In cross-country skiing, the legs are the main energy consumers (27). Surprisingly, this is also true in double poling (DP) (27), which often is considered primarily an upper-body exercise (8,11,15,20,29). A biomechanical analysis of DP (12), including both the upper and lower body, gave further support for the notion that the legs play a critical role in DP performance. There was marked electromyographic activity in several of the lower-body muscles and substantial flexion-extension movements in the hip, knee, and ankle joints. Moreover, the angle minima of these joints occurred very close to the time of peak pole force. This indicates that more accentuated movements in these joints enable a skier to more effectively use body mass and gravity, in addition to upper-body muscle activity, to optimize poling force and thus DP performance (12).
Another as yet uninvestigated advantage with the involvement of the lower body during DP is that it enables skiers to share a given workload over a larger amount of muscle mass, which may positively influence both the physiological response and DP performance. To examine the role of movement in knee and ankle joints concerning DP performance more specifically, a combined physiological and biomechanical analysis is needed. A comparison of a "normal" (DPFREE) with a "locked" (DPLOCKED) DP situation, in which both knee and ankle joints were restricted in angle displacement, would enable further insight into the functional importance of lower-body movement during DP.
The specific aims of the present study were to compare DPFREE and DPLOCKED, with the hypotheses that there are differences regarding 1) kinetic and kinematic DP cycle characteristics, 2) physiological response, and 3) DP performance.
Eleven elite cross-country skiers volunteered as subjects. The skiers' mean ± SD age was 21 ± 1.8 (range 20-25) yr, height was 179.1 ± 4.7 (171-185) cm, weight was 70.6 ± 8.0 (56-83) kg, pole length was 151 ± 4 (143-155) cm, and maximal oxygen uptake (V˙O2max DIA) was 72.3 ± 3.8 (65-80) mL·kg−1·min−1. All of the skiers were fully acquainted with the nature of the study before they gave their written informed consent to participate. The research techniques and experimental protocol were approved by the ethics committee of Umeå University, Umeå, Sweden (# 03-080).
Overall Design of the Study
Physiological and biomechanical measurement procedures were performed on the selected subjects. First, to characterize the subjects, all skiers performed a V˙O2max test, roller skiing on a treadmill using the diagonal skiing technique; and a test for measuring arm peak oxygen uptake (V˙O2peak ARM), using an arm ergometer. Secondly, two DP incremental tests were performed while roller skiing on a treadmill at 1° inclination, a) without and b) with locked knee and ankle joints (DPFREE vs DPLOCKED) (described more in detail below). Finally, kinetic and kinematic analyses were performed with each subject's DPFREE and DPLOCKED at the velocity corresponding to 85% of their maximal DPFREE velocity (V˙85% FREE). A similar inclination as in the present study has been used in previous studies investigating DP on a treadmill (1° vs 1.7%) (12,15). Physiological tests were carried out on separate days with 48 h in between, and the biomechanical comparison between the two DP situations was performed on the same day, separated by 2 h. All tests were performed in a random fashion within a 14-d period.
Instruments and Methods
Oxygen uptake (V˙O2) was measured using an ergospirometry system AMIS 2001 (Innovision A/S, Odense, Denmark) based on the mixed expired method with an inspiratory flowmeter. For the gas analyzer calibration, high-precision gases (16.00 ± 0.04% O2 and 4.00 ± 0.1% CO2; Air Liquide, Kungsängen, Sweden) were used. Before each test, ambient conditions were measured and the gas analyzers and inspiring flowmeter were calibrated. The calibration of the flowmeter was performed with a 3.0-L air syringe (Hans Rudolph, Germany) at a low, medium, and high flow velocities. Samples of blood (20 μL) were analyzed for blood lactate concentration by Biosen 5140 (EKF; Diagnostic Gmbh, Magdeburg, Germany). Calibration of the blood lactate analyzer was performed before each test and checked using a lactate standard of 12 mmol·L−1. Voltage was checked using a control solution of 4.8-6.4 mmol·L−1. Heart rate was measured using a heart rate monitor (model Polar S610, Polar Electro OY, Finland). Rating of perceived exertion (RPE) was measured using the 6-20 RPE scale (5).
