SCIENCE AND SKIING IV
SCIENCE AND SKIING IV
Edited by
Erich Müller
Stefan Lindinger
Thomas Stöggl
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Science and Skiing IV
Erich Müller, Stefan Lindinger, Thomas Stöggl (Eds.)
Maidenhead: Meyer & Meyer Sport (UK) Ltd., 2009
ISBN: 978-1-84126-255-0
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Contents
Introduction
PART ONE: KEYNOTE PAPERS
Vibration exposure in alpine skiing and consequences for muscle activation levels
P. Federolf, V. von Tscharner, D. Haeufle, B. Nigg, M. Gimpl and E. Müller
Importance of sensorimotor training for injury prevention and athletic performance
A. Gollhofer and M. Gruber
Eccentric exercise in alpine skiing
H. Hoppeler and M. Vogt
Alpine skiing technique - practical knowledge and scientific analysis
S. Loland
Stimuli and stimulation: hypoxia and mechanics
J. Mester, S. Achtzehn, M. de Marées, E. Engelmeyer, A. M. Liphardt and F. Suhr
PART TWO: INVITED PAPERS
Diet and muscle fatigue during two weeks of alpine ski training
D. W. Bacharach and K. J. Bacharach
Prediction of ankle ligament elongations in snowboarding using a kinematic model
S. Delorme, M. Lamontagne and S. Tavoularis
The competitive Cross - Country Skier - an impressive human engine
H.-C. Holmberg
Equipment development and research for more performance and safety 1
V. Senner, S. Lehner and H. Böhm
PART THREE: ALPINE SKIING
Alpine skiing and snowboarding training system using induced virtual environment
V. Aleshin, S. Klimenko, M. Manuilov and L. Melnikov
Heart rate recovery in young alpine skiers with congenital heart disease
M. Bernhörster, A. Rosenhagen, M. Castano, L. Vogt, R. Hofstetter and W. Banzer
Emotional experiences and FLOW in easy bump pistes
T. Brandauer, T. Felder and V. Senner
How to ski faster: art or science?
M. Brodie, A. Walmsley and W. Page
Modeling edge-snow interactions using machining theory
C. A. Brown
Predictors of falls in downhill skiing and snowboarding
M. Burtscher, R. Pühringer, I. Werner, R. Sommersacher and W. Nachbauer
Preliminary results on the role of the coach-athlete relationship in a developing ski nation
S. Chroni, I. Lefoussi and M. Kalomirou
Measurement of dynamical ski behavior during alpine skiing
M. Fauve, M. Auer, A. Lüthi, H. Rhyner and J. Meier
Analysis of skiers’ performance using GPS
P. J. Gómez-López, O. Hernán and J. V. Ramírez
System-Theory-Based-Model-Building for a practice and theory of alpine skiing
H. Haag
Calculation of the pressure distribution between ski and snow
D. Heinrich, P. Kaps, M. Mössner, H. Schretter and W. Nachbauer
Eccentric and concentric torque of knee extension and flexion in alpine ski racers
H. Hoshino, K. Tsunoda and T. Sasaki
Influence of the skier’s body geometry on the duration of the giant slalom turn
Z. Hraski and M Hraski
Description of race cources and estimation of ground reaction forces by GPS-Data and video
A. Huber, K.-H. Waibel and P. Spitzenpfeil
Identification of the physical characteristics that discriminate between competitive levels and specialties of alpine skiers
F.M. Impellizzeri, E. Rampinini, M. Freschi, N. A. Maffiuletti, M. Bizzini and P. Mognoni ΐ
Development of a measuring system on ski deflection and contacting snow pressure in turns
H. Kagawa, T. Yoneyama, D. Tatsuno, N. Scott and K. Osada
Not the skier - but the slope turns the skis
G. Kassat
Athlete responses to using a real time Optical Navigation Feedback System during ski training
R. Kirby
Influence of physical fitness on individual strain during recreational skiing in the elderly
S. Krautgasser, P. Scheiber, J. Kröll, S. Ring-Dimitriou and E. Müller
EMG signal processing by wavelet transformation - applicability to alpine skiing
J. Kröll, J. M. Wakeling, J. Seifert and E. Müller
Effects of a mock-up force plate on riding technique and perception - a prerequisite for comprehensive biomechanical analyses in mogul skiing
N. Kurpiers, U. G. Kersting and P. R. McAlpine
Robot imitating human skiing used for teaching and equipment testing
L. Lahajnar and B. Nemec
Applications of physics education research to skiing pedagogy for coaches and instructors
R. LeMaster
H:Q ratios in open versus closed kinetic chain: what is the relevance for alpine ski racers?
