All knee variables are shown in Table 4 and Figure 5B (including events 1-7). The KA at pole plant (KAPole_In; event 1) decreased between 15 and 27 km[middot]h−1 (P < 0.05), with no changes before and thereafter. Pole plant was followed by a considerable knee flexion across all velocities, where KA minimum during poling phase (KAmin_PP; event 3) gradually decreased from 148° ± 12° to 129° ± 13° through 27 km[middot]h−1 (all P values <0.05) and was then maintained. The knee flexion ROM during poling phase (ROMK_flex_PP; events 1-3) was unchanged at 16° ± 6° over all velocities, but the knee flexion time during poling phase (t) shortened through 27 km[middot]h−1 (all P < 0.05), resulting in an almost threefold increase of the knee flexion AV during poling phase (AVK_flex_PP) up to 143 ± 28°[middot]s−1 at V max (P < 0.05). The difference between KAPPF (event 2) and KAmin_PP (event 3) increased only between 15 km[middot]h−1 and V max from 2° ± 1° to 6° ± 3° (P < 0.05), and KAPPF occurred constantly 0.06 ± 0.03 s before KAmin_PP. The final part of the poling phase was characterized by a knee extension (events 3-4) of 4° to 8° across all velocities and was completed at a decreasing KA at the end of poling phase (KAPole_Out; P < 0.05) across V sm, with no further changes up to V max. The initial recovery phase showed a slight knee flexion (events 4-5) at higher velocities up to 6° followed by an accentuated extension phase in the knee joint (events 5-6). The KA maximum during recovery phase (KAmax_RP; event 6) was maintained at 165° ± 7° across all velocities and skiers. The knee extension ROM during recovery phase (ROMK_ext_RP) increased through 21 km[middot]h−1 (all P values <0.05), with no further change up to V max, whereas the knee extension time during recovery phase (t) showed a clear but inconsistent decrease from 9 km[middot]h−1 up to V max (P < 0.05). Altogether, this resulted in a fourfold increase of knee extension AV during recovery phase (AVK_ext_RP) up to 63 ± 17°[middot]s−1 through 27 km[middot]h−1 (P < 0.05), with no further changes up to V max. The final part of the recovery phase and the DP cycle was characterized by a knee flexion (events 6-7) that continued during the subsequent poling phase (events 1-3 of the next cycle). The knee flexion ROM during recovery phase (ROMK_flex_RP) increased through 27 km[middot]h−1 (P < 0.05), with no further shift up to V, whereas the knee flexion time during recovery phase (t) was maintained at 0.16 ± 4 s across all velocities and skiers. Regarding the total knee flexion phase from KAmax_RP to KAmin_PP (events 6-7/1-3), we found a consistently increasing knee flexion AV during C (AVK_flex_C) up to 124 ± 37°[middot]s−1 through 27 km[middot]h−1 (all P values <0.05), with no further shift up to V max.
The main findings of the study are as follows: 1) elite skiers control DP speed by increasing both Pf and CL; 2) increased CL at high speed is achieved by increased pole force, despite reduced poling time and time-to-peak pole force; 3) elbow joint kinematics during the poling phase adapt to higher speeds by decreasing angle minimum and increasing flexion and extension ROM as well as AV; and 4) leg work adaptations are characterized by a) decreasing HA and KA at pole plant and angle minima during the poling phase, b) increasing hip and knee flexion ROM and AV occurring around pole plant, and c) increasing hip and knee extension ROM and AV during the recovery phase.
The constant rise in Pf in the present study is in line with previous research into the DP technique (8,13,16) and has been proposed as the primary method of increasing DP speed (25). The significant increase in CL across V sm using the DP technique has not been reported in earlier studies. It had previously been shown that CL is maintained at V sm (8,13,16) and then remained unchanged (16) or decreased (8,13) at V max. So what can explain the fact that elite skiers in the present study adapted to increased V sm through a simultaneous increase of both CL and Pf with a consistent Pf/CL ratio?