All subjects used carbon-fiber racing poles. The right-hand pole, specially constructed for force measurements and adjustable in length from 140 to 165 cm, enabled the athletes to adjust the pole to their preferred individual length (84 ± 0.5% of body height). The ground-reaction force directed along the pole was measured at 2000 Hz by a strain-gauge force transducer (Hottinger-Baldwin Messtechnik GmbH, Darmstadt, Germany) weighing 60 g and installed in a lightweight (75 g) aluminum body, both mounted directly below the pole grip. The pole force transducer was calibrated using a specific calibration apparatus with 10 different standard weights (5-50 kg). A validation of pole force was then performed by 30 DP imitations on an AMTI force plate (2000 Hz) (AMTI, Watertown, MA). The mean absolute error over the entire poling phase was 3.8%. This is equivalent to maximum mean pole force deviations of −8% (−20 N) occurring around the force maxima and 10% (26 N) in the last third of poling phase. Relative peak pole force and relative impulse of pole force were determined. All relative values were expressed as percentages of body weight (BW). Elbow-, hip-, knee-, and ankle-joint angles were measured using goniometers (potentiometers: Megatron, Munich, Germany; strain gauges: Penny and Giles Controls Ltd, Cmwfelinfach, UK) at 2000 Hz. Calibration measurements were performed five times at 90, 180° (elbow, hip, and knee joint), and 110° (ankle joint), and angle values were calculated from the corresponding mean voltage data. One DP cycle was defined as the period from the start of the pole-ground contact to the start of the subsequent pole-ground contact. Each DP cycle was divided into a poling phase and a recovery phase. All data were averaged over five cycles for each subject. Cycle time (CT), poling time (PT), and recovery time (RT; determined by subtracting PT from CT) were determined for each DP cycle. Poling frequency (Pf) was calculated from the pole force data (Pf = 1 · CT−1). All data were collected by a complete measurement system (Biovision, Werheim, Germany) consisting of an input box with 16 channels connected to an A/D converter card (DAQ 700 A/D card-12 bit, National Instruments) and a portable pocket PC (Compaq iPAQ H3800) to store the kinetic and kinematic data for further offline analysis. The processing of all data was managed by IKE master (IKE-Software Solutions, Salzburg, Austria).
Treadmill and arm ergometer.
All roller-skiing tests were performed on a motor-driven treadmill (Rodby, Sodertalje, Sweden) specially designed for roller-ski tests. To exclude variations in rolling resistance, all subjects used the same pair of roller skis (Pro-Ski C2, Sterners, Nyhammar, Sweden). The roller skis and bearings were prewarmed for 10 min immediately before each test by an independent skier passively rolling at a velocity of 18 km·h−1. The subjects were secured with a safety harness suspended from the ceiling before all treadmill tests. All subjects were accustomed to training and testing on the treadmill using both diagonal skiing and DP. The arm ergometer was a mechanically braked cycle ergometer (Monark AB, Vansbro, Sweden), with the handgrips mounted on the cranks. The ergometer was calibrated before each test with a 4000-g weight in accordance with the manufacturer. The subjects were seated behind the ergometer, with the height of the chair adjusted so that the arms, when extended, were below the heart level. Subjects were required to minimize their use of lower-extremity musculature.
Fixation of knee and ankle joints.
The aim was to induce the DPLOCKED situation by restricting the knee- and ankle-joint movements. The full knee fixation was obtained by using two specially constructed light (538 g each) knee orthoses (MediRoyal, Sollentuna, Sweden). Their pin joints were seized, and the whole orthoses were additionally fixed by adhesive tape around the knee. The orthoses were worn in both situations, that is, during DPLOCKED (locked pin joint) and DPFREE (unlocked pin joint). In the ankle joint, the fixation was obtained by 1) strapping the heels down to the roller ski by a belt to avoid elevation of the heels (plantar flexion), and 2) adding adhesive tape around the ankle joint and the roller ski to limit lower-leg movement relative to the roller ski. The latter still allowed a small movement (~10°) in the ankle joint to ensure that the skiers could maintain a good balance. All fixation procedures were performed by an orthopedic engineer. After fixation, the subject's perceived feeling was registered during a 10-min period before the test started to ensure that the fixation was comfortable. During the first 8 min, the subjects were stood passively on the floor, followed by 2 min performing 40 DP simulations without poles. All subjects performed six DP training sessions on different days with the knee and ankle joints locked before the physiological and biomechanical measurements were performed, to accustom the subjects to the fixation during exercise (Fig. 1).
