Powerlifting is a sport designed to test dynamic maximal strength in 3 exercises: the back squat, bench press, and deadlift (15). Powerlifters are routinely engaged in rigorous training programs to improve and maximize strength for competition. Long-term participation in resistance training programs has been associated with decreased shoulder range of motion (ROM) in recreational weightlifters (18,20) and bodybuilders (3). Several factors have been shown to impact joint flexibility, including muscle hypertrophy (9,28), exercise execution through a full ROM (28), and static and dynamic stretching (2,29,34). Specifically, Morton et al. (28) found that static stretching and resistance training, with exercises performed across a full ROM, can improve hip and knee flexibility in healthy adults.
Only a single study has evaluated flexibility in powerlifters (5). Chang et al. (5) found that powerlifters displayed decreased total shoulder ROM, which includes motion at the glenohumeral (GH), sternoclavicular, acromioclavicular, and scapulothoracic joints. Although total shoulder motion is often used to measure the functional ROM of the shoulder, GH ROM has been shown to be important in diagnosing and treating pathologies within the shoulder complex (30). The GH joint is surrounded by a connective tissue capsule that attaches around the margins of the glenoid labrum, proximally, and the anatomical neck of the humerus, distally. Although this capsule is flexible and permits a wide ROM under normal conditions, exposure to repetitive stress and weight-bearing loads such as the bench press can lead to connective tissue changes, resulting in anterior instability (21) and posterior capsular stiffness (8). Reduced passive GH ROM from capsular stiffness or tightness in the rotator cuff musculature is often considered a causative factor in shoulder injuries related to repetitive stress (35,40). Shoulder injuries are the most common in powerlifters (1,17,33) with 53.1% of powerlifters reporting past episodes of shoulder pain (36); yet, no study to date has isolated shoulder ROM at the GH joint in powerlifters.
Injuries to the lumbar spine and knee are also common in powerlifters with a lifetime prevalence of 40.8 and 39.2%, respectively (36). Reductions in lower-extremity ROM at the hip and knee joints have been associated with knee joint dysfunction and injury (13,27). In particular, reduced hamstring flexibility has been associated with low back pain and greater knee joint loads (27,32). A paucity of research exists on the effects of resistance training on lower-extremity ROM. Chang et al. (5) found that powerlifters displayed improved flexibility in the sit-and-reach test, a nonspecific measure of low back and hamstring mobility. To understand this benefit, additional research is needed to isolate hamstring flexibility and monitor training patterns in powerlifters.
Therefore, the purpose of this study was to evaluate upper- and lower-extremity passive ROM in powerlifters using goniometric analysis at the GH, hip, knee, and ankle joints. Furthermore, no study has examined factors that may lead to ROM adaptations in powerlifters, such as stretching frequency, muscle hypertrophy, and exercise selection. Therefore, a secondary outcome measure of this study was to examine the relationship between ROM and training patterns (e.g., exercise selection and flexibility training frequency) using questionnaires to ultimately guide future recommendations for improving flexibility and preventing injury in chronic resistance training populations.
Experimental Approach to the Problem
This study provides a cross-sectional examination of passive ROM (PROM) in powerlifters relative to controls with minimal weight training experience. Passive ROM of the GH, hip, knee, and ankle joints was assessed using goniometry. The Apley scratch test and modified Thomas test were used to measure ROM across multiple joints. Exercise selection, training frequency, and stretching frequency were measured using questionnaires. After initial ROM comparison with age-matched controls, the entire powerlifting cohort was divided based on Wilks score to determine whether any existing ROM adaptations were present in elite powerlifters (Wilks >500).
