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Original Research

The Importance of Maximal Leg Strength for Female Athletes When Performing Drop Jumps

Barr, Matthew J.1; Nolte, Volker W.2

Author Information
Journal of Strength and Conditioning Research: February 2014 - Volume 28 - Issue 2 - p 373-380
doi: 10.1519/JSC.0b013e31829999af
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Abstract

Introduction

Drop jumps (DJs) are a popular and effective method used to increase jumping ability (22) and improve other athletic abilities dependent on powerful hip and knee extension. Drop jumps typically involve an athlete stepping off a preselected dropping height, a brief landing, and then quickly jumping upward again. Given the emphasis on brief landing times, DJs are typically considered as a method of improving the stretch-shortening cycle performance (3).

Although the majority of research has used male subjects, recent studies have shown that DJs enhance neuromuscular function (17–19) and athletic performance (8,16) in women. Although no research has examined the importance of leg strength on DJ performance in women, Lephart et al. (13) reported that reduced lower body strength caused female athletes to land with decreased knee flexion, which likely increases the risk of sustaining a knee injury. Such research highlights the need for a better understanding of the relationship between lower body strength and performance of plyometric exercises in female athletes.

A common suggestion in the coaching literature is that a predetermined level of lower body maximal strength is required before DJ training can begin. These recommendations are usually expressed as a measure of relative strength such as a back squat 150% of body mass (BM) or a leg press 200% of BM (5). High levels of maximal strength during DJs would intuitively seem important because peak eccentric forces during landings greatly increase as athletes increase the dropping height (25), which places higher loads on the tendons and muscles involved (10,23). It is unclear if these suggestions of prerequisite levels are based on preventing injury, enhancing the efficacy of DJ training, or a combination of both.

Radcliffe and Osternig (20) previously examined the relationship between leg strength and DJ performance. They found a weak correlation between maximal back squat relative to the BM and the drop height (0.30–1.05 m) that produced the highest jump height (JH) of all their trials. They did note that there was a significant trend for JHs to decrease with dropping height. They did not, however, examine the role that maximal strength played in attenuating the decrease in JH as the dropping heights increased.

The purpose of this study was to examine the relationship between maximal leg strength and DJ performance. If there is merit in the suggestions that high levels of leg strength are required before beginning DJ training, there may be an observable relationship between maximal strength and DJ performance, particularly at the higher dropping heights. It was hypothesized that female athletes with high levels of maximal leg strength relative to their own BM would be able to perform DJs from high dropping heights better than female athletes with low levels of leg strength. Understanding the relationship between maximal leg strength and DJ performance may allow for the design of more effective and safer training programs.

Methods

Experimental Approach to the Problem

In order to assess the importance of maximal leg strength for female athletes when performing drop jumps, a cross sectional design was used. The study consisted of having 15 female rugby players perform drop jumps at several heights between 0.24 m and 0.84 m. Using a force plate, participants were assessed for jump height, ground contact time, reactive strength index and relative peak eccentric force and relative peak concentric force during the drop jumps. The participants were also tested for predicted one repetition maximum front squat. In order to determine the importance of maximal leg strength for drop jumps, Pearson's correlations were calculated between maximal front squat relative to body mass and each of the drop jump parameters. The participants were also divided into a strong group and a weak group with two way ANOVAs and T-Tests used to compare performance between the two groups.

Subjects

The subjects involved in the study were 15 female university rugby players (age = 20.3 ± 0.5 years, height = 1.71 ± 0.5 m, BM = 71.6 ± 9.9 kg, and strength training background = 2.7 ± 1.1 years) who would be considered high-level athletes in their sport with 14 of the 15 subjects having been Canadian national team members at the senior or junior level. All the players had a minimum of 3 months of strength training background under the supervision of a strength and conditioning coach. Regular strength training sessions typically involved squats (front or back), bench, press, deadlift, clean and jerk, snatch, plyometrics (including DJs), and other exercises. Before any testing, all the subjects signed consent forms approved by the University of Western Ontario Research Ethics Board in compliance with human testing.

