THE EFFECT OF SUPERVISED STRENGTH TRAINING ON YOUNG ELITE MALE SOCCER PLAYERS PHYSICAL PERFORMANCE

Transcription

1 Serbian Journal of Sports Sciences ISSN Original article 2013, 7(4): ID Received: 14 Apr 2013 UDC Accepted: 16 Sept 2012 THE EFFECT OF SUPERVISED STRENGTH TRAINING ON YOUNG ELITE MALE SOCCER PLAYERS PHYSICAL PERFORMANCE Eystein Enoksen 1, Martin Staxrud 1, Espen Tønnessen 2 & Shaher A. I. Shalfawi 1,2 1 Norwegian School of Sport Sciences, Department of Physical Performance, Oslo, NORWAY. 2 Norwegian Olympic Sport Center (Olympiatoppen), Oslo, NORWAY. Abstract The aim of the present study was to measure the effect of direct supervision by a strength and conditioning coach on elite strength training sessions for soccer junior players compared to self strength-trained subjects. Twenty-six young well-trained elite male soccer players aged (Mean ± SD) (19.1 ± 3.5 years), body mass (72.6 ± 7.8 kg) and stature (178.6 ± 7.8 cm) volunteered to participate in the present study. Subjects were divided into 3 groups, a supervised strength training group (N = 9), an unsupervised strength training group (N = 8) and a control group with no strength training (N = 9). A 10-week strength training program was developed and the supervised strength training group performed the program under the supervision of a strength and conditioning coach. The unsupervised strength training group performed the same training program with no supervision and the control group followed only the regular team soccer training. To measure the effect of the program and the supervision of a strength and conditioning coach, all subjects were tested on maximum strength in leg muscles (1RM) according to leg press, sprinting speed (10 m and 40 m) and jumping performance (countermovement jump). The analyses between groups showed that the supervised strength training group showed a significant increase in CMJ (2.6 ± 1.1 cm) and leg press (26.3 ± 7.9 kg) performance compared to the unsupervised strength training group. The supervised strength training group also showed a significant increase in 10 m (-0.05 ± 0.02 s), 40 m (-0.12 ±.03 s) and leg press (37.2 ± 9.0 kg) performances compared to the control group. The findings of the present study indicate that a strength training program directly supervised by a strength and conditioning coach over a period of 10 weeks led to a significant improvement compared to self-implemented strength training. Key words: Periodization, coaching, strength and conditioning INTRODUCTION Several studies have been conducted on the role of coaches in different sports, indicating a positive correlation between the use of expert coaches and the development of performance parameters [8, 25, 28]. Others have shown that professional expert coaching is of vital importance for success in top-level, competitive sport [19]. Since experienced coaches develop a high degree of self-awareness regarding their instructional behavior through experience [11], McCullick et al [19] have identified five different character traits in expert coaches, such as comprehensive knowledge, specific competency, effective perceptions skills, systematic coaching routines and the ability to reflect critically, together with a strong desire for lifelong learning. However, McCullick et al [19] underlined that To know only about the character traits of a coach makes no one fully expert. Therefore, learning from deliberate practice is important for the development of a coach [9]. To develop coaching expertise, researchers have focused on the need for education [28], pre-coaching experience [7], practical situational contexts [6] and trainer experience [21]. However, the gender and the athletic ambitions of the trainer also influence the outcome of the training. It has been found that male athletes prefer working with a male strength coach, no matter how highly qualified the female coach might be, while female athletes are productive with any qualified strength coach, irrespective of sex [16]. The job of the strength and conditioning coach is to prepare athletes to achieve successful results in their sports. Athletes who are coached by expert coaches learn more and perform better than athletes coached by less experience coaches [23]. This could be a result of their ability to bring about excellent performances, and their ability to coach more athletes to a higher level in shorter time than coaches with less experience [23]. It has been shown that that the total time athletes spend with expert coaches is highly significant for the development 195

