of all subjects was characterized by a low variance in amplitude during the climbs (CV: 2.5 ± 0.8 % for the first uphill and 3.0 ± 1.0 % for the second uphill). For all the subjects, the average distribution of time spent in the 3 intensity zones (Z1, Z2, Z3) was 7.0 ± 4.8 %, 51.3 ± 4.7 %, and 42.0 ± 6.5 % of race time, respectively. ▶ Fig. 2 shows the relationship between performance and field ● test V˙ O2 variables expressed for body mass unit. Race time of ski mountaineers was negatively correlated with V˙ O2max(r = − 0.87, p < 0.001), VT1 (r = − 0.82, p < 0.001) and RCT (r = − 0.85, p < 0.001). There was no significant relationship between race time and absolute values of V˙ O2max (r = − 0.66, p = 0.46), VT1 (r = − 0.63, p = 0.51) and RCT (r = − 0.63, p = 0.52). ▶ Table 4 shows time, mean and maximal HR, and energy cost ● during the two uphill and the two downhill sections. The mean EC during the two uphills was 14.2 ± 2.5 J.kg − 1.m − 1 (CV = 17 %). Mean HR was significantly lower during the second uphill compared to the first uphill: 173 ± 6 vs. 170 ± 6 bpm and 93.4 ± 2.1 vs. 95.5 ± 1.8 % HRmax. Maximal HR decreased also significantly during the second uphill (175 ± 7 vs. 178 ± 5 bpm, respectively). EC increased significantly by ~15 % during the second uphill compared to the first uphill (15.2 ± 2.9 vs. 13.3 ± 2.2 J.kg − 1.m − 1, respectively).

Speed data ▶ Table 5 shows mean and maximal speed, mean and maximal ● climbing speed during the two uphill and downhill sections for the whole group. Mean and maximal linear speed decreased significantly during the second uphill (4.43 ± 0.90 vs. 5.44 ± 0.86 km. h − 1, and 9.11 ± 1.55 vs. 10.22 ± 1.58 km.h − 1, respectively). Mean and maximal climbing speeds were also significantly lower during the second uphill (0.891 ± 0.100 vs. 0.992 ± 0.100 km.h − 1, and 1.385 ± 0.212 vs. 1.572 ± 0.230 km.h − 1, respectively).

Discussion ▼ The aim of this study was to quantify and describe for the first time, the exercise intensity of ski mountaineering racing by recording continuously heart rate and speed. We have compared our results to similar studies on off-road cycling [13], crosscountry skiing [20] and running [6] in order to show that ski mountaineering racing can be considered as one of the most strenuous endurance sports.

ski mountaineers were classified during the “Trace Catalane

Duc S et al. Physiology of Ski Mountaineering Racing. Int J Sports Med 2011; 32: 856–863

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Table 2 Physiological data of field maximal test (mean ± SD).

860 Training & Testing

speed (km.h ) uphill first second downhill first second

Mean climbing −1

5.5 ± 0.8 4.6 ± 0.9 ° 47.8 ± 6.3 37.7 ± 4.2

−1

speed (km.h )

10.5 ± 1.3 9.1 ± 1.6#

0.992 ± 100 0.891 ± 93*

1.572 ± 0.230 1.385 ± 0.212*

78.5 ± 10.0 68.8 ± 8.3

– –

– –

200 180 160 140 120 100

2 200

Altitude (m)

2 100 2 000 1 900

80 60 40 20

1 800 1 700 1 600 0

10

20

30

40

50

60

70

80

90

100

0

time (% total race time)

Fig. 2 Smoothed overall plot of heart rate (bold line) and altitude (filled line) of one typical subject (MC) over the entire race. Dashed horizontal line indicates the HR value of VT1 and RCT.

120 115

Race time (min)

110 y = –1.314x + 182.9 2 R = 0.725

105 100 95

y = –1.260x + 185.0 2 R = 0.751

y = –1.408x + 171.8 2 R = 0.667

90 85 80 35.00

40.00

45.00

50.00

55.00

−1

speed (km.h )

2 300

1 500

Maximal climbing

speed (km.h )

HR (bpm)

−1

60.00

65.00

70.00

75.00

80.00

VO2 (l.min–1.kg–1)

Fig. 3 Race time as a function of maximal oxygen uptake (●), first ventilatory threshold (◆) and second ventilatory threshold (◇).