The subjects were tested for 1 repetition maximum (1RM) front squat strength, countermovement jump (CMJ), squat jump (SJ), and DJs from 0.24, 0.36, 0.48, 0.60, 0.72, and 0.84 m. Front squats were picked as the lower body maximal strength exercise because the subjects were technically adept at them from performing them regularly in training and regularly tested with it as part of the team protocol. One week before the testing, the subjects underwent a familiarization session for the DJs. The athletes were all familiarized to the other tests through their regular team testing. The testing took place on 2 separate days with the maximal front squat test happening on the first day followed by the jump testing several days later. In addition to the physical testing, the subjects’ heights were also measured using a measuring tape and their mass was taken using a Kistler force plate (model 9287BA; Kistler, Winterthur, Switzerland). Height and mass were measured on the day of the jump testing. All the tests took place in the late afternoon between 1500 and 1700 hours during regular team training times. The subjects were at the end of a 3-week transition period between the competitive season and the beginning of their off-season and were not currently undertaking any heavy physical training. The absence of any heavy physical training meant that the subjects showed up well rested to the testing sessions but detraining would be minimal. The testing sessions were also part of the subjects' regular team testing, so it could be assumed that the subjects were highly motivated for the testing.

Strength Testing

The strength testing involved the subjects doing a maximal number of front squats and using the Wathen equation

to predict the 1RM load (26). This equation has been shown to be a valid predictor of maximal squat strength in university-age women (14). The rationale for using an RM test was to allow for a single maximal effort set rather than multiple sets trying to determine a 1RM. The 2RM load used for the test was estimated in consultation with the participants' strength and conditioning coach and based on loads employed previously in training. The predicted 1RM squatting score was kept as an absolute value (1RM) and then expressed relative to BM (1RM/BM). While testing, all the subjects held the bar across their shoulders, hands grasping the bar, elbows pointed directly ahead and were required to squat down to the thighs below parallel position where their hamstrings would touch their calf muscles.

Jump Testing Data Collection

All DJs, CJs, and SJs (12) were performed and collected on a Kistler force plate (model 9287BA). The subjects were all familiar with this method from their regular team fitness testing. Before the jump testing, the subjects underwent a 15-minute dynamic warm-up involving dynamic stretching, skipping, and jumping movements. The subjects were given 3 trials for both SJ and CMJ with 30 seconds between each jump.

Two Kistler forces plates (models 9287BA and 9287B) were used to collect the DJ data using the method suggested by Baca (2). The 2 force plate method is useful because the actual landing velocity does not necessarily correspond to the dropping height and can cause errors in calculations if only 1 force plate is used (11). Wooden boxes at designated drop heights were placed on 1 of the force plates (Kistler 9287BA). The subjects were instructed to begin the DJ by “stepping off the wooden box without lifting their center of gravity and landing on the other force plate” (Kistler 9287B). The subjects were instructed to perform the jump by “jumping as high as they can while trying to minimize the ground contact time (GCT).” Each force plate was sampling at 1,000 Hz.

While testing the DJs, the subjects were given 3 trials at each dropping height with a 45-second rest between each jump (21). After completing the DJs at each of JHs, the subjects were given 3 minutes of recovery before their DJs at the next drop height. The order of drop heights tested progressively grew higher with the participants beginning at the lowest (0.24-m) dropping height and ending at the highest (0.84-m) dropping height.

Jump Testing Data Analysis

Force data were amplified, converted from analog to digital data and transferred to Bioware (Kistler, Winterthur, Switzerland) software on the attached computer. The data were then exported into Microsoft Excel (Microsoft, Seattle, WA, USA) for further analysis. Using a forward dynamics approach, the force data were integrated to calculate velocity and displacement. The guidelines of Street et al. (24) were used, which have been shown to allow for the most valid calculation of the JH.

The highest displacement of the center of gravity obtained during the CMJs and SJs was kept as the JH (CMJ height and SJ height). For the DJs, the trial that produced the highest vertical displacement of the center of gravity during the subsequent jump was kept for analysis. For each DJ, a reactive strength index (RSI) score was calculated by dividing the JH in centimeters by the time on the ground in seconds (4). The largest force measured during the DJs, when the velocity of the center of mass was positive, was kept as the peak concentric force and expressed relative to BM in Newtons to give it a measure of force times body weight produced (relative peak concentric force [RPCF]). Conversely, the largest force measured during the DJs, when the velocity of the center of mass was negative, was kept as the peak eccentric force and expressed relative to the BM in Newtons to give it a measure of force times the body weight produced (relative peak eccentric force [RPEF]).

