THE ROLE OF ECCENTRIC REGIME OF LEG MUSCLE WORK IN ALPINE SKIING

Alpine skiing is characterized by a great number of leg movements with muscle contractions in eccentric regime. The role of these movements is to absorb gravitation and inertial forces, manage skis more precisely and maintain balance. Recent studies have determined the volume, duration and intenisty of eccentric contractions as well as the basic characteristics of movement amplitudes and velocities. Based on the previous findings the experiments involving eccentric training using a bicycle ergometer confirmed a positive impact that this kind of training has on increasing maximum power, strength, endurance, coordination, injury prevention, metabolic work efficiency, more efficient work with longer muscle length and its role in miming skiers’ movements. This paper is an rewiev of the studies so far in the field of kinematics, skiing dynamics and the effect of eccentric training on the development of athletes’ performances.


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In the second case, during fast and cyclic movements (sprints, jumps), the absorbed energy is dissipated and enables a more efficient "stretch-schortening cycle" (SSC).The reasons for such effects of ECC contraction have not been fully explained.Some authors believe that the absorbed energy at the end of ECC contraction is released as elastic energy in tendons, during a fast movement which follows (in a srpinglike manner).In order to make use of a reverse effect, the time between the end of ECC and the beginning of CON contraction ("coupling time") needs to be short since otherwise elastic energy would be transformed into heat.Originally, it was believed that the time cannot be longer than 0.15 ms, and then Wilson, Elliot and Wood (1990) concluded that the effects of lower intensity may be used within the period of 4 seconds.Other authors believe that the enhanced effect of CON contraction is a result of the effect reflex has on stretching.Others still believe that ECC contraction allows muscles to establish their (pre)activation at a higher level and start CON contraction with a greater activation capacity (Linthorne, 2001).

Eccentric training and adaption processes
Compared with the movements in CON regime, the muscle work in ECC regime is more efficient in terms of energy and metabolism, even up to 50% (Isner-Horobeti, Dufour, Vautravers, Geny, Coudeyre, & Richard, 2013).Thus, in ECC training there is a possibility to load muscles significantly more and with lower metabolic requirements -"more efficiency with less effort" (LaStayo, Pierotti, Pifer, Hoppeler, & Lindstedt, 2000).During ECC ergometer cycling the consumption of О 2 may amount to 1/6-1/7 of the consumption in relation to the cycling in CON regime (Biglan-Ritchie & Woods, 1976).Vorgt (2011) achieved the total volume of work of 240 tons per training session in his experiment with ECC cycling on a bicycle ergometer, which is almost impossible to achieve by the cycling in CON regime.La Stayo et al. (2000) in their experiment involving cycling on a bicycle ergometer, with the work intensity of 65% VО 2 mаx, recorded the work with the loads four times greater in ECC regime (489 W) compared to a group working in CON regime (128 W).
Adaptation processes under the influence of ECC training reflect in the changes in nervous and muscle system.Muscle mass is increased due to the hypertrophy of fast-twitch muscle fibres of type Iа and IIx (Folland & Williams, 2007; Friedmann-Bette et all, 2010; Guilhem, Cornu, & Guevel, 2010; Baroni, Rodrigues, Franke, Geremia, Rassier, & Vaz, 2013).The adaptation changes upon ECC training result in an increase in contraction velocity and muscle spindle length as well as an increase in the capacity to use the accumulated elastic energy in muscle-tendon system (Lindstedt, et al., 2002) more efficiently, an increase in overall work efficiency and improved injury prevention.
Fatigue is an important factor of the manifestation of elastic muscle features, since it reduces the effect of muscle activity.By applying different training models it has be found that muscles need more than 24 hours to recover after hard ECC training and, due to a high volume of work, it is possible to apply the maximum of 2 or 3 ECC training sessions per week (LaStayo, 2000; Vogt & Hoppeler, 2009).

