CC BY
99
58 ПРАКТИЧЕСКАЯ МЕДИЦИНА '3 (88) апрель 2015 г. Том 1
УДК 616.718.5/.6:611.738.3:796
Е. ГАПЕЕвА12, в. ЛАBPEНТЬEBA3, Я. ЭрЕЛИНЕ12, M. ПЯЭСУКЕ12
1Институт спортивной биологии и физиотерапии, г. Тарту, Эстония 2Школа докторантов по социологии, поведенческой психологии и науке о здоровье, Тартуский университет, г. Тарту, Эстония 33АО «Медикум», г. Таллин, Эстония
Сила мышц голени и постуральная стабильность у юных спортсменов
Гапеева Елена — врач (физическая и медицинская реабилитация), доктор философии, научный работник в области кинезиологии и биомеханики, тел. +372 7376286, e-mail: [email protected]
Лаврентьева вероника — магистр наук, кинезитерапевт (ортопедия, травмы позвоночника), tel. +372 56158761, e-mail: [email protected]
Эрелине Яан — магистр наук, доктор философии, лектор по биомеханике и эргономике, тел. +372 7376286, e-mail: jaan. [email protected] Пяэсуке Мати — кандидат биологических наук, профессор кинезиологии, тел. +372 7376286, e-mail: [email protected]
Исследования показывают, что дисбаланс в продукции силы и недостаточная постуральная стабильность увеличивают риск повреждения голеностопного сустава у спортсменов. В статье сравниваются показатели силы мышц голени и постуральной стабильности у юных каратистов и баскетболистов в возрасте 15-17 лет. Демонстрируется разница в характеристиках постурального контроля при би- и монопедальном тесте для исследования статического равновесия в положении стоя на стабильной и нестабильной поверхностях, с открытыми и закрытыми глазами, а также с применением современной динамографической платформы и программного обеспечения. Приводятся практические рекомендации для спортивной тренировки.
Ключевые слова: изометрическая сила, статическое равновесие тела, карате, баскетбол, голеностопный сустав, гибкость.
H. GAPEYEVA12, V. LAVRENTJEVA3, J. ERELINE12, M. PДДSUKE1•2
11nstitute of Exercise Biology and Physiotherapy, University of Tartu, 5 Jakobi St., Tartu, Estonia, 51014 2Doctoral School of Behavioural, Social and Health Sciences, University of Tartu, 5 Jakobi St., Tartu, Estonia, 51014
3Medicum Ltd, 61 Punane St., Tallinn, Estonia, 13619
Calf muscle strength and postural stablity in young male athletes
Gapeyeva H. — MD (PRM), PhD, Researcher in Kinesiology and Biomechanics, tel. +372 7376286, e-mail: [email protected] Lavrentjeva V. — PT, MSc, physiotherapy specialist (orthopaedics ans spine injuries, tel. +372 56158761, e-mail: [email protected] Ereline Ja. — MSc, PhD, Lecturer in Biomechanics and Ergonomics, +372 7376286, e-mail: jaan. [email protected] Pflflsuke M. — PhD, Professor of Kinesiology, tel. +372 7376286, e-mail: [email protected]
Many studies have shown that not only force generation dysbalance, but also a deficit of postural stability increase the rate of ankle joint injuries in athletes. Calf muscle strength and postural stability during different static standing tests in young karate athletes and basketball players aged 15-17 years are compared in the study. Authors discuss the differences in postural sway characteristics during bi- and monopedal standing on stable and unstable ground, with and without visual input, using a modern force plate and software. Based on the results of the study, practical recommendations are given for sports training. Kew words: isometric strength, static body balance, karate, basketball, ankle joint, flexibility.
Karate is a kind of martial art that is practised worldwide [1]. In accordance with different studies, this sport event has a low risk for injuries [2, 3]. Another very popular kind of sport is basketball, the players of which frequently suffer from injuries of lower extremities. Compared to karate athletes, the ankle joint injuries occur in basketball players approximately fivefold more [4].