This study was approved by the University of Scranton Review Board, and written informed consent was obtained from each participant before testing. Flyers were sent to the e-mail addresses of those who participated in at least 1 U.S. powerlifting competition. For inclusion in the study, powerlifters were required to be actively competing and have at least 4 years of weight training experience. After initial screening, 15 male powerlifters from Northeast Pennsylvania were included (aged 20–62 years) along with carefully selected age- and weight-matched controls (aged 20–63 years), who were not actively participating in resistance training and had less than 6 months of total weight training experience. All study participants were free of injury at the time of testing. Multifrequency bioelectrical impedance analysis (MBIA) was performed to assess total body mass, body fat percentage, and lean body mass, as previously described (6). The Wilks coefficient, a validated measure of relative strength in powerlifting (39), was determined for each powerlifter based on published online tables from USA Powerlifting (38). To calculate Wilks score, Wilks coefficients were multiplied by total weight lifted according to self-reported, nonequipped maximum lifts from the most recent powerlifting event. Subject characteristics are summarized in Table 1.
After MBIA measurements, participants were instructed to perform a warm-up on a rowing machine (Concept2 indoor rower; Concept 2, Morrisville, VT, USA), as previously described (7). Because of positioning requirements, flexibility measurements were performed in the following order: the Apley scratch test, GH flexion, GH horizontal adduction, GH internal rotation, GH external rotation, hip flexion, hip abduction, hip adduction, KEA, GH extension, GH horizontal abduction, hip extension, modified Thomas test, ankle plantar flexion, and ankle dorsiflexion.
Powerlifters were instructed to complete a 32-item questionnaire before testing. Training and exercise frequency were measured in days per week and sets per session for the back squat, bench press, and deadlift. Frequency of flexibility training (i.e., upper- or lower-body static or dynamic stretching) was measured in days per week. To assess exercise selection, powerlifters were instructed to list frequency of all exercises used on a weekly basis. Frequency of bench press and bench press variations (i.e., barbell or dumbbell bench presses with any angle of incline) was calculated as a proportion to all upper-body exercises, and frequency of back squat and deadlift was calculated as a proportion to all lower-body exercises.
Passive Range of Motion
Upper- and lower-extremity PROM were assessed on the dominant arm and leg using a 12-in. plastic goniometer (Baseline; Medline Industries, Middletown, NY, USA). All PROM measurements involved a passive movement by the examiner with the participant's limb relaxed. The same examiner with more than 20 years of experience conducting ROM assessments as a certified athletic trainer and educator performed all PROM measurements. Three measurements were taken for each PROM, and the average was used for analysis. Common anatomical landmarks were used as reference for goniometric placement, according to Norkin and White (30).
Passive ROM of the GH joint was evaluated in all 3 planes of motion to assess any limitations that may be indicative of capsular tightness, and end-feel was determined when attempts to overcome resistance caused scapular movement. GH flexion and extension were assessed in the supine and prone positions, respectively, with the scapula stabilized, and end-feel was determined when scapular elevation or tilt became apparent (30). Measurements of GH internal rotation and external rotation were taken in the supine position at 90° of shoulder abduction with pressure placed across the coracoid process (25,30). GH horizontal adduction measurements were taken in the supine position, where the arm was neutrally rotated at 90° of shoulder abduction and passively horizontally adducted to a soft end-feel, as previously described (24). GH horizontal abduction was measured in the prone position, where the neutrally rotated arm was passively horizontally abducted to a firm end-feel.
Lower-extremity PROM was assessed at the hip, knee, and ankle using established methods (30). Hip flexion was measured in the supine position with the knee flexed, and the pelvis stabilized, until a soft end-feel was reached. Hip extension measurements were taken in the prone position with the knee extended, until a firm end-feel was reached. Any rotation in the pelvis marked the end ROM for hip flexion and extension. Hip abduction and adduction measurements were taken in the supine position with the pelvis stabilized, until a firm end-feel was reached or rotation or tilting of the pelvis became apparent. KEA measurements were performed in the supine position: the nondominant limb was secured at 0° hip flexion, while the dominant hip was passively flexed to 90° hip flexion, and the knee was passively extended, until the end-feel was observed (Figure 1). Ankle plantar flexion and dorsiflexion measurements were taken in the seated position at 90° knee flexion with the ankle suspended in the air, and the distal tibia and fibula stabilized, until a firm end-feel was reached.
