The relationship between isokinetic performance of hip and knee and jump performance in university rugby players

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1 Isokinetics and Exercise Science 21 (2013) DOI 10s.3233/IES IOS Press The relationship between isokinetic performance of hip and knee and jump performance in university rugby players Benjamin Harrison, Will Firth, Sean Rogers, Joshua Tipple, Jon Marsden, Jennifer A. Freeman, Alan D. Hough and Gary L.K. Shum School of Health Professions, Plymouth University, Plymouth, UK Abstract. OBJECTIVE: The aim of this study was to investigate how isokinetic lower limb strength and velocity of knee and hip extension correlate to vertical jump performance in university team rugby players. METHODS: Twenty asymptomatic university team rugby players performed a maximal vertical jump test, in which the vertical displacement was measured from maximum standing reach height to maximal height attained from a countermovement jump. A dynamometer assessed the maximal isokinetic strength and velocity of hip and knee extension in the dominant leg. Peak torque was measured when participants moved at angular velocities of 60 and 120 /s for hip extension, and 120 and 240 /s for knee extension. The highest velocity achieved during isokinetic hip and knee extension up to a maximum level of 360 /s was recorded. RESULTS: Moderately strong correlations were found between knee extension strength and vertical jump height, particularly when testing at faster angular velocities of 240 /s (R = 0.609, p = 0.002). There was also a significant moderate correlation between maximum knee extension speed and vertical jump height (R = 0.540, p = 0.007). CONCLUSIONS: The results suggest that focusing on strength/power training of the knee extensors at a high speed may result in improved vertical jump performance. Keywords: Isokinetics, knee, hip, rugby, biomechanics 1. Introduction Rugby union is a popular 15 player invasion game played across the world by professionals and amateurs [1]. Rugby players must have sufficient anaerobic capacities to achieve high levels of muscular strength, power, and speed for heavy physical body contacts [2]. Vertical jumping, which is a fundamental skill for certain set pieces and open play, is becoming ever more important in the response to modern kicking tactics in rugby. It is reported that 15% of game time is char- Corresponding author: Dr. Gary Shum, Ph.D., Plymouth University, School of Health Professions, Peninsula Allied Health Centre, Derriford Road, Plymouth PL6 8BH, UK. Tel.: ; Fax: ; gary.shum@plymouth.ac.uk. acterized by high intensity activities, such as vertical jumping [3 5]. Maximal jumping performance is associated with higher levels of speed, strength and power [3 5]. Consequently, before and during the rugby season, players typically participate in resistance training programs designed to improve and maintain optimal performance during the entire competitive season [6]. Previous studies have investigated the relationship between vertical jump height and isokinetic strength of the lower limbs. Vanezis and Lees [7] demonstrated that the strength contribution in relation to vertical jump ability from the ankle, knee and hip were 28%, 29%, and 43% respectively in football players. They suggested that the muscle strength characteristics of the lower limbs, rather than jumping technique, are the main determinants of vertical jump performance [7]. ISSN /13/$27.50 c 2013 IOS Press and the authors. All rights reserved

2 176 B. Harrison et al. / The relationship between isokinetic performance of hip and knee and jump performance Table 1 Subject characteristics (mean ± standard deviation) Age (years) 22.6 ± 4.1 Height (cm) ± 6.20 Weight (kg) ± 9.5 Body Mass Index 26.4 ± 2.5 Vertical jump Height (cm) 59.6 ± 6.8 Tsiokanos et al. [8] found little evidence to support the role of ankle plantar flexor strength, but reported that knee and hip extensor strength measured isokinetically at 60, 120 and 180 /s had a moderate to strong correlation to vertical jumping performance. Positive correlations have also been reported between vertical jump performance and lower limb strength in women [9]. However, no previous study has looked at the correlation between the isokinetic performance of the hip and knee and vertical jumping performance in rugby players. Higher angular velocities are considered to favour game related performance and thus provide useful information to strength and conditioning coaches on areas to target to improve jumping ability [8,10]. However, no previous study has determined the maximum angular velocities of the hip and knee extensors in rugby players and its relation to vertical jump performance. The objectives of this study were therefore to investigate the relationships between isokinetic performance (peak torque and maximum angular velocities) of the hip and knee extensors and the vertical jump performance in university team rugby players. 