2 of talented athletes [3]. Amongst the elite division of strength and conditioning coaches, the most commonly reported behavior included silent monitoring (observing their athletes (21.99%), management (organization of weight activities (14.62%), and hustle (verbal feedback intended to intensify efforts (11.12%) [17]. The importance of strength training for soccer players is well documented in the literature. Wisløff et al [27] found that 1RM in half squats had a strong correlation to maximal speed and elastic power in toplevel Norwegian elite soccer players. Hoff et al [13] pointed out that 1RM half squat increased from an average of 161 kg to 215 kg with a training frequency of 3 times per week consisting of performing 5 reps x 4 sets over a period of 8 weeks, with a training intensity of 85% of 1RM. In another training intervention Hoff and Helgerud [14] showed that soccer players increased their performances in 1RM half squats from 116 kg to 176 kg, 1.87 s to 1.81 s in 10 m sprint, 3.13 s to 3.08 s in 30 m sprint, and from 57.2 cm to 60.2 cm in counter movement jump (CMJ) over a period of 8 weeks with 4 reps x 4 sets twice week. Several studies have also shown how strength training affects the performance of soccer players [1, 10, 20, 22]. Nevertheless, no study to date has investigated the effect of direct supervision by a strength and conditioning coach on elite athletes. A study by Mazzetti et al [18] investigated The influence of direct supervision of resistance training on strength performance, however, though their subjects had resistance training experience, they were not elite athletes. Also, a study by Coutts et al [5] investigated the Effect of direct supervision of a strength coach on measures of muscular strength and power in young rugby league players, their subjects were healthy, young talented rugby players, but not at an elite level. Both studies found, however, that supervised training by a strength and conditioning coach had a greater effect on strength performance compared to unsupervised training. Therefore, the aim of the present study is to measure the direct effect of strength and conditioning coach supervision on the physical performance of elite young soccer players. METHODS AND MATERIALS EXPERIMENTAL APPROACH TO THE PROBLEM To measure the effect of direct supervision by a strength and conditioning coach on elite junior soccer players, a 10-week strength training program was developed, implemented and followed up by a national expert strength and conditioning coach working at the Norwegian Olympic Sport Center. A pre- and posttest were conducted before and after implementing the strength training program, measuring maximum strength in leg muscles (1RM) using leg press, sprinting speed (10 m and 40 m) and jumping performance (countermovement jump). According to their pre-test results, the subjects were divided into 3 groups, a strength training group with expert coach supervision (N = 9), a strength training group without expert coach supervision (N = 8) and a control group with no strength training (N = 9). The hypothesis in this study was that supervised strength training would lead to better performance development when compared to that of the unsupervised strength training group and the control group. Jump performance was estimated at the laboratory of the Norwegian Olympic Sport Center using force-platform-based determinations of impulse and thus velocity at takeoff. The force platform used was AMTI (model SG-9, advanced mechanical technologies, Newton, MA, USA), sized 122 x 62 cm. The data was amplified (AMTI Model SGA6-3), digitalized (DT 2801) and saved on a computer with the aid of the special software program Biopack MP 100. The force platform had a time resolution of 1000 Hz and a force resolution of 0.1 N. All testing instruments were checked and approved by testing experts at the Norwegian Olympic Committee and Confederation of Sports before testing took place. The 10 m and 40 m sprints were measured at the Norwegian Olympic Sport Center using a start mat and two pairs of double infrared photocells, which were connected by cables and linked to a computer which measures time to s. The photocells were mounted on a 50 m sprint running track. The leg press was tested using a Hammer strength linear leg press machine (Life fitness, IL, USA). SUBJECTS Twenty-six young, well-trained elite young male soccer players