The effect of eccentric training on strength development
The advantages of ECC training in relation to CON training method have been proven in several experiments in untrained subjects.Several experiments lasting 7-10 weeks have shown a positive effect on increasing maximum isometric power of 36% (LaStayo et al., 2000) and strength in sets of high jumps by 7% (Elmer, Hahn, McAllister, Leong, & Martin, 2012) as well as that the method of "high volume-low intensity" exercises in ECC regime has shown better results compared to the "high intensity-low volume" method.In two experiments of similar duration (number of trainings x number of sets x number of repetitions), (Guilhem, et all., 2010; Guilhem, Cornu, Maffiuletti, & Guevel 2013) applied two methods in ECC regime exercises: isotonic and isokinetic.Finally, it has been determined that the participants in the isokinetic experiment performed twice the volume of work on average, yet a greater effect on strength increase was achieved by the isotonic method (2.4%).
A few studies, in trained athletes, also indicated a positive effect of ECC training.Various experiments where two groups applied CON and ECC training regime lasted 3-10 weeks, 3-4 training sessions per week.Despite different methodology of the experiments, the groups with ECC regime showed better results than the groups with traditional CON training regarding the enhancement of: jumping height (8%), movement frequency by 11% (Lindstedt, et al., 2002), maximum isometric hamstring muscle power by 11%, torque at the velocity of 60°/s by 0.4% per session (Mjolsnes, Arnason, Osthagen, Raastad, & Bahrl, 2004); jumping height by 11% (Sheppard, et al., 2008); percent of IIa fibre type; strength and jumping height by 5% (Cook, Beaven, & Kilduff, 2013).
The effect of eccentric training on coordination CON and ECC muscle contractions differ in the process of movement programming at the level of the central nervous system (CNS).The study by Guilhem et al. (2010) has shown a higher level of EEG activity of the cortex with ECC contraction in relation to CON contraction.At muscular level, with similar power intensity a lower level of EMG activity was recorded during ECC compared to CON contraction (Biglan-Ritchie & Woods, 1976).The authors suppose that a smaller number of motor units are activated during ECC contraction in relation to the CON contraction of the same intensity.This results in an increase in mechanical load per an activated motor unit.Therefore it is harder to control and coordinate movements during ECC contraction.

Eccentric training modalities
According to the studies so far there are two models of ECC regime training that are frequently applied (Roig, et  1. "High intensity -low volume" approach is a training with heavy loads (around 1 repetition maximum (RM), and a small number of sets, mainly performed in a "pure eccentric" or "mixed eccentric and concentric contraction" manner, with "isotonic" or "isokinetic" exercises.It is generally performed using ergometers with additional load.2. "Low intensity -high volume" approach is a training characterized by a greater number of repetitions and submaximal load.It is most frequently performed with ECC ergometers.ECC training with supramaximal and maximal load (≥1RM) increases maximum power and muscle mass.By submaximal loads (≤1RM), the enhancement of power and muscle tone is achieved.In order to develop these performances the training includes exercises with overload (≥1RM), plyometric exercises, exercises using one's own body (e.g."Nordic hamstring exercises") or exercises using ergometers (inertial load ergometers and bicycle ergometers).

CHARACTERISTICS OF LEG MUSCLE ACTIVITY IN SKIERS
Contrary to the belief that alpine skiing represents a dynamic activity with lots of explosive movements, the angular velocities in knee and hip joints show that skiing is a sport which is not characterized by maximum movement velocity (Berg, Eiken, & Tesch, 1995;Patterson, Raschner, & Platzer, 2009).Average angular velocities in the knee joint range between 20-40 º/s and the maximum recorded velocities of 69 º/s are far from the angular velocities in the knee joint of sprinters and jumpers (1000º/s) or cyclists (200-400º/s) (Berg & Eiken 1999.The speed of power generation is lower and maximum values are reached later on and what makes skiing different from the majority of other sports is that a greater part of a turn is dominated by ECC contractions, due to the movement down a slope.Body moving continuously down a slope with an increased velocity (30-100 km/h) generates kinetic energy and thus a heavy load to the knee joint extensors.The loads that skiers endure during a turn are 2-4.5 G (Hoppeler, 2014), i.e. 58-112% of maximum voluntary isometric contraction (MVC).The duration of ECC contraction phase is 84.5% in slalom (SL) and 88.2% in giant slalom (GSL) out of the total turn duration, which indicates a large volume of ECC regime of muscle work in order to manage inertial forces and maintain position (Berg, Eiken, & Tesch, 1995).
Regarding the role of certain fibre types, the research by Vogt & Hoppeler (2012) indicated a small but significant dominance of slow-twitch (I) in relation to fast-twitch fibres (II) in the sample of elite skiers as well as correlation between successfulness in competition results (FIS competition points) and slow-twitch fibre type (I).It is assumed that the velocity and intensity of the entire ECC and CON contraction cycle depends on the type of muscle fibres and their length.
Movement amplitudes, knee joint angles, duration, velocity and frequency of movements as well as the characteristics of muscle activity indicate that the movements in skiing may be classified between slow SSC and shock absorbing function.The specified characteristics represent a challenge for planning and implementation of ECC exercises in strength training of alpine skiers.