Methodological considerations Several methodological points of our study must to be considered before comparing our results with the other studies. Concerning the regression linear results, it is important to note that the size group was small (only 10 subjects) although this is common when investigating elite athletes. Moreover, due to the mixed gender of the studied groups (4 women and 6 men), the physiological variables are lower in females (body weight: 56 ± 3 kg; V˙ O2max: 3.5 ± 0.4 l.min − 1; VT1: 2.5 ± 0.3 l.min − 1; RCT: 3.3 ± 0.3 l.min − 1) compared to males (body weight: 65 ± 8 kg; V˙ O2max: 4.4 ± 0.4 l.min − 1; VT1: 3.3 ± 0.4 l.min − 1; RCT: 4.1 ± 0.5 l. min − 1). These differences, quite logically, are most likely related to the higher relative amount of “passive” fat tissue and less “active” muscle mass in females, even when they are lean. That is why the race time is nearly 20 % higher in females compared to ▶ Table 3). These 2 characteristics (mixed genders and males (● small size) have probably affected the relationship between race

Table 5 Mean and maximal speed, mean and maximal climbing speed during each uphill and downhill section of the race.

performance and relative physiological variables (one can see 2 ▶ Fig. 3), and thus results of linear regressions distinct groups in ● must be treated with caution. Concerning the HR during the race, several potential uncontrolled factors during the study can affect the results. Firstly, the light snow cover during winter 2006–2007 induced a shorter and easier “Trace Catalane”. Thus, the results obtained in the present study may not be applied to a typical international ski mountaineering race, notably for %HRmax and time sprint in HR zones and for training suggestions. Secondly, the maximal test could be shorter for ski mountaineers (less than 10 min) to reach their real HRmax. Therefore, it might be possible that average intensity was slightly overestimated when expressed as % HRmax. Moreover, because the maximal test was performed in field conditions, it is more difficult to control workload than in lab conditions on a treadmill or an ergocyle. In particular, measurements in the field do not allow constancy of the subject speed and the snow friction. Thirdly, the HR responses, notably during the climb could be affected by cardiac drift due to fatigue and/or dehydration. We did not control the status of dehydration of subjects during and after the race. Nevertheless, each subject had drunk ~0.5 l of personal carbohydrate solution to prevent dehydration. This amount was probably moderate because the temperature during the race was cold ( − 5 to 0 ° C). The cardiac drift (HR difference between the start and the end of the climb) during the first uphill was less than 8 ± 4 beats.min − 1 and very similar during the second uphill (7 ± 2 beats.min − 1). Moreover, intra-individual variation of HR did not differ between the 2 uphills (2.5 ± 0.8 % vs. 3.0 ± 1.0 %). The slight increase of the CV during the second uphill is partially caused by the decrease of the HR during the time taken by the racers to change the ski configuration before and after the foot section (to attach the cramps to the feet and the skis onto the rucksack). Regarding these 3 points, the EC results should be used cautiously because we did not measure the real V˙ O2 but only estimated it by the HR/V˙ O2 relationship obtained during the field maximal test and the anaerobic contribution to energy expenditure was unknown. It is therefore possible that EC calculated was overestimated.

Physiological demand of ski mountaineering Our first hypothesis that physiological demand in ski mountaineering racing, assessed by mean %HRmax sustained through time and HR exercise profile, is similar to other exhaustive endurance sports like off-road cycling, cross-country skiing and running is confirmed by our results. The average HR measured during the “Trace Catalane” competition was 93.4 ± 1.8 % of HRmax. The variability of HR during uphill was less than 5 % and mean HR was very close to RCT during a race. The mean race time of the ten ski mountaineers studied, was about 101 min. Mognoni et al. [20] found similar average HR during cross-country skiing races (94–95 % HRmax) but for

Duc S et al. Physiology of Ski Mountaineering Racing. Int J Sports Med 2011; 32: 856–863

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Maximal

Mean

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the energy expenditure, which can be reflected by the elevated HRmean during the downhill (in the present study, HRmean was 83.8 and 86.8 % HRmax, for the 2 downhills, respectively). Fourthly, ski mountaineering is characterised by lack of recovery periods, except maybe during easy descent sections and transition periods between ascents and descents when ski mountaineers change the configuration of skis and boots. However, time taken by the best ski mountaineers to perform these transitions is lower than 60 s, and thus HR can only decrease slightly.