aged (Mean ± SD) (19.1 ± 3.5 years), body mass (72.6 ± 7.8 kg) and stature (178.6 ± 7.8 cm) volunteered to participate in this study. The participants were all highly committed to training, putting in 14 ± 3 hours per week. All participants gave their written, voluntary informed consent and the local ethics committee at the Norwegian School of Sport Sciences approved the study. The duration of regular soccer training sessions was 2 hours, where about 30 minutes were used for warming up and cooling down. Approximately one and half hours were spent on pure soccer training during the intervening period. TEST PROCEDURES All participants received relevant information about test procedures and took part in preliminary training for all tests two weeks before the pre-test took place. Following the subjects warm-up sessions (The warmingup sessions before the pre-and post-tests were standardized to 15 min easy jogging and 3 x 40 m strides), they were asked to perform three maximum attempts in the following order; maximal running velocity over 196

3 40 m sprint; Countermovement jump and maximal leg press strength (1RM). The sprint test was executed on an indoor sprint lane; the lane has a start mat and photocells. The 40 m sprint was performed from a standing position on the starting mat. Time for the acceleration phase (0-10 m) and full speed phase (0-40 m) was measured on the same run. Measurement of the 40 m sprint started when the participant took off from the starting mat (Time Zero) and stopped when the player crossed the 10 m line and the 40 m line. Five minutes after the sprinting test, the CMJ was performed with hands placed on hips. The hands-on-hips method was adopted to restrict performance to leg and hip explosiveness, and minimize differences in jumping technique. The jumps were performed from a standing position with the plantar part of the foot making contact with the force platform, a counter-movement down was performed until the knee angle was approximately 90º. Five minutes after the vertical counter movement jump, the 1RM leg-press was executed. We chose the leg-press instead of half squats because of the participation of many young and inexperienced athletes, and to reduce the risk of injury. All participants had to perform 2 x 10 reps with a light weight before they started to press 1-4 reps with progressively heavier weights, until they reached their individual 1RM on the leg-press machine. The slightest weight increase was 5 kg. The following criteria were set as a base for accepting the trials: performing the trials as described in the procedure, accepted force platform registration, and accepted Laboratory Photocells registration. When the participants fulfilled the three requirements, the best result was retained for analysis. INTERVENTION PROGRAM The training intervention lasted 10 weeks and was applied during the pre-season period for players. A maximal strength training program was developed and followed up by a national expert strength and conditioning coach at the Norwegian Olympic sport center. A general weight lifting course in the first week was implemented for all participants in this study. Between the first and sixth week, the program focused on hypertrophy training (Table 1 and 2), and from week 7-10, it focused on maximal strength training (Table 3 and 4). The intervals between sets were 2-4 minutes depending on degree in terms of workload, and intensity. The supervised strength training group performed the maximal strength training program with the supervision of a strength and conditioning coach at the Norwegian Olympic training centre for 80 minutes, twice a week for 10 weeks. The unsupervised strength training group performed the same training program without the help of an expert coach at the Norwegian training centre for 80 minutes twice a week for 10 weeks. The control group took part only in regular soccer training, without strength training. It was the duty of the expert coach to follow up every strength training session throughout the 10 weeks, providing technical instruction, advice on training methodology, motivation and optimal social and mental support. Table 1. Day one of the periodized strength and conditioning program from week 1 week 6 Week 1 Week 2 Week 3 Week 4 Week 5 Week 6 Exercise S R I S R I S R I S R I S R I S R I Back Squat % % % % % % Leg press % % % % % % Back Squat jump % % % % % % Barbell stiff leg dead lift % % % % % % Barbell bench press % % % % % % Behind the neck