Introduction
The barbell hip thrust, introduced in the literature by Contreras et al. (13 4,13,18,19 16 Figure 1 ), the force vector hypothesis states that it may better transfer to sports that are dependent on horizontal force production, because, when standing, horizontal force vectors are anteroposterior. Sprinting is particularly relevant in this context, as horizontal force, horizontal force times horizontal velocity (often misappropriated as “horizontal power”), and horizontal impulse have strong associations with sprint running, both at maximal speed and during acceleration (7,8,35 41 34 34 34
Figure 1.: Hip thrust technique.
The squat is one of the most well-studied and used exercises in strength and conditioning. A recent meta-analysis on the squat found that increases in back squat strength transfer positively to sprint performance (r = −0.77) (43 11,42 6 26,29 9,25,39,47 24 21,51 14
Research examining specificity has shown that during 1 repetition maximum (1RM) testing, training specificity is a primary factor (37,48 38 14 12
Therefore, the purpose of this study was to compare the effects of 6-week hip thrust and front squat training programs on 10- and 20-m sprint times, horizontal jump distance, vertical jump height, isometric midthigh pull performance, and both 3RM front squat and 3RM hip thrust strength in adolescent males. It was hypothesized that (a) the hip thrust group would improve 3RM hip thrust to a greater extent than the front squat group, because of specificity; (b) the front squat group would improve 3RM front squat to a greater extent than the hip thrust group, because of specificity; (c) the hip thrust group would improve 3RM front squat, but not as much as the front squat group; (d) the front squat group would improve 3RM hip thrust, but not as much as the hip thrust group; (e) the hip thrust group would improve 10- and 20-m sprint times to a greater extent than the front squat group, as hip thrusts elicit greater gluteus maximus and hamstring activation; (f) the front squat group would improve vertical jump better than the hip thrust group, as the front squat involves a vertical load vector and displays greater quadriceps activation; (g) both groups would improve horizontal jump distance to a similar degree, as the horizontal jump uses both vertical and horizontal external force vectors and display similar levels of gluteus maximus and quadriceps activity; and (h) both groups would improve the isometric midthigh pull force to a similar degree, as both heavily rely upon the hamstrings and gluteus maximus.
Methods
Experimental Approach to the Problem
This was a single-center, investigator-blinded, parallel-group, randomized controlled trial with equal randomization (1:1). Each group was assigned to perform the hip thrust or front squat twice per week for 6 weeks, for a total of 12 sessions. Performance variables were collected before, and after, the 6-week training period.
Subjects
Eligible participants were all adolescent athletes, ages 14–17 years, and were enrolled in a New Zealand rugby and rowing athlete development program (Table 1 ). All subjects had 1 year of squatting experience and no hip thrusting experience. An a priori power analysis was performed for increases in acceleration (α = 0.05; β = 0.80; Cohen's d = 2.44) (30
Table 1.: Comparison of baseline characteristics of the front squat and hip thrust groups.
Procedures
On the first day, subjects completed the necessary forms (Informed Consent, Assent and PAR-Q) and completed a familiarization protocol for the hip thrust and isometric midthigh pull. Three days later, subjects performed a 10-minute lower-body dynamic warm-up before undertaking baseline testing. This included the recording of physical characteristics before progressing to measurement of vertical jump, horizontal jump, and sprinting. On the second day, after the 10-minute lower-body dynamic warm-up, the subjects' front squat and hip thrust 3RM were assessed, followed by their isometric midthigh pull.
Familiarization Protocol
Three days before baseline testing, familiarization protocols were completed for the hip thrust and isometric midthigh pull, as the subjects were not familiar with these movements or testing procedures. For the hip thrust, subjects performed sets with 10, 6, and 4 repetitions with 20, 40, and 60 kg, respectively. Isometric midthigh pull familiarization was completed by having subjects perform three 5-second pulls of increasing intensity (50, 70, and 90%) with 30 seconds between each pull; finally, a 5-second isometric midthigh pull was performed at 100% intensity.
Dynamic Warm-up
A 10-minute lower-body dynamic warm-up was performed, consisting of 2 sets of 10 repetitions of the following movements: standing sagittal plane leg swings, standing frontal plane leg swings, body weight squats, and hip thrusts. In this study, all references to a 10-minute lower-body dynamic warm-up refer to this procedure.