2. Materials and methods 2.1. Subjects Twenty men, university team rugby players, were recruited. Subjects were excluded if they had sustained an injury to the knee/hip/lower back in the past year, were over 30 years of age, and were not regularly competing for the University First or Second rugby team. Table 1 details the demographic characteristics of the subjects. The experiments were undertaken with the understanding and written consent of each subject, and that the study conforms to The Code of Ethics of the World Medical Association (Declaration of Helsinki), with the approval of the Ethics committee of Plymouth University, United Kingdom. All subjects nominated their right leg as the dominant side, determined by the leg with which they would instinctively kick a ball. Prior to testing, subjects were led through a warm up involving moderate intensity jogging indoors over a 10-m distance for the vertical jump station and use of a cycle ergometer at a moderate load for ten minutes for the isokinetic testing station. In both cases warm up included dynamic stretches of the lower limbs and lasted five minutes in duration. To minimize bias, an allocated instructor/motivator and a data collector were set for each station. Data collectors were blinded from each other s results until testing was completed and conjoined for data analysis thereby increasing the reliability and internal validity of the testing procedure [11]. All subjects successfully completed all aspects of the study protocol pain free. The order of isokinetic testing and vertical jump performance was randomized with at least sixty minutes rest period separating the tests within the same day Isokinetic testing The concentric strength of the hip and knee extensors of the dominant leg of each subject was measured using a Biodex dynamometer (Model III, Biodex Medical Systems, Shirley, New York, USA), which provides reliable measures of torque and velocity [12 15]. In keeping with the concentric velocities used in previous studies [15,16], the hip extensor isokinetic concentric strength was tested at 60 /s and 120 /s while the knee extensor isokinetic concentric strength was tested at 120 /s and 240 /s [17,18]. The maximal angular velocities of hip and knee extensors were determined by asking the subjects to perform maximal contractions as fast as possible when the speed of the dynamometer was set at its maximum (360 /s) and resistance set at 500 Nm. Subjects were instructed to carry out three submaximal and one maximal practice attempt at each testing velocity, each separated by a 20-s rest interval. Subjects then performed five maximal attempts, each separated by a 40-s rest interval. The maximal value of the peak torque was taken as a measure of maximal strength. Throughout testing subjects were instructed to keep their arms folded against their chest to prevent unintentional body movements. Testing of knee extension followed the protocol of Wrigley and colleagues with the subject tested in a sitting position [15]; however hip extension testing was adapted, so that the concentric hip extensor strength could be determined in a more functional standing position with minimal lumbar extension (Fig. 1). The dynamometer chair was set up with the back of the chair flat and a triangular wedge placed at the foot of the

3 B. Harrison et al. / The relationship between isokinetic performance of hip and knee and jump performance 177 chair. Testing in the standing position involved bending at the waist with the participant lying over the wedge with the chair raised to the level of their anterior superior iliac spine. The mechanical axis of the dynamometer was aligned to the anatomical axis of the hip joint which was identified at the greater trochanter as established by the International Society of Biomechanics [19], and straps were pulled taut across the lumbar spine and around the chair with a towel provided for comfort. Using the contralateral knee attachment to the tested leg, the pad of the leg attachment was strapped tightly in place above the patella (Fig. 1) Vertical jump performance The countermovement jump and the squat jump [18] are commonly used for assessing vertical jump height. The countermovement jump test was chosen for use in this study as it focuses on the function of the lower limb extensor muscles [20], and produces a maximal contraction that is easier to control and is both reliable [11] and functionally relevant to rugby [21]. It commonly attains greater jump heights than the squat jump. This is believed to be due to muscle elastic recoil, triggering of a stretch reflex, provision of a more natural movement pattern and greater magnitudes of movement at the knee, hip and ankle [22]. Using this method vertical jump height was assessed by measuring the distance between standing reach height and maximal height when carrying out the countermovement jump. This entailed moving from a natural stance position to a squatting position which allows subjects to propel themselves vertically with the allowance of arm swing, to achieve maximal jump height. The starting position was standardised with Fig. 1. Testing position for isokinetic hip extensor strength. subjects