S. Lembert, C. Raschner, H.-P. Platzer, C. Patterson and E. Mildner
Physiological profile of Swiss elite alpine skiers - a 10-year longitudinal comparison
N. A. Maffiuletti, K. Jordan, H. Spring, F. M. Impellizzeri and M. Bizzini
Effects of ski stiffness in a sequence of ski turns
M. Mössner, D. Heinrich, P. Kaps, H. Schretter and W. Nachbauer
Power endurance testing in alpine ski racing
C. Patterson, C. Raschner, H.-P. Platzer and S. Lembert
Acquisition of EMG signals during slalom with different ski boots 399 N. Petrone, G. Marcolin, M. De Gobbi, M. Nicoli and C. Zampieri
Loading conditions and neuromuscular activity during vertical knee flexion-extension and turn-like movements in a new skiing simulator under vibration conditions
R. Pozzo, F. Zancolò, A. Canclini and G. Baroni
Turn characteristics and energy dissipation in slalom
R. Reid, M. Gilgien, T. Moger, H. Tj0rhom, P. Haugen, R. Kipp and G. Smith
Transfer from inline-skating to alpine skiing learning in physical education
B. Román, Ma. T. Miranda, M. Martínez and J. Viciana
Quantitative assessment of physical activity during leisure alpine skiing
A. Rosenhagen, C. Thiel, L. Vogt and W. Banzer
Guided Alpine Skiing - Physiological demands on elderly recreational skiers
P. Scheiber, S. Krautgasser, J. Kröll, E. Ledl-Kurkowski and E. Müller
Biomechanical basis for differential learning in alpine skiing
W. I. Schöllhorn, P. Hurth, T. Kortmann and E. Müller
Cumulative muscle fatigue during recreational alpine skiing
J. Seifert, J. Kröll and E. Müller
Mechanical load and muscular expenditure in alpine ski racing and implications for safety and material considerations
P. Spitzenpfeil, A. Huber and K. Waibel
Relationship between vertical jumps and different slalom courses
V. Strojnik and A. Dolenec
A step forward in 3D measurements in alpine skiing: a combination of an inertial suit and DGPS technology
M. Supej
Measurement of ski deflection and ski-snow contacting pressure in an actual ski turn on the snow surface
D. Tatsuno, T. Yoneyama, H. Kagawa, N. Scott and K. Osada
Physiologic characteristics of leisure alpine skiing and snowboarding
C. Thiel, A. Rosenhagen, L. Roos, M. Huebscher, L. Vogt and W. Banzer
The influence of the laterality of the lower limbs on the symmetry of connected carving turns
F. Vaverka and S. Vodickova
The method of time analysis of a carving turn and its phases
S. Vodickova and F. Vaverka
Respiratory and metabolic demands of field versus laboratory tests in young competitive alpine ski racers
S. P. von Duvillard, D. Bacharach and F. Stanek
Assessment of timing and performance based on trajectories from low-cost GPS/INS positioning
A. Waegli, F. Meyer, S. Ducret, J. Skaloud and R. Pesty
Performance analyses in alpine ski racing regarding the characters of slopes and course settings
K. Waibel, A. Huber and P. Spitzenpfeil
PART FOUR: CROSS COUNTRY SKIING
3D analysis of the technique in elite ski-touring and cross-country skiers engaged in world cup races and on a treadmill
A. Canclini, G. Baroni, R. Pozzo, M. Pensini and G. Rossi
Individual modeling of the competition activities for elite female ski races during the 2006-2007 season
A. Cepulénas
Intra- and inter-individual variations in the daily haemoglobin and hematocrit concentration in elite cross country skiers caused by different body positions, state of hydration, exercise and altitude
E. Engelmeyer, M. de Marées, S. Achtzehn, C. Lundby, B. Saltin and J. Mester
Visibility and availability of GPS in cross-country skiing
A. Krüger, K. Sikorski, J. Edelmann-Nusser and K. Witte
Simulation of classical skiing using a new ski tester
V. Linnamo, V. Kolehmainen, P. Vähäsöyrinki and P. Komi
Balance performance of elderly cross-country skiers - standing still or swaying smart?