Intrarater reliability of PROM measurements using a universal goniometer have been established by several studies, reviewed in Norkin and White (30). Intraclass correlation coefficient (ICC) values ranged from 0.87 to 0.99 for shoulder PROM measurements and 0.81–0.98 for lower-extremity PROM measurements (30).
Range of motion was evaluated across multiple joints using the Apley scratch test and modified Thomas test. For the Apley scratch test, each participant was instructed to raise the dominant arm over the head and place the hand at the lowest-possible position on the back, while the other arm reached behind the back in an attempt to touch fingertips, as previously described (3). The distance between fingertips was recorded, and the test was repeated for the nondominant arm. For the modified Thomas test, each participant began in the seated position on the edge of a table while flexing and holding the nondominant hip and knee. The examiner rolled the participant to a supine position with both knees flexed and passively lowered the dominant limb to the resting position, after which hip and knee angles were recorded (31) (Figure 2). Intrarater ICC values for the Apley scratch test and modified Thomas test were above 0.94 and 0.82, respectively (3,31).
Data were organized using Microsoft Excel, and statistical analyses were performed in GraphPad Prism 7 software. All data are represented as mean ± SD unless noted otherwise. Paired 2-tailed t-tests with an alpha-level of 0.05 were used to determine statistical significance. Minimal detectable change at the 90% confidence level (MDC90) was determined for intrarater measurements, using an established algorithm (19): MDC90 = 1.65 * SEM * where the SEM = SD. MDC90 values are presented in Table 2.
Single-Joint Passive Range of Motion
Single-joint PROM in the upper extremity was significantly restricted in powerlifters, particularly in GH extension, internal rotation, and external rotation (Table 3). Interestingly, powerlifters did not demonstrate any PROM limitations in the lower extremity and displayed greater hamstring flexibility, as measured by KEA (Table 3).
Multijoint Range of Motion
The Apley scratch test and modified Thomas test were used to assess multijoint ROM in the upper and lower extremities, respectively. Powerlifters displayed significant ROM limitations in the Apley scratch test in both dominant and nondominant arms (Figure 3A). No significant differences in hip and knee angle measurements were observed between powerlifters and controls in the modified Thomas test (Figure 3B).
Training Assessment in Entire Powerlifting Cohort
Powerlifters trained on 4.29 ± 1.33 days per week and incorporated flexibility training on 1.87 ± 0.54 days per week. Powerlifters performed the bench press on 2.07 ± 1.06 days per week, the back squat on 1.67 ± 1.08 days per week, and the deadlift on 1.40 ± 0.71 days per week. Bench press and bench press variations accounted for 74.8% of all upper-body exercises, whereas the back squat and deadlift accounted for 79.7% of all lower-body exercises in powerlifters' training programs. No significant differences between back squat training frequency and deadlift training frequency were observed, as measured by days per week (p = 0.451) or sets/session (p = 0.690).
Range of Motion in Elite Powerlifters
To determine whether ROM limitations in the upper extremity were seen in elite powerlifters, the powerlifting cohort was split into 3 groups according to Wilks score: <400 (low, n = 6), 400–500 (intermediate, n = 5), and >500 (high, n = 4). Upper-extremity ROM discrepancies seen in entire cohort were diminished in the low Wilks group and, interestingly, were more pronounced in the high Wilks group compared to controls with minimal weight training experience (Table 4).
Training Assessment in Elite Powerlifters
The high Wilks group had more than a 2-fold increase in powerlifting experience (i.e., years actively competing) and a significantly higher lean body mass compared with the low Wilks group (Table 5). No differences in training frequency, bench press frequency, or flexibility training were observed between low and high Wilks powerlifters (Table 5).
The primary aim of this study was to determine the presence of ROM limitations associated with chronic resistance training by studying the powerlifting population. In this study, we found that powerlifters displayed reduced GH ROM, particularly in GH extension, internal rotation, and external rotation, as determined by both single-joint goniometric measurements and the Apley scratch test. Chang et al. (5) found similar upper-extremity ROM restrictions in powerlifters, demonstrating a loss of mobility in shoulder flexion, extension, and internal rotation, and external rotation using shoulder complex ROM measurements. Previous studies also reported ROM restrictions in shoulder internal rotation in recreational weightlifters (18) and bodybuilders (3). The results of this study suggest that the shoulder ROM discrepancies seen in previous studies (3,5,18) are attributed, at least in part, to decreased mobility in the GH joint, which suggests capsular tightness and reduced flexibility in the rotator cuff musculature (8,10).