Vertical and Horizontal Jumps
Vertical jump height was measured by calculating the difference between standing reach height and maximum jump height from a Vertec (Jump USA, Sunnyvale, CA). Horizontal jump distance was measured by calculating the difference between the starting heel position and the landing heel position of the most rearward landing foot, measured using a tape measure. The vertical and horizontal jumps were performed using a countermovement jump with arm swing; that is, athletes were allowed to flex at the hips, knees, and ankles to a self-selected depth to use the stretch-shortening cycle during triple extension. Subjects were given 3 trials for each test, separated by 3 minutes of rest. The highest and farthest jumps from the 3 trials of each respective jump were analyzed.
Sprinting Performance
After the vertical and horizontal jump testing, subjects were given 10 minutes of rest before performing 20-m sprint testing. Three warm-up 20-m sprint trials at approximately 70, 80, and 90% of maximum sprinting speed were performed before testing. Data were collected using 3 sets of single beam timing lights (SmartSpeed, Fusion Sport, Coopers Plains, Australia), placed at 0 (start), 10-m, and 20-m distances, respectively, wherein 0–10 m and 0–20 m split times from the fastest 20-m trial were used for analysis. All timing lights were set to a height of 60 cm (17
Front Squat and Hip Thrust 3 Repetition Maximum Strength Testing
Subjects first performed a 10-minute lower-body dynamic warm-up. First, 3 progressively heavier specific warm-up sets were performed (∼60, 70 and 80% of predicted 3RM), for the front squat, followed by 2–3 sets of 3RM testing sets. Three repetition maximum was chosen over 1RM because of safety concerns. During the front squat, subjects' feet were slightly wider than shoulder width apart, with toes pointed forward or slightly outward. Subjects descended until the tops of the thigh were parallel with the floor (40 13
Isometric Midthigh Pull
Subjects, still warm from strength testing, performed an isometric midthigh pull while standing on a triaxial force plate (Accupower, AMTI, Watertown, MA) within a squat rack sampled at a frequency of 400 Hz. Each subject held onto an adjustable bar using an alternate grip (power grip) that was locked at a height situated halfway between (midthigh position) each subject's knee (top of the patella) and top of the thigh (inguinal crease). Each subject was permitted to self-select his own joint angles, so long as the bar was situated halfway between his knee and inguinal crease. On the command “go,” the subjects were instructed to pull the fixed bar “hard and fast” and maintain maximal effort for 5 seconds, with the intention of generating maximum vertical ground reaction force. Peak vertical ground reaction force was recorded from 2 trials separated by 3 minutes of rest. The force-time data were filtered using a second-order low-pass Butterworth filter with a cutoff frequency of 16 Hz. The maximum force generated during the 5-second isometric midthigh pull was reported as the peak force. The highest peak force from both trials was used for analysis. Peak force was used as it was the most reliable variable (coefficient of variation [CV] = 3.4%; intraclass correlation coefficient [ICC] = 0.94). Other variables, such as time-to-peak force (ICC = 0.71; CV = 16%) and average rate of force development (ICC = 0.64; CV = 23%), were unreliable, possibly because of the 400 Hz sampling frequency. For rate-dependent variables, 1,000 Hz or higher is recommended (23,33
Training Protocol
Subjects were matched according to total strength and then randomly allocated to one of the 2 training groups (front squat or hip thrust) by a coin flip. Statistical analysis (t -test) was performed to ensure that there were no statistical differences between groups (p ≤ 0.05) in the measured baseline variables (Table 1 ). For lower body, one group performed front squats only while the other group performed hip thrusts only. The repetition scheme used for the front squat and hip thrust is presented in Table 2 . In addition to lower-body training, both groups performed upper-body and core exercises, consisting of 4 sets of incline press or standing military press; 4 sets of bent-over rows, bench pull, or seated rows; and 4 sets of core exercises for the abdominals/lower back. Each week, on 2 separate days spaced at least 72 hours apart, the front squat group performed 4 sets of front squats and the hip thrust group performed 4 sets of hip thrusts in a periodized fashion (Table 2 ). The aforementioned 10-minute dynamic warm-up followed by 3 progressively heavier specific warm-up sets was performed before each session. Three-minute rest periods in between sets were used throughout the duration of the training. During week 1, 60% 3RM loads were used. Loads were increased gradually each week, assuming the subject completed all repetitions with proper form.
Table 2.: Sets, repetition schemes, and loads used for the front squat and hip thrust.*
Training records were kept to analyze loading progressions. During the week after the 6 weeks of training, posttesting was conducted in the same fashion as the pretesting. Subjects were instructed to maintain their current diet and to abstain from performing any additional resistance training.