V. Lippens and V.Nagel
Biomechanics in classical cross-country skiing - past, present and future
W. Rapp, S. Lindinger, E. Müller and H.-C. Holmberg
Biomechanical factors of biathlon shooting in elite and youth athletes
G. Sattlecker E. Müller and S. Lindinger
Effectiveness of ski and pole forces in ski skating
G. Smith, B. Kvamme and V. Jakobsen
Competition analysis of the last decade (1996 – 2008) in crosscountry skiing
T. Stöggl, J. Stöggl and E. Müller
PART FIVE: SNOWBOARDING
Jump landings in snowboarding: an observational study
P. McAlpine, N. Kurpiers and U. Kersting
Influence of physical fitness parameters on performance in elite snowboarding
H.-P. Platzer, C. Raschner, C. Patterson and S. Lembert
Optimizing snowboard cross and ski cross starts: a new laboratory testing and training tool
C. Raschner, H.-P. Platzer, C. Patterson, M. Webhofer, A. Niederkofler, S. Lembert and E. Mildner
Lift generation in soft porous media with application to skiing or snowboarding
Q. Wu and Q. Sun
PART SIX: SKI JUMPING
Kinematic analysis of the landing phase in ski jumping
F. Greimel, M. Virmavirta and H. Schwameder
Stability during ski jumping flight phase
F. Hildebrand, V. Drenk and S. Müller
The relationship between the timing of take-off action and flight length by using the doll-model
T. Sasaki, K. Tsunoda, H. Hoshino, S. Miyake and M.Ono
PART SEVEN: HIGH ALTITUDE TRAINING IN SKIING
Effect of hypoxic training on angiogenetic regulators and mesenchymal stem cells
M. de Marées, P. Wahl, F. Suhr, S. Achtzehn, A. Schmidt, W. Bloch and J. Mester
The anatomical hazards of the grip wrist tunnel syndromes
S. Provyn, P. Van Roy and J. P. Clarys
Oxygen uptake during local vibration and cycling
B. Sperlich, H. Kleinöder, M. de Marées, D. Quarz and J. Mester
Effects of short term high-intensity exercise under noroxic and hypoxic conditions and warming up on erythrocyte and plasma lactate concentrations after a giant slalom simulation
P. Wahl, C. Zinner, E. Lenzen, M. Hägele, S. Achtzehn, W. Bloch and J. Mester
Introduction
The Fourth International Congress on Science and Skiing was held at St. Christoph a. A., Tyrol, Austria. It was the follow up conference of the first two International Congresses on Skiing and Science, which were also held in St. Christoph a. A., Austria, in January 1996 and in January 2000 and of the Third International Congress on Science and Skiing, which was held in Aspen, Colorado, USA, in April 2004.
The conference was organized and hosted by the Department of Sport Science at the University os Salzburg, Austria, and by the Christian Doppler Laboratory “Biomechanics in Skiing”, Salzburg, Austria. It was also again part of the programmes of the steering group “Science in Skiing” of the World Commission of Sports Science.
The scientific programme offered again a broad spectrum of current research work in Alpine and Nordic skiing and in snowboarding. The highlights of the congress were eight keynote and four invited lectures. The scientific programme of the congress was completed by 2 work shops, 82 oral presentations and 76 poster presentations.
In the proceedings of this congress, the keynotes and invited lectures as well as the oral presentations are published. The manuscripts were subject to peer review and editorial judgement prior to acceptance.
We hope that these congress proceedings will again stimulate many of our colleagues throughout the world to enhance research in the field of skiing so that at the Fifth International Congress on Science and Skiing, which will be organized in the winter 2010/11, many new research projects will be presented.
Erich Müller
Stefan Lindinger
Thomas Stöggl
We would like to express our cordial thanks to Elke Lindenhofer for the time and the energy which she invested in the editting of this book.
Part One
Keynote Papers
Vibration exposure in alpine skiing and consequences for muscle activation levels
1 Introduction
Vibration exposure is known to affect muscle physiology and neuromuscular activity. The effect of whole body vibrations on muscle activation has been studied in the context of balance and postural control, passenger safety in vehicles, and vibration training in sports. Mester et al. (1999) have shown that strong ski vibrations are generated at the ski-snow interface that propagate through the whole body of the skier. The vibrations create resonance phenomena in soft tissue compartments. Vibrations are especially harmful for the brain, the eyes or ears and organs sensitive to vibrations (Griffin, 1975; Zou et al., 2001). They further hamper motion control (steering quality) and increase the risk of falls and injuries.
To prevent the damaging effects of vibrations different damping mechanisms are used by the human body. The main damping is believed to occur in the leg joints: passively by the cartilage and soft tissue attached to the bones or actively by stiffening the joints by muscle contraction (co-contraction). Vibration dependent muscle tuning has been proposed as damping mechanism (Nigg, 1997; Nigg and Liu, 1999). Muscle tuning has been studied for walking and running using accelerometers for measuring the vibrations (Wakeling et al., 2003; Boyer & Nigg, 2007). However, vibrations and possible muscle tuning has not been studied in skiing, an activity with high soft tissue vibrations. Thus, the purpose of this study was
1. to characterize intensity and frequency content of equipment vibrations for different skiing techniques and for different snow conditions using the wavelet analysis method,
2. to quantify simultaneously vibration intensities and muscle activation signals on four muscles of the lower extremities during alpine skiing,
3. to characterize vibration damping within the body,
4. to determine resonance frequencies of the soft tissue compartments of calf, thigh, and hamstrings and
5. to determine whether muscle activation levels change for conditions with different vibration exposure.
2 Methods
Ten experienced skiers completed 24 runs performing 5–7 short turns, 6 carving turns, and gliding in tuck position. The 24 trials of each subject were conducted between 9am, and 12.30pm. For eight of the ten subjects, snow conditions changed from hard frozen to soft snow during this time.