Due to frequent ankle joint injuries, athletes have to make pauses in the training cycle. If appropriate rehabilitation does not restore the joint function, the injury can become chronic and end the athlete's career. Calf muscle strength and postural control play a significant role in the stabilization of ankle joint and prevention of injuries. Systematic review of literature shows that general joint laxity is a predictive measure for leg injuries. Greater strength of the plantar flexors as well as increased postural sway may also be a predictive measure for sustaining an ankle injury [5]. Ankle joint injuries are the most frequently occurring ones due to which athletes turn to emergency medicine or to the orthopedist [6-10]. Lateral ankle sprain occurs in 95% of all ankle joint injuries and 12% of all injuries [6, 7, 11]. Ankle joint injuries occur more frequently in athletes of such sports where the main activity includes running, jumps, and rapid movements or landing on an uneven ground [12]. According to different authors, out of the total number of injuries the ankle joint injuries make up 40% in basketball (7), 31% in soccer (7), and 16.7% in volleyball (13,3). In martial arts this index is the lowest — 9.1% (3). The risk of injury in karate increases with the duration of training, training load per week and raise in the sport level (2).
Balance is the ability of keeping the body stable in different conditions. Ankle joint is closely bound with the body's center of mass and is therefore essential in maintaining postural control during standing and at movements [14]. It has been shown that sport training improves proprioception and increases strength of muscle surrounding the ankle joint, which furthers joint stability and keeps body balance [14]. Also it was demonstrated that not only muscle disbalance but also body postural control deficit increases the number of injuries in athletes [15, 14].
A previous study of healthy adults confirmed that Tai Chi practitioners have greater isokinetic thigh muscle strength and better balance confidence by Activities-specific balance confidence scale [16]. In the study of Violan et al. [17] six months of karate training (twice a week) evoked in boys aged 8-14
years a greater improvement in balance (Flamingo test with eyes closed) as compared to age- and gender matched recreational sport participants. The recent study of Vando et al. [18] confirmed that short-term high-intensity karate training decreased postural sway between bi- and monopodalic open eyes tasks as compared to non-athletes in the age of 10.2±1.7 years (mean±SD). The majority of studies focusing on karate and postural control concern only adults [19] and no appropriate sources have been found by the authors if this article in regard to the leg muscles strength and postural control in young karate and basketball athletes. The aim of the present study was to compare calf muscle strength, ankle joint flexibility and postural stability during bi- and monopedal standing on stable and unstable ground, with and without visual input, and with decreased support area in young athletes traned in different kinds of sport: karate and basketball.
Methods
Participants
Seventeen male adolescent athletes aged 15-17 years volunteered for this study. The first group included 10 karatekas and the second group 7 basketball players; their training period (mean±SE) was 7.7±0.8 years and 8.4±0.5 years, respectively. All athletes trained for 8-12 hours (5-6 times) per week and trainings were performed in sport clubs, supervised by one coach for each group. The anthropometric data of participants are presented in Table 1. None of the participants complained of joint pain. The exclusion criteria of study participants were orthopaedical and neurological diseases; vestibular or visual disorders; history of injury in the past 12 months; and regular training in other kinds of sport. Study partcipants and their parents gave their written informed consent before the study, following the explanation of the experimental protocol and procedures. The study was approved by the Human Subjects Ethics Committee of the University of Tartu in accordance with the Declaration of Helsinki.
Measurement of anthropometric
characteristics and active range of motion
The participants' body height was measured in stretch posture by anthropometer attached on the wall (Soehnle Professional, Germany). The body mass was measured with the subject standing without support by scale (Circle Balance, Soehnle, Germany). The foot length was measured in its maximum distance by Campbell Caliper 20 (Rosscraft, USA) bilaterally.