Statistical Analyses
All data were reduced and entered into Stata (StataCorp, College Station, TX, USA), wherein Shapiro-Wilk tests were performed to ensure normality, where p ≤ 0.05 in a Shapiro-Wilk test is indicative that the data are nonparametric. For normal data, effect sizes (ESs) were calculated using Cohen's d (between group: M 1 and M 2 are the mean changes (M post − M pre ) for each group, and s pooled is the pooled SD of changes from each group; within group: M d is the mean difference from pre-to-post and s d is the SD of differences between subjects), which was defined as small, medium, and large for 0.20, 0.50, and 0.80, respectively (10 d better represents changes due to the intervention, as it uses within-subject differences rather than between-subject differences (5,36,45 p -value of less than or equal to 0.05 in the Shapiro-Wilk test, ES were reported in terms of Pearson's r (z is the z -score from a Wilcoxon signed-rank or rank-sum test, for within- and between-subject comparisons, respectively), which was defined as small, medium, and large for 0.10, 0.30, and 0.50, respectively (10 28 3 27
Results
Of the 29 athletes recruited for this experiment, a total of 24 athletes completed the training protocol, as 3 athletes were removed because of nonadherence and 2 athletes were removed because of injury, not because of the training protocol. Thirteen subjects successfully adhered to the hip thrust protocol and 11 subjects successfully adhered to the squat protocol for all 6 weeks.
Within-Group Outcomes for the Hip Thrust Group
Within the hip thrust group, very likely beneficial effects were observed for 20-m sprint time (Δ = −1.67%; d = 1.14 [0.67–1.61]); peak force during the isometric midthigh pull (Δ = +10.22%; d = 1.01 [0.52–1.51]); and 3RM hip thrust strength (Δ = +42.76%; d = 2.20 [1.71–2.69]). A likely beneficial effect was observed for the normalized peak force during the isometric midthigh pull, which increased by 7.67% (d = 0.77 [0.27–1.27]). Possibly beneficial effects were observed for 3RM front squat strength (Δ = +7.10%; d = 0.64 [0.15–1.13]); vertical jump (Δ = +3.42%; d = 0.43 [−0.07 to 0.93]); horizontal jump (Δ = +2.38%; d = 0.51 [0.02–1.00]); and 10-m sprint times (Δ = −1.05%; d = 0.55 [0.06–1.04]) (Figure 2 and Table 3 ).
Figure 2.: Within-subject effect sizes (Cohen's d ± 90% confidence limit) after 6 weeks of hip thrusting.
Table 3.: Premeasures, postmeasures, differences, and percent changes of all performance measures.
Within-Group Outcomes for the Front Squat Group
Within the front squat group, most likely beneficial effects were observed for 3RM front squat strength (Δ = +12.85%; d = 1.66 [1.10–2.22]) and 3RM hip thrust strength (Δ = +21.06%; d = 1.59 [1.03–2.15]). A very likely beneficial effect was observed for vertical jump height, which increased by 7.30% (d = 1.11 [0.56–1.66]). A likely beneficial effect was observed for horizontal jump (Δ = +1.71%; r = 0.39 [−0.17 to 0.76]). Possibly beneficial effect was observed for peak force (Δ = +1.90%; r = 0.32 [−0.24 to 0.72]) and normalized peak force (Δ = +1.98%; r = 0.27 [−0.30 to 0.69]) during the isometric midthigh pull. Finally, unlikely beneficial effects were observed for 10-m (Δ = +0.10%; d = −0.02 [−0.54 to 0.40]) and 20-m (Δ = −0.66%; d = 0.19 [−0.34 to 0.72]) sprint times (Figure 3 and Table 3 ).
Figure 3.: Within-subject effect sizes (ES ± 90% confidence limit) after 6 weeks of front squatting. Black diamond = Cohen's d , open diamond = Pearson's r .