Normative values differ between shoulder complex ROM and GH ROM measurements because of the degree of scapular motion (26). For adult men, mean values for total shoulder flexion, extension, internal rotation, and external rotation are 167, 62, 69, and 104°, respectively (4). These measurements correspond with previous studies assessing total shoulder ROM (3,5,18). Mean values for GH flexion, extension, internal rotation, and external rotation are 106, 20, 49, and 94°, respectively (23). These measurements correspond with age-matched controls in this study and explain the differences in ROM values from previous studies. In addition, differences in normative values may be attributed to sample size and the age of the population studied (30). The controls and powerlifters in this study had a mean age of 34.9 and 35.3 years, respectively, and these values are 7–8 years higher than previous studies (3,5,18), indicating that the age of the population studied may have led to slightly lower ROM values because of decreased joint laxity associated with aging (16). Finally, the characteristics of the target population can have a significant impact on ROM adaptations. Kolber et al. (18) found that recreational weight lifters display reductions in active shoulder ROM, with the exception of external rotation ROM, which was improved. This benefit is likely due to the incorporation of more exercises that require end-range external rotation ROM, such as latissimus pull-downs and shoulder presses. Powerlifters place more focus on exercises related to the bench press—as 74.8% of all upper-body exercises among powerlifters involved the bench press and bench press variations—and less focus on exercises that require end-range external ROM, and this training pattern may explain the absence of this benefit in the powerlifting population.
Because nearly 75% of all upper-body exercises among powerlifters involved the bench press and bench press variations in training, it is likely that the observed GH ROM limitations in powerlifters may be attributed to a chronic focus on the bench press. Biomechanically, the movement phase of the bench press is performed primarily in the horizontal abduction/horizontal adduction plane, while the shoulder is fixed at a certain degree of motion in GH flexion and rotation (16). Interestingly, we found no significant ROM limitations in the movement plane (i.e., horizontal abduction/adduction), while severe ROM restrictions were observed in the supportive planes of motion (i.e., flexion/extension and rotation), especially among elite powerlifters. These data support the findings of Morton et al. (28) and suggest that resistance training through a joint's full ROM is an important factor in preventing negative ROM adaptations. Overall, these results suggest that an overemphasis on the bench press may lead to shoulder ROM adaptations that specifically impact motions with fixed degrees of movement in exercise execution.
Both passive and active stretching used in flexibility training programs have been shown to improve flexibility (2,34). Powerlifters in this study incorporated flexibility training in less than half of their training sessions (i.e., 1.87 ± 0.54 days per week), which is considered insufficient according to current recommendations for healthy adults (12). These data indicate that GH ROM adaptations may be dependent not only on the degree of motion involved in exercise execution, but also on the frequency of flexibility training. Stretching protocols addressing GH extension (16), internal rotation (25), and external rotation (37) should be used in training programs with special attention to internal and external rotation, as these limitations are commonly seen in weightlifters with impingement syndrome (22). In addition to stretching protocols, strength training professionals must also prescribe exercises to maintain proper strength balance of the rotator cuff musculature to prevent unfavorable postural adaptations (7) and anterior instability (21) in populations that expose the shoulder joint to chronic, repetitive loading.
Powerlifters in this study displayed no significant ROM limitations in the lower extremity. Chang et al. (5) found that powerlifters displayed better flexibility on the sit-and-reach test, a nonspecific measure of hamstring, low back, and gluteal flexibility. Correspondingly, we hypothesized that these reported benefits were primarily caused by improved hamstring flexibility, as the hamstring muscle groups are activated through a wide ROM in powerlifting training and competitions (11). The results of this study supported our hypothesis, demonstrating improved hamstring flexibility as measured by the KEA. In addition, Chang et al. (5) reported ROM limitations in hip flexion; yet, our data indicate no significant limitations in hip flexion through both goniometric measurements and the modified Thomas test. We suppose that the larger sample size in this study may account for these differences; however, as these are the only studies to date assessing lower-extremity ROM adaptations in any resistance training cohort, further research is warranted in this area.