Predictors of ski mountaineering performance The group of ski-mountaineers studied have similar absolute V˙ O2max (4.1 l.min − 1) compared to cross-country skiing athletes investigated by Mognoni et al. (3.9 l.min − 1), but lower compared to mountain bikers investigated by Impellizzeri et al. (4.8 l.min − 1). The lower level of V˙ O2max can be due to the conditions of field maximal test, which was performed at moderate altitude (~2000 m). Several studies [14, 25, 26] showed that V˙ O2max in endurance trained athletes decreased by 6–14 % per 1 000 m altitude gained above 1 500 m sea level. This change explains to a great extent the decrease of human performance, notably the endurance capacities [14], and this effect seems to occur above 580 m altitude in highly trained athletes [7]. Ski-mountaineers have similar VT1 compared to off-road cyclists (75.5 vs. 75 % V˙ O2max; [13]) but higher RCT (93.4 vs. 87 % V˙ O2max; [13]). We observed that HRmax during the maximal test was lower than the often used age-predicted HRmax (182 vs. 188 bpm). It is well known, like V˙ O2max, that HRmax decreases significantly with increasing altitude [21]. Nevertheless, we can suppose that HR responses of ski-mountaineers could not be greatly affected by altitude because all subjects lived 1 000 m above sea level and trained at moderate altitude (1 500–2 500 m). The body mass of the ski-mountaineers studied (~61 kg) are similar to cross-country skiers mass (~62 kg reported by Mognoni et al.) but slightly lower than off-road cyclists mass (~64 kg reported by Impellizeri et al.). Ski-mountaineers purchase to decrease as soon as possible their body mass (and also to use lighter material) because they spend very much time in ascents during races (i. e., ~85 % for the present race studied) whereas downhill total time represents only ~40–60 % during a Olympic off-road race. We found that race time was significantly correlated with V˙ O2max (r = − 0.87), VT1 (r = − 0.82) and RCT (r = − 0.85) expressed for body mass unit, but not with the absolute values. Thus, our second hypothesis that physiological measures relative to mass are better predictors of ski mountaineering performance than absolute measures is confirmed. Several studies [8, 12] have also reported that physiological measures in off-road cycling (peak power, V˙ O2max, power at anaerobic threshold) relative to total rider mass were more correlated to performance (race time or average course speed) than absolute variables. On the contrary, performance in cross-country skiing seems to be strongly correlated to absolute V˙ O2max [16] and upper body power [19]. This suggests that like in off-road biking, ski mountaineering training programs should focus upon improving relative physiological values rather than focusing upon maximizing absolute values to improve performance.

Energy cost of ski mountaineering Based on linear regression between HR/V˙ O2 during maximal field test, we estimated the energy cost of each ski mountaineer during the race. The mean EC of the two uphill sections was 14.2 ± 2.5 J.kg − 1.m − 1. Recently, Tosi et al. [23] reported a lower EC

