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Original Research

No Differences in O2-Cost between V1 and V2 Skating Techniques During Treadmill Roller Skiing at Moderate to Steep Inclines

Losnegard, Thomas; Myklebust, Håvard; Hallén, Jostein

Author Information
Journal of Strength and Conditioning Research: May 2012 - Volume 26 - Issue 5 - p 1340-1347
doi: 10.1519/JSC.0b013e318231a69e
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Abstract

Introduction

Since the official appearance of the ski skating technique in the middle of the 1980s, remarkable technique development has occurred. Introduction of new types of races as the sprint event and mass start and better track preparation, improved equipment, and changes in course profiles have contributed to this development (11,29). Ski skating consists of several different techniques that can be considered as a gear system wherein the skiers can freely choose their technique according to the terrain, snow condition, and speed. Their choice will be influenced by their experience, beliefs, and knowledge concerning the optimal technique for performance in that particular situation.

The speed achieved in competition depends on several physiological and mechanical factors. One of these factors is the O2-cost of locomotion, defined as the amount of energy spent per unit of velocity (6), and there have been reports about a close relation between O2-cost and performance in several endurance sports (5,21), including crosscountry skiing (16,17,20). According to the rules of the International Ski Federation (FIS), one-third of a crosscountry ski race must be uphill with a gradient between 4.5 and 9° (7), and approximately half of the skier's race time is spent in uphill sections (1). Hence, the O2-cost of different techniques on uphill terrains will probably influence the skiers' performance.

The primary ski skating technique used on flat terrains and moderate uphill terrains is V2 (also called “gear 3” or “double dance”), whereas V1 (also called “gear 2” or "paddling) is traditionally used on steeper inclines, and this choice has been supported by research showing that on these inclines, V1 is more economical than V2 (14). However, recently, it has been observed that some elite crosscountry skiers use the V2 technique even on steep uphill terrain (1). In the V1 technique, skiers use their poles on every second leg push-off (left or right) in contrast to the V2 technique, where the poles are used on every leg push-off.

It is also evident that the cycle rate (CR) varies substantially among skiers (18), and routine testing has shown a large interindividual variation in O2-cost. From a kinematic point of view, CR has been shown to influence O2-cost in different sports such as bicycling and running (9). However, it is less known if CR relates to some of the differences in O2-cost between skiers in V1 and V2 ski skating techniques.

Although previous studies have investigated physiological, kinetic, and kinematic differences between V1 and V2 ski skating techniques (2–4,14,17,19,27), there is little updated knowledge on the physiological and kinematical aspects in elite skiers using different ski skating techniques on uphill terrains. The purpose of the study was therefore to compare V1 with V2 with regard to O2-cost, V[Combining Dot Above]O2max, and kinematic aspects during treadmill roller skiing at moderate to steep uphill inclines in elite crosscountry skiers. Based on the previous literature, we hypothesize that the O2-cost will be lower for the V1 compared with the V2 ski skating technique on steep inclines. Second, we hypothesize that the variance in CR would be associated with variance in O2-cost among subjects.

Methods

Experimental Approach to the Problem

This study used a within-subject repeated-measures design to determine if V1 and V2 differ in O2-cost and V[Combining Dot Above]O2max at moderate to steep inclines. To do this, elite male crosscountry skiers were tested at 3 submaximal loads with V1 and V2 to determine O2-cost and maximal bouts with both techniques to determine V[Combining Dot Above]O2max. All testing was conducted over 2 days near the beginning of the season, from mid-September to late November. Day 1 included submaximal tests at 4 and 5° inclines with both techniques and a V[Combining Dot Above]O2max with one of the techniques. Day 2 was similar, with submaximal tests at 4° (to measure the coefficient of variation) and 6° inclines (instead of 5°), and the skating technique during the V[Combining Dot Above]O2max test was changed. The 4° submaximal test on day 2 is reported in the final Results section. For 10 of the subjects, there was at least 1 rest day between days 1 and 2. However, for 4 subjects, it was not possible to have a rest day, and they were tested consecutive to day 1. Testing was counterbalanced for the type of ski technique to eliminate the potential influence of testing order, and this order of technique was consistent for each subject between inclines and tests. Each subject was tested at the same time of the day.

