|
nif:isString
|
-
Ten male national and international levels cross-country skiers (male, age: 21–28; height: 1.88 ± 0.03 m; body mass: 81.2 ± 7.0 kg; International Ski Federation (FIS) points: 90 ± 37 [mean ± SD]) volunteered to participate in the study. All participants signed an informed consent form before the experiment and were made aware that they could withdraw from the study at any point without providing an explanation. The study was approved by the Norwegian Data Protection Authority and was conducted in accordance with the Declaration of Helsinki.
All athletes used their personal ski boots and poles (with a length ~90% of body height). Total equipment mass was 3.7 ± 0.1 kg, including the kg 2.05 kg roller skis and the remaining weight due to shoes and poles. To minimize variations in rolling resistance, all skiers used the same pair of roller skis (IDT Sports, Lena, Norway). Lead bars weighing 0.25 kg were added to the underside of the roller ski in increments to add appropriate weight according to the protocol described in detail below. The main idea in our approach was to test skiers at two distinct inclines, while allowing the skiers to choose the most efficient technique while roller skiing. However, we chose inclines where all skiers were expected to use the same sub-technique. Specifically, the G2 and G3 skating techniques were used during this research project; The G2 skating technique (also referred to as offset and V1 skate) is used in steep uphill terrain and involves an asymmetrical double poling action together with every second leg push. Hence, a strong side with synchronized double poling and a leg push-off, as well as a weak side where only the legs push-off is executed. The G3 skating technique (also referred to as V2, 2-skate, and double dance) is used in slight to moderate uphill terrain and involves a symmetrical double poling action together with a single leg push on each side.
After body-weight measurement and marker placement, the athletes performed a 20-minute low-intensity warm-up and treadmill familiarization, followed by four 7-minute submaximal intervals. The intervals were performed either with normal-unloaded ski equipment (0 kg) or with the addition of 0, 0.5, 1.0, and 1.5 kg evenly distributed below the whole roller skis in blinded randomized order. The addition or removal of weight was done by the test leader and not observed by the athlete, and the skis were placed on the athlete prior each interval by the test leader. On two separate days, the steep (12%) and moderate (5%) terrain techniques were tested at 7 km/h and 14 km/h, respectively. The speed and incline were matched to obtain comparable work rates and to assure that all skiers used G2 at 12% incline and G3 at 5% incline as their self-chosen techniques. The duration and exercise intensity were set to ensure submaximal aerobic steady state occurred.
Ventilatory parameters were assessed by employing open-circuit indirect calorimetry with an Oxycon Pro apparatus (Jaeger GmbH, Hoechberg, Germany) for two minutes at the end of each interval. Prior to each measurement, the VO2 and VCO2 analyzers were calibrated using a known mixture of gases (16.00 ± 0.04% O2 and 5.00 ± 0.1% CO2, Riessner-Gase GmbH & Co., Lichtenfels, Germany) and the expiratory flow meter calibrated with a 3-L syringe (Hans Rudolph Inc., Kansas City, MO). Heart rate (HR) was recorded throughout the entirety of each test using the skier's own heart rate monitors. Blood lactate concentration was analyzed using the Biosen C-Line Sport lactate measurement system (EKF Industrial Electronics, Magdeburg, Germany) from 5-μL of fingertip blood collected at the end of each interval. Rating of perceived exertion (RPE) was assessed immediately after each stage. During the four interval trials, three-dimensional movement data were captured from a ten-camera Qualisys Oqus system (Qualisys AB, Gothenburg, Sweden) with a sampling rate of 250 Hz. Retro-reflective markers were placed on the lateral epicondyle, malleolus, and on the boots in positions correspondent to malleolus and toe, on both body sides.
Work rate was calculated as the sum of power against gravity (Pg = m · g · sin α · v) and friction (Pf = m · g · cos α · μ · v), where m is the body mass of the skier (including equipment and additional weight for each interval), g is the gravitational acceleration, α is the angle of treadmill incline, v is the speed of the treadmill belt, and μ is the frictional coefficient [5, 17]. The rolling friction force (Ff) of the skis was determined prior to each test day by using a towing test, while the friction coefficient (0.026) was calculated by dividing the friction force by the normal force ((Fn): μ = Ff · Fn-1). The metabolic rate was calculated as the product of VO2 and the oxygen energetic equivalent using the associated respiratory exchange ratio and standard conversion tables [18]. Gross efficiency was calculated as the work rate divided by the metabolic rate and presented as a percentage.
The ski cycle was defined from ski lift-off to the successive ski lift-off. This was determined from the vertical displacement of retro-reflective marker placed on the lateral epicondyle of the skier (placed on the boot). These points served as trigger points, marking the beginning and end of each skate cycle for both G2 and G3 techniques. For each interval, 10 skating cycles were extracted for the analysis of cycle kinematics. The ski-cycle consists of the lift-off, recovery, and ground contact phases (i.e. gliding and push-off). The lift-off time is the time necessary for the foot to reach the highest point (vertical displacement) within the cycle, while the recovery time is the time it takes for the foot to travel from the highest point until the ski is planted onto the ground. The ground contact time was defined as the time between ski plant and ski lift-off. The analyzes were performed using custom-written Matlab™ (The MathWorks Inc., Version R2014a, Natwick, MA, USA) codes. The skis’ position in time was re-sampled to 500 data points in order to compare the four weighted conditions and ten participants. The cycle rate was calculated as the number of cycles per second.
All data were presented as mean ± SD in tables and as mean ± SEM in Fig 1. To examine the effects of distal loading within inclines and differences of loading between inclines, a marginal model (population-averaged model) was performed using the Statistical Package for Social Sciences 11.0 (SPSS Inc., Chicago, Illinois, USA). A repeated statement is used to specify covariance structures for longitudinal data on participants, where compound symmetry was used in fitting covariance structure for the residuals across techniques. For fixed effects (regression coefficients) we entered technique and weight (with an interaction term) into the model in order to describe the relationship between the dependent variable and predictor variables for the entire population. P-values < 0.05 were considered statistically significant.
Figure data removed from full text. Figure identifier and caption: 10.1371/journal.pone.0197592.g001 Effect of added weight on efficiency.Metabolic rate (A) and gross efficiency (B) for normal (0 kg, no added weight) and loaded (0.5, 1.0, and 1.5 kg) conditions for 10 elite cross-country skiers while roller skiing at steep (G2 at 12% incline) and moderate (G3 at 5% incline) slopes in the skating technique (mean ± SEM).
|
![[This page as RDF]](http://data.gesis.org/somesci/static/rdf-icon.gif){kind=icon}