an investigation into the acute effects of depth jumps

AN INVESTIGATION INTO THE ACUTE EFFECTS DEPTH JUMPS ON MAXIMAL STRENGTH PERFORMANCE NATHAN BULLOCK1

AND

OF

PAUL COMFORT1,2

1

Manchester City Football Club, Platt Lane, Greater Manchester, United Kingdom; and 2The Directorate of Sport, Exercise and Physiotherapy, University of Salford, United Kingdom

ABSTRACT

INTRODUCTION

Bullock, N and Comfort, P. An investigation into the acute effects of depth jumps on maximal strength performance. J Strength Cond Res 25(11): 3137–3141, 2011—Research has demonstrated that high-load low-velocity (HLLV) exercises ($85% 1 repetition maximum [1RM]) increase performance in subsequent low-load high-velocity (LLHV) exercises, when separated by a rest period $4 minutes. To date, few studies have investigated LLHV exercises on subsequent HLLV exercises. The purpose of this study was to compare the effects of 2, 4, or 6 depth jumps (DJs) on subsequent 1RM back squat performance. Fourteen subjects (age 22 6 4 years, height 177 6 10 cm, body mass 80.3 6 14.4 kg) completed five 1RM back squat testing sessions, either control, retest, or 1 of 3 interventions (2, 4, or 6 DJs from a height of 33 cm, 4 minutes before the first 1RM attempt), in a counterbalanced order. Intraclass correlation coefficients demonstrated a high test–retest reliability for the 1RMs (r = 0.989, p , 0.001). Repeated-measures analysis of variance with Bonferroni post hoc analysis revealed significantly greater 1RM performance (140.71 6 35.68 kg: p = 0.004, 140.50 6 33.77 kg: p , 0.001, 141.43 6 34.39 kg: p = 0.002, respectively) for each intervention (2, 4, or 6 repetitions, respectively) compared to the control condition (132.43 6 34.56 kg). No significant differences were found between interventions (p . 0.05). The findings of this investigation demonstrate that the inclusion of 2, 4, or 6 DJs, 4 minutes before a maximal squat, enhances subsequent strength performance.

A

KEY WORDS potentiation, 1 repetition maximum, plyometrics

Address correspondence to Paul Comfort, [email protected]. 25(11)/3137–3141 Journal of Strength and Conditioning Research Ó 2011 National Strength and Conditioning Association

number of mechanisms explaining postactivation potentiation (PAP) have been proposed, including increases in the phosphorylation of myosin regulatory light chains (26), increased Ca2+ sensitivity of the myofilaments (12), and by use of the H-Reflex enhancement which increases neurotransmission (30). Postactivation potentiation has been shown to increase performance by acutely optimizing muscle force and power production more than using traditional training techniques (22,24). This contractile movement at the cellular level creates PAP and fatigue. It is the balance between PAP and fatigue that influences the consequential contractile response as to whether performance increases, decreases, or remains the same (13). Originally, PAP was investigated as the effect of high-load low-velocity (HLLV) activity on subsequent low-load highvelocity (LLHV), with much research demonstrating that an HLLV ($85% 1RM) PAP stimuli, with $4 minutes of rest, to ensure the dissipation of fatigue, increases performance in the subsequent LLHV activity (4,6,15,17,18,25,28). Rahimi (18) found that 2 sets of 4 repetitions at 60, 70, or 85% of 1RM for a back squat showed improvements in sprint times of 1.9, 1.77, and 2.98%, respectively, in well-trained athletes (squat .2 times body mass [BM]). Duthie et al. (4) compared 3 sets of 3 repetitions of traditional (jump squats, 30% 1RM, then half squats, 95% 1RM), complex (half squats, 95% 1RM, then jump squats, 30% 1RM) and contrast (alternating sets of half squats, 95% 1RM, and jump squats, 30% 1RM) training methods to see which method would be preferable for an acute increase in performance. Duthie et al. (4) analyzed and categorized the data into stronger and weaker groups and found that stronger athletes elicited an increase in performance and therefore suggested that a PAP response is elicited in well-trained but not in recreationally trained individuals. This increase, only in elite athletes, may be because of a higher amount of type 2 fibers; subjects with higher type 2 fast twitch muscle fibers have increased subsequent performance (4,7,21,27). Plyometrics, such as depth jumps (DJs), used as a preload exercise have also been shown to increase subsequent exercise VOLUME 25 | NUMBER 11 | NOVEMBER 2011 |

3137

Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.

