Lower-body strength and power are key physical qualities that underpin performance in a multitude of sports (26,28). It has been well established that the force-generating capacity of muscle is highly influenced by the structural arrangement of the fascicles (1,19). Furthermore, it has been noted that the interaction between lower-body muscle and tendon structures largely determines an athlete's ability to express power during a stretch-shortening cycle (SSC) activity (11,21). As muscle structure demonstrates large plasticity, and therefore is highly adaptive to training, understanding the relationships between muscle and tendon structures and physical performance is of great importance to strength and conditioning practitioners (1,8,23). Because of this, it is necessary to first identify the specific lower-body muscle structures, which may be related to strength and power variables of interest, through cross-sectional analyses. Identification of the muscle structures that may be related to more highly developed lower-body strength and power qualities may greatly assist practitioners with talent identification and to provide a basis of rationale for longitudinal studies. Such studies would allow for determination of whether changes in the muscle structure lead to enhancements in strength and power (7).
Previous cross-sectional studies have identified that there are significant relationships between specific lower-body muscle structures and performance in the back squat, countermovement jump (CMJ), and squat jump (SJ). Research by both Brechue and Abe (3) and Nimphius et al. (23) have reported significant relationships between vastus lateralis (VL) thickness and 1 repetition maximum (1RM) back squat (r = 0.82, p < 0.01) or relative 1RM back squat (r = 0.57). Although these studies suggest that greater lower-body strength, as measured with a 1RM squat, is related to increased thickness in the VL, no research to date has investigated whether similar relationships are present with the isometric midthigh pull (IMTP). Additionally, it has been identified that better performance in the CMJ and SJ was related to decreased fascicle length and greater pennation of the lower-body musculature (8). Earp et al. (8) reported that in 25 resistance-trained male subjects, lateral gastrocnemius (LG) muscle structure could predict jumping ability. Although significant relationships have been reported between LG pennation angle and jumping performance (8), it is yet to be determined whether similar relationships exist within elite athletic populations. Once it is established whether specific lower-body muscle structures are related to performance in the CMJ, SJ, and IMTP in elite athletic populations, longitudinal analyses can be performed to determine the influence of training-specific adaptations in lower-body muscle structure on strength and power qualities.
Fukashiro et al. (11) suggested that the fascicles are the force generator of the muscle-tendon unit, whereas the tendon structures act as an energy redistributor and a power amplifier. It has previously been noted that in better performers, the LG will typically perform a powerful concentric or isometric contraction during the eccentric phase of an SSC (11). This allows a larger magnitude of elastic strain energy to be developed within the tendon, because of the reduced deformation of the muscle, and hence increased tendon deformation (7,11). Fukashiro et al. (11) indicated that the tendons' ability to effectively store and redistribute this energy will largely determine performance in the CMJ. As a result, the stiffness of the lower-body muscle-tendon complex (MTC) is of particular importance to practitioners because this may greatly influence the magnitude of elastic strain energy that can be stored and released during an SSC (10). Understanding the muscle structures that may be related to lower-body MTC stiffness and the amount of explained variance this stiffness has on strength and power qualities may provide highly useful information to practitioners and coaches who work with athletes requiring high levels of lower-body power.
Although it has been identified that specific muscle structures are related to performance in the squat, CMJ and SJ, in resistance-trained males, to our knowledge no research to date has determined whether similar relationships are present in elite male athletes and with the IMTP. Furthermore, no previous research has investigated the relationships between lower-body muscle structure and lower-body MTC stiffness. As a result, it is necessary to identify any relationships that may be present between lower-body muscle structure and strength and power qualities, as well as with the mechanical properties of the lower-body. This will provide strength and conditioning practitioners and coaches with a sound rationale for talent identification and an understanding of the muscle structures related to an enhanced expression of lower-body strength and power.
Experimental Approach to the Problem
The purpose of this study was to determine whether any significant relationships were present between lower-body muscle structure and lower-body strength and power qualities. Furthermore, it was also an aim to identify any relationships that were present between the lower-body strength and power variables measured within this study. This study involved a cross-sectional analysis, whereby subjects were required to have their lower-body muscle structure assessed using ultrasonography, before completing a battery of lower-body strength and power tests that were all completed during a single session. Strength and power tests included the CMJ, SJ, and IMTP.