Powerlifters in this study focused the majority of their lower-body training on the back squat and the deadlift (about 80%) with no significant differences in the frequency of these 2 exercises. To meet the requirements for the back squat in competition, the knee joint must be above the hip joint at the bottom phase, and the knees must be fully locked at the top phase of the lift (15) allowing for hamstring activation along a wide ROM (11). A study by Hales et al. (14) has shown that the deadlift elicits different hip and knee joint angles than the back squat, requiring hamstring activation along an even wider ROM. Therefore, the balanced combination of the back squat and deadlift in powerlifting requires the hamstrings to generate concentric and eccentric forces along a wide ROM, and this tendency may have improved hamstring flexibility in this study. This phenomenon—hereafter termed the full ROM hypothesis—has been the basis for strength training recommendations for decades (9,16), but has not been extensively studied in the literature.
We found that elite powerlifters (Wilks >500) have more pronounced upper-extremity ROM restrictions that are also seen exclusively in the supportive planes of motion (i.e., GH flexion/extension and rotation). Although exercise and flexibility training frequency did not differ between low and high Wilks groups, elite powerlifters possessed significantly greater lean body mass, an indirect measure of muscle hypertrophy, which has been shown to structurally limit mobility, especially in the degree of shoulder and elbow flexion required for the Apley scratch test (3,16,41). Furthermore, elite powerlifters possessed more than a 2-fold increase in powerlifting experience, measured by years of active competition. As most of the upper-body training in powerlifting involves the bench press, chronic focus on bench press and limited focus on exercises that stress full ROM in GH flexion/extension (e.g., rows, pull-downs, and military presses) may gradually limit shoulder mobility, as explained by the full ROM hypothesis. Overall, these data suggest that both increased muscle girth and a chronic overemphasis on the bench press produce more pronounced limitations in GH ROM.
While the findings of this study have direct implications for methods to improve flexibility and potentially prevent injury in powerlifters, the limitations of this study must be acknowledged. Although powerlifters represent an ideal population to study the effects of chronic resistance training because of their strict adherence to training programs, this study was limited with the relatively small sample size, especially with elite powerlifters. In addition, although GH ROM measurements provide insight into joint mobility and susceptibility to injury, they are limited in their functional approach compared with total shoulder ROM measurements. Goniometric measurements in future studies should be extended to include nondominant limbs, as ROM may differ between dominant and nondominant limbs. Finally, this study was cross-sectional in nature, and training patterns may change over time. Prospective studies using large sample sizes can address these issues and provide confirmative insight into the full ROM hypothesis.
The bench press, back squat, and deadlift are important exercises to include in any strength training regimen. When designing strength programs that use the bench press, strength training professionals should carefully evaluate ROM and prescribe stretching exercises to maintain GH ROM—particularly in GH extension, internal rotation, and external rotation—and strengthening exercises for the rotator cuff musculature to maintain strength balance and prevent injury. To elicit improvements in hamstring flexibility and reduce the risk of knee and low-back pathologies, the practitioner can also use the back squat and deadlift, while paying close attention to proper exercise form and execution through a full ROM.
This study was funded through an internal research grant from The University of Scranton. The authors have no conflict of interest to declare. The results of this study do not constitute the endorsement of any product by the authors or the National Strength and Conditioning Association.
1. Aasa U, Svartholm I, Andersson F, Berglund L. Injuries among weightlifters and powerlifters: A systematic review. Br J Sports Med 51: 211–220, 2017.
2. Ayala F, de Baranda A. Effect of 3 different active stretch durations on hip flexion range of motion. J Strength Cond Res 24: 430–436, 2010.