Acute Effects of Depth Jumps

TABLE 1. Order of testing. Session 1 Group A Group B Group C

Control Control Control

Session 2 (repetitions)



2 4 6

4 6 2

6 2 4

performance (8,16,21,29). Hilfiker et al. (8) and Terzis et al. (27) used 5 DJs to induce PAP preceding an LLHV exercise and found a 6% increase in jump performance and a 4.6% increase in shotput distance, respectively. In contrast, Massamoto et al. (14) and Rahimi (19) studied the effect of DJ on subsequent HLLV performance and found significant increases in 1RM back squat performances of 4.5 and 2.74%, respectively. Massamoto et al. (14) commented that the low volume of DJ used in their study may have limited the PAP effect and that a greater volume may result in a greater increase in performance. The purpose of this study was to determine if 2, 4, or 6 DJs can acutely increase performance, for a 1RM back squat and to determine the differential effects of 2, 4, or 6 DJs on subsequent 1RM back squat performances. It was hypothesized that 6 DJs would result in the greatest increase in 1RM back squat performance, because this higher volume may elicit a greater neurological response.

METHODS Experimental Approach to the Problem

The aim of this research was to investigate the acute effects of DJ on subsequent 1RM back squat performance and to determine if the volume of DJ has a differential effect on subsequent 1RM back squat performance. A repeatedmeasures design, to identify a within-subjects effect, was used (Table 1). Subjects performed 1RM protocols on 5 different occasions, at least 3 days apart, using a standardized warm-up procedure (1,14), with the only addition during the interventions being the inclusion of 2, 4, or 6 DJs (Figure 1).

Session 5 Control Control Control

ensure performances between subjects were standardized as much as possible. Subjects were asked not to perform any physical activity for 24 hours before testing, because a mean reduction of 3.1% in performance has been found in subjects who had trained in the 24 hours before testing (6). Subjects were also asked to consume the same diet on each day of testing and avoid alcohol consumption for the 24 hours before testing. Data collection was conducted under the supervision of a certified strength and conditioning coach. Test conditions were block randomized and counterbalanced to reduce a training effect (Table 1). Subjects performed 1RM protocols on 5 different occasions, at least 3 days apart, to ensure that fatigue did not affect the testing (30). The 1RM testing was used for its high reliability (r . 0.92), as previously reported by Jackson et al. (10). One Repetition Maximum Testing

Before the 1RM attempts, a warm-up protocol was performed by all subjects (Figure 1). The warm-up included 4 sets of squats, for 10, 8, 4, and 2 repetitions, at progressively heavier loads, including standardized dynamic stretches, as previously adopted by Massamoto et al. (16). Four minutes of rest was provided before each 1RM attempt or DJ, to allow for full recovery of phosphagen energy stores (6,17,20,30).

Subjects

Fourteen trained ($2 years experience; $3 times a week) male (age 22 6 4 years; height 177 6 10 cm, body mass 80.3 6 14.4 kg) collegiate level athletes volunteered for this study. Subjects had regularly performed back squats and plyometric training, had no musculoskeletal injuries or illness in the previous 6 months, and all subjects provided written informed consent before participation. This study was approved by the University of Salford institutional review board. Procedures

Before performing the tests, subjects attended a minimum of 3 familiarization sessions with the investigators (4), which included instruction of both backs squat and DJ techniques to

3138

the

Figure 1. Flow diagram of order of sessions.

TM

Journal of Strength and Conditioning Research


the

TM


| www.nsca-jscr.org

A standardized 1RM protocol (1) was followed to assess 1RM back squat strength. The squat depth was also standardized to a depth of 90° knee flexion. This was measured using a goniometer during the warm-up and a band placed behind the subjects, which provided feedback to subjects as to when to finish the descent phase and to begin the ascent phase (Figure 2). Verbal encouragement was given by test administrators. All 1RM tests were conducted by the same administrators using the same equipment (Multirack, Power Lift, Jefferson, IA, USA). Depth Jumps

The DJ was performed by stepping off a 33-cm box (Technique boxes, Power Lift), landing on the floor with both their feet and then immediately jumping as high as possible, as previously described by Massamoto et al. (14). To prevent arm swing from being a confounding variable, subjects were instructed to place both their hands on their iliac crest throughout the DJ (21). Statistical Analyses