Table 1. Age and anthropometric data of young male athletes (mean±SE)
Characteristics Karatekas Basketball players
Age (year) 16.60±0.34 16.57±0.20
Body height (cm) 179.74±2.23 189.03±0.48*
Body mass (kg) 68.90±3.50 79.77±2.28*
BMI (kg-m"2) 21.20±0.67 23.91±0.63
Foot length (cm) 26.8±0.28 28.4±0.52*
n 10 7
Note: BMI — body mass index; * — p<0.05 as compared to karatekas. Foot length data are for the dominant leg
Figure 1. Measurement of plantar and dorsal flexion strength by hand-held dynamometer Lafayette Manual Muscle Test System
The active range of motion (ROM) of ankle joint's dorsal (DF) and plantar flexion (PF) was measured by standard mechanical goniometer (Gollehon Extendable Goniometer, Lafayette Instrument, USA) and guidelines [20]. Three measurements were taken for each motion and the best result was accepted for analysis.
Isometric maximal voluntary strength measurement
Calf muscle isometric maximal voluntary contraction (MVC) strength at dorsal and plantar flexion was measured by hand-hald dynamometer Lafayette Manual Muscle Test System (Lafayette Instrument Company, USA) for both lower extremities. Hand-held dynamometer is a reliable and valid assessment tool for muscle strength measuring [21]. During testing the participant was supine. Before measurement the participant was asked to keep the foot in neutral position. Dynamometer was positioned on dorsal distal metatarsal part of foot for measuring the DF strength (Fig. 1A) and on plantar part for measuring the PF strength, respectively (Fig. 1B). The participant was asked to push against the dynamometer for approximately 2-3 s with maximal effort and the instruction «push as hard as possible» was given before each contraction. The best result from three trials was taken to analysis. A rest period of one minute was allowed between trials. All measurements were performed by the same physiotherapist. The MVC force-to-body mass ratio MF (kg): BM (kg) was calculated.
Postural sway measurement
The body postural stability was investigated by sway amplitude of centre of pressure (CoP) in anterior-posterior (AP) and mediolateral (ML) direction during 30-s standing on a force plate Kistler 9286A (Switzerland) with the dimensions of 60x40 cm and SWAY® software of the movement analysis system Elite Clinic (BTS S.p.A., Italy). In order to establish the influence of the visual input, the unstable ground and the increased support area, the following data were registered barefoot at five conditions:
— bipedal standing on force plate with eyes open (EO) (1) and eyes closed (EC) (2);
— bipedal standing on balance pad (Alcan Airex AG,
Switzerland; dimensions 39.5x48.5 cm and thickness 6 cm) placed on force plate EO (3) and EC (4);
— unipedal standing (on the dominant leg) on force plate EO. During the last test, the support leg was straight and the contralateral leg was flexed in hip and knee joints by 90 degrees (Fig. 2).
During OE trials the participant looked at a picture (black point in the centre of circle with the diameter of 14 cm) placed on the wall at the eye level. The distance between this mark and the participant was 3.5 m. The pause between trials was one minute (seating on standard chair with back support). All the measured
Figure 2. Body postural stability testing on balance pad (Alcan Airex AG, Switzerland) using force plate (Kistler, Switzerland). The participant is standing on the dominant leg, the contralateral leg is flexed in hip and knee joints by 90 degrees
characteristics were adjusted to the subject's height (m) and foot length (mm) [22].
Testing Protocol
The study was performed in the Laboratory of Kinesiology and Biomechanics of the University of Tartu during the competition period of the training cycle for all athletes. They were asked to avoid participation in any training activity one day before the testing. Before the testing, the athletes had a rest for about 25 min, sitting on the chair with back support, to minimize the effect from walking to the laboratory. The testing was performed in the following sequence: (1) anthropometric measurements; (2) ROM of ankle joint of DF and PF; (3) strength measurement of DF and PF; after 5-min seated rest (4) postural stability testing.