The skiers were equipped with 1-D acceleration sensors (Analog Devices™ (ADXL series), range: 35 to 120g) placed in axial direction on the shaft of the ski boot (parallel to the tibia), on the muscle compartments of triceps surae, quadriceps and hamstrings, and on the skin covering bones close to the ankle, knee hip and neck joints. Muscle activation was measured using bipolar surface EMG sensors on the gastrocnemius, vastus medialis, vastus lateralis, and semitendinosus. All sensor signals were recorded with a mobile EMG measurement device (Biovision™) carried in a backpack. EMG and acceleration signals were collected at a frequency of 2000 Hz.
For each run three specific movements were selected for further analysis: four consecutive short turns (2.8 ± 0.3 sec.), four consecutive carving turns (6.4 ± 0.6 sec.) and two seconds of gliding in tuck position. Short turns are highly dynamic movements in which the muscles act mainly concentric. Carving turns are executed at high speeds with little body motion. Due to centripetal forces the skiers’ muscles are loaded mainly eccentrically. Gliding was executed in the tuck position, characterized by relatively small hip and knee angles. In this position the muscles are mainly isometrically contracted.
The recorded acceleration signal was resolved with a wavelet transformation into intensities calculated for a set of 22 center frequencies between 0.6 and 80 Hz. EMG data was resolved using 13 wavelets with center frequencies between 6.9 and 542 Hz. In both cases, wavelet transformations (von Tscharner, 2000) were used, because the intensity calculated for each wavelet (each frequency range) is normalized with respect to the energy content of the analyzed signal. At a given time, the total intensity was calculated by adding the intensities of all wavelets. The square root of the total intensity is proportional to the signal amplitude and was called magnitude of the signal. The mean total intensity of a signal (EMG or acceleration) during a specific movement was calculated by averaging the total intensity over time. The mean total intensity characterized the vibration intensity or muscle activation level of the selected movement and can be compared between trials. The spectrum of a signal (EMG or acceleration) was calculated by integrating the intensity of each wavelet over time. Hence, the frequency range and frequency resolution of the spectrum derived from the wavelet analysis depended on the number wavelets and on their center frequencies.
Vibration damping within the skier’s body was characterized by dividing the vibration magnitudes determined at hip and neck by the vibration magnitudes measured at the ankle. Ankle vibrations were considered input vibrations for the body. This procedure provided only approximate values for the damping because all vibration signals were recorded with 1-D acceleration sensors.
Resonances of soft tissue compartments were determined by dividing the intensity spectrum measured at a soft tissue compartment by the intensity spectrum measured at the ankle (vibration input). The resonance frequency was determined as the frequency at which this quotient was maximal. Frequencies below 10 Hz were not considered, because the skiing movements are in this range.
To determine if muscle activation levels change if the intensity of the vibration exposure changes, Pearson’s correlation coefficient r between mean total intensity of the EMG signal and the mean total intensity of the acceleration signal was calculated for the 24 trials of each subject.
3 Results
Equipment vibrations measured at the ski boot showed peak accelerations between 20 to 30 g in the steering phase in short turns. Vibrations were small during turn initiation (~2 g) and during gliding (~5 g). In carving vibration amplitudes were in the range of 5 to 20 g. It seemed that vibrations were high when the ski skidded, and when the ground reaction forces were high. Frequency spectra were highly subject specific, but in all subjects the peak intensities were found in the range of 5 to 30 Hz. As the snow turned softer in the course of the day, frequencies above 15 to 20 Hz were increasingly damped.
All subjects showed strong vibration damping within the body. At 10 Hz, mean vibration magnitudes measured at the hip and neck decreased to 30% and 20%, respectively, compared to vibration amplitudes measured at the subject’s ankle. With increasing frequency these vibration amplitudes decreased further. Above 60 Hz the vibration amplitudes were less than 12% for the hip and less than 5% for the ankle.
Resonances frequencies of the muscle compartments were subject, muscle and movement specific. During short and during carving turns resonance frequencies occurred typically in the range of 10 to 30 Hz, in gliding the resonance frequencies were for most subjects and must muscles higher, typically in the 20 to 40 Hz range.
For eight subjects the snow turned significantly softer during the course of the day. In these cases the intensity of the vibrations the skiers were exposed to decreased significantly. The mean total intensity measured at the ankle decreased for short turns and carving by a factor between 2 and 3.5. In straight gliding the vibration intensities decreased by a factor of about 1.5. For short turns and carving, a concurrent decrease of muscle activation levels was observed. In short turns, the correlation coefficient r between the mean total intensity of the vibration exposure and the mean total intensity of the EMG signal was between 0.4 and 0.9. For the muscles the biceps femoris, gastrocnemius, vastus lateralis, and vastus medials the correlation was statistically significant for 6, 7, 6, and 5 of the subjects, respectively. In carving, statistically significant correlations were found for 7, 8, 6, and 5 subjects. Although the variability of
the vibration intensity in straight gliding was much smaller, significant correlations were found for 1, 2, 2, and 3 subjects.
4 Discussion
Vibration intensities observed at the equipment level (at the ski boot) differ for different skiing styles. This can be explained by different speeds, different skisnow interaction mechanisms (e.g. lateral skidding vs. cutting of the snow surface vs. gliding), or different equipment resonances (e.g. in case of edged skis torsional modes are excited, in case of flat skis mainly bending modes are excited). The results of this study also clearly indicate that the snow properties have substantial effect on the vibration intensity.