Fifteen elite competitive male (21.7 ± 5.0 years; 176.6 ± 5.5 cm; 71.0 ± 8.5 kg) surfing athletes participated in this study. Inclusion criteria involved the following: (a) actively competing at an international level, (b) aged 16–35 years, and (c) currently free of any injury or medical condition, as per a health screening questionnaire. The study and procedures were approved by the Edith Cowan University Human Ethics Committee (approval number: 10228) and were conducted according to the Declaration of Helsinki. All participants were provided with information detailing the study before providing informed consent and were screened for medical contraindications before participation. Age range of subjects was 16.4 years to 31.0 years. In the event that a subject was under 18 years of age a parent or guardian signed the informed consent form.
Muscle structure was measured with real-time B-mode ultrasonography (SSD-1000; Aloka Co., Tokyo, Japan) with a 7.5-MHz linear probe (17,18,21). A water-soluble gel was placed on the probe to allow for acoustic contact, with no depression of the dermal layer (4). After at least 10 minutes of sitting in a chair, to allow for a fluid shift, each subject was placed in a supine position, with the legs resting on a bench, to measure the muscle thickness and pennation angle of the VL. Measures of the VL were assessed at 50% of the distance between the greater trochanter and lateral epicondyle of the femur (8,23). To measure the LG, subjects were in a prone position, with legs fully extended on the bench, and the probe placed over the LG. Measures of the LG were assessed at two-thirds of the distance between the lateral epicondyle of the femur and the lateral malleolus (8,23). Fascicle length of the VL and LG were calculated from the equation reported by Fukunaga et al. (12) (fascicle length = muscle thickness × [sin pennation angle]−1). Two images were recorded of the VL and LG from the left (LVL and LLG, respectively) and right (RVL and RLG, respectively) legs of each subject (Figures 1 and 2). One image was used to assess muscle thickness with the other for pennation angle and fascicle length. All images were analyzed using a free public imaging software (ImageJ 1.40g; National Institutes of Health, Bethesda, MD, USA). Each image was assessed 3 times, and the average value for muscle thickness, pennation angle, and fascicle length from the 6 measures were used for analysis (4). Furthermore, the intraclass correlation coefficient (ICC) and coefficient of variation percent (CV%) were 0.99–1.00 and 0.8–1.2% for LG muscle thickness, 0.87–0.91 and 6.3–6.5% for LG pennation angle, 1.00–1.00 and 0.8–0.9% for VL muscle thickness, and 0.94–0.96 and 5.3–6.0% for VL pennation angle, respectively.
Lower-Body Strength and Power
After a 10-minute whole-body warm-up, consisting of squats, lunges, and dynamic mobility movements, subjects completed the physical testing in the following order: CMJ, SJ, and IMTP. To perform the CMJ, subjects were required to stand in an upright position on a portable force plate (400 Series Performance Force Plate; Fitness Technology, Adelaide, Australia) with a wooden dowel placed across their backs. The force plate was connected to a portable laptop, running an analysis software package (Ballistic Measurement System; Fitness Technology), and sampled at 600 Hz. Subjects performed 3 trials of the CMJ, from a self-selected depth, with instructions to jump as high and quickly as possible (31). Subjects were then required to perform 3 trials of the SJ. Subjects held the wooden dowel across their upper back, with a linear position transducer (PT9510; Fitness Technology) attached to the dowel. Subjects were instructed to be in a position, whereby the top of thighs were parallel with the ground, and were required to hold the position for 3 seconds before jumping as high as possible on the command “go” (15,22,29). The linear position transducer was interfaced with the portable force plate and was attached to the portable laptop running the analysis software package. Each subject's best trial was used for analysis. The best trial for the CMJ and SJ were determined by the vertical jump height. Additionally, for the SJ, trials were omitted in the event of a small amplitude countermovement of greater than 2 cm, as observed by the displacement-time trace on the analysis software package (15,29). All jumps were analyzed for the following variables: peak force (PF), peak velocity (PV), and jump height. The ICC and CV% for these variables of the CMJ and SJ were as follows: PF (0.98 and 3.9%, and 0.97 and 2.1%, respectively), PV (0.99 and 1.0%, and 0.92 and 3.2%, respectively), and jump height (0.94 and 4.0%, and 0.82 and 6.8%, respectively). Furthermore, a measure reflective of lower-body MTC stiffness was calculated from the CMJ, with the equation of Fpeak/ΔL, whereby Fpeak is the peak ground reaction force and ΔL is the vertical displacement of the center of mass (9,25,30). The ICC and CV% for lower-body MTC stiffness was 0.96 and 4.5%, respectively. Additionally, to discriminate the effect of the SSC, the eccentric utilization ratio (EUR) was determined with the following equation: EUR = CMJ jump height/SJ jump height (22).