Figure 3. Active range of motion of dorsal and plantar flexion in young male karatekas and baskeballers (mean±SE)
Right Left Right Left
Dorsal flexion Plantar flexion
*p<0.05
Statistical analysis
Data are means and standard errors of means (±SE). Student's unpaired t-test was used to find differences between the groups and between the tests for postural stability measurements. The normal distribution of data was checked by Kolmogorov-Smirnov's test and all measured variables had p>0.05. A level of p<0.05 was selected to indicate the lowest statistical significance.
Results
Anthropometric characteristics and active ROM of ankle joint
Basketball players were taller than karatekas (p<0.05) and had greater body mass (Table 1). No significant differences were noted in BMI (p>0.05).
Plantar flexion active ROM was greater (p<0.05) in karatekas (the right leg) (Fig. 3). In total, the tendency for greater ROM by 17-29% was noted in karatekas as compared to basketball players, but as regards the major data — dorsal flexion and left leg plantar flexion — the difference was not statistically significant (p>0.05). The data of the right and left leg inside one group of athletes did not differ singnificantly (p>0.05).
Figure 4. Isometric maximal voluntary contraction force of dorsal and plantar flexion in young male karatekas and baskeballers: absolute values (A) and (B) (mean±SE)
*p<0.05; **p<0.01; ***p<0.001
MVC strength of calf muscles
Isometric MVC force of DF and PF was significantly greater in basketball players as compared to karatekas (Fig. 4A). This characteristic for DF was greater by 11% (p<0.05) for the right leg and by 13% (p<0.001) for the left leg of baskeball players as compared to karatekas.
The difference for PF force as compared to DF was by 2-3 times greater in basketball players and they also had greater PF force of the right leg by 31% (p<0.001) and of the left leg by 29% (p<0.01) as compared to karatekas. No significant differences (p>0.05) were noted between the data of the right and left leg inside each group of athletes.
When we normalised the data of MVC force to body mass, no significant differences were found between MF:BM ratio for DF and PF strength characteristics between the groups and between the right and left leg (Fig. 4B).
Figure 5. The centre of pressure sway amplitude in the anterior-posterior (AP) (A) and mediolateral directions (ML) (B) during three postural tests: quiet bipedal standing eyes open (EO) and eyes closed (EC) on stable ground (1); on balance pad (EOpad; ECpad) (2); and during unipedal standing EO on the dominant leg on stable ground (DL) and on balance pad (DLpad) in young male karatekas and basketball players (mean±SE). Data are normalised to body height (m) and foot length (mm)
AP
Б 0.24 и
0.20 -
— 0.16-
EO EC EOpad ECpad DL DL pad
ML
*p<0.05; **p<0.01; ***p<0.001
Postural stability
Data of CoP sway in AP and ML direction ajusted to body height and foot length are presented in Figure 4, A and B, respectively.
Bipedal standing on stable ground. Karatekas demonstrated by 45% lower (p<0.05) CoP sway amplitude in ML direction of EO test during bipedal standing on stable ground as compared to basketball players.
No significant differences (p>0.05) were noted between groups in this characteristic of EC test. CoP sway in AP direction did not differ significantly either between EO and EC test within each group or between karatekas and basketball players.
Bipedal standing on unstable ground. During bipedal standing on balance pad at EO test, CoP sway amplitude in both AP and ML directions did not differ significantly between groups. At EC test, basketball players had greater CoP sway in AP by 14% (p<0.05) and in ML direction by 32% (p<0.001) as compared to karatekas.
When comparing the EO and EC test during standing on unstable ground, the athletes of both groups demonstrated significantly greater (p<0.01) CoP sway at EC test in both AP and ML directions — karatekas by 36% and 22%, respectively and basketball players by 51% and 43%, respectively.