The vibrations present at the equipment level can be considered as the input vibrations the skier’s body is exposed to. Even at slow speeds and simple skiing styles used in this study, which are typical for recreational skiing, substantial and potentially hazardous vibration exposure levels were found. Accelerations measured during straight gliding exceeded the recommended limits of vibration exposure of international standards for work place safety (ISO Standard 2631) by a factor 10. Accelerations measured during carving or short turns exceeded these limits by a factor 20 or 30, respectively.
The high acceleration values found in this study at the ski level (input) may suggest that in addition to the motion control issues created by vibrations, the skier may also be at risk of sustaining injuries directly induced by vibrations. The most sensitive organs for vibrations injuries are the sensory systems of eyes (Griffin 1975) and ears (Zou et al., 2001). However, vibration injuries due to skiing were so far not reported and professions involving skiing, such as ski instructor or mountain guide, have not been associated with chronic injuries of the sensory organs. A possible explanation for this discrepancy is that the body posture assumed in skiing, with angled ankle, knee and hip joints, is well suited for damping the vibrations the body is exposed to. As a result, the sensitive organs in the head are well protected from the vibrations originating at the feet. This explanation is supported by the data found in this study: The vibration intensities measured at ankles, hip, and neck of the subjects indicate that vibrations are effectively damped within the body.
There are different damping mechanisms which might be used by the body including joint damping and damping through muscle tuning. In both cases, muscle action is needed to either maintain a desired joint angle, or to tune muscle stiffness. Consequently, it was expected that muscle activation levels increase if the intensity of the vibration exposure increases. During short turns and during carving the majority of the correlation coefficients calculated between EMG and acceleration data in fact showed a clear and statistically significant positive correlation. However, while all skiers showed this correlation in two or more of their muscles, most skiers also had muscles whose activity did not correlate with the vibration exposure levels. Two points should be considered when interpreting this result: (a) the primary function of muscle activity is the execution of the movement task. Although the movement task was similar during the 24 trials of each subject, fluctuations from changing choice in the route, from different snow surface conditions, and other influencing factors will create fluctuations in the muscle activation and might obscure the impact of vibration exposure levels. (b) when considering muscle tuning: only one muscle is required to change the resonance properties of the whole muscle compartment. It might be a more effective damping strategy, if the body avoids resonance by activating only specific muscles instead of all muscles in the muscle compartment. In gliding, only a small fraction of the subjects showed the expected correlation between muscle activation levels and vibration exposure levels. This can be explained by two additional points: (a) in gliding, compared to the other analyzed movements, the differences in vibration levels was much smaller, hence, fluctuations have a more severe effect on the calculated correlation. (b) In executing gliding the skiers have more “degrees of freedom” in their choice of body position and movements compared to short turns or carving turns which are well trained, automated movements, executed very similarly each time. In gliding, skiers may for example shift their weight between left leg and right leg to prevent fatigue. Such voluntary movements will cause strong fluctuations in the muscle activation levels, which render a correlation to vibration levels impossible.
References
Boyer K., Nigg B., 2007, Quantification of the input signal for soft tissue vibration during running, Journal of Biomechanics, 40 (8): 1877–1880.
Griffin, M. J., 1975, Levels of whole body vibration affecting human vision. Aviation Space and Environmental Medicine, 46 (8): 1033–1040.
Mester, J., Spitzenpfeil, P., Schwarzer, J., Seifritz, F., 1999, Biological reaction to vibration - implications for sport. Journal of Science and Medicine in Sport 2 (3): 211–226.
Nigg B., Liu W., 1999, The effect of muscle stiffness and damping on simulated impact force peaks during running. Journal of Biomechanics, 32 (8): 849–856.
Von Tscharner (2000). Intensity analysis in time-frequency space of surface myoelectric signals by wavelets of specified resolution, Journal of Electromyography and Kinesiology 10: 433–445.
Wakeling J., Liphardt A.-M., Nigg B., 2003, Muscle activity reduces soft-tissue resonance at heel-strike during walking, Journal of Biomechanics, 36 (12): 1761–1769.
Zou J, Bretlau P, Pyykkö I, Starck J, Toppila E., 2001, Sensorineural hearing loss after vibration: an animal model for evaluating prevention and treatment of inner ear hearing loss, Acta Oto-Laryngologica, 121 (2): 143–148.
Importance of sensorimotor training for injury prevention and athletic performance
1 Introduction
A vast number of public campaigns propagate that active life style and participation in sports has many benefits for health. Alpine skiing is popular and attractive and definitely the most important winter sport activity at all. From elite Alpine Skiing we receive fascinating impressions about the flexibility, the powerful actions as well as the highly skilled compensative reactions necessary to preserve movement control in difficult situations. Bodily fitness and highly developed motor coordination is a prerequisite to exert and/or counteract rapid forces changes and to activate and control the limbs and the body during a downhill race or slalom competition.