Statistical Analyses

The reliability, as estimated with intraclass correlations coefficients, of all measurements was high (0.80–0.97). A 1-way analysis of variance (ANOVA) was used to compare the differences in DJ parameters for each of the dropping heights. A Fisher's least squares difference test was used as a post hoc test.

To examine the effect that maximal leg strength had on DJ performance, Pearson's correlation was calculated to determine the relationship between each of the DJ parameters, from each of the dropping heights, with 1RM/BM. Ninety percent confidence intervals were calculated for each correlation and correlations. Using the guidelines of Hopkins (9), the magnitude of positive correlations was considered trivial for being <0.1, small for being ≥0.1 and <0.3, moderate for being ≥0.3 and <0.5, large for being ≥0.5 and <0.7, very large for being ≥0.7 and <0.9, and nearly perfect for >0.9. The same system was used for negative correlations.

For further analysis, the subjects were divided into 2 groups depending on whether they could squat a barbell loaded to a weight more (high strength [HS]) or less (low strength [LS]) than their BM. Seven subjects were placed in the HS, and 8 were placed in the LS group. Independent t-tests were used to examine the differences between the HS group and the LS group for CMJ, SJ, 1RM, and 1RM/BM. Two-way ANOVAs (strength group × drop height) were used to examine the difference in scores at each of the dropping heights for DJ JH, DJ GCT, DJ RPCF, and DJ RPEF. Fisher's least squares difference test was used as a post hoc test. To characterize the differences between groups, Cohen's d effect sizes were calculated with the following classification system used to determine the magnitude (9). A difference was considered trivial for being <0.2, small for ≥0.2 and <0.6, moderate for ≥0.6 and <1.2, and large for ≥1.2. All statistical analyses were conducted with XLSTAT (New York, NY, USA) software.

Results

The strength of the correlations between DJ JH and 1RM/BM showed an increase from small to large as the subjects progressed from 0.24 up to 0.84 m (Table 1). Ground contact time seemed to show a similar pattern progressing from trivial at the 0.24-m drop height to large at 0.72-m height but only small at the 0.82-m height. The RSI scores between 0.36 and 0.84 m were moderate and large.

Table 1
Table 1:
Correlation and 90% confidence intervals between the front squat relative to the body mass and drop jump parameters at each of the different dropping heights.*

Once the subjects were divided into HS and LS groups, there were moderate differences in CMJ, SJ, and 1RM and large differences in 1RM/BM (Table 2). An overall difference, as detected by an ANOVA, between the HS and LS groups for DJ JH (p = 0.02) was found with the post hoc analysis finding a significant difference (p = 0.029) at the 0.84-m drop height (Table 3 and Figure 1). The reactive strength index also had a significant overall difference (p = 0.004) between the 2 groups with post hoc analysis finding a between group difference (p = 0.017) at the 0.72-m drop height (Figure 2 and Table 3). No significant difference for GCT between the HS and LS groups were found for the DJs.

Table 2
Table 2:
Group scores for squat, CMJ, and SJ and differences between the HS and LS groups for those measures.*
Table 3
Table 3:
Differences in drop jump parameters, combined between all dropping heights, between the HS and LS groups as calculated by a 2-way analysis of variance and a Fisher's LSD post hoc analysis.*
Figure 1
Figure 1:
Jump height during drop jumps at each of the dropping heights. The solid line represents the high strength (HS) group, and the dashed line represents the low strength (LS) group. Error bars denote SD. * Indicates an overall significant difference between the HS and LS groups as calculated by a 2-way analysis of variance and a Fisher's post hoc test. Significant differences at individual drop heights between groups as calculated by a Fisher's post hoc test are denoted with an arrow.
Figure 2
Figure 2:
Reactive strength index during drop jumps at each of the dropping heights. The solid line indicates the high strength (HS) group and the dashed line indicates the low strength (LS) group. Error bars denote SD. * Indicates an overall significant difference between the HS and LS groups as calculated by a 2-way analysis of variance and the Fisher's post hoc test. Significant differences at individual drop heights between groups as calculated by the Fisher's post hoc test are denoted with an arrow.

A comparison of the effect of the different drop heights on DJ parameters showed significant differences between dropping heights with the exception of JH (Table 4). Ground contact time and RPEF showed significant increases with increases in dropping height and RSI and RPCF showed significant decreases with increases in dropping height (Table 4).