shoulder press % % % % % % Dumbbell side lateral raise % % % % % % Sit-Ups with medicine ball 2 Max 4kg 2 Max 4kg 2 Max 4kg 2 Max 4kg 2 Max 4kg 2 Max 4kg Medicine ball lying throw-up 2 Max BW 2 Max BW 2 Max BW 2 Max BW 2 Max BW 2 Max BW S = Number of sets, R = number of reps, I = Intensity based on 1RM, BW = Body weight, Recovery between sets were set to 2 min, and recovery between exercises were between 4 5 min Table 2. Day two of the periodized strength and conditioning program from week 1 week 6 Week 1 Week 2 Week 3 Week 4 Week 5 Week 6 Exercise S R I S R I S R I S R I S R I S R I Back Squat % % % % % % Back Squat jump % % % % % % Back Squat jump 3 8 BW 3 8 BW 3 8 BW 3 8 BW 3 8 BW 3 8 BW Pull downs % % % % % % Seated row machine % % % % % % Biceps curls % % % % % % Dumbbell biceps curls % % % % % % The bridge exercise 3 Max BW 3 Max BW 3 Max BW 3 Max BW 3 Max BW 3 Max BW Back extension 3 Max BW 3 Max BW 3 Max BW 3 Max BW 3 Max BW 3 Max BW Body bar standing twist 3 Max BW 3 Max BW 3 Max BW 3 Max BW 3 Max BW 3 Max BW 197

4 Table 3. Day one of the periodized strength and conditioning program from week 7 week 10 Week 7 Week 8 Week 9 Week 10 Exercise S R I S R I S R I S R I Back Squat % % % END Leg press % % % % Back Squat jump % % % % Barbell stiff leg dead lift % % END Barbell bench press % % % % Behind the neck shoulder press % % % % Dumbbell side lateral raise % % % % Sit-Ups with medicine ball 2 Max 4kg 2 Max 4kg 2 Max 4kg 2 Max 4kg Medicine ball lying throw-up 2 Max BW 2 Max BW 2 Max BW 2 Max BW Table 4. Day two of the periodized strength and conditioning program from week 7 week 10 Week 7 Week 8 Week 9 Week 10 Exercise S R I S R I S R I S R I Back Squat % % % END Back Squat jump % % % % Back Squat jump 4 6 BW 4 6 BW 4 4 BW 4 2 BW Pull downs % % % % Seated row machine % % % % Biceps curls % % % % Dumbbell biceps curls % % % % The bridge exercise 3 Max BW 3 Max BW 3 Max BW 3 Max BW Back extension 3 Max BW 3 Max BW 3 Max BW 3 Max BW Body bar standing twist 3 Max BW 3 Max BW 3 Max BW 3 Max BW 40 m sprint 2 100% 2 100% 2 100% CMJ 2 100% 2 100% 2 100% STATISTICAL ANALYSIS All statistical analysis was carried out using SPSS 17.0 for Windows (SPSS Inc., Chicago). Reliability was calculated using two-way mixed Intra-class Correlation (ICC) for all the dependent measures in this study. The normality of data distribution was then explored by histogram plot and tested using the Shapiro-Wilk test for all groups. Descriptive statistics were then calculated and reported as mean and standard deviations (SD) of the mean for each group of players on each variable. The intra-group differences were examined using the paired samples t-test to test differences in central location (mean). If the data was then found to follow a normal distribution, the differences between groups in terms of central location (mean) were examined using One-way ANOVA followed by Tukey s post-hoc test. For the data that did not follow a normal distribution, the non-parametric Kruskal-Wallis test was used for assessment, followed by the Mann- Whitney Test. To determine whether the effect size was trivial (d < 0.2), small (d = ), moderate (d = ), large (d = ) or very large (d > 2.0), the scale developed by Hopkins et al [15] was used. Significance was accepted at the p 0.05 level. A 95% Confidence Interval (95% CI) was also calculated for all measures. RESULTS The test re-test reliability for 10 m time was intra-class correlated (ICC) (ICC = 0.91, p < 0.01), for the 40 m time (ICC = 0.95, p < 0.01), for the CMJ (ICC = 0.95, p < 0.01), for the leg press (ICC = 0.92, p < 0.01) and for the body weight (ICC = 0.95, p < 0.01). The intra-group results indicate that the supervised strength training group demonstrated a significant improvement in 40 m sprinting time (-0.06 ± 0.07 s), CMJ (2.7 ± 2.7 cm) and leg press (39.4 ± 16.9 kg). Furthermore, the unsupervised strength training group showed a significant improvement in leg press performance (13.1 ± 15.3 kg) and a significant increase in body weight (2.9 ± 2.9 kg). The control group, on the other hand, evinced a significant decrease in 10 m sprint time performance (0.03 ± 0.03 s), 40 m sprint time performance (0.06 ± 0.06 s), but a significant improvement in CMJ (2.0 ± 1.7 cm). Finally, the control group showed a significant decrease in body weight (-1.9 ±1.6 kg) (Table 5). The intra-group analyses showed that the supervised strength training group showed a significant increase in CMJ (2.6 ± 1.1 cm) and leg press (26.3 ± 7.9 kg) performance compared to the unsupervised strength training group. The supervised strength training group also demonstrated a significant increase in 10 m (-0.05 ± 0.02 s), 40 m (-0.12 ± 0.03 s) and leg press (37.2 ± 9.0 kg) performances compared to the 198