Participation in sports, however, can also be harmful (Bahr et al 2003). In Alpine Skiing the technical development of the skiers and the bindings reduced a larger number of injuries by approximately one half (Duncan et al 1995). However, the rate of the knee injuries, more specifically, the rate for anterior cruciate ligament (ACL) injuries is increasingly high. Pujol et al (2007) provided evidence that 10% of all injuries in snow sports are injuries to the ACL. In their comparison of the top 30 athletes in the world, they reported an injury rate for males and females of 50% for ACL injuries during 25 years of competitive skiing.
Due to the long lever arms of the thigh and the shank and due to the fixation of the skiboot on the skies, the knee joint complex has especially in Alpine Skiing to tolerate and compesate huge rotational moments. For the ACL injuries in skiing two mechanisms are discussed: (A) A combination of hyperextension and anterior pivot shift when landing on the ski tails with extended knees and (B) a coincident valgus-flexions-external rotation moment applied typically occurring when the skier straddles the gate.
In the past, knee injuries have been related to deficits in muscular strength and strength training was consequently applied to meet the requirements of the sport discipline. In recent years an increasing number of papers showed that intra- and inter-muscular coordination of the muscles encompassing the knee joint complex is of high importance. Thus, in order to minimize the injury risks, preventive training setups are designed to address coordinative adaptation. For Alpine Skiing, however, an assessment of their potential role is still not investigated under controlled conditions.
2 Mechanical and/or sensory function of the ACL?
In the present chapter the functional neuromuscular properties of the hamstring muscles and the ACL are discussed in order to “motivate” specific training programs for an improved active knee joint control.
The mechanical function of the ACL in conjunction with PCL to ensure passively knee joint stability has been intensively investigated (Woo et al 1991). Quite a few histological research papers revealed that the ACL contains also mechanoreceptors (Freeman/Wyke 1967; Haus/Halata 1990) and it has been discussed whether these receptors may have functional importance as a feedback loop to secure and control the integrity of the knee joint. It has been argued that the hamstring muscles need to provide effective tension in order to avoid excessive anterior tibia displacements (Johansson et al 1990, 1991; Solomonow et al 1987). More recent studies confirmed the existence of a reflex arc between the ACL and the hamstrings in humans (Friemert et al 2005a; Friemert et al 2005b). Functionally, an intact reflex connexion between ACL and hamstring muscle could lead to a quick muscular reaction if the ligament is stretched. This would ensure a muscular security against excessive anterior tibia displacements. The segmented hamstring reflex activity found in the experiments was attributed to a short latency response (SLR) and a medium latency response (MLR) (Friemert et al 2005b).
Based on biomechanical and neurophysiological research Melnyk et al (2007) could show, that the MLR reflex response is functionally specific: In patients with a history of ACL rupture, the excitability of the stretch evoked reflex was considerably changed. Compared to the healthy leg, the latency of the MLR was prolonged and the amplitude of the anterior tibia translation which was measured with a standardized test stimulus significantly increased. From subjects reporting “giving way” symptoms after ACL injury they presented data that this “feeling” is not simply related to the decrease in mechanical joint stability. For the MLR component of the stretch response a significant prolonged latency could be observed. Thus, “giving way” is also associated with altered stretch reflex excitability that takes place on the spinal level. In their paper they concluded, that sensorimotor function may be influenced by appropriate training stimuli. It was suggested that sensorimotor training early after ACL rupture might be promising for a rapid restoration of joint function.
3 Benefits from sensorimotor training
Sensorimotor training (SMT) has been applied in various sport disciplines as a training tool. In 2004, Gruber/Gollhofer demonstrated that SMT is not only associated with improvements in balance control but also with a significant enhanced rate of force development during isometric voluntary contractions. These improvements are closely associated with an increased neuromuscular activation at the onset of muscle action. It was concluded that the adaptations after SMT enables the neuromuscular system to earlier and more powerful activate the muscles. Gruber et al (2006) examined the effects of a SMT training on knee joint stability. In order to examine the effects on the ankle and/or knee joint, subjects were divided randomly into three subgroups. The first group conducted the SMT barefooted (n = 21), the second group with an ankle brace (Air-Stirrup® Plus, Aircast®, Summit, USA), which was used as a semirigid fixation of the ankle joint (n = 21), and the third group trained with a ski boot as a rigid fixation of the ankle joint (n = 21). Training was performed over a period of 4 weeks with a total of sixteen training sessions.
Knee joint stiffness of the right leg was measured with an apparatus (Fig.1) allowing application of fast impulses to induce an anterior tibia translation in the knee joint. By two linear potentiometers tibia displacement calculated as the difference between the thigh and shank against a mounting frame.
Fig. 1: Device to induce anterior tibia displacements at the knee joint. With two linear potentiometers the relative displacement of patella and tibia plateau with respect to the mounting frame allow quantification of the anterior displacement of the shank relative to the thigh.