acromion process positioned 30 cm from the wall. Subjects performed two practice attempts before completing three maximal jumps, interspersed with a 30 second interval during which they returned to the initial starting position. The vertical jump height was measured by the chalk marking on each participant s middle finger Statistical analysis All results were normalised using the body mass (kg) of each participant. To determine concentric hip extensor strength in a functional standing position, a two-way mixed intraclass correlation coefficient (ICC 3,k ) was calculated with a 95% confidence interval to determine the consistency of data of each subject over the 5 isokinetic test trials for the hip extensors within each session [23]. Statistical analysis was conducted using the Statistical package for Social Sciences 19.0 (SPSS. IBM, New York). Spearman s one tailed correlation analysis was used to determine the relationship between maximum isokinetic strength, maximum velocity (when tested at 360 /s) and jump performance, with the significance level set at Results The ICC of the concentric hip extensor isokinetic peak torque values at 60 /s and 120 /s was (95% CI ) and (95% CI ) respectively. The ICC of the maximal angular velocities of hip extensor (tested at 360 /s) was

4 178 B. Harrison et al. / The relationship between isokinetic performance of hip and knee and jump performance Table 2 Isokinetic strength values of the hip and knee extension Mean ± standard deviation Range Normalised Peak Torque knee extension at 120 /s (Nm/kg) 2.21 ± Absolute Peak Torque knee extension at 120 /s (Nm) ± Normalised Peak Torque knee extension at 240 /s (Nm/kg) 1.57 ± Absolute Peak Torque knee extension at 240 /s (Nm) ± Normalised Peak Torque hip extension at 60 /s (Nm/kg) 2.03 ± Absolute Peak Torque hip extension at 60 /s (Nm) ± Normalised Peak Torque hip extension at 120 /s (Nm/kg) 1.93 ± Absolute Peak Torque hip extension at 120 /s (Nm) ± Maximum Angular Velocity of hip extension tested at 360 /s ( /s) ± Maximum Angular Velocity of knee extension tested at 360 /s ( /s) ± Nm: Newton metre; kg: kilogram. Table 3 Correlation (Spearman s one tailed) between vertical jump height and peak torque and maximum velocities of hip and knee extension Knee Peak Torque Knee Peak Torque Hip Peak Torque Hip Peak Torque Maximum hip Maximum knee 120 /s (Nm/kg) 240 /s (Nm/kg) 60 /s (Nm/kg) 120 /s (Nm/kg) extensor velocity ( /s) extensor velocity ( /s) Vertical jump (cm) P P<0.05, significant correlation; Nm: Newton metre. The concentric peak torque values of the knee and hip extensors and the maximum velocities of hip and knee extension when tested at 360 /s are shown in Table 2. There was a significant moderate positive correlation between the knee extensor peak torque and vertical jump height (Table 3), with stronger correlations found at an angular velocity of 240 /s (R = 0.609) than at 120 /s (R = 0.540) (Table 3). Correlations between hip extensor peak torque and vertical jump height were weak and did not reach statistical significance at angular velocities of 60 and 120 /s (Table 3). There was a significant moderate correlation between maximum knee extension speed and vertical jump height (R = 0.516, p = 0.010, Table 3) but there was a non-significant weak correlation between hip extension speed and vertical jump height (R = 0.127, p = 0.296, Table 3). 4. Discussion In keeping with previous work [18,24], the correlation between knee extension strength and vertical jump height was higher when strength was tested at faster angular velocities, up to 240 /s. This may reflect the similarity between the strength test and the explosive action of the vertical jump test as test speed increases. However, although knee angular velocities of up to 500 /s have been reported at the instant of toe-off during a one-leg vertical jump [25], the measurement of isokinetic torque at even higher velocities may not result in greater correlations [16] in which a lower association was found between knee extension peak torque at jump height at 360 /s when compared to 240 /s. In our study only a few subjects were able to achieve velocities of 360 /s and it is unclear if this was the case for the subjects of aforementioned study [16]. If all of subjects were not able to achieve the target speed [16], as was seen in the current study (Table 2), it would reduce the validity of the torque measurement. It is because isokinetic dynamometry is only reliable in measuring the peak torque if the predetermined angular velocity can be reached [14,15]. The fact that the maximal knee velocity achieved in the dynamometer was correlated to vertical jump height and to the peak knee extensor torque measured at 240 /s (R = 0.609, p = 0.002, Table 3) highlights the fact that the development of torque at high speeds is an important determinant of jump height. This greater correlation of jump height with knee extension rather than hip extension may be attributed to several factors. It may reflect that there is a larger magnitude of movement at the knee joint, when compared to the hip during the countermovement jump [22] and that the knee extensor tendons and muscle are highly elastic [26] and thereby able to generate additional force through elastic recoil. This is also in keeping with biomechanical models of jumping indicating that the knee extensors play a significant role in shifting the force from the legs to the trunk [27]. Analysis of jump performances [7] suggests that individuals adopt