Intraclass correlations coefficients were performed to determine reliability between the 2 control 1RMs. Intraclass correlations coefficients for 1RM test and 1RM retest demonstrated a high test–retest reliability (r = 0.989; p , 0.001); therefore, during further analysis, the 3 interventions (2, 4, and 6 repetitions of DJs) were analyzed compared only to the initial 1RM values. Repeated-measures analysis of variance (ANOVA), with Bonferroni post hoc analysis was performed to determine if there were any significant differences in maximal squat strength between the control (initial 1RM performance) and after each intervention (2, 4, or 6 DJs). All data were analyzed using SPSS 16.0 (SPSS, Inc., Chicago, IL, USA). An a priori level of significance was set at an alpha level of p # 0.05.

Figure 2. Subject squatting to 90° knee flexion.

p = 0.001; respectively) compared to the initial 1RM results achieved during the control condition (132.43 6 34.56 kg). No significant difference was found between interventions (p . 0.05), and all statistical power values were .0.92. There was, however, a trend observed that the stronger athletes (squat $ 2 times BM) demonstrated a greater increase in 1RM performance after 6 repetitions of DJ.

RESULTS Intraclass correlation coefficients demonstrated a high test–retest reliability for the control 1RMs (r = 0.989, p , 0.001). Repeated-measures ANOVA revealed that there were significant differences between conditions (p , 0.05). Further post hoc analysis (Bonferroni) revealed that each of the interventions (2, 4, and 6 DJs) resulted in significantly greater (Figure 3) 1RM performance (140.71 6 35.68 kg, p = 0.004; 140.50 6 33.77 kg, p , 0.001; 141.43 6 34.39 kg,

Figure 3. Comparison of 1 repetition maximum performances.

VOLUME 25 | NUMBER 11 | NOVEMBER 2011 |

3139


Acute Effects of Depth Jumps DISCUSSION The results from this study demonstrate that 2, 4, or 6 DJs performed 4 minutes before a 1RM back squat significantly (p , 0.001) increases the subsequent 1RM by 6.26, 6.09, and 6.80%, respectively, which is in line with previous findings (2.74–4.6%) (14,19). However, there was no significant difference (p . 0.05) between the number of repetitions of DJ, although there was a trend for the stronger subjects (squat .2BM) to show greater increases after performing 6 DJs. The slight discrepancies between this study and prior research are likely to be because of the slightly differing methods. Hilfiker et al. (8) used modified DJs, from a 43.2-cm box, which only included the landing component and not the subsequent concentric rebound. Massamoto et al. (14) used a 60-cm box with a 30-second rest period between the potentiating exercise and subsequent activity, and Rahimi (19) compared 15-, 30-, and 60-second rest. No research, however, has compared to see if an optimal rest period lies between the rest periods investigated (15–240 seconds) for 2, 4, or 6 repetitions of DJs. Higher box heights used by Massamoto et al. (14) and Hilfiker et al. (8) may have affected muscle recruitment and forces that are accepted during the eccentric phase. The higher the box, the greater the number of muscle fibers recruited (5) because of higher maximal forces on landing (29); therefore, the ability to jump high while maintaining a short ground contact time may be more fatiguing using higher boxes compared to lower boxes. The DJ may create PAP and fatigue; whether performance increases or decreases is because of a combination of 2 main factors, the rest period and volume of DJ relative to the subject’s strength (11,12). Because PAP stimulating exercises primarily use the phosphagen energy system, it has been suggested that full resynthesis of the phosphagen stores is warranted post DJ and before1RM attempt (7,25). However, if the rest period is too long, there may be no positive effect in performance associated with PAP; the precise timeframe of PAP induced via DJ is currently not known. Despite no significant difference being found between the 2 4, and 6 repetitions, there was a trend that stronger subjects, with a 1RM squat greater than twice body mass, responded better to a higher volume (6 repetitions) of DJs, because this may elicit a greater PAP response. This is in line with the findings of other studies that have found that stronger athletes demonstrate a greater increase in subsequent performance, which has been attributed to a greater number of type 2 muscle fibers (4,6,7,15,17,21,27). Specifically, Ruben et al. (23) found that athletes who were able to squat .2BM increased subsequent exercise significantly (plyometric bounds over hurdles) and therefore may elicit PAP the most. Ruben et al. (23) also noted that those who squatted ,2 BM but .1.5 BM also increased performance in subsequent exercise (plyometric bounds over hurdles) and therefore may also elicit a PAP response, similarly to the findings of this study.