5 control group (Table 6). The results also show that the control group experienced a significant drop in body weight (-2.6 ± 1.0 kg) compared to the supervised strength training group (Table 6). Table 5. Mean results of 10m, 40m, CMJ, leg press and body weight within groups from pre to post test Variable Supervised Strength training Group Change 95% CI Control group Pretest Posttest Unsupervised Strength training Group Pretest Posttest Change 95% CI Pretest Posttest Change 10m (s) 1.58± ± ± to ± ± ± to ± ± ± to 0.05* 40m (s) 5.27± ± ± to -0.01* 5.09± ± ± to ± ± ± to 0.11* CMJ (cm) 33.7± ± ± to 4.78* 40.1± ± ± to ± ± ± to 3.28** Leg press (kg) 237.2± ± ± to 52.4** 233.1± ± ± to 25.9* 245.6± ± ± to 18.4 Weight (kg) 74.7± ± ± to ± ± ± to 5.3* 72.1± ± ± to -0.7** * = p < 0.05, ** = p < % CI Table 6. Mean results of the performance differences between groups Between SSG and SG Between SSG and CG Between SG and CG Velocity SGW SG Diff. 95% CI SGW CG Diff. 95% CI SG CG Diff. 95% CI 10m (s) -0.02± ± ± to ± ± ± to -0.02** 0.01± ± ± to m (s) -0.06± ± ± to ± ± ± to -0.05** 0.01± ± ± to 0.03 CMJ (cm) 2.7± ± ± to 4.83* 2.7± ± ± to ± ± ± to -0.22* Leg press (kg) 39.4± ± ± to 43.06* 39.4± ± ± to 56.29** 13.1± ± ± to Weight (kg) 0.7± ± ± to ± ± ± to 4.64* 2.9± ± ± to 7.09** SSG = Supervised Strength training Group, SG = Unsupervised Strength training Group, CG = Control-group, * = p < ** = p < 0.01 The results further indicate that the control group showed a significant increase in CMJ (1.8 ± 0.8 cm) and a significant drop in body weight (4.7 ± 1.1 kg) compared to the unsupervised strength training group (Table 6). DISCUSSION The main findings from this investigation show that the supervision of a strength and conditioning specialist in implementing a strength training program performed over a ten-week period led to a significant improvement in performance for almost all measured variables compared to the unsupervised strength training group and the control group. The intra-group results indicated that the supervised training group demonstrated a notable improvement in 40 m sprint time, CMJ, and 1RM leg press from pre- to post-test. The unsupervised training group showed a notable improvement in 1RM leg press and a marked increase in body mass, but a decrease in terms of their 10 m and 40 m sprint times. Furthermore, the control group witnessed a decrease in 10 m and 40 m sprint time performance and an increase in CMJ performance, with a significant reduction in body mass. The inter-group results indicate that the supervised training group evinced a significant improvement in CMJ and 1RM leg press compared to the unsupervised training group. The supervised training group also showed a significant improvement in 10 m, 40 m, 1RM leg press and a rise in body weight compared to the control group. Finally, the unsupervised strength training group saw a significant improvement in CMJ and an increase in body weight compared to the control group. The significant increase for the 1RM leg press was greater for the supervised strength training group than the unsupervised training group, indicating that this significant increase was caused by the direct supervision of the strength and conditioning specialist. Since the training program implemented in this study was aimed to target both hypertrophy and maximal strength training respectively, we believe that the improvement in strength was caused by both hypertrophy and the heavier training loads that resulted in the activation of a higher number of fast twitch motor units, which according to several studies are essential for inducing optimal increases in strength [3, 12]. It is not surprising that the supervised strength training group showed an improvement in their CMJ and 40 m performance; studies have revealed a significant relationship between 40 m sprint time and CMJ performance compared to 10 m sprint time and CMJ performance [2, 24]. However, the improvement in CMJ performance could be attributed to the significant improvement in leg press performance, resulting in a higher force production applied on the ground [4, 26] relative to the supervised strength training group s body weight. On the other hand, the explanation for the lack of CMJ and sprint performance improvement in the unsupervised strength training group could be attributed to the slight but significant increase in leg press performance combined with a significant increase in body weight. Finally, the significant improvement in the control group s CMJ performance might be attributed to the significant drop in body weight, combined with maintaining the same level of leg press 199