V˙O2max was measured during diagonal skiing with roller skis on a treadmill. The velocity was set to 10.5 km·h−1, with a progressive inclination of 1°·min−1 starting at 4° and up to 11°. If the subject could continue after 11°, the velocity increased 0.3 km·h−1 every 30 s. Voluntary exhaustion occurred between 6 and 8 min. V˙O2peak ARM was measured during arm cranking. The frequency was metronome paced at 70 rpm, and the work rate was increased by 15 W·min−1. The initial workload was selected so that the subjects would become exhausted within 6-8 min. The incremental DP tests (DPFREE and DPLOCKED) were performed using a continuous progressive protocol at a constant inclination of 1°, starting at 9 km·h−1 and increasing 3 km·h−1 every work period of 4 min with 1 min of rest in between until voluntary exhaustion. V˙O2, V˙CO2, ventilation (V˙E), tidal volume (VT), breathing frequency (fb), and heart rate were measured during the last minute of every work period. Blood lactate concentration was measured directly after the treadmill was stopped, at which point the subjects also were asked to rate their RPE. Blood samples for determination of blood lactate concentration were collected 1, 3, and 5 min after maximal exercise. V˙max was calculated using the formula V˙max = Vf + ((t/240) · 3 km·h−1), where Vf is the velocity of the last completed workload (km·h−1), t is the duration of the last workload (s), and 3 km·h−1 the velocity difference (ΔV) between the last two. V˙O2max DIA measured using the diagonal stride technique, V˙O2peak DPFREE, and V˙O2peak DPLOCKED using the DP technique in the two compared DP situations were calculated by averaging the three highest 10-s consecutive measurements of O2 uptake. Peak value was defined if two of the following three criterion were reached: a respiratory exchange ratio > 1.10, a blood lactate concentration > 8-9 mmol·L−1 (1), and a rate of perceived exertion > 17.
The kinetic and kinematic analyses of DPFREE and DPLOCKED were performed at each individual's V85% FREE, which was 24.5 ± 1.4 (22.3-26.3) km·h−1. This velocity was chosen for this study because it was close to individual racing velocities. Relative velocity was chosen to provide similar relative exercise intensity for interindividual comparison to equalize the situation for all skiers.
All data were checked for normality and presented as means (x¯), ranges (x¯min−x¯max), and standard deviations (SD). Physiological, kinematic, and kinetic data were compared between the two measuring situations, DPFREE and DPLOCKED, at V85% FREE using Student's t-tests for paired samples. Statistical significance was set at P < 0.05. All statistical tests were processed using SPSS 11.0 Software (SPSS Inc, Chicago, IL) and Microsoft Office Excel 2003.
Peak exercise values.
Maximal DP velocity (V˙max) was 9.4% higher during DPFREE than during DPLOCKED (29 ± 1.9 (26.1-30.8) vs 26.5 ± 1.5 km·h−1 (24.5-28.4)), and time to exhaustion in the incremental test was 11.7% longer (37 min 49 s (32:45-41:06) vs 33 min 51 s (30:43-37:15)) (both P < 0.05) (Fig. 2). The skiers obtained 7.7% higher V˙O2peak during DPFREE than during DPLOCKED (4.65 ± 0.6 (3.7-5.3) vs 4.29 ± 0.6 L·min−1 (3.3-5.1)) (P < 0.05) (Fig. 3). The V˙O2peak during DPFREE corresponded to 91.1% of their V˙O2max. The skiers reached a V˙O2peak ARM of 3.98 ± 0.4 L·min−1 during arm cranking, corresponding to 78% of their maximal oxygen uptake obtained during diagonal skiing (Fig. 3). Oxygen pulse at maximal exhaustion was 4.9% higher during DPFREE compared with DPLOCKED (19.1 ± (15.7-24.7) vs 18.2 ± mL·beat−1 (14.2-23.2)) (P < 0.05) (Fig. 4C). V˙E/V˙O2 was higher at maximal exhaustion during DPLOCKED, whereas there were no differences in heart rate, blood lactate concentration, V˙E, VT, or fb between the two exercise modes (Figs. 4 and 5).