3140

the

Because of the nature of a 1RM test, subjects did not always attain their 1RM on their first attempt; this meant that for those subjects who succeeded and managed to lift more than their 1RM, they may have lost some of the potentiating effect because of the timeframe, because they performed the DJ. If an attempt was made at a greater weight earlier, then the 1RM may have been higher because of lower levels of accumulative fatigue. Current literature shows that the positive effects of PAP cease within 12 minutes (2,3) but can be seen to deteriorate after 5 minutes in power athletes (3,16). There were a large number of subjects who failed on the fourth or fifth attempt, by these attempts it was between 12 and 16 minutes after the DJ, so the PAP effect may have been lost by this stage. For future research, the methodology as used by Massamoto et al. (14) could be applied so that before each lift DJs were performed, this may ensure that the performanceenhancing effects from the DJ occur for each 1RM attempt; however, this method may also induce more fatigue, because of the accumulative number of exercises. It is recommended that future research investigate the optimal time period between LLHV exercise and the subsequent bout of HLLV exercise because time periods in the current literature range from 20 seconds to 5 minutes (4,7,8,21). Furthermore, research should also aim to identify the exact mechanism by which LLHV exercises (such as DJs) enhance subsequent HLLV activity, along with determining the chronic effects of incorporating DJ in the interset rest period during a maximal strength phase.

PRACTICAL APPLICATIONS From the results of this study, it appears that the inclusion of DJs (2, 4, or 6 repetitions) 4 minutes before a maximal strength activity significantly enhances performance in the strength-based activity. It is therefore recommended that during a maximal strength phase of a periodized program, DJ may be effectively incorporated into the interset rest period to elicit greater force production during the subsequent set. It is also recommended that when performing maximal strength tests, athletes should be prevented from performing plyometric activities between attempts to prevent inflation of their performance.

REFERENCES 1. Baechle, TR, Earle, RW, and Wathen, D. Resistance training. In: Essentials of Strength Training and Conditioning. Baechle, TR, and Earle, RW, eds. Champaign, IL: Human Kinetics, 2008. pp. 381–412. 2. Batista, MAB, Ugrinowitsch, C, Roschel, H, Lotufo, R, Ricard, MD, and Tricoli, VAA. Intermittent exercise as a conditioning activity to induce postactivation potentiation. J Strength Cond Res 21: 837–840, 2007. 3. Chiu, LZF, Fry, AC, Weiss, LW, Schilling, BK, Brown, LE, and Smith, SL. Postactivation potentiation response in athletic and recreationally trained individuals. J Strength Cond Res 17: 671–677, 2003. 4. Duthie, GM, Young, WB, and Aitken, DA. The acute effects of heavy loads on jump squat performance: An evaluation of the complex and contrast methods of power development. J Strength Cond Res 16: 530–538, 2002.

TM



the

TM

Journal of Strength and Conditioning Research 5. Ebben, WP, Fauth, ML, Kaufman, CE, and Petushek, EJ. Magnitude and rate of mechanical loading of a variety of exercise modes. J Strength Cond Res 24: 213–217, 2010. 6. Gullich, A and Schmidtbleicher, D. MVC-induced short-term potentiation of explosive force. New Stud Athl 4: 67–81, 1996. 7. Hamada, T, Sale, DG, MacDougall, JD, and Tarnopolsky, MA. Postactivation potentiation, fiber type, and twitch contraction time in human knee extensor muscles. J Appl Physiol 88: 2131–2137, 2000. 8. Hilfiker, R, Hubner, K, Lorenz, T, and Marti, B. Effects of drop jumps added to the warm-up of elite sport athletes with a high capacity for explosive force development. J Strength Cond Res 21: 550–555, 2007.

| www.nsca-jscr.org

19. Rahimi, R. The influence of varied rest intervals after plyometric exercise on maximum squat performance. Int J Fitness 4: 45–50, 2008. 20. Read, MM and Cisar, C. The influence of varied rest interval lengths on depth jump performance. J Strength Cond Res 15: 279–283, 2001. 21. Rixon, KP, Lamont, HS, and Bemben, MG. Influence of type of muscle contraction, gender, and lifting experience on postactivation potentiation performance. J Strength Cond Res 21: 500–505, 2007. 22. Robbins, DW. Postactivation potentiation and its practical applicability: A brief review. J Strength Cond Res 19: 453–458, 2005.