First, our subjects performed DP at a higher level, with a 30-50% higher V max compared with the earlier studies (Figs. 1A and 2B). The skiers reached a V max of 29.5 km[middot]h−1 (8.2 m[middot]s−1) with a CL of 7.5 m at a Pf of 1.08 Hz (64.8 poling cycles per minute). From our point of view, it is unrealistic to attain such high velocities without increasing both Pf and CL. If we assume that skiers who performed at a CL of 4.4-5.7 m at their V max in earlier studies (8,13,16) were to ski at a speed of 27 km[middot]h−1, they would have had to increase their Pf by 32-72% (79-102 cycles[middot]min−1) to reach this target speed. This exceeds the reported frequencies that skiers use at racing velocities (11). One suggested explanation as to why skiers seem to limit their increase in Pf is to save energy. Each DP cycle involves a large amount of oxygen-demanding internal work in repositioning the skier between each cycle; this is reported to represent approximately 30% of the total energy cost (V˙O2peak_DP) during DP (9).
A second possible reason for the observed parallel increase in CL and Pf is that a new technical pattern has been developed among world class skiers over the last decade, leading to a more complex movement that increasingly involves the lower body (hip, knee, and ankle joints) and different arm and shoulder patterns (11). This new DP technique was found to be characterized by smaller joint angle minima (elbow, hip, knee, and ankle) and higher flexion AV, more abducted arm position ("wide elbow" pattern) (11), and less trunk flexion. This enables skiers to produce higher pole force and impulse (11) and thereby probably increase their CL. In addition, the longer recovery phase and lower Pf (Figs. 1C and 2A) of this modern form of DP has been shown to be related to a lower heart rate response and blood lactate concentration as well as to longer time to exhaustion (10).
If we look at other sports, we find different solutions to the CL-frequency relationship. Swimmers increase speed by increasing stroke rate (SR) accompanied by a decrease in stroke length (SL) with an increase in the SR/SL ratio (see (24,29)). This strategy can be explained by the high drag forces (F = constant × V) because water is 1000 times denser than air (6). The frequency increase helps to limit the greater reduction in speed during nonpropulsive phases compared with, for example, running (only air resistance) where most of the energy applied to the ground ends up as forward motion against less drag (6). In running, speed increases are controlled by increases in both step frequency and length, accompanied by greater ground reaction forces and rates of force development along with a decrease in ground reaction times (see (12)). This strategy of speed adaptation is in line with our results from examining DP on a treadmill, although in this situation skiers had to overcome rolling resistance without air resistance. The lack of air resistance has to be taken into consideration when interpreting the different results because in the current and previous studies (8,13,16) using roller skis or skis over ground, CL changes with speed. During a DP cycle, air drag forces vary continuously due to the change from a tucked body position during poling to a more upright position during the recovery phase (25). On the basis of data from Svensson (28), air resistance can be estimated as being between approximately 10 N for a tucked position and approximately 20 N for an upright position for the highest DP speed in the current study (8.2 m[middot]s−1). Consequently, the influence of increased air resistance at the highest speeds might have altered our CL versus velocity graph somewhat in comparison to DP over ground. Altogether, today's elite skiers, using a modernized DP technique, follow a speed increase strategy that is similar to various other forms of human terrestrial and aquatic locomotion. These are characterized by a curvilinear relationship between cycle rate versus speed (concave-up shape) and CL versus speed (concave-down shape), with the CL maxima attained at the upper end of the speed range ranging between 60% and 100% of the corresponding maximum speed (5).
The CT and therefore the absolute poling and recovery times decreased across V sm, with no further change at V max in our study. This is in contrast to previous studies that have reported a continuous decrease over velocities (13,16) (Fig. 1A-C). Interestingly, these studies also showed only slightly longer (0.30 s) poling times at each V max compared with our study (0.26 s), whereas maximum velocities were approximately 9 km[middot]h−1 lower. As regards absolute recovery time, we found distinctly higher values in all V sm and V max when compared with the aforementioned studies. Of note is that a longer recovery phase has been shown to be one criterion for a modern and fast DP strategy in elite cross-country skiers with lower blood lactate and heart rates (10,11).