Subjects

Fourteen elite senior male crosscountry skiers (age: 24 ± 3 years, height: 184 ± 6 cm, weight: 79 ± 7 kg) volunteered to participate in the study. The subjects had regularly participated in roller ski treadmill testing over the previous 1–3 years and were therefore familiar with the testing. All the skiers competed at the Norwegian national level at a high standard. Among the subjects, there were one World-cup winner, 6 skiers with top 30 results in World-cup races, 3 skiers with several top 10 results in FIS long distance races, and all 14 skiers had top 30 results in the Norwegian championship. The study was approved by the Regional Ethics Committee of Southern Norway, and the subjects gave their written consent before study participation.

Procedures

Submaximal Oxygen Cost

Submaximal tests were all performed at a speed of 3 m·s−1, lasted 6 minutes, and with 2-minute breaks between bouts. The speed was chosen to be high enough to induce a relevant technique at moderate inclines but low enough to obtain a steady state V[Combining Dot Above]O2 (<90% of V[Combining Dot Above]O2max). Blood plasma lactate concentration was measured 30 seconds into the break after each 6-minute effort. Because of the O2 measurement apparatus, the subjects were unable to express their rating of perceived exertion (RPE) during the trial. Therefore, at 4 minutes into the trial, they were asked to choose their RPE, which they then reported at the end of the trial. O2-cost in this study was defined as the average oxygen uptake (milliliters per kilogram per minute) between 3.5 and 5.5 minutes at each submaximal bout. The reliability coefficient (typical error expressed as CV) for the O2-cost from the 4° (days 1 and 2) was 2.5% (V1) and 2.3% (V2).

Maximal Oxygen Consumption

Eight minutes after the last submaximal effort, the subjects performed the V[Combining Dot Above]O2max test. The starting incline and speed were 6° and 3 m·s−1. The initial speed was kept constant, and the incline was subsequently increased by 1° every minute until 8°. Thereafter, the speed was increased by 0.25 m·s−1 every minute. Skiing to exhaustion and a plateau in V[Combining Dot Above]O2 were used as criteria to indicate that V[Combining Dot Above]O2max was reached. Oxygen uptake was measured continuously, and the highest mean values over 1 minute was taken as V[Combining Dot Above]O2max. A safety harness with autostop (in the case of a fall) secured the subjects during the V[Combining Dot Above]O2max tests. The reliability coefficient (typical error expressed as CV) for the V[Combining Dot Above]O2max tests has been shown to be 2.2% (V1) and 2.7% (V2) (n = 11; 2 trials) in our laboratory.

Assessment of Individual Variability

Interindividual variation in O2-cost at a specific workload was calculated as the SD of the O2-cost of the skiers divided by the mean value. Delta economy was measured as the increase in oxygen uptake per degree change in incline. Interindividual coefficient of variance in delta economy was calculated as the SD in delta economy divided by the mean value. The intraindividual coefficient of variance in the gross economy between the 2 techniques was calculated as the SD of the difference in O2-cost between techniques, divided by the mean value.

Kinematic Parameters

Cycle rate and CL were calculated using video analysis. One cycle was defined as the time between consecutive right pole plants for V1, and the time between every other right pole plant for V2 (19). An average of 9 ± 2 consecutive cycles was used for calculating the CR. Mean CL was calculated as treadmill velocity divided by CR.

Apparatus

Oxygen consumption (V[Combining Dot Above]O2) was measured by an automatic system sampling in 30-second epochs (Oxycon Pro Jaeger Instrument, Hoechberg, Germany) evaluated by Foss and Hallén (9). Heart rate (HR) was measured between 3.5 and 5.5 minutes with a Polar S610i™ (Polar electro OY, Kempele, Finland), and blood lactate concentration was measured in unhemolyzed blood, collected from capillary fingertip samples. Blood was collected in a heparinized capillary tube, and 25 μL was injected with a pipette into the mixing chamber of the lactate analyzer (YSI 1500 Sport, Yellow Springs Instruments, Yellow Springs, OH, USA). The lactate analyzer and Oxycon Pro Jaeger Instrument were calibrated according to the instruction manual and described in detail by Losnegard et al. (15). All testing was performed on a roller ski treadmill with belt dimensions of 3 × 4.5 m (Rodby, Sodertalje, Sweden). Inclines and speed were calibrated before the start of the study and checked during and after the testing period. Swix CT1 poles (Swix, Lillehammer, Norway) with customized treadmill roller skiing tips were used (pole length 167.5 ± 5.5 cm, corresponding to 91% of body height). Two different pairs of Swenor Skate skis (Swenor, Sarpsborg, Norway) with wheel type 1 were used depending on the binding system the skiers normally used (NNN, Rottefella, Klokkarstua, Norway or SNS, Salomon, Annecy, France). The rolling friction coefficient (μ) of the skis was tested before, during, and after the project using a towing test, previously described by Hoffman et al. (10). All the tests were performed after warming up with 15 minutes of treadmill roller skiing at a 3° incline at constant individually selected speed (2.25–2.75 m·s−1 corresponding to ∼60–75% of the HRmax). This served as a warm-up for both the athlete and the roller skis, which acquired a friction coefficient of 0.018 (NNN) and 0.020 (SNS). The breaks between warm-up, recovery, and tests trial were limited to 2 minutes to maintain stable rolling friction. The subject's body weight was measured before each treadmill test (Seca, model 708 Seca, Hamburg, Germany). The motions of the skiers were filmed with a stationary 50-Hz video camera (Sony DCR-TRV900E, Sony, Tokyo, Japan). The distance between the camera and skiers was 5 m. The camera was positioned perpendicular to the skiing direction and, Dartfish Connect 4.5 (Dartfish Ltd., Fribourg, Switzerland) was used for counting frames in a cycle.