Between-Group Comparisons
For all between-group comparisons, a positive ES favors the hip thrust. Between the front squat and hip thrust groups, both the vertical jump (d = −0.47 [−1.20 to 0.23]) and front squat 3RM strength squat (d = −0.55 [−1.25 to 0.15]) possibly favored the front squat. It is unlikely that one intervention was better than the other for improving horizontal jump (d = 0.15 [−0.57 to 0.87]). Changes in both 10-m (d = 0.32 [−0.39 to 1.03]) and 20-m (d = 0.39 [−0.31 to 1.09]) sprint times possibly favored the hip thrust. Changes in normalized peak force during the isometric midthigh pull strength were likely superior in the hip thrust (r = 0.28 [−0.07 to 0.57]). Finally, very likely benefits to the hip thrust were observed in both hip thrust strength (d = 1.35 [0.65–2.05]) and peak force during the isometric midthigh pull (r = 0.46 [0.14–0.69]) (Figure 4 and Table 3 ).
Figure 4.: Magnitude-based effect sizes (ES ± 90% confidence limit) of performance measures. Black diamond = Cohen's d , open diamond = Pearson's r .
Discussion
The purpose of this study was to examine and compare the effects of a 6-week squat or hip thrust program on performance measures in male adolescent athletes. Hip thrust within-group analyses revealed possibly to most likely beneficial effects for all outcomes. The large ES noted for hip thrust strength changes (d = 2.20) is in line with the principle of specificity. Clearly beneficial effects for the hip thrust group to improve front squat strength were noted (d = 0.64). Because the hip thrust has been shown to elicit similar quadriceps electromyographic (EMG) amplitude as compared with, and greater hip extensor EMG amplitude than, the squat, these results are intuitive (14 d = 0.55) and 20-m (d = 1.14) sprint times are in line with the force vector hypothesis, as the hip thrust likely develops an anteroposterior force vector, and sprint performance is highly correlated with horizontal force output, which is directed anteroposteriorly (35 d = 1.02; normalized d = 0.77) were observed as hypothesized. These effects are likely due to the position-specific adaptations of end-range hip extension, which is required during the isometric midthigh pull, in addition to the high EMG amplitudes of the hip and knee extensors during the hip thrust (14 d = 0.43) and horizontal (d = 0.51) jump measures were observed, but with small-to-medium ES. These outcomes are likely due to the ability of the hip thrust to place mechanical demands on the hip and knee extensors (14 50
Numerous within-group effects were observed in the front squat group. As per our hypotheses, increases in both front squat (d = 1.66) and hip thrust (d = 1.59) 3RM were observed. These increases are likely due to the front squat's hip and knee extension moment requisites (22 15 14 r = 0.39) and vertical (d = 1.11) jumps, respectively. The axial force vector of the front squat may have helped subjects develop larger vertical force during jumping, thus increasing vertical impulse, which is directed axially and is a key factor for both horizontal (50 1,49 r = 0.32) and normalized peak force (r = 0.27) during the isometric midthigh pull, respectively, were also observed. Again, these adaptations may be due to the vertical force vectors of both the front squat and isometric midthigh pulls. It is surprising, however, that the front squat only elicited unclear or trivial effects in 10-m (d = −0.02) and 20-m (d = 0.19) sprint performance, as previous research has shown the squat to be an effective intervention for increasing speed (43
The primary purpose of this investigation was to compare the 2 interventions, the front squat and barbell hip thrust, on the aforementioned performance outcomes. Possibly beneficial effects for the hip thrust were noted for 10-m (d = 0.32) and 20-m (d = 0.39) sprint times, which provides further support for the force vector theory. The hip thrust was also very likely beneficial in increasing hip thrust 3RM strength (d = 1.35) and peak force during the isometric midthigh pull (r = 0.46), whereas likely beneficial effects were observed for normalized peak force during the midthigh pull (r = 0.28). Although the former was to be expected, as per the principle of specificity, the latter result was unexpected, as the isometric midthigh pull uses a vertical external force vector. This may have to do with the hip extension moment requisites of the isometric midthigh pull, which the hip thrust may be more effective in improving. As per our hypotheses, the front squat was possibly beneficial for improving vertical jump (d = −0.47) and front squat 3RM strength (d = −0.55) over the hip thrust, which also supports the force vector theory. Finally, as per our hypothesis, no clear effect was observed for horizontal jump performance (d = 0.15). This may be because both horizontal and vertical components are important for the horizontal jump (50
To the authors' knowledge, only one other study has demonstrated transfer from one resisted hip extension exercise to another. Speirs et al. (46
In both groups, absolute hip thrust 3RM strength and changes in hip thrust 3RM were much greater than absolute front squat 3RM strength and changes in front squat 3RM. The front squat group increased their hip thrust 3RM by 23.5 ± 14.7 kg (111 ± 20.9 to 134 ± 11.2 kg), whereas their front squat 3RM increased by 9.64 ± 5.80 kg (75.0 ± 10.4 to 84.6 ± 10.0 kg). The differences in the hip thrust group were even more pronounced, in that their front squat 3RM increased by 5.50 ± 8.53 kg (77.6 ± 12.3 to 83.1 ± 13.7 kg), whereas their hip thrust 3RM increased by 49.5 ± 22.4 kg (115 ± 23.5 to 165 ± 33.0 kg). These differences are likely due to the nature of the hip thrust exercise, in that there is more stability and decreased coordination requirements. However, a full kinetic analysis of the hip thrust is needed for further insight.