Physiological comparison at submaximal velocities.
Heart rate was 3-6% higher during DPLOCKED than during DPFREE (P < 0.05), with no difference in V˙O2 at submaximal velocities (Fig. 4A,B). The oxygen pulse was 3-10% lower in DPLOCKED compared with DPFREE at all submaximal velocities with significant differences at 9 and 12 km·h−1 (P < 0.05) (Fig. 4C). V˙E was higher in the locked situation at 18, 21, and 24 km·h−1 (P < 0.05), with no difference in VT and fb at 18 km·h−1 but with significantly higher values at the two highest submaximal velocities (21 and 24 km·h−1; both P < 0.05) (Fig. 5A-C). V˙E/V˙O2 was higher during DPLOCKED at 18, 21, and 24 km·h−1 (all P < 0.05) (Fig. 5D). Blood lactate concentration was higher during DPLOCKED at submaximal velocities (12-24 km·h−1; all P < 0.05) (Fig. 4D).
Biomechanical comparison of DPFREE and DPLOCKED at V85% FREE.
The skiers reached 9.1% lower impulse of pole force (4.9 ± 0.5% BW (4.1-5.8) vs 4.5 ± 0.7% BW (3.5-5.4)) and 10.9% lower peak pole force (32.1 ± 7.5% BW (22-44) vs 28.7 ± 9.5% BW (21-47)) during DPLOCKED (P < 0.05). CT decreased 11.1% from 1.13 ± 0.09 s (1.01-1.30) to 1.01 ± 0.09 s (0.89-1.14) (P < 0.05) (Fig. 6). The shortened CT was accompanied by a 4.9% shortened PT from 0.30 ± 0.03 s (0.25-0.34) to 0.29 ± 0.03 s (0.25-0.33) (P < 0.05) and a 13.3% shortened RT from 0.83 ± 0.09 s (0.77-0.99) to 0.72 ± 0.08 s (0.63-0.88) (P < 0.05). The Pf during DPLOCKED (1.03 ± 0.14 Hz (0.88-1.31)) was 13.6% higher compared with DPFREE (0.89 ± 0.07 Hz (0.77-0.99)) (P < 0.05) (Fig. 6).
The main findings in the present study comparing free and locked knee and ankle joints during DP are that the locked situation elicited 1) a higher blood lactate concentration and heart rate response at submaximal intensities, with no differences in oxygen consumption; 2) a lower pole force and a higher Pf at 85% V˙max; 3) a lower V˙O2peak and V˙max; and 4) a shorter time to exhaustion. Altogether, these data demonstrate that movements of the knee and ankle joints are an integrative part in the skillful use of the DP technique and that restrictions of the motion in these joints during DP markedly affect both biomechanical and physiological parameters and impair DP performance.
Biomechanical comparison at submaximal work (V85% FREE).
The fixation of the knee and ankle joints during DPLOCKED caused a 9.1% lower relative impulse of pole force (IPFrel) during the poling phase at the same absolute velocity (Fig. 6). This is explained by a 10.9% lower relative peak pole force (PPFrel) and a 4.9% shorter absolute PT (PTabs) (IPFrel = PFrel · PTabs). A possible mechanism as to why the generation of pole force was impaired when the free movement in the leg joints was restricted was first indicated by Holmberg and coworkers (12). They found a relationship between the dynamic of extension-flexion movement in the knee and ankle joints to pole force and DP velocity, respectively. This was explained by a "high" DP starting position with extended knee and ankle joints, followed by a quick flexion in these joints during the poling phase, allowing the skiers to enhance their use of body mass and gravity (12). This also reinforces the production of impulse of pole force by an increase of peak pole force and a lengthening of PT. Furthermore, a shortening of the RT by 13% during DPLOCKED, together with a slightly shortened PT (3.3%), resulted in shorter total cycle duration. The longer PT during DPFREE and the more active flexion in hip, knee, and ankle joints might have been accompanied by a slightly more acute pole angle to the ground towards the end of poling phase compared with DPLOCKED. This might have optimized force application because of a slightly larger horizontal force component. However, because the resultant pole force towards the end of poling phase is small compared with the first third of ground contact and goes towards zero (12), we regard this as having a minor influence on the higher performance during DPFREE. Future studies combining kinetic and kinematic analyses are required to investigate this aspect of DP more deeply. Consequently, from all discussed aspects, to maintain the same DP velocity during DPLOCKED, the skiers had to compensate by increasing their Pf, indicating that the poling phase was less efficient without active leg work.
