To improve performance and prevent injury during competition, the athlete commonly prepares by performing sports-like training. To tailor such sport-specific training models, the pattern of muscle use during the competitive event needs to be revealed. The present study examined the pattern of muscle use in elite alpine skiing. We hypothesized that muscle control during alpine skiing is influenced by the accelerative forces that result from the fast downhill displacement and the consecutive sharp turns performed through a race course.
Thus, we performed kinematic studies of world class alpine skiers in their competitive events. The different disciplines were compared with reference to concentric and eccentric muscle use in relation to positions and angular movement velocities of the hip and knee joints. The main focus was on giant slalom (described in (2)) and slalom skiing. However, to get an impression of the range of variation among the different disciplines of alpine skiing, we also performed measurements on selected individuals performing super-G and freestyle mogul skiing.
MATERIALS AND METHODS
Sixteen male alpine skiers, all members of the Swedish National Senior or Junior Teams, including two World Cup winners and two World Championship medalists, were studied during the performance of either super G (SG), giant slalom (GS), slalom (SL), or freestyle mogul (FM) skiing. All subjects provided informed consent. Data were obtained in ski slopes used for World Cup competitions. Results obtained during GS and a detailed description of the methods and equipment used have been reported (2).
Briefly, a portable data acquisition system, including a data tape recorder, amplifiers, electrogoniometers, and electromyographic (EMG) electrodes, was worn by the skier while performing two trials in a race course for any specific event. Unilateral knee and hip-joint angles and rectified and filtered EMG signals from the knee extensor muscles (v lateralis, v medialis) were recorded during skiing. Subsequently, angular velocities of the hip and knee joints were calculated. Maximum isometric EMG obtained at 90° (1.57 rad) knee angle (180° = straight leg) during indoor laboratory trials, before and after skiing, was used as the reference for maximal voluntary contraction (100% MVC). EMG signals were subsequently normalized and expressed in percent of EMG at MVC.
To quantify knee-extensor activity and to relate it to angular positions and velocities, data was analyzed in the following manner. Movement cycles (one left plus one right turn) were identified and subdivided from knee joint angle maxima and minima (for details see (2)). EMG activity was averaged for each subdivision (time sequence) of the movement cycle. Joint angles and EMG were then averaged across 5-11 such movement cycles from each individual run and subsequently across the two consecutive runs.
Differences between or within GS and SL skiing were compared using unpaired or paired t-test. No statistical comparisons were performed for SG or FM.
In all disciplines minima and maxima values of the hip and knee-joint angle coincided indicating closely correlated movements in these joints. Overall knee-angle minima invariably related to the inside, usually less activated, leg during SG, GS, and SL skiing. Knee angle of the outside (main load-bearing) leg varied between 83-96° in SG, 86-114° in GS, and 98-111 in SL, the range for SL being smaller (P < 0.05) than for GS. Minimum knee angle for the outside leg was smaller (P < 0.05) in GS than SL (Fig. 1B-1C) and appeared to be even smaller in SG (Fig. 1A; as compared in one five-fold World Cup winner who performed the SG and GS within the same measurement session). In FM skiing, the range of knee-angle movements was much larger (62-133°) than in the other disciplines, and less relative difference between knee angles of the outside and inside leg was observed for this discipline (Fig. 1D; as assessed in one Olympic finalist).
Movement-cycle time of a right and a subsequent left turn was 4.1 s in SG, 3.5 ± 0.6 s (mean ± interindividual SD) in GS, and 1.6 ± 0.2 s in SL. Knee-angle velocity of the outside leg averaged 17°·s−1 in SG, 34 ± 2°·s−1 in GS, and 69 ± 11°·s−1 in SL, the difference between GS and SL being statistically significant (P < 0.05). The FM movement cycles were markedly shorter (0.8 s) resulting in angular velocities exceeding 300°·s−1.
The maximum EMG intensity of any given time sequence of the turn attained similar values of 74 ± 33% and 73 ± 21% MVC in the GS and SL disciplines. Average EMG was greater (P < 0.05) during the eccentric than the concentric phase of the outside leg in both GS and SL. Because of large interindividual variations in EMG, quantitative comparisons with SG and FM, respectively, must be exercised with caution. In all disciplines, however, EMG intensity of some turns exceeded 100% of EMG attained during MVC. In SG, GS, and SL, the EMG peaked in the late phase of the eccentric muscle action. In FM skiing EMG peaked during the transition phase from concentric to eccentric muscle action.