6 strength throughout the intervening period. However, we believe that the main factor behind the different levels of improvement between the supervised strength training group and the other groups in this study was the direct supervision of a strength and conditioning specialist. Several studies have supported the effect of supervised training during training sessions, Hoff and Helgerud [14] refer to a supervised study with top-level soccer players obtaining a 35% improvement in maximal leg-press strength. These results indicate that expert coaching in strength training is beneficial for the development of important physical performance parameters in the sport of soccer. The findings of Gallo and De Marco [11] also support the idea that efforts by strength and conditioning coaches efficiently improve the quality of training athletes. Mazzetti et al [18] found that directly supervised strength training resulted in greater improvement in terms of maximal strength gain compared with unsupervised training. Furthermore, a positive correlation has been observed between the use of expert coaches in the training process and the development of player performance [8, 19, 23, 25, 28]. The practical knowledge of the strength and conditioning specialist is often said to be essential for technical procedures, workload and intensity regulation in strength training sessions [19]. Other factors, such as different psychological factors brought about by constant supervision of the supervised strength training group could have influenced competitiveness and motivation [18]. Furthermore, the amount of time athletes spend with expert coaches is found to be significant for the performance development of young talented athletes [2, 9]. CONCLUSION & PRACTICAL APPLICATIONS The finding of the present study indicate that a strength training program directly supervised by a strength and conditioning specialist over a period of 10 weeks led to a significant improvement in measured variables, compared to an unsupervised strength training group and a control group of well-trained soccer players. Even though the supervised and unsupervised strength training groups implemented the same training program, the physiological explanation for the improvement of one over the other cannot be explained by this study. However, we believe that the main cause behind the differences between the two groups lies with constant supervision, which could have effected motivation and competitiveness. From the results obtained through this study, we strongly recommend the direct supervision of a strength and conditioning specialist during strength and conditioning sessions for soccer players, since it has been shown that it is effective in developing the physical performance of players. REFERENCES 1. Baker, D. (1996). Improving vertical jump performance through general, special, and specific strength training: A brief review. The Journal of Strength and Conditioning Research, 10 (2), Baker, D., & Nance, S. (1999). The relation between running speed and measures of strength and power in professional rugby league players. The Journal of Strength and Conditioning Research, 13 (3), Campos, G. E., Luecke, T. J., Wendeln, H. K., Toma, K., Hagerman, F. C., Murray, T. F., et al. (2002). Muscular adaptations in response to three different resistance-training regimens: specificity of repetition maximum training zones. European Journal of Applied Physiology, 88 (1-2), Carlock, J. M., Smith, S. L., Hartman, M. J., Morris, R. T., Ciroslan, D. A., Pierce, K. C., et al. (2004). The relationship between vertical jump power estimates and weightlifting ability: a field-test approach. The Journal of Strength and Conditioning Research, 18 (3), Coutts, A. J., Murphy, A. J., & Dascombe, B. J. (2004). Effect of direct supervision of a strength coach on measures of muscular strength and power in young rugby league players. The Journal of Strength and Conditioning Research, 18 (2), Culver, D., & Trudel, P. (2006). Cultivating coaches communities of practice: Developing the potential for learning through interactions. In R. Jones (Ed.), The Sports Coach as Educator: Reconceptualising Sports Coaching (pp ). London: Routledge. 