Unipedal standing on stable and unstable ground. During unipedal standing at EO test on stable ground, basketball players demonstrated by 14% greater (p<0.05) CoP sway amplitude in AP direction and no differences were noted in ML direction between the groups. When standing on unstable ground, CoP sway amplitude did not differ significantly (p>0.05) between karatekas and basketball players.
All athletes showed significantly increased (p<0.001) sway amplitude in both AP and ML directions standing on balance pad as compared to stable ground, but this fifference was greatest for AP direction: 53% in karatekas and 46% in basketballers.
Discussion
The novelty of the present study is comparison of the maximal isometric voluntary strength of calf muscles and body stability during different static postural tasks between young male karate practitioners and basketball players. Several authors have shown that balance disorders and the strength deficit or disbalance of calf muscles can result in falls and ankle injuries [10, 14]. Good control over body postural muscle function helps to prevent falls and avoid injuries.
Anthropometric characteristics and flexibility of ankle joint
Young male athletes, training with similar loads per week in Tartu karate and basketball clubs, participated in this study. The BMI did not differ significantly between the groups, but basketball players were taller and had longer feet. Also they had lower flexibility of ankle joint for PF as compared to karatekas. It was shown that jount stiffness influences on standing stability [23].
Isometric strength of calf muscles
The present study showed that basketball players had greater isometric MVC force in comparison to karate practitioners — in PF on average by 31% and in DF by 13%.
The multiannual (perennial) training is conventionally divided into four stages. The goal of stages I-II is the improvement of health, comprehensive physical development, acquiring movement skills and basic skills of the respective sport. The training in stages III-IV is more specific: the competition method and tactical preparation are very important and the share of specialized preparation and competition is increasing [24]. The participants of the study were advanced karate practitioners and basketball players with the experience of 6-9 years in the respective sport. There were no statistically significant differences in the duration of experience.
In the training process of karate practitioners, the gaining of tactical knowledge dominates during preparation for competitions. However, the difference in strength between karate practitioners and basketball players may also be due to the greater height and muscle mass of basketball players. The longer are the bone levers, the greater is the force that can be applied to a segment of the body [25]. Thus the differences in muscular strength can be attributed to specificity of training of basketball players, with more attention
paid to the development of lower extremities muscular strength, as well as to the greater height and muscle mass. However, when comparing MVC maximum force-to-BM data, no significant differences were noted between the groups in calf muscle strength. Therefore the influence of body mass should be taken into account in the analysis of sports training effect and our previous study has also confirmed this [26].
Postural stability
In this study, the static equilibrium of the body was measured with the use of force plate. During the study, the athletes were standing on the platform and then on the polyurethane foam pad with a thickness of 6 cm, placed on the platform. Karate trainings and competitions take place on soft mats (tatami) with the thickness of 2-4 cm [27], so the testing situation was similar to the training conditions. All athletes managed to perform these tests, however, some of them needed additional attempts.
The study showed that in EO tests basketball players had greater postural sway amplitude in ML direction during bipedal static standing on stable ground as compared to karatekas. In AP direction basketball players also had greater postural sway amplitude standing on dominant leg without balance pad in comparison to karate practitioners.
It can be presumed that better parameters of postural stability for karate practitioners can be attributed to the specificity of their training, where it is necessary to develop specific motor skills to achieve good result at competitions: the movement speed in different directions, the frequency of kicks with lower extremities, the speed of the blocks [28]. To be able to kick with lower extremities the karatekas need to have good flexibility and body balance, as well as to be able to stand on one foot with lower postural sway. In addition, basketball players have greater height, which gives the advantage to karate practitioners when performing the test. In basketball, good postural stability is significant for movement speed and accuracy of throw.
Another possible reason why the postural stability of karate practitioners was better compared to the basketball players is that all athletes performed the tests barefoot; however, basketball players train in footwear and karate practitioners without it. Wu [29] showed in his study that Tai Chi training has positive effects on the body postural stability and makes it more effective as compared to the group without the corresponding training.