To perform the IMTP, subjects were required to stand on the portable force plate, gripping a customized pull rack, with their shoulders placed over the bar, in a position similar to that of the second pull of a power clean (13). Subjects performed 2 trials of the IMTP, with 2 minutes of rest between each trial. In the event of a difference in the PF between the 2 trials of greater than 250 N, a third trial was performed (20). Knee angle was measured with a goniometer to ensure a range of 125–140° (14), with subjects instructed to push as hard as possible into the force plate (27,31). This large range in knee angle was used to account for individual differences in relative limb length, with Comfort et al. (5) recently identifying that changes in knee angle do not significantly alter PF. The force plate was connected to a portable laptop, running the analysis software package, and sampled at 600 Hz. Each subjects' best trial, as determined by the trial with the highest PF, was used to determine PF and relative PF (rPF) (N·BW−1). The ICC and CV% for PF for this cohort were 0.97 and 5.4%, respectively. To reflect an athlete's ability to rapidly apply force during a dynamic movement, relative to their maximal force capacity, the dynamic strength deficit (DSD) ratio was calculated using the following formula: DSD = CMJ PF/IMTP PF (32).
Mean and SD were reported for all muscle structure measures and lower-body strength and power variables (Tables 1 and 2). Normality of data was assessed with the Shapiro-Wilk statistic. In the event of the assumption of normality being violated, the data was log-transformed for analysis. Pearson product-moment correlation coefficients (r) were performed on all measures to identify whether any significant relationships were present between the muscle structure measures and lower-body strength and power variables, as well as within the strength and power variables. To interpret the magnitude of relationships, the strength of the Pearson correlation coefficients were classified as 0.0–0.1 (trivial), 0.1–0.3 (small), 0.3–0.5 (moderate), 0.5–0.7 (large), 0.7–0.9 (very large), and 0.9–1.0 (near perfect) (16). Furthermore, the coefficient of determination (r2) was calculated for all significant relationships to demonstrate the explained variance. Additionally, 90% confidence intervals were calculated for all statistically significant relationships. All statistical analyses were performed using a statistical analysis package (SPSS, version 22.0; IBM, Chicago, IL, USA), with statistical significance set at p ≤ 0.05.
Significant relationships were identified between LVL and RVL thickness and PF, PV, and jump height in the CMJ and SJ, as well as with PF in the IMTP (Table 3). Similarly, significant relationships were found between LLG pennation angle and PV in the CMJ, PF in the SJ, and PF and relative PF in the IMTP (Table 4). However, no significant relationships were identified between RLG pennation angle and any lower-body strength and power variable (Table 4). Furthermore, IMTP PF demonstrated a very large relationship with PF in the CMJ (r = 0.76, r2 = 0.57, p < 0.01) and SJ (r = 0.81, r2 = 0.65, p < 0.01), and large relationships with jump height in the CMJ and SJ (p < 0.01 and p = 0.02, respectively) (Figures 3 and 4). Additionally, lower-body MTC stiffness exhibited large relationships with DSD ratio (p < 0.01), RLG pennation angle (p < 0.01) (Figures 5 and 6), PF (r = 0.60, r2 = 0.36, p = 0.02), and jump height (r = 0.53, r2 = 0.28, p = 0.04) in the CMJ.
The purpose of this study was to determine whether any significant relationships were present between specific lower-body muscle structures and lower-body strength and power qualities, as well as within the strength and power qualities. The results of this study indicate that VL thickness of both the left and right leg was significantly related to performance in the CMJ, SJ and IMTP. Furthermore, LLG pennation angle exhibited significant relationships with SJ and IMTP PF, and IMTP rPF. Additionally, lower-body MTC stiffness was significantly related to DSD ratio, RLG pennation angle, and PF and jump height in the CMJ.