THE EFFECT OF ECCENTRIC REGIME EXERCISES ON SKIERS' PERFORMANCES
In accordance with the previous findings, some studies on the effect of ECC training on skiers' power, endurance and coordination enhancement have been conducted.Vogt (2011) applied a 6-wk training using an eccentric cycle ergometer in his experiment with skiers.The control group performed a traditional CON training 3 times a week lasting 60 min, while the experiment group had 40 CON + 20 ECC minutes of training with cycle ergometer.Both groups recorded an increase of 10% in ISO power, whereas only the experiment group recorded an increase of 7.9% in a high jumping test.Gross, Luthy, Kroell, Muller, Hoppeler and Vogt (2010) and Vogt & Hoppeler (2012) carried out the 5-week experiments with elite skiers.In addition to a traditional strength training (without jumping), the skiers also applied a set of 20 minutes on ECC cycle ergometer1-2 times a week.The load was gradually increased and finally it was 140% of the initial load (from 404 to 965 W).An increase was recorded in isometric power (12%), leg muscle mass (1.9%), maximum strength in high jumps (8.8%), squat jumps (92%) and power in ECC contraction as well (100% BM).Despite the intense but gradual load increase, the occurrence of "muscles soreness" was not recorded.The above mentioned studies indicate that the implementation of ECC training affects the enhancement of maximum strength and power, without jumps.The research by Vogt, Dapp, Blatter, Weisskopf, Suter and Hoppeler (2003), using an ECC ergometer showed that more successful skiers have better coordination of movements (the relation between load and demonstrated strength).The correlation between these results was recorded in the skiers in SL and GSL but not in DH (downhill).This result is understandable when we take into account the movement dynamics (longer and slower movements in DH) and the protocol of the test used (higher movement frequency).The results of ECC training indicate that good coordination and power manifested in ECC contractions are the main attributes of elite skiers, especially in SL.Having in mind that parts of the brain spheres activated during ECC contraction are different from those active during the movements involving CON contraction, from the control aspect, it is necessary to practice the tasks involving ECC contractions un-der specific conditions and exercises that mime the movements in the snow (Fang, Siemionow, Sahgal, Xiong, & Yue, 2004).

PRACTICAL APPLICATIONS OF ECCENTRIC TRAINING IN SKIING
Alpine skiing represents a complex integration of various physiological systems neither of which may be more important than the other in their total capacity.When developing and assesing the success factors, the specificities of skiing kinematics and dynamics must be taken into account (Ropret, 2015).One of the specific features of skiing is a considerable volume of ECC regime of leg muscle work where the movement mechanics significantly differs from other sports.
Intense muscular strains that require a high level of maximum muscle power, strength endurance lasting for 1-2 min, high level of coordination of the movements (balance and precise leg movements) on unstable surface, under specific conditions of increased altitudes (rarefied air, cold) are undisputable.The loads of 2-4.5 G require training with additional load for the development of maximumn power and particularly in ECC contraction regime.A greater number of repetitions of the movements and duration of the activity (30-70 turns during 1-2 min) require the development of great endurance.
Effects of ECC training have manifold significance for skiers' training: increase in maximum power and strength, optimum ratio between strength and muscle length, intra and intermuscular coordination enhancement, injury prevention, explosive strength increase without application of intense jumping exercises, more efficient metabolic work, development of great endurance, larger volume of work, work involving the movements similar to skiing, improvement of the relations between agonist and antagonist muscle power.
The ability to absorb kinetic energy and good neuro-muscular movement control in ECC regime must be practised in specific conditions or conditions as similar to skiing as possible.Preference is given to the exercises using ergometers compared to traditional plyometric exercises for several reasons: skiing movements are slower, amplitudes of the knee joint movements are higher, power generation lasts longer and coupling phase is not that important in relation to plyometric exercises.In addition, stretching and fold-ing movements in skiing are not ultimately aimed at high takeoff, but at absorption and dosage of the load caused by gravitation and inertion, along with control of fine leg movements at such heavy loads.Unlike the exercises using ergometers, plyometric exercises are performed in motion of low amplitude with small changes in joint angles, which is not characteristic of skiing.Because of this, the exercises in ECC regime using ergometers have an advantage over jumping exercises.
Using metabolically favorable ECC method provides the opportunity to apply a "high volume-low intensity" method, which is particularly important for the development of an endurance component in leg strength in skiers.
The studies so far have included a variety of training and measuring protocols.The experiments lasted 5-10 weeks, with 2-3 training sessions per week in a form of continous duration of between 15 and 30 minutes or 3-4 sets with 7-10 repetitions with a oneday break between workouts.The loads were gradually increased, starting from the initial load of 130 W in untrained skiers to 1200 W in the elite ones.Despite the specified differences in methodology, all the studies have indicated a positive effect of ECC training on the development of skiers' performances regarding power, strength and endurance enhancement.It is necessary to conduct further research related to determining the optimum volume, intenisty, velocity and protocol of this type of training with a special focus on the specificities of alpine skiing.