Physiological comparison at submaximal velocities.
Blood lactate concentration and heart rate were higher at 12-24 km·h−1 during DPLOCKED, with no differences in oxygen consumption (i.e., work economy). We suggest three possible explanations why DPLOCKED elicited a higher blood lactate concentration at the same absolute velocities. First, and most importantly, the work during DPLOCKED was performed at higher relative work intensities, closer to the skiers' mode-specific V˙O2peak (Figs. 2 and 3). It can be assumed that this caused a higher relative load on the muscles used for DP propulsion. This results in an earlier recruitment of the more glycolytic type II fibers (26) and a higher intramuscular pressure, which may have limited the perfusion to the working muscles (21). Secondly, the higher lactate response may be a result of the markedly shorter RT during DPLOCKED because blood flow to exercising muscles is largest during the relaxation phase between muscle contractions (17). Thirdly, the higher lactate accumulation during DPLOCKED is an effect of the skiers being prevented from moving their legs during the DP movement. This is supported by studies showing that skeletal muscle is the most important tissue for lactate production and clearance during exercise and also by studies showing that active skeletal muscle can be a substantial net lactate consumer (9,26). This has been recently shown during DP (27). In addition, several studies have shown that use of lactate by skeletal muscle appears to be higher when light exercise is performed compared with at complete rest (9,26). This suggests that even a small activation of lower body muscles, as shown in the DPFREE situation (12), may be important for the total lactate clearance during DP. The higher HR and ventilatory response during DPLOCKED at submaximal velocities might reflect the more pronounced metabolic acidosis during this exercise mode.
Peak oxygen uptake.
Compared with arm cranking, the skiers in the present study obtained 7.8 and 16.8% higher V˙O2peak during DPLOCKED and DPFREE, respectively. This could be explained mainly by differences in the amount of active muscle mass (4,23), suggesting that DP is more than just upper-body exercise (12). Arm cranking is traditionally regarded as an upper-body exercise, primarily involving arm and torso muscles and, to a minor extent, leg muscles to stabilize the torso during more intensive exercise (24,25). The significantly higher V˙O2peak in DPLOCKED compared with arm cranking may be explained by the crucial role the leg muscles play in balancing the body during stance in this exercise mode. It is also possible that this difference was influenced, to some minor extent, by the fact that arm cranking is an unspecific exercise mode for a skier. In addition, during DP, the rectus femoris assists hip flexion (12), and the hamstrings and gluteal muscles act eccentrically to first decelerate the rapid forward movement of the trunk during the poling phase and then to assist in the extension movement of the upper body to a new starting position during the recovery phase.
The 7.7% lower V˙O2peak in the DPLOCKED situation compared with the DPFREE situation could be the result of a smaller amount of active muscle mass, which could be assumed to decrease or at best maintain the arteriovenous O2 difference at the same level as during DPFREE. It should be mentioned that this difference in relative amount of active muscle mass is only an assumption because it is not possible at present to quantify the activity level of each individual skeletal muscle during exercise (22). Therefore, we cannot rule out the possibility that the total active muscle mass was similar. However, the highly dynamic movement pattern and large amplitude in the leg joints during DPFREE (12) compared to DPLOCKED, and the substantial difference in V˙O2peak, suggest the opposite. Another possible factor in the lower V˙O2peak during DPLOCKED is that the restricted motion in the knee and ankle joints might have negatively influenced the peripheral muscle pump (18). The consequence of this would be an impaired venous return and thereby a decreased stroke volume. Both of these possibilities are supported by the lower oxygen pulse at V˙max during DPLOCKED. A third factor to consider is that the fixation of the ankle and knee joints in a decisive manner may have disturbed the skiers' normal technical pattern, hindering them from reaching the same absolute velocity as in DPFREE. Although the subjects were accustomed to the DPLOCKED situation, it still may have had a minor influence on the results.
