Ten recreational skiers took part in the study, eight male and two female, four of whom had previous experience of ski-touring. Their mean age was 31.9 (range 20–65) years. Written informed consent was obtained from each subject.
We used Wedze W500RT 158cm hybrid resort-touring skis (Decathlon) fitted with Tyrolia Ambition 10 bindings. These bindings permit the use of conventional downhill ski boots. A uni-directional “skin” was attached to the underside of each ski. The heel riser of the ski binding was set to its highest setting, so that the boots were effectively horizontal on the treadmill (see Fig. 1).
We used a conventional running treadmill (h/p/cosmos, Pulsar 4.0, Nussdorf-Traunstein, Germany), set at its maximum slope of 25 percent (14 degrees) throughout the study. In order to protect the belt of the treadmill, ski poles were not used.
Subjects skied for a one minute warm-up at 0.5 km/hr. Recording was started with a treadmill speed of 1 km/hr. The speed was increased by 0.3 km/hr every minute until they could no longer keep pace with the treadmill. Our aim was to assess maximum exercise capacity, with a test lasting 8–12 minutes. During the test, subjects selected their own step rate.
Ventilation \(\dot{{(\text{V}}_{\text{E}})}\), oxygen consumption (VO2) and carbon dioxide output (VCO2) were measured breath by breath using a metabolic cart (JAEGER™ Vyntus™ CPX, incorporating SentrySuite software Version 2.21.1; Vyaire Medical Products Ltd, Chineham, Basingstoke) connected to an oronasal mask (Hans Rudolph). The mask was connected to a low-resistance (< 1.0 cm H2O·L − 1·s− 1 at < 15 L·s− 1) digital volume transducer with a combined dead space of 270 mL. The flow sensor was calibrated automatically using a built-in flow generator, producing precise constant airflows. Gas concentrations were sampled (50mL·min− 1) at the mouth via a 2.4 m sample line and analysed using a high speed digital O2 sensor based on an electrochemical principle, and a fast response digital CO2 sensor based on the principle of infrared absorption. These were calibrated using ambient air and gases of known concentration (5% CO2, 15% O2, balance N2; BOC, Guilford, UK). During the test, breath-by-breath data were averaged over 10 seconds.
Heart rate was recorded using a heart rate monitor (Polar Electro, Kempele, Finland). Predicted maximum HR was estimated as 220 – age in years. Peripheral oxygen saturation (SpO2) was recorded continuously using a finger probe (Ohmeda). When the subject reached their peak effort and the treadmill was stopped, a finger-prick blood sample was taken and analysed for lactic acid (Biosen, EKF diagnostics).
At peak effort we noted HR, VO2 (VO2max) and treadmill speed. Anaerobic threshold (AT) was determined automatically by the SentrySuite software, using the V-slope method. The respiratory compensation point (RCP) was determined automatically from the VE/VCO2 plot.
Having determined the speed of the treadmill at AT during the maximal test at sea level, five subjects returned on a different day to ski at this speed for 40 minutes. As for the maximal test, the slope of the treadmill remained constant at 25%. After 40 minutes, a finger-prick blood sample was analysed to see if there had been any significant accumulation of lactic acid.
The combined weight of each ski with skin and binding was 2.5 kg. With these bindings, subjects were able to use conventional downhill ski boots, each boot also weighing around 2.5 kg. In order to see the effect of equipment weight, we studied five subjects with and without a one kilogram ankle weight strapped to each boot (Fig. 1). VO2 and HR were recorded whilst the subject skied for ten minutes at their AT speed (as previously determined from the maximal protocol). Subjects were studied with and without the added weights in random order, leaving a ten minute break between the two conditions.
To study the effect of altitude, each subject underwent two ski tests to exhaustion on separate days, in random order: one at sea level and one in an environmental chamber (WIR52-20HS, Design Environmental Ltd., Gwent, Wales, U.K) with an oxygen content of 14% (equivalent to an altitude of 3000m). The temperature and humidity of the chamber were set to the ambient settings of the day when the subject performed their sea-level test. SpO2 was monitored continuously, and the test was terminated if it fell below 75%.
Comparisons between sea-level and simulated altitude were made using paired t-tests on the SPSS statistical package, taking 0.05 as the level of statistical significance. For any data that were not normally distributed, we used non-parametric Wilcoxon sign-rank tests.
The study was approved by the Nottingham Trent University Human Ethics Committee, and all procedures conformed to the standard set by the Declaration of Helsinki. No specific funding was received for the study.