Statistical Analyses

All data were checked for normality with a Shapiro-Wilk test and presented as mean and SD. Paired t-tests were used for detecting significant differences in O2-cost at each incline during submaximal testing and V[Combining Dot Above]O2max, between V1 and V2 techniques. A one-way repeated-measure analysis of variance was calculated to analyze the changes in CR over the 3 different inclines. When global significance over time was determined, Bonferroni post hoc analysis was used to determine changes over inclines. Pearson's product moment correlation analysis was used for correlation analyses. The magnitude of differences between techniques was also expressed as standardized mean differences (Cohen's D effect size; ES). The criteria to interpret the magnitude of the ESs were as follows: 0.0–0.2 trivial, 0.2–0.6 small, 0.6–1.2 moderate, 1.2–2.0 large, and >2.0 very large (13). Statistical calculations were performed using Microsoft Excel and SigmaPlot 11 software. A p-value ≤0.05 was considered statistically significant.

Results

Submaximal Oxygen Cost and Maximal Oxygen Consumption

The mean O2-cost at 4, 5, and 6° inclines corresponded to approximately 67, 77, and 87% of V[Combining Dot Above]O2max values, respectively. The mean HR was 80, 90, and 94% of HRmax as measured during the V[Combining Dot Above]O2max tests. There were no differences in O2-cost or HR, La, and RPE during submaximal tests between ski skating techniques at any incline (Figure 1). Time to exhaustion and all the physiological variables during the maximal tests were similar between V1 and V2 (Table 1). The magnitude of the differences in physiological responses between techniques, expressed as standardized mean differences (effects size), was trivial to small. Effect sizes in O2-cost between V1 and V2 at 4, 5, and 6° were 0.24, 0.08, and 0.05, whereas in V[Combining Dot Above]O2max the effect size between V1 and V2 was 0.26.

Table 1
Table 1:
Physiological responses and time to exhaustion during the V[Combining Dot Above]O2max tests.
Figure 1
Figure 1:
Physiological responses during V1 and V2 at 4, 5, and 6° with a constant speed at 3 m·s−1. A) Oxygen uptake (3.5–5.5 minutes), B) heart rate (3.5–5.5 minutes), C) blood lactate accumulation 30 seconds after each bout, and D) rate of perceived exertion according to Borg scale (6–20). Error bars for V1 has a negative direction and for V2 a positive direction.

Assessment of Individual Variability

There was a large interindividual variation in the O2-cost at a specific workload. There were also some consistent intraindividual differences between techniques (Figures 2 and 3) meaning that the same subject had a higher O2-cost with one of the techniques at all inclines. The absolute difference in the O2-cost (milliliters per kilogram per minute) at a specific load between the most and the least economical skiers was typically >15% (Figure 2). The interindividual coefficient of variance in O2-cost was between 3 and 4.7% and decreased with increasing incline in V1 but stayed constant in V2. This is in line with the finding that the interindividual coefficient of variance in delta economy, measured as the increase in oxygen uptake per degree change in incline, was larger in V2 than in V1 (11.5 vs. 7.5%). The intraindividual coefficient of variance in gross economy between the 2 techniques was 1.6%. There was no difference in the average V[Combining Dot Above]O2max between V1 and V2, but the intraindividual coefficient of variance in V[Combining Dot Above]O2max between the 2 techniques was 3.4%.