Table 4
Table 4:
Mean and SD scores for jump height, GCT, RSI, concentric peak force relative to BW and eccentric peak force relative to BM for drop jumps at each of the dropping heights.*

Discussion

The results of this study seem to support the hypothesis that female athletes with a greater 1RM/BM are able to outperform their weaker peers during DJs, particularly at the highest dropping heights. The HS group displayed better JH scores during the DJs, CMJs, and SJs (Tables 2 and 3). When comparing the HS and LS groups, there was an overall difference in favor of the HS group but the only drop height that was significantly different between the 2 was the 0.84-m drop height (Figure 1, Table 3). In addition, the relationship between 1RM/BM and DJ JH for the whole group was the strongest at the highest drop height (Table 1) with a large correlation (r = 0.56) between the 2. An examination of the differences between HS and LS for RSI showed a similar pattern as JH (Figure 2 and Table 3) with an overall difference between the groups and a significant difference at the 0.72-m dropping height. The correlations between RSI and 1RM/BM were not quite as consistent across dropping heights as JH, but much of this can be attributed to the large effect that GCT time has on the RSI score. The subjects were encouraged to primarily maximize JH and to minimize GCT secondarily, so the results could have been much different if the subjects had been given instruction to primarily minimize GCT or maximize RSI (27). The results for both RSI and JH indicate that strong female athletes outperform their weaker peers during DJs.

The likely reason that the 1RM/BM was important for the female athletes in this study was the large RPEF they were exposed to at the higher levels. The RPEF nearly doubled from the 0.24-m dropping height to the 0.84-m dropping height with a force >8 times that of the body weight (Table 4). The athletes adjusted to the increase in the RPEF by increasing GCT (Table 4). The increase in the GCT led to a decrease in the RSI but not to a decrease in the JH (Table 4). The small correlations between the JH and 1RM/BM at the lower dropping heights would suggest that it is different physical qualities that make an athlete successful in DJs and squatting. However, the higher forces the athletes had to tolerate when landing from a 0.84-m drop height likely increased the importance of being able to display high levels of maximal strength because of changes in the pattern of muscle contraction (23). Previously, it has been shown that Finnish weightlifters (Olympic style) with very high levels of maximal leg strength saw no decline in the JH during DJs as they progressed from 0.2 to 1 m (7).

An implication of the results would be that female athletes should focus on increasing maximal leg strength relative to body weight if the intention is to use DJs in their training program. Experimentally, it has been shown that 16 weeks of squat training emphasizing maximal strength improved DJ JH at several drop heights ranging from 0.2 to 1 m, but the gains were much greater at the higher dropping heights (6). This is not surprising given that RPEF, in this study, doubled by moving the drop height from 0.24 to 0.84 m (Table 4), which would increase the need for high levels of maximal leg strength. Incorporating maximal strength training into a program that is gradually increasing the dropping height used during DJs is likely an effective way to get the benefit of DJ training while building up a tolerance to the high eccentric forces that DJs place on athletes. Two studies (1,15) that showed excellent improvement in the CMJ JH (10.67 and 5.6 cm) used a program of progressive overload in DJs moving from low drop heights to high drop heights (0.51–1.14 and 0.2–0.6 m) while simultaneously using a squatting program to develop maximal leg strength. The fact that these studies found greater improvements in the CMJ JH than in using either method alone suggests that there is a synergistic effect between the 2 methods. The ability of maximal strength training to improve the tolerance to high eccentric loads during DJs may be the reason for this synergistic effect.

Practical Applications

A key finding of this study was that female athletes with high levels of maximal relative leg strength were able to better tolerate the high eccentric forces that come from DJs when compared with their weaker peers. The importance of maximal strength was most noticeable during DJs involving a high drop height (>0.8 m). The large eccentric forces that come from high (>0.8-m) drop heights may facilitate better performance adaptations, so increasing relative strength for female athletes is important if they want to use these dropping heights. The relationship between drop height tolerance and relative strength is perhaps the reason that studies that use maximal strength training and DJs typically produce better results than does either modality in isolation. It is suggested that female athletes focus on increasing their maximal relative leg strength before beginning a DJ program and continue to develop it once they begin to include DJs in their training programs.

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Keywords:

plyometrics; drop height; eccentric strength

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