7. Cushion, C. J., Armour, K. M., & Jones, R. L. (2003). Coach education and continuing professional development: Experience and learning to coach. Quest, 55 (3), Douge, B., & Hastie, P. (1993). Coach effectiveness. Sports Science Review, 2, Ericsson, K. A., & Charness, N. (1994). Expert performance: Its structure and acquisition. American Psychologist, 49 (8), Fatouros, I. G., Jamurtas, A. Z., Leontsini, D., Taxildaris, K., Aggelousis, N., Kostopoulos, N., et al. (2000). Evaluation of plyometric exercise training, weight training, and their combination on vertical jumping performance and leg strength. The Journal of Strength and Conditioning Research, 14 (4), Gallo, G. J., & De Marco, G. M., Jr. (2008). Selfassessment and modification of a division I strength and conditioning coach's instructional behavior. The Journal of Strength and Conditioning Research, 22 (4), Hakkinen, K., Alen, M., Kallinen, M., Newton, R. U., & Kraemer, W. J. (2000). Neuromuscular adaptation during prolonged strength training, detraining and restrength-training in middle-aged and elderly people. European Journal of Applied Physiology, 83 (1), Hoff, J., Gran, A., & Helgerud, J. (2002). Maximal strength training improves aerobic endurance performance. Scandinavian Journal of Medicine and Science in Sports, 12 (5), Hoff, J., & Helgerud, J. (2004). Endurance and strength training for soccer players: physiological considerations. Sports Medicine, 34 (3), Hopkins, W. G., Hawley, J. A., & Burke, L. M. (1999). Design and analysis of research on sport performance enhancement. Medicine and Science in Sports and Exercise, 31 (3), Magnusen, M. J., & Rhea, D. J. (2009). Division I athletes' attitudes toward and preferences for male

7 and female strength and conditioning coaches. The Journal of Strength and Conditioning Research, 23 (4), Massey, C. D., Maneval, M. W., Phillips, J., Vincent, J., White, G., & Zoeller, B. (2002). An analysis of teaching and coaching behaviors of elite strength and conditioning coaches. The Journal of Strength and Conditioning Research, 16 (3), Mazzetti, S. A., Kraemer, W. J., Volek, J. S., Duncan, N. D., Ratamess, N. A., Gomez, A. L., et al. (2000). The influence of direct supervision of resistance training on strength performance. Medicine and Science in Sports and Exercise, 32 (6), McCullick, B. A., Cumings, R., & DeMarco, G. M. (1998). The road to expert coaching. Georgia Association for Health, Physical Education, Recreation, and Dance, 32 (1), Moore, E. W., Hickey, M. S., & Reiser, R. F. (2005). Comparison of two twelve week off-season combined training programs on entry level collegiate soccer players' performance. The Journal of Strength and Conditioning Research, 19 (4), Nash, C., & Collins, D. (2006). Tacit knowledge in expert coaching: Science or art? Quest, 58 (4), Rønnestad, B. R., Nymark, B. S., & Raastad, T. (2011). Effects of In-season strength maintenance training frequency in professional soccer players. The Journal of Strength and Conditioning Research, 25 (10), Schempp, P., McCullick, B., & Mason, S. (2006). The development of expert coaching. In R. Jones (Ed.), The sports coach as educator: Re-conceptualising sports coaching (pp ). London: Routledge. 24. Shalfawi, S. A., Sabbah, A., Kailani, G., Tonnessen, E., & Enoksen, E. (2011). The relationship between running speed and measures of vertical jump in professional basketball players: a field-test approach. The Journal of Strength and Conditioning Research, 25 (11), Werner, G. (2010). Strength and conditioning techniques in the rehabilitation of sports injury. Clinical Journal of Sport Medicine, 29 (1), Weyand, P. G., Sternlight, D. B., Bellizzi, M. J., & Wright, S. (2000). Faster top running speeds are achieved with greater ground forces not more rapid leg movements. Journal of Applied Physiology, 89 (5), Wisløff, U., Castagna, C., Helgerud, J., Jones, R., & Hoff, J. (2004). Strong correlation of maximal squat strength with sprint performance and vertical jump height in elite soccer players. British Journal of Sports Medicine, 38 (3), Woodman, L. (1993). Coaching: A science, an art, an emerging profession. Sport Science Review, 2 (2), Address for correspondence: Prof. Eystein Enoksen Norwegian School of Sport Sciences Department of Physical Performance Postboks 4014 Ullevål Stadion 0806 Oslo, NORWAY Tel Fax eystein.enoksen@nih.no 201