The front squat's ability to increase vertical jump height is quite intuitive, as both the front squat and vertical jump use the same external force vector direction (vertical). In addition, the substantial utilization of the quadriceps in both the front squat and vertical jump (22,31,51 6 32,50
It is surprising that, although squats have been shown to improve sprint performance (43 30 7,8,35 41 14
Hip thrust training resulted in greater improvements in the isometric midthigh pull peak force compared with squat training, even though the pull involved a vertical force vector. It is proposed that this is due to the hip extension moment-angle curves of the squat vs. that of the hip thrust, in that the hip thrust likely has a greater hip extension moment requisite at the angle at which the isometric midthigh pull is performed, but these joint-specific kinetic hypotheses require further investigation.
There are a number of limitations that must be borne in mind when interpreting the results from this study. Adolescent males have changing hormone levels and a large number of life stressors (2,44 4,6
Future research should duplicate these methods in other populations, such as females, adults, and athletes from various sports. In addition, these findings cannot necessarily be extrapolated to those without squatting experience and with hip thrusting experience, as novelty may bias the hip thrust. Furthermore, finding a proper protocol to maximize transference is imperative, as, for example, light, explosive hip thrusts may be better for improving power production, but heavy hip thrusts may be better for improving the contribution of the hip joint to horizontal force production. The dichotomization of exercise selection in this study must be eliminated from future research, as combining exercises tends to elicit greater adaptations than one exercise (20
Practical Applications
In line with previous literature, specificity is critical for improving the strength in a lift. This indicates that athletes that participate in sports like basketball and volleyball, which are predicated on vertical jump, may benefit more from the front squat rather than the hip thrust. However, in sports such as rugby and American football, it may be more beneficial for athletes to perform the hip thrust, because of its carryover to acceleration. Because the hip thrust does seem to increase front squat performance, it is possible that the hip thrust may be a viable option to perform during times of injury to maintain or increase front squat strength. The direction of the resistance force vector relative to the body appears to play a role in transference, in that axially resisted movements (front squat) appear to better transfer to vertical-based activities (vertical jump), and anteroposterior-resisted movements (hip thrust) appear to better transfer to horizontal-based activities (20-m sprint). The carryover of the hip thrust to peak isometric midthigh pull force is indicative that the hip thrust may have carryover to deadlift lockout, even though the positions are slightly different. Finally, it is likely best to perform a combination of movements rather than just one; it is recommended that athletes incorporate both the squat and hip thrust for complementary improvements in performance. Future studies are needed in adults and female populations, as these findings cannot be extrapolated.
Acknowledgments
The lead author, BC, would like to disclose a potential conflict of interest. He is the patentee and inventor of The Hip Thruster (US Patent Number US8172736B2), which is an apparatus designed to allow for comfortable performance of the hip thrust variations.
References
1. Adamson G, Whitney R. Critical Appraisal of Jumping as a Measure of Human Power. Basel, Switzerland: Karger Publishers, 1971.
2. Arnett JJ. Adolescent storm and stress, reconsidered. Am Psychol 54: 317–326, 1999.
3. Batterham AM, Hopkins WG. Making meaningful inferences about magnitudes. Sportscience 9: 6–14, 2005.
4. Beardsley C, Contreras B. The increasing role of the hip extensor musculature with heavier compound lower-body movements and more explosive sport actions. Strength Cond J 36: 49–55, 2014.
5. Becker BJ. Synthesizing standardized mean-change measures. Br J Math Stat Psychol 41: 257–278, 1988.
6. Bloomquist K, Langberg H, Karlsen S, Madsgaard S, Boesen M, Raastad T. Effect of range of motion in heavy load squatting on muscle and tendon adaptations. Eur J Appl Physiol 113: 2133–2142, 2013.