Figure 2
Figure 2:
Individual and average oxygen uptake at 4, 5, 6° by using V1 and V2 techniques. The line of identity is shown as a solid line meaning that the values below the line result in lower O2-cost in V2 vs. V1 and those above the line result in lower values in V1 vs. V2. Dots with error bars represent mean ± SD.
Figure 3
Figure 3:
Example of A) average of 2 subjects that ski more economically by using V1 and B) average of 2 subjects that ski more economically by using V2 at 4, 5, and 6°.

Kinematic Parameters

Cycle rate increased significantly between inclines (p < 0.05), and the values at 4–6° were 0.72 ± 0.05, 0.75 ± 0.05, and 0.78 ± 0.04 Hz in V1 and 0.51 ± 0.05, 0.53 ± 0.05, and 0.55 ± 0.04 Hz in V2, respectively. The interindividual coefficient of variance in CR, measured as the average CR at 4, 5, and 6°, was 5.8% (V1) and 8.6% (V2). Despite the large variation in both CR and O2-cost, there was no significant correlation between the 2 variables (Figures 4 and 5).

Figure 4
Figure 4:
Individual cycle rate at 4–6° with a constant speed at 3 m·s−1.
Figure 5
Figure 5:
Cycle rate in relation to O2-cost at V1 (A) and V2 (B) at 4, 5, and 6°. Solid lines are trend lines for the respective inclines.

Discussion

We found no significant differences between V1 and V2 in the O2-cost at moderate to steep inclines (4–6°) which is in contrast to the findings of previous studies (14,17). Millet et al. (17) compared V1 and V2 on snow on different types of terrain and reported a higher O2-cost in V2 than in V1. However, this study is not directly comparable with our study because of the differences in terrain and level of skiers. Kvamme et al. (14) studied well-trained Nordic combined and biathlon athletes on treadmill inclines and durations that were identical to those of this study. They suggested that V1 has a lower O2-cost at steeper inclines (>4°) than V2 has. A striking difference between the results of the 2 studies is the blood lactate response. In Kvamme et al. (14), the difference in postexercise lactate between the 2 techniques increased with increasing incline, with the lactate concentration being highest during V2 at inclines >4°. In the present study, there was no difference in the lactate response at any incline (Figure 6). It has previously been shown that in double poling, where most of the total work is achieved from the upper body musculature, the arms have a net release of lactate whereas the legs have a net uptake of lactate (30). Hence, a major upper body contribution to forward propulsion will probably lead to a higher lactate production, compared with a greater use of the legs in ski skating techniques. This may indicate that the balance between leg and upper body contribution to the propulsion shifted more to upper body work as the incline increased for the subjects reported by Kvamme et al. (14). The lack of such a lactate response in this study may indicate that our subjects were able to preserve the high reliance on leg propulsion even at steep inclines. This concurs with the finding that the difference in blood lactate between techniques correlates with the differences in O2-cost (data not shown). It should be noted that the skiers in the study of Kvamme et al. (14) probably have less ski skating experience, because most of the subjects were well-trained Nordic combined athletes and not elite crosscountry skiers. These factors together could explain some of the different results between studies.

Figure 6
Figure 6:
Delta La− V2–V1 in this study and in the study of Kvamme et al. (14) at different inclines. Positive values indicate higher blood lactate concentrations during V2 than during V1.

Skiing techniques have evolved with the development of sprint skiing and mass start abilities (28,29). The change in training focus because of the change of competition formats might have led to a more adjustable V2 technique, such as timing in leg push vs. pole push and direction of skis in the set down phase of the skis, as described by Stöggl et al. (29). Optimal timing between arms and legs may have compensated for the possible suboptimal force direction of the V2 technique.

This study was carried out with roller skis on a large treadmill, and it should be noted that there is limited information on how the findings observed in the laboratory could be applied to skiing on snow. Nevertheless, elite crosscountry skiers include high volumes of roller ski training in the precompetition phase and during this training our elite skiers aim to simulate on-snow skating more than achieving an optimal roller skating technique. Also, previous studies on treadmill roller skiing show a strong correlation with on-snow skiing performance (16,25). Interestingly, some skiers prefer either V1 or V2 during on-snow skiing, and we noted that in some of the most extreme skiers, these preferences concur with the most economical technique during treadmill skiing. Millet et al. (17) compared V1 and V2 on snow on different types of terrains and reported a higher O2-cost in V2 than in V1 in nonelite crosscountry skiers, the same conclusion as that reached by Kvamme et al. (14) in a similar group of skiers during roller skiing. Hence, we think that the results from roller ski testing are valuable for on-snow skiing, but the results should of course be used with caution.