To the best of our knowledge, this is the first study to report on the relationships between lower-body muscle structure and performance in the IMTP. Although previous research has identified that VL thickness demonstrates significant relationships with lower-body strength, through performance in the squat, it is yet to be reported whether similar relationships were present with the IMTP (3,23). The results of this study indicate that increased thickness in the VL is related to greater PF and rPF production during the IMTP. It has been well established that the maximal force that can be produced by a muscle is determined by the activity of the subunits of the muscle, namely the muscle fibers, sarcomeres, and myofibrils (33). Therefore, it is proposed that larger thickness of the VL muscles reflects great hypertrophy of the extensors in general and thereby allows for a greater production of force, because of a potentially greater number of actin and myosin filaments within the muscle (33). This would allow for increased cross-bridging within the muscle fibers, and hence, it is apparent that larger muscles would be capable of producing greater forces when compared with smaller muscles (33).
Interestingly, VL thickness was also significantly related to all performance variables (PF, PV, jump height) of the CMJ and SJ. Furthermore, the results of this study identified that PF in the IMTP was significantly related to PF, PV, and jump height in the CMJ, as well as PF and jump height in the SJ. This suggests that the subjects with greater maximal lower-body strength, as measured with the IMTP, were capable of producing better performances in the CMJ and SJ. It appears likely that this is part due to increased muscle thickness, allowing for a greater production of force, directly influencing the isometric tests and underpinning performance in the dynamic tests of power. This is evidenced by VL thickness explaining approximately 30% of the variance in IMTP PF, and 35 and 58% of the variance in CMJ PF and SJ PF, respectively. These results are in agreement with previous research that suggests that lower-body strength underpins lower-body power in a range of sport relevant activities (6,24).
The LLG pennation angle exhibited significant relationships with PV in the CMJ, PF in the SJ, and PF and rPF in the IMTP. The CMJ and SJ data agree with that of Earp et al. (8), which identified that a significant amount of variance was explained by LG pennation angle for CMJ jump height and relative power (β = 0.47, r2 = 0.19, p = 0.02; β = 0.77, r2 = 0.42, p < 0.01, respectively) and SJ jump height and relative power (β = 0.46, r2 = 0.21, p = 0.02; β = 0.42, r2 = 0.17, p = 0.03, respectively). However, this is the first time relationships have been identified between LG pennation angle and performance in the IMTP. Furthermore, LLG pennation angle explained 48% of the variance in IMTP PF and 40% in IMTP rPF. Although it is was surprising that significant relationships were not also identified between RLG pennation angle and strength and power variables, it is important to note that except for 2 athletes, all the subjects performed surfing with a “natural” stance (left foot forward). As such, this indicates that for most subjects in this study, their dominant foot was their left foot, which may help explain the disparities is the relationships between the left and right leg.
It has previously been reported that larger pennation angles within a muscle allow for a greater physiological cross-sectional area (PCSA) (17,19). Because of the higher PCSA, there is a greater concentration of muscle subunits, and hence an increase in the maximal magnitude of force that can be produced (8,19). In combination with the relationships identified between VL thickness and CMJ, SJ, and IMTP performance, this study suggests that the athletes with greater thickness in the VL and increased pennation in the LG exhibit higher levels of lower-body strength and power. These findings, in combination with the previous research that has shown that changes in VL thickness explains 64% of changes in speed performance in highly trained athletes (23), indicate that future research should determine whether increases in VL thickness and LG pennation angle can also transfer to associated improvements in CMJ, SJ, and IMTP performance.
Lower-body MTC stiffness was identified to exhibit significant relationships with DSD ratio, RLG pennation angle, and PF and jump height in the CMJ. Interestingly, this study is the first to identify significant relationships between lower-body MTC stiffness and DSD ratio. Furthermore, the data of this study suggest that 46% of the variance in DSD ratio is explained by lower-body MTC stiffness. This indicates that athletes with greater lower-body MTC stiffness have the ability to use a greater proportion of their maximal isometric force during a dynamic movement. In combination with the relationships identified between LG pennation angle and performance in the CMJ, this study provides further support to research that has suggested that the performance of dynamic lower-body activities are strongly related to the stiffness of the lower-body musculature (2). Although the concept that lower-body stiffness underpins performance in dynamic lower-body movements is not novel, to the best of our knowledge, this is the first study to report relationships between these variables using the equations presented in this study.