The short pole ground contact phases at high velocities (<0.28 s) consisted of short elbow flexion times (<0.11 s), short time-to-peak pole force (<0.08 s), and elbow extension times (<0.17 s). It could be argued these short absolute times represent a critical limit for the full recruitment of both type I and type II fibers for sufficient force generation. Studies on force-velocity characteristics regarding contraction velocity on single fibers in vivo demonstrated that type I fibers need 100-140 ms and type IIA fibers need approximately 55-85 ms to create maximal tension and thereby force (1-3). These time gaps for slow twitch fibers are longer than the elbow flexion time and time-to-peak pole force found in the present study at the highest velocities. Thus, skiers might not be able to fully recruit both fiber types at high velocities. To overcome this, elite skiers use distinct muscle preactivation before pole plant at higher velocities to reinforce the generation of pole force (11). In running, higher preactivity is related to greater stiffness of the tendomuscular system in the subsequent eccentric phase of the foot ground contact, and this relationship becomes even stronger as running speed increases (see (12)). Preactivity is assumed to increase the sensitivity of the muscle spindle through enhanced alpha-gamma coactivation resulting in a potentiation of stretch reflexes, which are able to enhance force production not only in the eccentric but also in the concentric and propulsive phases of running (4,15). These are phenomena that may also play a similar role in cross-country skiing and especially in DP.
As regards to short ground contact times in human locomotion, Minetti (14) has stated that the inverse relationship between contact time and speed means the ability to store and release mechanical energy to and from the elastic structures is a limiting factor. A second problem is the constraint of stopping the foot or the pole with respect to the ground, which would imply that muscle contraction generates less force for propulsion, as it is directly related to progression speed. To circumvent these limitations, Minetti (14) suggests searching for a new pole design that would allow the pushing portion of the pole to slide. This way, muscle contraction speed would be much lower while continuing to slide on the medium.
The ability to produce short ground contact times in combination with high force production seems to be crucial, not only in attaining high maximum velocities but also for movement economy in cross-country skiing. Several studies (e.g., [18,20,22]) have found that endurance performance and peak treadmill performance on level terrain are influenced not only by central factors related to V˙O2 but also by so-called muscle power factors related to neuromuscular characteristics (e.g., voluntary and reflex neural activation, rate and force of myofibrillar cross-bridge cycle activity, muscle force, and running mechanics).
Relative poling and recovery times decreased and increased, respectively, across V sm and thereafter leveled off. Conversely, Millet et al. (13) demonstrated a preservation of relative poling and recovery time over V sm with a subsequent increase up to V max, accompanied by a shortening of relative recovery time (Table 1 and Fig. 1D). In addition, Nilsson et al. (16) did not report any difference over velocities, suggesting increased ground contact time as a potential strategy for increasing speed. Altogether, this is contrary to our results, which clearly demonstrate skiers' adaptation by a shortening of poling time from approximately 40-30% CT (recovery time: ∼60-70% CT) through 21 km[middot]h−1 and by maintaining both variables as speed increased up to V max. In the present study, the constant relative poling time in the last three velocities is approximately 25% shorter and consequently has a longer relative recovery time when compared with the above mentioned studies (Table 1 and Fig. 1D). Such differences between faster and slower athletes have already been reported by Holmberg et al. (10). This new pattern emphasizes that modern competitive skiers use a technical strategy across velocities, in which they first reduce poling time to a "critical" threshold and thereafter try not to fall below a duration that might limit a sufficient production of force.
In accordance with Millet et al. (13), peak pole force in the current investigation increased with V sm. In addition, the decrease of time-to-peak pole force from 0.21 to 0.07 s over the various velocities caused a fourfold increase in the rate of force development. Our results add to the understanding of the effect of speed increases on force characteristics during DP. Due to the aspect of distinctly reduced poling times at high velocities, as discussed above, it is important to have not only fast but also substantial force production to create a high CL and to not only work with increases in Pf. It has been shown that the better skiers had a higher peak pole force and a shorter time-to-peak pole force at racing speed (11). In the present study, the importance of fast and large pole force production at maximum speed was clearly observed. In comparison to pole force, plantar force showed less alteration with speed. However, a clear load shift from the whole foot to the rear of the foot was observed, whereas forefoot force was unchanged regardless of speed. It can be postulated that the increased rearfoot force during the poling phase was caused by an emphasized pushing-forward movement in the lower leg (slight knee extension) across increased velocities (Fig. 5B). We interpret this as a strategy where the skiers get in a hunched, forward-leaning position with the aims of 1) maintaining forward-backward balance, 2) creating a slight counteraction (action-reaction) in opposition to the backward-directed final pole thrust to generate higher horizontal pole forces, and 3) shifting pressure from the grip wax zone to the gliding zone at the rear part of the ski, thereby reducing gliding friction when DP on snow.