In the present study the parameters of postural sway were measured in laboratory conditions for both open
and closed eyes. Basketball players demonstrated greater postural sway also during EC bipedal standing on unstable ground in both AP and ML direction. It has been shown that sway of the center of gravity of the body increases with age and with the difficulty of the test [30], which was also noted in this study. The more difficult the test was, the worse became the postural stability for both basketball players and karate practitioners. A recent study of the effect of intensive one-week karate training camp of preadolescent karate athletes showed improvement of postural stability — approximately 3-fold decrease of CoP sway length and velocity for static bipedal standing tests (both EO and EC); and two-fold decrease of these characteristics for monopedal standing test EO [18]. Better postural stability in karatekas could be explained by training specificity as it was demonstrated for gymnasts, for example. Findings of the study of Vuillerme and Nougier [31] suggest a decreased dependency on attentional processes for regulating of postural sway during unipedal standing (on stable and 7-cm thick foam surface) in gymnasts with respect to non-gymnasts.
The major limitations of our study were: (1) small number of subjects, although the cohort under study was homogeneous; (2) the investigation was performed with static postural tasks without using the dynamic tasks for posture. Future studies could establish the training effect of postural control, in comparison to age- and gender matched healthy untrained controls.
Conclusions
Isometric MVC force production of calf muscles was significantly greater in young male basketball players then in karatekas — by 30% for PF muscles and by 11% for DF muscles.
Postural stability was better in karate practitioners in three tests out of six — during bipedal standing on stable ground EO (less CoP sway amplitude in ML direction); without using the visual input during bipedal standing on unstable ground (less sway amplitude in AP and ML direction); during monopedal standing (less sway in AP direction). Based on the results of this study, it can be recommended for basketball players to perform exercises that improve postural control; and for karate practitioners to strengthen the muscles participating in ankle joint function, having a positive effect on stabilizing the joints.
Acknowledgements
The study was partly supported by the Estonian Ministry of Education and Research projects SF0180041s12 and IUT20-58.
REFERENCES
1. Burke D.T., Al-Adawi S., Lee Y.T., Audette J. Martial arts as sport and therapy. J Sports Med Phys Fitness, 2007; 47: 97-102.
2. Zetaruk M., Violan M., Zurakowski D., Micheli L. Karate injuries in children and adolescents. Accident analysis & Prevention, 2000; 32: 421-425.
3. Destombe C., Lejeune L., Guillodo Y., Roudaut A., Jousse S., Devauchelle V., Saraux A. Incidence and nature of karate injuries. Joint Bone Spine, 2006; 73: 182-188.
4. Kujala U., Taimela S., Antti-Poika I., Orava S., Tuominen R., Myllynen P. Acute injuries in soccer, ise hockey, volleyball, basketball, judo, and karate: analysis of national registry data. BMJ, 1995; 311: 1465-468.
5. Dallinga J.M., Benjaminse A., Lemmink K.A. Which screening tools can predict injury to the lower extremities in team sports? A systematic review. Sports Med, 2012; 42: 791-815.
6. Kavanagh J. Is there a positional fault at the inferior tibiofibular
joint in patients with acute or chronic ankle sprains compared to normals? Manual Ther, 1999; 4: 19-24.
7. Caulfield B. Functional instability of the ankle joint. Physiotherapy, 2000; 86: 401-411.
8. Westlin N., Vogler H., Albertsson M., Arvidsson T., Montgomery F. Treatment of lateral ankle instability with transfer of the extensor digitorum brevis muscle. J Foot Ankle Surg, 2003; 42: 183-192.
9. Pontaga I. Ankle joint evertor-invertor muscle torque ratio decrease due to recurrent lateral ligament sprains. Clin Biomech, 2004; 19: 760-762.
10. Akbari M., Karimi H., Farahini H., Faghihzadeh S. Balance problems after unilateral ankle sprains. J Rehabil Res Dev, 2006; 43: 819-824.