5 B. Harrison et al. / The relationship between isokinetic performance of hip and knee and jump performance 179 different strategies for jumping; with some favouring greater input from the knee while others favour the hip. Thus, there may be sport specific differences in jumping style between footballers and rugby players that reflect the different correlations found. Previous work has found a moderate correlation between jump height and isokinetic hip extensor strength in footballers when measured at 60 and 120 /s [8,15]. In contrast, although identical velocities were used in the present study, no such correlation was found. This may reflect differences in the testing procedure and/or the population assessed. Previous studies have measured hip strength in the supine position [8,15], however pilot work indicated that it was very difficult to stabilize the pelvis in rugby players who have strong physique, rendering the elimination of a lumbar extension component very difficult to control. For this reason an alternative testing protocol that enabled better pelvic fixation was developed (Fig. 1). This testing position not only avoids the pitfalls of supine testing but allows assessment in the upright functional position. To our knowledge, this is the first study to measure concentric hip extensor strength using this test position and the high ICC values vindicate its implementation. Another reason why the correlations between jump height and strength were moderate for the knee and yet weak for the hip may be due to the velocity of the isokinetic tests adopted. The results indicated a trend for a stronger correlation with test protocols using higher velocity movements (at the knee), and higher velocities were used to test the knee extensors (180 and 240 /s) than the hip extensor (60 and 120 /s). A further exploration of this possibility could be addressed by including test velocities that are set closer to the subjects maximal attainable velocity, which in this study was on average 353 /s and 319 /s for the knee and hip respectively. Limitations to the work include the fact that the vertical jump is a closed chain activity whilst isokinetic testing is an open chain activity involving assessment of an isolated muscle group with a restricted angular velocity [8,28]; thus factors such as the action of a muscle over multiple joints is not tested. Also, the strength of the correlations found particularly between hip strength and jump height clearly indicate that concentric strength is only one factor determining jump height and future work should explore additional factors such as eccentric strength, flexibility, muscle-tendon properties, jump technique and player physical characteristics. The high ICCs only indicate high within-session repeatability [23]. Future studies are suggested to determine the reproducibility over different days. The standardised protocol suggested in this study would be amenable to minimize error by precise repositioning and therefore good reproducibility [29]. 5. Practical implications This is the first study to determine concentric hip extensors strength in a new position in order to eliminate the contribution from lumber extensors and the high ICC values indicate excellent repeatability within each session. The moderately strong relationship between higher normalised isokinetic strength values of the knee extensors and vertical jump height suggests that focusing on power training of the knee extensors may result in improved vertical jump performance and enhanced power outputs. The maximal knee velocity achieved in the dynamometer was correlated to vertical jump height and to the peak knee extensor torque measured at 240 /s (R = 0.609, p = 0.002). This finding supports the significance of testing at high isokinetic angular velocities but that said, with the proper precautions attributed to the validity of such tests. Acknowledgements We would also like to thank all the university rugby team players who took part in the study. No benefits in any form have been or will be received form a commercial party/grant body related directly or indirectly to the subjects of this study. Conflict of interest The authors on this manuscript report no conflict of interest. This study does not involve any external research grant support. References [1] Gabbett TJ. Physiological and anthropometric characteristics of amateur rugby league players. Br J Sports Med. 2000; 34(4): [2] Babault N, Cometti G, Bernardin M, Pousson M, Chatard JC. Effects of electromyostimulation training on muscle strength and power of elite rugby players. J Strength Cond Res. 2007; 21(2):

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