It could be asked what level of V˙O2peak is possible during DP by training. Based on the results of another study on skiers with a similar V˙O2max (5.1 L·min−1) (7), it can be assumed that the skiers in the present study could, at most, reach a cardiac output of 30 L·min−1. If approximately 40% of the blood during submaximal DP is distributed to the arms and approximately 35% is distributed to the legs as shown by Calbet et al. (7), it could be estimated that the arms would demand a blood flow of approximately 6.0 L·min−1 per arm to approach V˙O2max during DP. Theoretically such blood flow is extremely high compared with what has been reported in the literature (13,28). Furthermore, it is well above 5.2 L·min−1, which is, at present, the highest reported arm blood flow for skiers (6,7). The latter implies that some of today's elite cross-country skiers already may perfuse their upper body maximally during DP. The critical question is whether it is possible to further increase upper-body blood flow and/or O2 extraction by training to enhance aerobic yield and performance during cross-country skiing with the DP technique. Three potential ways to achieve a higher V˙O2peak closer to V˙O2max would be i) to increase upper-body skeletal muscle mass (3) through hypertrophy strength training, ii) to increase peripheral vasodilatory capacity and O2 extraction by exercising upper-body skeletal muscles (6,7), and iii) to involve a larger total amount of body muscle mass during DP through a technical modification of DP movement, for instance, more active motion in the knee and ankle joints (12). Among these, the latter would be the most time-efficient intervention.
Maximal DP velocity and time to exhaustion.
The skiers in the present study obtained 9.4% higher V˙max and 11.7% increased time to exhaustion during DPFREE compared with DPLOCKED. By allowing the skiers to use a specific technical pattern, both a higher pole force and a higher impulse of pole force were obtained. The increased force generation enabled the skiers to reach a higher maximal DP velocity, the importance of which has been shown in previous studies on DP performance (12,14). At lower velocities, a lower impulse of pole force can be compensated for by an increase in Pf. However, this adaptation reaches an upper limit with increasing DP velocity because PT becomes too short and Pf too high to produce enough pole force to maintain a high DP velocity and obtain a high V˙max. Because time to exhaustion has been shown to be a function of relative exercise intensity (19), the longer time to exhaustion during DPFREE may be explained by a lower relative workload at submaximal DP velocities attributable to the higher V˙max. Moreover, it is reasonable to assume that it may also be an effect of an increased amount of active muscle mass in the DPFREE situation. This assumption is indirectly supported by early data from Åstrand and Saltin (2), who demonstrated that the time to exhaustion for combined arm and leg cycling was markedly improved compared with only leg cycling at the same work rate. The prolonged time to exhaustion in the present study was achieved without any improvement in DP economy. This has previously been shown as a potential factor in prolonging work duration during high-intensity DP (10,11,16). However, this does not exclude a reduced relative workload on the upper-body muscles during DPFREE because measuring total V˙O2 does not give information about regional oxygen consumption. This is essential for full evaluation of the relative activity level in different body regions during DP (6,7).
The present study shows that the DPFREE condition resulted in 1) biomechanical differences, examined at 85% of maximal DP velocity, with a higher impulse of pole force and peak pole force, a longer RT during each poling cycle, and a lower Pf; 2) a lower heart rate response and blood lactate concentration, with no difference in oxygen consumption at submaximal velocities; 3) a higher peak oxygen uptake and maximal DP velocity; and 4) a longer time to exhaustion. These results confirm the formulated hypotheses of this study. Therefore, it can be recommended that cross-country skiers consider the potential of involving more muscle mass in DP with a minor technique modulation towards a more dynamic use of the legs to improve DP performance and decrease cardiovascular and metabolic response at submaximal velocities. Altogether, these findings demonstrate the functional importance of involving the lower body during DP to optimize DP performance.
The authors would like to express their appreciation for the helpful data analysis advice supplied by Prof. Bengt Saltin at Copenhagen Muscle Research Centre, Denmark and the support by Prof. David W. Bacharach at St. Cloud State University, USA. The authors also thank the athletes and trainers for their participation, enthusiasm, and cooperation in this study and the National Wintersport Centre in Östersund for providing the facilities. This study was supported by the Swedish Olympic Committee.
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Keywords:©2006The American College of Sports Medicine
CROSS-COUNTRY SKIING; KINEMATICS; KINETICS; PEAK OXYGEN UPTAKE; UPPER BODY; WORK ECONOMY