11. Papadopoulos E.S., Nicolopoulos C., Anderson E., Curran M., Athanasopoulos S. The role of ankle bracing in injury prevention, athletic performance and neuromuscular control: a review of the literature. Foot, 2005; 15: 1-6.
12. Dayakidis M.K., Boudolos K. Ground reaction force data in functional ankle intability during two cutting movements. Clin Biomech, 2006; 21: 405-411.
13. Wright I., Neptune R., van den Bogert A., Nigg B. The influence of foot positioning on ankle sprains. J Biomech, 2000; 33: 513-519.
14. Lin W.-H., Liu Y.-F., Hsieh C., Lee A. Ankle eversion to inversion strength ratio and static balance control in the dominant and nondominant limbs of young adults. J Sci Med Sport, 2009; 12: 42-49.
15. McGuine T.A, Greene J.J., Best T., Leverson G. Balance as a predictor of ankle injuries in high school basketball players. Clin J Sport Med. 2000; 10: 239-244.
16. Tsang W.W., Hui-Chan C.W. Comparison of muscle torque, balance, and confidence in older tai chi and healthy adults. Med Sci Sports Exerc, 2005; 37: 280-289.
17. Violan M.A., Small E.W., Zetaruk M.N., Micheli L.J. The effect of carate training on flexibility, muscle strength and balance in 8 to 14-year-old boys. Pediatr Exerc Sci, 1997; 9: 55-64.
18. Vando S., Filingeri D., Maurino L., Chaabene H., Bianco A., Salernitano G., Foti C., Padulo J. Postural adaptations in preadolescent karate athletes due to a one week karate training camp. J Hum Kinet, 2013; 38: 45-52. eCollection 2013.
19. Filingeri D., Bianco A., Zangla D., Paoli A., Palma A. Is karate effective in improving postural control? Arch Budo, 2012; 8: 203-306.
20. Clarcson H.M. Joint motion and function assessment. Lippincott, Williams &Wilkins, Philadelphia, 2005.
21. Arnold C.M., Warkentin K.D., Chilibeck P.D., Magnus C.R. The reliability and validity of handheld dynamometry for the measurement
of lower-extremity muscle strength in older adults. J Strength Cond Res, 2010; 24: 815-824.
22. Chiari L., Rocchi L., Cappello A. Stabilometric parameters are affected by anthropometry and foot placement. Clin Biomech (Bristol, Avon), 2002; 17: 666-677.
23. Edwards W.T. Effect of joint stiffness on standing stability. Gait Posture, 2007; 25: 432-439.
24. Koik H., Brock C. Course for basketball trainers. (In Eston.) Tallinn, Estonian Olympic Committee, 2006.
25. Watkins J. Structure and function of the musculoskeletal system. Champaign, Human Kinetics, 1999.
26. Paasuke M., Ereline J., Gapeyeva H. Knee extensor muscle strength and vertical jumping performance characteristics in pre- and post-pubertal boys. Pediatr Exerc Sci, 2001; 13: 60-69.
27. Neemre I., Zokov A., Luhamaa M., Saag T. Course for trainers. Karate (in Eston.) Tallinn, Estonian Olympic Committee, 2006.
28. Blazevic S., Katie R., Popovic D. The effect of motor abilities on karate performance. CollAntropol, 2006; 30: 327-333.
29. Wu G. Evaluation of the effectiveness of Tai Chi for improving balance and preventing falls in the older population — a review. J Am Geriatr Soc, 2002; 50: 746-754.
30. Rogind H., Lykkegaard J., Bliddal H., Danneskiold-Samsoe B. Postural sway in normal subjects aged 20-70 years. Clin Physiol Functs Im, 2003; 23: 171-176.
31. Vuillerme N., Nougier V. Attentional demand for regulating postural sway: the effect of expertise in gymnastics. Brain Research Bulletin, 2004; 63: 161-165.