The coefficient of variance in the O2-cost between subjects, measured as the average oxygen uptake at 4, 5, and 6° was 3.4% (V1) and 4.2% (V2). The coefficients of variance in O2-cost (for running) and gross efficiency (for cycling) have been shown to be 1.8–5.0 and 3.0–6.2%, respectively (12,22,23,26). Hence, the individual variations in work economy in crosscountry skiing seem to be similar to cycling and running. This may be somewhat surprising because crosscountry skiing synchronizes both the upper and lower body and may be a more challenging technique than either running or cycling. Another striking finding is the close relationship between the O2-cost in the 2 techniques. For example, the subjects who were most economical in V1 were also relatively economical in V2 (Figure 2). Because all of our subjects are elite skiers and high performers, this indicates that, in crosscountry skiing, the O2-cost is partly determined by intrinsic factors.

It has been observed that elite crosscountry skiers and biathlon athletes use different ski skating techniques under similar conditions and have preferences when it comes to choice of technique. Despite the fact that there was a close relationship between O2-cost of the 2 techniques, there were individual differences, with some athletes being most economical in V1 and some in V2. In general, the coefficient of variance in the difference in O2-cost between the 2 techniques was 1.6% and not very different from the test-retest reproducibility (4°, day 1 to day 2). However, there are 3 factors that indicate that this is because of individual differences rather than measurement variability. First, we tested the different inclines and techniques over several days in a balanced order, and the differences in these skiers were consistent between days. Second, there was a close relationship between the difference in blood lactate concentration and the difference in O2-cost. Third, most of the subjects in this study (n = 11) have been participating in a testing program during the last 1–3 years using the same testing protocol. For instance, 4 of these subjects have repeatedly tested the same technique as the most economical over the last 2 years (Figure 3). Their most economical technique is also their preferred technique during skiing on snow. Hence, choice of technique may nevertheless not be insignificant for the individual skier.

Cycle rate increased linearly from 4 to 6° with a constant speed at 3 m·s−1 (Figure 4). Furthermore, CR was highest and CL shortest during V1, which is in agreement with the findings of previous studies (2,3,17,19,24,27). It is well known that CR (or cadence) influences the O2-cost in other endurance sports, such as bicycling and running (9) and that this phenomenon has also been observed in crosscountry skiing (18). However, despite the large interindividual variance in CR (V1: 5.8%, V2: 8.6%) and gross economy (V1: 3.4%, V2: 4.2%), no significant correlations between CR and O2-cost (Figure 5), CR and anthropometric data (weight, height), or CR and pole length data were found in this study. This shows that CR alone is not a significant variable with respect to O2-cost between subjects in ski skating at submaximal loads.

Practical Applications

This study revealed similar oxygen costs between V1 and V2 ski skating techniques, across multiple inclines. Our results also indicate that elite skiers' technique selections are not determined by degree of incline, because the velocity was constant at all inclines. The differences in O2-cost between techniques were consistent at all inclines for the individual skier, and consequently, some skiers may prefer V1 and some V2 on moderate to steep inclines. These observations can contribute to optimization and evaluation of training preparation and competition strategies for individual athletes. Thus, coaches should exercise caution when recommending the technique choice for a whole group, because individual variation must always be considered. It is probably essential to take into account a skier's training history and technique focus when selecting a technique for various terrains. In addition, skiers usually shift between techniques even on constant uphill terrain. Therefore, it is important for the athletes to improve their skill and physiological capacity in both techniques, thus improving their weakest technique.

Acknowledgments

The authors thank the athletes and coaches for their participation, enthusiasm, and cooperation in this study. They would also like to acknowledge Head Coach Kåre Tønne and engineer Bjarne Rud for helpful advice and assisting during testing. No funding was received for this work from any of the following organizations: National Institutes of Health (NIH), Welcome Trust, Howard Hughes Medical Institute (HHMI), and other(s). No conflicts of interest, financial or otherwise, are declared by the authors. The results of this study do not constitute endorsement by National Strength and Conditioning Association.

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Keywords:

crosscountry skiing; elite skiers; interindividual variations; skiing techniques; V[Combining Dot Above]O2max; work economy

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