Our data are the first to describe joint kinematics during DP at different velocities and are discussed below using an integrative approach. Elbow position at pole plant remained constant at all velocities compared with a distinct decrease in HA and KA across V sm (Figs. 4 and 5A and B). Skiers adapted to speed increases with a more flexed body position at pole plant, which perhaps placed the poles in a more advantageous position for generating propulsive force. All the joints investigated showed an emphasized flexion during the poling phase through 27 km[middot]h−1, with decreasing angle minima and flexion times (Tables 2-4) and with increasing flexion AV with the highest values in the elbow joint (344°[middot]s−1). In total, this helped to increase the peak pole force and the rate of force development during the poling phase. Elbow extension during the poling phase, another important propulsive factor, was characterized by a decrease in extension time and an increase in extension ROM, causing a remarkable increase in AV (173-548°[middot]s−1). In comparison, hip and knee extensions were much less distinct, which might be explained by the fact that the lower body only has an assisting function (stabilization and counteraction) regarding the generation of pole force during the final part of the poling phase. It could be suggested that the small knee extension (Fig. 5B; events 3-4) causes a minor pushing-forward movement in the lower leg at the end of the poling phase that may act as a slight counteraction, reverse to the pole push-off direction.
In contrast to the elbow, the hip and the knee joints also have important roles during the recovery phase 1) to perform a body extension, thereby raising the CoG and positioning the body ready for the next pole plant, and 2) to enable the use of the whole body with coactions between arms, trunk, and legs for pole force generation by an extensive hip (trunk) and knee flexion shortly before pole plant (10,11). These important extension-flexion patterns that match pronounced force production increase in range across velocities. It is noteworthy that the maximal HA and KA during the recovery phase did not change across velocities, whereas the extension ROM, starting from the low crunched DP position at the end of the poling phase, increased. This can be explained by the decreasing joint angles at the event "pole-out" (Fig. 5A and B; event 4). The hip and the knee extensions AV of >100°[middot]s−1 during the first two thirds of the recovery phase at racing velocities >25 km[middot]h−1 may be considered a challenge from a coordinative-technical point of view. An accentuated trunk extension requires a high level of timing and back extensor muscle strength; particularly at high velocities, the maximal hip and knee extensions must be reached early enough in the DP cycle to be ready for the subsequent, effective joint flexion toward the next pole plant. From an energetic perspective, this repetitive up-and-down movement is a challenge because the upper body represents approximately 70% of total body mass (30). The hip and the knee flexion with increased ROM and AV continued down to the angle minima during the poling phase and has been shown to be correlated to the production of high pole forces and thus to DP performance (10,11).
The limitations of cross-country skiing research on an indoor treadmill include the estimation of the potential effects of air resistance. One possibility for solving this would be to perform a study in a wind tunnel, which would enable researchers to specifically examine the influence of air resistance on CL, which is likely to increase at higher speeds. Another constraint in the current study is the lack of 3D data that preclude a more detailed analysis of some specific aspects of DP. One of these is the movement of the shoulder joint with the typically more abducted arm position used in modern DP (11) and its possible relation to the generation of pole force. Another aspect is a more complex examination of trunk and pole angles as well as trajectories of CoG in different planes during DP.
In summary, the present study shows that elite skiers are using a strategy to generate higher velocities by increasing both Pf and CL. The increase in CL is achieved by an increased force production during a shortened poling time, approaching physiologically critical values with a limited time for the recruitment of both slow and fast twitch fibers for sufficient force generation. This is managed by increased ROM, shortened flexion-extension times, and increased AV. It is notable that the functional importance of the recovery phase increases with speed due to the required 1) faster and more extensive repositioning of the body before the next poling phase and 2) larger and faster hip and knee flexion toward the next pole plant, thus providing a pronounced and rapid force production across speeds. The strategy described above needs a well-developed ability to produce great pole force over limited time, emphasizing the demands for specific explosive strength and highly developed motor skills.
The authors thank the support of Prof. Dave Bacharach and Prof. John Seifert at St. Cloud State University, St. Cloud, MN. The authors also thank the athletes and the trainers for their participation, enthusiasm, and cooperation in this study. This study was supported by the Swedish Olympic Committee. The results of the present study do not constitute endorsement by ACSM.
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Keywords:© 2009 American College of Sports Medicine
CYCLE LENGTH; FREQUENCY; JOINT ANGLES; PERFORMANCE; PLANTAR FORCE; POLE FORCE