In order to study the reflex regulation, mechanical stimuli were adjusted which displaced the shank in anterior direction. Within 50 ms following the initial onset in displacement the maximum force of 358.2 ± 52.1 N was reached.
Following SMT the group training with ski boots showed increased knee joint stiffness for a given tibia displacement concomitant with enhanced emg responses of the hamstring muscles (Fig. 2). These findings underline the importance of fast hamstring actions to stabilize the knee joint which was reported for ACL-injured subjects during isometric leg extensions (McNair et al 1992). In the same line, the simulations based on a 3D model of the lower limb showed that increased hamstring force was superior to reduced quadriceps force in order to stabilize the ACL-deficient knee during gait (Shelbourne et al 2005).
Fig. 2: Individual and mean alterations of knee joint stiffness and hamstring reflex size (mean amplitude voltage of M. biceps femoris and M. semitendinosus from 30–90 ms after mechanical stimulus) before and after a 4 week sensori-motor training.
Considering knee joint injuries, it is of major importance that the enhanced activities of the hamstring muscles get directly feedback about the actual ACL load. Therefore, the adaptations following SMT have been taken to explain the reduced risk of suffering an ACL injury. However, an overall preventive effect of SMT on knee joint injuries is not yet proved according to prospective studies. There were no preventive effects reported after balance board training, considering acute knee joint injuries, in handball (Wedderkopp et al 1999) and volleyball players, respectively (Verhagen et al 2004).
Whether a SMT with a fixed ankle joint will increase the preventive effect in Alpine Skiing has urgently to be answered in future prospective training studies.
References
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Eccentric exercise in alpine skiing
1 Background
In normal human movement muscles are used both to provide positive and negative external work. Positive work is a consequence of concentric muscle contractions, i.e. contractions in which activated muscles shorten (“shortening contraction”) and thus provide the external work. Typical dominant concentric muscle activities are observed during cycling, swimming or uphill walking. In cycling and swimming, the positive work of concentric exercise is dissipated mainly to overcome the frictional resistance of air or water. In the case of uphill walking, concentric muscle shortening leads to upward movement of the body’s center of mass, thus increasing the body’s potential energy against gravity. Eccentric contractions are defined as muscle activities that occur when the force applied to the muscle exceeds the force produced by the muscle. Under these conditions the activated muscle is lengthened (“lengthening contraction”). Eccentric contractions are generally used to decelerate or brake. This is classically illustrated by downhill walking during which eccentric contractions dissipate the potential energy gained by uphill walking. Eccentric muscle contractions are also used to tension tendons upon impact in order to allow tendons to be lengthened and thus to store energy. This energy can subsequently been released by elastic recoil (thus providing positive external work) to reduce the energetic cost of locomotion. The roles of concentric and eccentric muscle contractions in locomotion have succinctly been reviewed by (Dickinson et al. 2000). It is of interest to note that the topics of eccentric muscle contractions or eccentric exercise is massively under researched. A cursory glance at PubMed indicates that there are at least one order of magnitude more research papers published using concentric than eccentric exercise modalities.
The interest of eccentric exercise in alpine skiing stems from the fact that alpine skiing is dominantly eccentric in nature (Berg and Eiken. 1999). These authors (Berg et al. 1995) had shown for giant slalom turns that the eccentric lengthening phase of the knee extensors lasted twice as long as the concentric push-off phase (1.0 vs. 0.5 sec). Moreover, EMG activity on the outside leg during eccentric muscle actions significantly exceed that of concentric actions and was similar to the maximum isometric knee extension as estimated in laboratory tests. The maximum eccentric torque developed on the outside leg at the peak of a turn acts against the combined gravitational and centrifugal forces experienced by the skiers (Hintermann et al. 1995). Maximal eccentric torque and dosage of eccentric contraction by knee extensors may be the key limiting factors determining the maximum speeds at which gates can be passed in competitive alpine skiing. In view of the fact that it is common knowledge that training procedures should closely match competitive activity - there is surprisingly little emphasis put on specifics of eccentric training modalities for alpine skiers.
The current review reports some of the main physiological conditions of eccentric muscle exercise and eccentric muscle training. It further describes the properties of an eccentric training device (eccentric ergometer) primarily designed for rehabilitation as well as the use of this eccentric ergometer as a training adjunct in competitive alpine skiers.
2 Physiology of eccentric exercise
Comparing eccentric to concentric muscle contractions, four main distinctions can be made:
1. At the same shortening velocities (i.e. same angular velocities of limb movement) eccentric contractions are observed to produce far greater forces than concentric contractions (Asmussen 1953). ). For the case of the human knee extensors the difference in peak torque between concentric and eccentric contractions at 2.6 rad/s angular velocity can be as much as twofold (Colliander and Tesch 1989) It is further of note that the decrease of force production with increasing speed of isotonic contraction described by Hill (1938) is absent for eccentric contractions. This circumstance is putting muscle tissue at risk in particular at high negative angular velocities.