3. Barlow J, Benjamin B, Birt P, Hughes CJ. Shoulder strength and range-of-motion characteristics in bodybuilders. J Strength Cond Res 16: 367–372, 2002.
4. Boone DC, Azen SP. Normal range of motion of joints in male subjects. J Bone Joint Surg 61: 756, 1979.
5. Chang DE, Buschbacher LP, Edlich RF. Limited joint mobility in power lifters. Am J Sports Med 16: 280–284, 1988.
6. Cutrufello PT, Dixon CB. The effect of acute fluid consumption following exercise-induced fluid loss on hydration status, percent body fat, and minimum wrestling weight in wrestlers. J Strength Cond Res 28: 1928–1936, 2014.
7. Cutrufello PT, Gadomski SJ, Ratamess NA. An evaluation of agonist: Antagonist strength ratios and posture among powerlifters. J Strength Cond Res 31: 298–304, 2017.
8. Dashottar A, Borstad J. Posterior glenohumeral joint capsule contracture. Shoulder Elbow 4: 1758–5740, 2012.
9. deVries HA, Housh TJ, Weir LL. Energetics of Muscular Contraction and Adaptations to Training at the Cellular Level. In: Physiology of Exercise for Physical Education, Athletics and Exercise Science. (5th ed.). Dubuque, IA: Brown, 1995. pp. 72–125.
10. Duzgun I, Tugut E, Cinar-Medeni O, Kafa N, Tuna Z, Elbasan B, et al. The presence and influence of posterior capsule tightness on different shoulder problems. J Back Musculoskelet Rehabil 30: 187–193, 2017.
11. Ebben W. Hamstring activation during lower body resistance training exercises. Int J Sports Physiol Perform 4: 84–96, 2009.
12. Garber CE, Blissmer B, Deschenes MR, Franklin BA, Lamonte MJ, Lee IM, et al. Quantity and quality of exercise for developing and maintaining cardiorespiratory, musculoskeletal, and neuromotor fitness in apparently healthy adults: Guidance for prescribing exercise. Med Sci Sports Exerc 43: 1334–1359, 2011.
13. Gomes JL, de Castro JV, Becker R. Decreased hip range of motion and noncontact injuries of the anterior cruciate ligament. Arthroscopy 24: 1034–1037, 2008.
14. Hales ME, Johnson BF, Johnson JT. Kinematic analysis of the powerlifting style squat and the conventional deadlift during competition: Is there a cross-over effect between lifts? J Strength Cond Res 23: 148–157, 2009.
15. IPF. International Powerlifting Federation. Available at: http://www.powerlifting-ipf.com/
. Accessed October 19, 2017.
16. Jeffries I. Administration, Scoring, and Interpretation of Selected Tests. In: Baechle, TR and Earle Roger, W, ed. Essentials of Strength Training and Conditioning. (4th ed.). Champaign, IL: Human Kinetics, 2008. pp. 296–309.
17. Keogh J, Hume PA, Pearson S. Retrospective injury epidemiology of one hundred one competitive Oceania power lifters: The effects of age, body mass, competitive standard, and gender. J Strength Cond Res 20: 672–681, 2006.
18. Kolber MJ, Beekhuizen KS, Cheng MS, Hellman MA. Shoulder joint and muscle characteristics in the recreational weight training population. J Strength Cond Res 23: 148–157, 2009.
19. Kolber MJ, Vega F, Widmayer K, Cheng MS. The reliability and minimal detectable change of shoulder mobility measurements using a digital inclinometer. Physiother Theory Pract 27: 176–184, 2011.
20. Kolber MJ, Beekhuizen KS, Cheng MS, Hellman MA. Shoulder injuries attributed to resistance training: A brief review. J Strength Cond Res 24: 1696–1704, 2010.
21. Kolber MJ, Corrao M, Hanney WJ. Characteristics of anterior shoulder instability and hyperlaxity in the weight training population. J Strength Cond Res 27: 1333–1339, 2013.
22. Kolber MJ, Hanney WJ, Cheatham SW, Salamh PA. Shoulder joint and muscle characteristics among weight-training participants with and without impingement syndrome. J Strength Cond Res 31: 1024–1032, 2017.