Importantly, the relationship between RLG pennation angle and lower-body MTC stiffness increasingly supports the data suggesting that increases in pennation angle result in greater passive resistance, and therefore increased stiffness and isometric-like qualities during lengthening (8). It is well established that during a CMJ, the tendon produces and stores the majority of the elastic strain energy. The greater the inherent stiffness of the LG muscle, the more deformation is produced within the Achilles tendon during an SSC. When there is an increase in the tendon deformation, there is greater production, storage, and redistribution of elastic strain energy (7,11). Therefore, it is proposed that increases in LG pennation angle may underpin the relationships between MTC stiffness, DSD ratio, and CMJ performance. Future research should aim to identify whether training-specific adaptations in LG pennation angle and lower-body MTC stiffness are related to changes in DSD ratio and CMJ performance.
Although a limitation of this study is the small sample size, the identification of the muscle structures that are related to a greater expression of lower-body strength and power qualities in this study should still assist practitioners with talent identification and to provide a sound basis of for future training studies. Such training studies should investigate whether changes in these specific muscle structures are associated with a concomitant change in the strength and power qualities. Furthermore, as a result of the significant relationships between muscle structure variables and lower-body MTC stiffness, changes in DSD with training should be further investigated. These analyses would provide strength and conditioning practitioners with the ability to prescribe effective training programs and to evoke specific structural changes, as opposed to merely mimicking movement velocities and patterns (7).
The results of this study suggest that greater thickness in the VL and increased pennation in the LG muscles may be related to improved performance in the CMJ, SJ, and IMTP. It is proposed that together, these specific structures are the result of increased hypertrophy within the muscles, which improve the force producing capabilities. The stiffness of the lower-body MTC was related to DSD ratio, RLG pennation angle, and CMJ performance. This indicates that increased pennation in the RLG appears to be related to greater lower-body MTC stiffness, which allows the athlete to apply a greater magnitude of force in a dynamic movement, in relation to their maximal strength.
1. Blazevich AJ, Cannavan D, Coleman DR, Horne S. Influence of concentric and eccentric resistance training on architectural adaptation in human quadriceps muscles. J Appl Physiol (1985) 103: 1565–1575, 2007.
2. Bojsen-Moller J, Magnusson SP, Rasmussen LR, Kjaer M, Aagaard P. Muscle performance during maximal isometric and dynamic contractions is influenced by the stiffness of the tendinous structures. J Appl Physiol (1985) 99: 986–994, 2005.
3. Brechue WF, Abe T. The role of FFM accumulation and skeletal muscle architecture in powerlifting performance. Eur J Appl Physiol 86: 327–336, 2002.
4. Brughelli M, Cronin J, Nosaka K. Muscle architecture and optimum angle of the knee flexors and extensors: A comparison between cyclists and Australian Rules Football players. J Strength Cond Res 24: 717–721, 2010.
5. Comfort P, Jones PA, McMahon JJ, Newton RU. Effect of knee and trunk angle on kinetic variables during the isometric mid-thigh pull: Test-retest reliability. Int J Sports Physiol Perform 10: 58–63, 2015.
6. Comfort P, Stewart A, Bloom L, Clarkson B. Relationships between strength, sprint, and jump performance in well-trained youth soccer players. J Strength Cond Res 28: 173–177, 2014.
7. Earp JE, Kraemer WJ, Cormie P, Volek JS, Maresh CM, Joseph M, Newton RU. Influence of muscle-tendon unit structure on rate of force development during the squat, countermovement, and drop jumps. J Strength Cond Res 25: 340–347, 2011.
8. Earp JE, Kraemer WJ, Newton RU, Comstock BA, Fragala MS, Dunn-Lewis C, Solomon-Hill G, Penwell ZR, Powell MD, Volek JS, Denegar CR, Hakkinen K, Maresh CM. Lower-body muscle structure and its role in jump performance during squat, countermovement, and depth drop jumps. J Strength Cond Res 24: 722–729, 2010.
9. Farley CT, Gonzalez O. Leg stiffness and stride frequency in human running. J Biomech 29: 181–186, 1996.
10. Foure A, Nordez A, Cornu C. Plyometric training effects on Achilles tendon stiffness and dissipative properties. J Appl Physiol (1985) 109: 849–854, 2010.
11. Fukashiro S, Hay DC, Nagano A. Biomechanical behavior of muscle-tendon complex during dynamic human movements. J Appl Biomech 22: 131–147, 2006.
12. Fukunaga T, Ichinose Y, Ito M, Kawakami Y, Fukashiro S. Determination of fascicle length and pennation in a contracting human muscle in vivo. J Appl Physiol (1985) 82: 354–358, 1997.