9. Hultman, E, Bergstrom, J, and McLennan-Anderson, N. Breakdown and resynthesis of phosphorylcreatine and adenosine triphosphate in connection with muscular work in man. Scand J Clin Lab Invest 19: 56–66, 1967.

23. Ruben, RM, Molinari, MA, Bibbee, CA, Childress, MA, Harman, MS, Reed, KP, and Haff, GG. The acute effects of an ascending squat protocol on performance during horizontal plyometric jumps. J Strength Cond Res 24: 358–369, 2010.

10. Jackson, AW, Watkins, M, and Patton, R. A factor analysis of twelve selected maximal isotonic strength performances on the Universal Gym. Med Sci Sports Exerc 12: 274–277, 1980.

24. Stone, MH, Sands, WA, Pierce, KC, Ramsey, MW, and Haff, GG. Power and power potentiation among strength-power athletes: Preliminary study. Int J Sports Physiol Perf 3: 55–67, 2008.

11. Jo, E, Judelson, DA, Brown, LE, Coburn, JW, and Dabbs, NC. Influence of recovery duration after a potentiating stimulus on muscular power in recreationally trained individuals. J Strength Cond Res 24: 343–347, 2010.

25. Sweeney, HL, Bowman, BF, and Stull, JT. Myosin light chain phosphorylation in vertebrate striated muscle. Regulation and function. Am J Physiol 264: 1085–1095, 1993.

12. MacIntosh, BR. Role of calcium sensitivity modulation in skeletal muscle performance. News Physiol Sci 18: 222–225, 2003. 13. MacIntosh, BR and Rassier, DE. What is fatigue? Can J Appl Physiol 27: 42–55, 2002. 14. Massamoto, N, Larson, R, Gates, T, and Faigenbaum, A. Acute effects of plyometric exercise on maximum squat performance in male athletes. J Strength Cond Res 17: 68–71, 2003. 15. McBride, JM, Nimphius, S, and Erickson, TM. The acute effects of heavy-load squats and loaded countermovement jumps on sprint performance. J Strength Cond Res 19: 893–897, 2005. 16. Morana, C and Perry, S. Time course of postactivation potentiation during intermittent submaximal fatiguing contractions in enduranceand power-trained athletes. J Strength Cond Res 23: 1456–1464, 2009. 17. Radcliffe, JC and Radcliffe, JL. Effects of different warm-up protocols on peak power output during a single response jump task. Med Sci Sports Exerc 38: 189, 1996. 18. Rahimi, R. The acute effects of heavy versus light-load squats on sprint performance. Phys Educ Sport 5: 163–169, 2007.

26. Szczesna, D, Zhao, J, Jones, M, Zhi, G, Stull, J, and Potter, J. Phosphorylation of the regulatory light chains of myosin affects Ca2+ sensitivity of skeletal muscle contraction. J Appl Physiol 92: 1661–1670, 2002. 27. Terzis, G, Spengos, K, Karampatsos, G, Manta, and Georgiadis, G. Acute effect of drop jumping on throwing performance. J Strength Cond Res 23: 2593–2597, 2009. 28. Tillin, NA and Bishop, D. Factors modulating post-activation potentiation and its effect on performance of subsequent explosive activities. Sports Med 39: 147–166, 2009. 29. Walsh, M, Arampatzis, A, Schade, F, and Bruggemann, GP. The effect of drop jump starting height and contact time on power, work performed, and moment of force. J Strength Cond Res 18: 561–566, 2004. 30. Yetter, M and Moir, GL. The acute effects of heavy back and front squats on speed during forty-meter sprint trials. J Strength Cond Res 22: 159–165, 2008. 31. Young, WB, Jenner, A, and Griffiths, K. Acute enhancement of power performance from heavy load squats. J Strength Cond Res 12: 82–84, 1998.

VOLUME 25 | NUMBER 11 | NOVEMBER 2011 |

3141