23. Lannan D, Lehman T, Toland M. Establishment of Normative Data for the Range of Motion of the Glenohumeral Joint [master of science thesis]. Lowell, MA: University of Massachusetts, 1996.
24. Laudner KG, Stanek JM, Meister K. Assessing posterior shoulder contracture: The reliability and validity of measuring glenohumeral joint horizontal adduction. J Athl Train 41: 375–380, 2006.
25. Linter D, Mayol M, Uzodinma O, Jones R, Labossiere D. Glenohumeral internal rotation deficits in professional pitchers enrolled in an internal rotation stretching program. Am J Sports Med 35: 617–621, 2007.
26. McCully SP, Kumar N, Lazarus MD, Karduna AR. Internal and external rotation of the shoulder: Effects of plane, end-range determination, and scapular motion. J Shoulder Elbow Surg 14: 602–610, 2005.
27. Messier SP, Legault C, Schoenlank CR, Newman JJ, Martin DF, DeVita P. Risk factors and mechanisms of knee injury in runners. Med Sci Sports Exerc 40: 1873–1879, 2008.
28. Morton SK, Whitehead JR, Brinkert RH, Caine DJ. Resistance training vs. static stretching: Effects on flexibility and strength. J Strength Cond Res 25: 3391–3398, 2011.
29. Nishikawa Y, Aizawa J, Kanemura N, Takahashi T, Hosomi N, Maruyama H, et al. Immediate effect of passive and active stretching on hamstrings flexibility: A single-blinded randomized control trial. J Phys Ther Sci 27: 3167–3170, 2015.
30. Norkin CC, White DJ. Measurement of Joint Motion, A Guide to Goniometry. Philadelphia, PA: FA Davis Company, 2009.
31. Page P, Frank C, Lardner R. Assessment and Treatment of Muscle Imbalance: The Janda Approach. Chicago, IL: Human Kinetics, 2010.
32. Radwan A, Bigney KA, Buonomo HN, Jarmak MW, Moats SM, Ross JK, et al. Evaluation of intra-subject difference in hamstring flexibility in patients with low back pain: An exploratory study. J Back Musculoskelet Rehabil 28: 61–66, 2015.
33. Raske A, Norlin R. Injury incidence and prevalence among elite weight and power lifters. Am J Sports Med 30: 248–256, 2002.
34. Roberts JM, Wilson K. Effect of stretching duration on active and passive range of motion in the lower extremity. Br J Sports Med 33: 259–263, 1999.
35. Shanley E, Rauh MJ, Michener LA, Ellenbecker TS, Garrison JC, Thigpen CA. Shoulder range of motion measures as risk factors for shoulder and elbow injuries in high school softball and baseball players. Am J Sports Med 39: 1997–2006, 2011.
36. Siewe J, Rudat J, Rollinghoff M, Schlegel UJ, Eysel P, Michael JW. Injuries and overuse syndromes in powerlifting. Int J Sports Med 32: 703–711, 2011.
37. Starrett K, Cordoza G. Mobilization Techniques. In: 2nd ed. Becoming a Supple Leopard: The Ultimate Guide to Resolving Pain, Preventing Injury, and Optimizing Athletic Performance. Las Vegas, NV: Victory Belt Publishing Inc, 2015. pp. 309–334.
39. Vanderburgh PM, Batterham AM. Validation of the Wilks powerlifting formula. Med Sci Sports Exerc 31: 1869–1875, 1999.
40. Wilk KE, Macrina LC, Fleisig KT, Aune KT, Porterfield RA, Harker P, et al. Deficits in glenohumeral passive range of motion increase risk of shoulder injury in professional baseball players: A prospective study. Am J Sports Med 43: 1–7, 2015.
41. Ye X, Loenneke JP, Fahs CA, Rossow LM, Thiebaud RS, Kim D, et al. Relationship between lifting performance and skeletal muscle mass in elite powerlifters. J Sports Med Phys Fitness 53: 409–414, 2013.