13. Haff GG, Carlock JM, Hartman MJ, Kilgore JL, Kawamori N, Jackson JR, Morris RT, Sands WA, Stone MH. Force-time curve characteristics of dynamic and isometric muscle actions of elite women Olympic weightlifters. J Strength Cond Res 19: 741–748, 2005.
14. Haff GG, Stone M, O'Bryant HS, Harman E, Dinan C, Johnson R, Han KH. Force-time dependent characteristics of dynamic and isometric muscle actions. J Strength Cond Res 11: 269–272, 1997.
15. Hasson CJ, Dugan EL, Doyle TLA, Humphries B, Newton RU. Neuromechanical strategies employed to increase jump height during the initiation of the squat jump. J Electromyogr Kinesiol 14: 515–521, 2004.
16. Hopkins WG. A scale of magnitudes of effects sizes. 2006. Available at: http://www.sportsci.org/resource/stats/effectmag.html
. Accessed October 29, 2014.
17. Kawakami Y, Abe T, Fukunaga T. Muscle-fibre pennation angles are greater in hypertrophied than in normal muscles. J Appl Physiol (1985) 74: 2740–2744, 1993.
18. Kawakami Y, Abe T, Kuno S, Fukunaga T. Training-induced changes in muscle architecture and specific tension. Eur J Appl Physiol 72: 37–43, 1995.
19. Kawakami Y, Ichinose Y, Kubo K, Ito M, Imai M, Fukunaga T. Architecture of contracting human muscles and its functional significance. J Appl Biomech 16: 88–98, 2000.
20. Kraska JM, Ramsey MW, Haff GG, Fethke N, Sands WA, Stone ME, Stone MH. Relationship between strength characteristics and unweighted and weighted vertical jump height. Int J Sports Physiol Perform 4: 461–473, 2009.
21. Kubo K, Kanehisa H, Takeshita D, Kawakami Y, Fukashiro S, Fukunaga T. In vivo dynamics of human medial gastrocnemius muscle-tendon complex during stretch-shortening cycle exercise. Acta Physiol Scand 170: 127–135, 2000.
22. McGuigan MR, Doyle TA, Newton M, Edwards DJ, Nimphius S, Newton RU. Eccentric utilization ratio: Effects of sport and phase of training. J Strength Cond Res 20: 992–995, 2009.
23. Nimphius S, McGuigan MR, Newton RU. Changes in muscle architecture and performance during a competitive season in female softball players. J Strength Cond Res 26: 2655–2666, 2012.
24. Peterson MD, Alvar BA, Rhea MR. The contribution of maximal force production of explosive movement among collegiate athletes. J Strength Cond Res 20: 867–873, 2006.
25. Secomb JL, Farley ORL, Lundgren L, Tran TT, King A, Nimphius N, Sheppard JM. Associations between the performance of scoring manoeuvres and lower-body strength and power in elite surfers. Int J Sports Sci Coach In Press.
26. Secomb JL, Tran TT, Lundgren L, Farley ORL, Sheppard JM. Single-leg squat progressions. Strength Cond J 36: 68–71, 2014.
27. Sheppard JM, Chapman DW. An evaluation of a strength qualities assessment for the lower body. J Aus Strength Cond 19: 14–20, 2011.
28. Sheppard JM, Cronin J, Gabbett TJ, McGuigan MR, Extebarria N, Newton RU. Relative importance of strength and power qualities to jump performance in elite male volleyball players. J Strength Cond Res 22: 758–765, 2007.
29. Sheppard JM, Doyle TLA. Increasing compliance to instructions in the squat jump. J Strength Cond Res 22: 648–651, 2008.
30. Sheppard JM, Newton RU, McGuigan MR. The effects of depth-jumping
on vertical jump performance of elite volleyball players: An examination of the transfer of increased stretch-load tolerance to spike jump performance. J Aus Strength Cond 16: 3–10, 2008.
31. Sheppard JM, Nimphius S, Haff GG, Tran TT, Spiteri T, Brooks H, Slater G, Newton RU. Development of a comprehensive performance-testing protocol for competitive surfers. Int J Sports Physiol Perform 8: 490–495, 2013.
32. Young KP, Haff GG, Newton RU, Sheppard JM. Reliability of a novel testing protocol to assess upper body strength qualities in elite athletes. Int J Sports Physiol Perform 9: 871–875, 2014.
33. Zatsiorsky VM, Kraemer WJ. Science and Practice of Strength Training (2nd ed.). Champaign, IL: Human Kinetics, 2006.