7. Brughelli M, Cronin J, Chaouachi A. Effects of running velocity on running kinetics and kinematics. J Strength Cond Res 25: 933–939, 2011.
8. Buchheit M, Samozino P, Glynn JA, Michael BS, Al Haddad H, Mendez-Villanueva A, Morin JB. Mechanical determinants of acceleration and maximal sprinting speed in highly trained young soccer players. J Sports Sci 32: 1906–1913, 2014.
9. Channell BT, Barfield JP. Effect of Olympic and traditional resistance training on vertical jump improvement in high school boys. J Strength Cond Res 22: 1522–1527, 2008.
10. Cohen J. Statistical Power Analysis for the Behavioral Sciences. New York, NY: Routledge Academic, 1988.
11. Comfort P, Bullock N, Pearson SJ. A comparison of maximal squat strength and 5-, 10-, and 20-meter sprint times, in athletes and recreationally trained men. J Strength Cond Res 26: 937–940, 2012.
12. Comfort P, Jones PA, McMahon JJ, Newton R. Effect of knee and trunk angle on kinetic variables during the isometric midthigh pull: Test-retest reliability. Int J Sports Physiol Perform 10: 58–63, 2015.
13. Contreras B, Cronin J, Schoenfeld B. Barbell hip thrust. Strength Cond J 33: 58–61, 2011.
14. Contreras B, Vigotsky AD, Schoenfeld BJ, Beardsley C, Cronin JA. Comparison of gluteus maximus, biceps femoris, and vastus lateralis EMG activity in the back squat and barbell hip thrust exercises. J Appl Biomech 31: 452–458, 2015.
15. Contreras B, Vigotsky AD, Schoenfeld BJ, Beardsley C, Cronin JA. Comparison of gluteus maximus, Biceps Femoris, and Vastus Lateralis EMG amplitude in the parallel, full, and front squat variations in resistance trained females. J Appl Biomech 32: 16–22, 2016.
16. Contreras BM, Cronin JB, Schoenfeld BJ, Nates RJ, Sonmez GT. Are all hip extension exercises Created equal? Strength Cond J 35: 17–22, 2013.
17. Cronin JB, Templeton RL. Timing light height affects sprint times. J Strength Cond Res 22: 318–320, 2008.
18. de Lacey J, Brughelli ME, McGuigan MR, Hansen KT. Strength, speed and power characteristics of elite rugby league players. J Strength Cond Res 28: 2372–2375, 2014.
19. Eckert RM, Snarr RL. Barbell hip thrust. J Sport Hum Perform 2: 1–9, 2014.
20. Fonseca RM, Roschel H, Tricoli V, de Souza EO, Wilson JM, Laurentino GC, Aihara AY, de Souza Leao AR, Ugrinowitsch C. Changes in exercises are more effective than in loading schemes to improve muscle strength. J Strength Cond Res 28: 3085–3092, 2014.
21. Gullett JC, Tillman MD, Gutierrez GM, Chow JW. A biomechanical comparison of back and front squats in healthy trained individuals. J Strength Cond Res 23: 284–292, 2008.
22. Gullett JC, Tillman MD, Gutierrez GM, Chow JW. A biomechanical comparison of back and front squats in healthy trained individuals. J Strength Cond Res 23: 284–292, 2009.
23. Haff GG, Ruben RP, Lider J, Twine C, Cormie P. A comparison of methods for determining the rate of force development during isometric midthigh clean pulls. J Strength Cond Res 29: 386–395, 2015.
24. Hartmann H, Wirth K, Klusemann M, Dalic J, Matuschek C, Schmidtbleicher D. Influence of squatting depth on jumping performance. J Strength Cond Res 26: 3243–3261, 2012.
25. Hoffman JR, Cooper J, Wendell M, Kang J. Comparison of Olympic vs. traditional power lifting training programs in football players. J Strength Cond Res 18: 129–135, 2004.
26. Hoffman JR, Ratamess NA, Kang J. Performance changes during a college playing career in NCAA division III football athletes. J Strength Cond Res 25: 2351–2357, 2011.
27. Hopkins WG. A spreadsheet for deriving a confidence interval, mechanistic inference and clinical inference from a P value. Sportscience 11: 16–21, 2007.
28. Hopkins WG, Marshall SW, Batterham AM, Hanin J. Progressive statistics for studies in sports medicine and exercise science. Med Sci Sports Exerc 41: 3–13, 2009.