From the present EMG recordings, it appears that the super G (SG), giant slalom (GS), and slalom (SL) events are characterized by a marked predominance of eccentric over concentric muscle actions both in terms of relative intensity and duration. Since force production is inherently greater during eccentric than concentric muscle actions for any given EMG intensity (1,5,10), muscle strain and force output must have been considerably higher during eccentric than concentric actions in these events. To our knowledge, a dominance of sustained eccentric muscle actions has not been reported previously for any other competitive sports activity. In most dynamic exercises requiring high muscular force output, each movement cycle typically includes a forceful push-off phase which serves to lift or propel a body against gravity, air resistance, or surface friction. In the powerlift squat, for example, the concentric load during lifting will roughly equal the preceding eccentric load. In speed skating, the concentric force production is the dominating mode of muscle use (8).
The unique feature of alpine skiing that makes a high sustained eccentric force production possible is the continuous vertical displacement of body mass by which the skier generates speed and kinetic energy. The leg extensor muscles are activated mainly during the turning phase and, hence, the muscular load may primarily be governed by an accelerative force proportional to the weight and speed of the skier and inversely proportional to the radius of the turn. In this way, performance during alpine skiing could, theoretically, be controlled by executing resisting (eccentric) muscle actions only. In all alpine disciplines, muscle activation reached near maximal levels during the course of almost every turn, and in SG and GS, high levels of muscle activation were sustained during prolonged periods of the turn. To maximize force output, it appears appropriate that the skier predominantly relies on eccentric muscle actions. Because eccentric muscle actions not only have a larger force capacity, but also are less energy consuming (4) and less fatiguing (10) than concentric actions, it may be speculated that the preponderance of such actions would improve the capacity to maintain force output throughout the actual race.
Compared with the other skiing disciplines, freestyle mogul (FM) skiing showed a different pattern of muscle use. In the FM discipline the skier rides down a steep slope with irregular large bumps (moguls) that are passed under time constraint. It seems that the skier initiates the movement cycle with an active push-off (concentric muscle action) before the impact (landing phase) onto the mogul. Upon impact the legs are compressed by the thrust and the knee extensors are forced into resisted lengthening (eccentric muscle action). During this phase of energy absorption (and probably energy dissipation into the snow) a rapid abatement in EMG activity was recorded, implying an idle phase before the start of the next movement cycle. Thus, in FM there was no dominance of eccentric muscle activation. In fact, the sequence of knee-extensor activation during an FM movement cycle in certain aspects resembles that of a running stride. Thus, it is likely that the high knee-extensor activity observed during the late concentric phase of an FM movement cycle reflects a preactivation of muscles preceding impact at landing. Such concentric preactivation increases muscle stiffness before the impact and is assumed to be essential for optimal running (9) and jumping (3) performance. The sharp drop in EMG following impact may, at least in part, result from reflex-mediated inhibition, as encountered in some drop jumps (3).
The low body position, already reported for the GS (2), was even more striking in SG skiing, whereas a slightly more erect position was assumed in the SL discipline. Thus, during the course of a turn the minimum knee angle of the outside leg (main load-bearing leg) was generally smaller in the high speed disciplines (i.e., knee angle SG<GS<SL). The capacity of the knee extensors to transfer muscular tension into external force in this closed-chain movement is markedly reduced at low knee angular positions (5). That the skier, despite evidence of a call for near-maximal effort during the turns, chooses to perform at a stance position approaching full squat may reflect the advantage of attaining a low position for reducing air resistance and improving balance.
Also, with regard to the knee-angular velocity, indirectly reflecting muscle shortening/lengthening velocity, there appeared to be a hierarchy with slower knee-angular movements in the high-speed disciplines (i.e., knee angular velocity: SG<GS<SL). The velocities ranged from less than 20°·s−1 in SG to about 70°·s−1 in SL. Even 70°·s−1 is extraordinarily slow in comparison with joint angular velocities in most other sports activities. In running, for example, knee angular velocities may approach 1000°·s−1 (7). FM skiing also differed from the other disciplines with respect to knee-angular velocity which was intermediately slow (200-400°·s−1) and similar to knee-angular velocities in cycling (6).
That FM skiing exhibited a pattern of muscle activation different from the other alpine disciplines illustrates that the influence of the downhill displacement of body mass on muscle control is not readily extrapolated from one discipline to the other, and hence, confirms our notion that the pattern of muscle activation of each specific discipline should be monitored before designing sport-specific training models.
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