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competitions lasting below 30 min. In contrast, lower values have been reported during off-road races [13] or running competitions [6] but for longer time durations. Impellizzeri et al. [13] found a mean intensity equal to 90 ± 3 % of HRmax during off-road cross-country races. However, the mean time of these competitions was almost 50 % higher compared to the “Trace Catalane” race (147 ± 15 min). It is well known that mean HR decreases with exercise time [6]. The mean HR of our ski mountaineers would have been lower if the original “Trace Catalane” track had been performed. Race time of our ski mountaineers group would have been probably between 160 and 200 min, as in the previous year, on a similar track, the winner performed the “Trace Catalane 2006” in ~149 min. Recently, Esteve-Lanao et al. [6] performed a review on physiological strain during running competitions ranging from 5–100 km in 211 subjects. The mean % HRmax decreased with increasing race distance: from ~94 % for 5 km race to 87 % for marathon race. It is interesting to note that the range of race time reported for the half-marathon (72– 132 min) is very similar to the “Trace Catalane” time range (86– 115 min). Independent of the race time, runners performed a half-marathon at about 90 % % HRmax and some of them at 92–93 %, which is very close to our results. Time spent in 3 HR zones determined on ventilatory threshold data, i. e., Z1 (below VT1), Z2 (between VT1 and RCT), and Z3 (above RCT), was computed for each subject. This method has previously been used to describe off-road cycling [13], crosscountry skiing races [20] and running [6]. Ski mountaineers spent 7.0 ± 4.8 % of race time in the Z1, 51.3 ± 4.7 % in the Z2 and 42 ± 6.5 % in the Z3. Similar results have recently been reported in the half-marathon [6]. Runners spent less than 5 % in Z1, nearly 55 % in Z2 and about 40 % in Z3. Once more, our results differ slightly from off-road biking. Impellizzeri et al. [13] found in off-road competition that 18 ± 10 % of race time was spent in EASYzone (similar to Z1), 51 ± 9 % in MODERATEzone (similar to Z2) and 31 ± 16 % in HARDzone (similar to Z3). Similar to the mean % HRmax difference, the time increase in EASYzone and the time decrease in HARDzone can be related to the difference in race time (147 vs. 101 min). The method used to determine the 3 intensity zones can also involve a slight difference in computed time. Our method was based on expired respiratory gases whereas Impellizzeri et al. [13] used the blood lactate – intensity curves. Some authors have shown that ventilatory and lactate thresholds can be found at different intensities in some athletes [15]. Finally, glycogen depletion and dehydration can involve greater HR during endurance exercise [11]. To prevent these effects, each subject drank ~0.5 l of carbohydrate solution (50–70 g/L) throughout the race. Other specific ski mountaineering factors can involve greater energy consumption, and thus HR responses, compared to running and off-road biking. Firstly, unlike off-road biking, ski mountaineers cannot use draft of other ski-mountaineers to reduce energy expenditure, and cannot preserve a HR reserve, because speed in uphill is generally lower by 10 km.h − 1. Secondly, similar to cross-country skiing, ski mountaineering involves greater trunk and arm muscles for propulsion compared to off-road cycling. It is well known that energy expenditure depends on the total activated muscle mass. Furthermore, intense and repeated isometric contractions of leg muscles are necessary to absorb shocks and vibrations caused by the snow conditions in descents. Thirdly, psychological factors like stress due to steep downhill in all snow conditions and mental focusing during descents to choose the best trajectory can increase

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Implications for training Most of the ski mountaineers used a HR monitor to control the training intensity. The reported ski-mountaineering exerciseintensity profile in the present study can be used by coaches as a starting point to design effective training programs. Because they spend nearly 50 % of total race time between VT1 and RCT, ski-mountaineers should focus their training on increasing intensity and sustained time at RCT. For example, they could include sub-maximal interval-training (10–30 min) in order to bring RCT close to V˙ O2max. This study also shows that 40 % of total

race time was spent above the RCT. Consequently, they must include in their training, exercise bouts between RCT and V˙ O2max, in order to be able to sustain high intensity with high level of acidosis. For example, they can repeat several short maximal interval-training bouts (30 s to 2 min) to increase buffering capacity.

Conclusion ▼ In this study, we carried out field measurements of HR and speed for assessment of physiological demand during a ski mountaineering race. Our data indicate that ski-mountaineers performed a ~100 min race at a very high intensity (mean of 93.4 % HRmax) and they spent nearly 50 % of time between VT1 and RCT and more than 40 % above RCT. These results are slightly higher compard to those observed in cross-country skiing, off-road biking and running racing. In spite of the limited number of subjects and conditions investigated, our data suggest that ski-mountaineering racing can be considered as one of the most strenuous endurance sports like off-road biking and cross-country skiing. To achieve the best performance, ski-mountaineers and coaches should focus on increasing RCT and buffering capacity with interval-training sessions. In addition to maximal aerobic capacities, body mass seems to appear as a key factor given that performance in ski mountaineering is strongly correlated to relative VT1, RCT and V˙ O2max values expressed per mass unit. The decrease of speed and HR and the increase of energy cost during the second uphill of the race point out that significant fatigue occurs in racers, improving the exhaustive character of ski mountaineering. Future studies with a larger number of subjects should confirm these hypotheses, notably the strain of ski mountaineering and the prevalence in fatigue of ski-mountaineers during racing.

Acknowledgements ▼ The authors thank the athletes, especially the Andorran and Catalan teams for their cooperation. We also acknowledge the Matsport Company for their technical support on the field.

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