29. Jacobson BH, Conchola EG, Glass RG, Thompson BJ. Longitudinal morphological and performance profiles for American, NCAA Division I football players. J Strength Cond Res 27: 2347–2354, 2013.
30. Lockie RG, Murphy AJ, Schultz AB, Knight TJ, Janse de Jonge XA. The effects of different speed training protocols on sprint acceleration kinematics and muscle strength and power in field sport athletes. J Strength Cond Res 26: 1539–1550, 2012.
31. Mackala K, Stodolka J, Siemienski A, Coh M. Biomechanical analysis of squat jump and countermovement jump from varying starting positions. J Strength Cond Res 27: 2650–2661, 2013.
32. Mackala K, Stodolka J, Siemienski A, Coh M. Biomechanical analysis of standing long jump from varying starting positions. J Strength Cond Res 27: 2674–2684, 2013.
33. McMaster DT, Gill N, Cronin J, McGuigan M. A brief review of strength and ballistic assessment methodologies in sport. Sports Med 44: 603–623, 2014.
34. Mendiguchia J, Martinez-Ruiz E, Morin JB, Samozino P, Edouard P, Alcaraz PE, Esparza-Ros F, Mendez-Villanueva A. Effects of hamstring-emphasized neuromuscular training on strength and sprinting mechanics in football players. Scand J Med Sci Sports 25: e621–629, 2015.
35. Morin JB, Edouard P, Samozino P. Technical ability of force application as a determinant factor of sprint performance. Med Sci Sports Exerc 43: 1680–1688, 2011.
36. Morris SB. Estimating effect sizes from Pretest-Posttest-control group designs. Organizational Res Methods 11: 364–386, 2008.
37. Morrissey MC, Harman EA, Johnson MJ. Resistance training modes: Specificity and effectiveness. Med Sci Sports Exerc 27: 648–660, 1995.
38. Nagano A, Komura T, Fukashiro S. Optimal coordination of maximal-effort horizontal and vertical jump motions–a computer simulation study. Biomed Eng Online 6: 20, 2007.
39. Otto WH III, Coburn JW, Brown LE, Spiering BA. Effects of weightlifting vs. kettlebell training on vertical jump, strength, and body composition. J Strength Cond Res 26: 1199–1202, 2012.
40. Pierce K. Basic back squat. Strength Cond J 19: 20–21, 1997.
41. Randell AD, Cronin JB, Keogh JW, Gill ND. Transference of strength and power adaptation to sports performance—horizontal and vertical force production. Strength Cond J 32: 100–106, 2010.
42. Requena B, Garcia I, Requena F, de Villarreal ES, Cronin JB. Relationship between traditional and ballistic squat exercise with vertical jumping and maximal sprinting. J Strength Cond Res 25: 2193–2204, 2011.
43. Seitz LB, Reyes A, Tran TT, Saez de Villarreal E, Haff GG. Increases in lower-body strength transfer positively to sprint performance: A systematic review with meta-analysis. Sports Med 44: 1693–1702, 2014.
44. Sizonenko PC. Endocrinology in preadolescents and adolescents. I. Hormonal changes during normal puberty. Am J Dis Child 132: 704–712, 1978.
45. Smith LJW, Beretvas SN. Estimation of the standardized mean difference for Repeated measures designs. J Mod Appl Stat Methods 8: 27, 2009.
46. Speirs DE, Bennett M, Finn CV, Turner AP. Unilateral vs bilateral squat training for strength, sprints and Agility in Academy rugby players. J Strength Cond Res 30: 386–392, 2016.
47. Tricoli V, Lamas L, Carnevale R, Ugrinowitsch C. Short-term effects on lower-body functional power development: Weightlifting vs. vertical jump training programs. J Strength Cond Res 19: 433–437, 2005.
48. Wilson GJ, Murphy AJ, Walshe A. The specificity of strength training: The effect of posture. Eur J Appl Physiol Occup Physiol 73: 346–352, 1996.
49. Winter EM. Jumping: Power or impulse? Med Sci Sports Exerc 523, 2005.
50. Wu WL, Wu JH, Lin HT, Wang G-J. Biomechanical analysis standing long jump. Biomed Eng Appl Basis Commun 15: 186–192, 2003.
51. Yavuz HU, Erdag D, Amca AM, Aritan S. Kinematic and EMG activities during front and back squat variations in maximum loads. J Sports Sci 33: 1058–1066, 2015.