2. At similar force developments, eccentric contractions require substantially lower electromyographic activities (Asmussen 1953; Bigland-Ritchie and Woods 1976). This indicates that fewer motor units are recruited to produce the same tension as in concentric contractions. With fewer motor units active, eccentric motor tasks are inherently more difficult to control and coordinate.
3. The energetic cost of producing negative work during eccentric exercise is considerably lower than the cost of producing positive work during concentric exercise (Asmussen 1953; Bigland-Ritchie and Woods 1976; Knuttgen 1986). It is reported that there is an up to six-fold difference of the metabolic cost of producing concentric vs. eccentric work. As a consequence, the physiological responses such as pulmonary ventilation, heart rate, cardiac output and muscle blood flow are reduced accordingly with eccentric vs. concentric work.
4. Eccentric exercise can produce substantial damage to muscle cell structure (Friden et al. 1981; Friden et al. 1991), resulting in delayed onset of muscle soreness (DOMS) temporary decrease in muscle performance and an increase of muscle creatine kinase in the peripheral blood (see Clarkson et al. 1987). The structural and muscle functional deteriorations observed after eccentric exercise are fully reversible over a period of weeks and repeated bouts of eccentric exercise induce a protection of muscle tissue from the damaging consequences of unaccustomed eccentric exercise (Clarkson et al. 1992, McHugh 2003). (all refs in NFP)
In view of the very specific conditions of eccentric exercise outlined above (high mechanical load at low metabolic requirements and challenging coordination task) we felt that eccentric exercise should be evaluated as a training adjunct for alpine skiers.
3 Design and applications of an eccentric ergometer
We custom built an eccentric recumbent ergometer designed to deliver in excess of 3000 Watts of mechanical power (Fig. 1).
Fig. 1: An alpine junior skier training on our custom build eccentric ergometer
The ergometer is computer controlled and can be programmed to drive pedals forwards or backwards. Desired power output and cadence can freely be selected from a menu. The necessary torque to be supplied by the subject in order to achieve a selected power output at a selected cadence is displayed on the computer screen overlaid with the trace indicating the actual instantaneous torque delivered by the subjects. The subject is required to match the two curves as closely as possible. This is a visual - motor coordination task of considerable difficulty requiring constant attention. The software offers the option to estimate the difference of the executed vs. the target torque by the root mean square (RMS) of the difference of the two signals (indicated by the hatched area in Fig. 2). The ergometer can thus provide a numerical assessment of the quality of the coordination of the eccentric performance of a subject.
Fig. 2: Visualized real time feedback on the computer screen of the eccentric ergometer. Green line: target load = 100%. Dosage of eccentric muscle action can be quantified as the area under the curve, indicated as % standard deviation. Top = bad coordination, bottom: good coordination.
The same ergometer was previously used in a rehabilitation setting involving heart infarct patients (Steiner et al. 2004). In this setting, the ergometer was used to maximize mechanical load on lower extremity muscles within the low metabolic capacity of these patients. Maximal average mechanical loads attained during rehabilitation of heart infarct patients were up to 380 Watts.
4 Eccentric exercise with alpine skiers
Training protocols
In patients, elderly, untrained subjects or in endurance athletes, a negative work exercise program on the eccentric ergometer has to be started very carefully (Gerber et al. 2006, Daepp et al. 2007). When applying a low load of only 130 watts for 15 minutes untrained subjects get strong muscle soreness within 24 hours post exercise (Klossner et al. 2007). As in alpine skiing eccentric muscle action is very dominant during sport specific tasks (Berg et al. 1999), a higher initial training load can applied on elite skiers. Typically training on the eccentric bike can be started at around 400 watts for 20 minutes without getting muscle soreness in skiers (Weisskopf and Vogt 2007). Within 5 training sessions, alpine skiers were able to double the eccentric load up to 800 watts.
Effect of eccentric exercise on jumping performance
In competitive sports, exercise training on an eccentric ergometer was first applied to high school basket ball players (Lindstedt et al. 2002). These athletes trained for 6 weeks (30 min three times per week). During the training period the eccentric load was gradually increased until they were working at nearly 500 watts during the last three weeks. A weight training control group was drawn from the same high school basket ball players. All eccentric-trained subjects increased their jump height, with an overall mean increase by 8% (+5cm). In response to eccentric training, hopping frequency increased by 11%, suggesting an enhanced strain energy storage and recovery possibly related to an increased stiffness of the muscle-tendon unit.
We applied a similar 6-week eccentric training protocol on eight junior alpine skiers. They trained for 20 minutes on the eccentric ergometer in addition to 40 minutes concentric weight training during the same session three times per week. 7 subjects served as a concentric weight trained control group (60 min per session). Counter movement jumping height increased exclusively in the eccentric-trained group by 7.9% (+4.1cm, see also Fig. 3), similar to the results of the basket ball player study (Lindstedt et al. 2002). For the eccentric trained group, no changes were measured in peak and mean power during the concentric