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Original Research

Does Performing Drop Jumps With Additional Eccentric Loading Improve Jump Performance?

Aboodarda, Saied J.1; Byrne, Jeannette M.1; Samson, Michael1; Wilson, Barry D.2; Mokhtar, Abdul H.3; Behm, David G.1

Author Information
Journal of Strength and Conditioning Research: August 2014 - Volume 28 - Issue 8 - p 2314-2323
doi: 10.1519/JSC.0000000000000498
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Abstract

Introduction

In many sporting activities involving jumping movements, performing a fast muscle stretch (eccentric phase) immediately before an explosive shortening contraction (concentric phase) has been shown to enhance mechanical work output (3–6,13,22,26,33,36,41). This phenomenon known as stretch-shortening cycle (SSC) has been attributed to rapid lengthening of the muscle-tendon complex, which in turn facilitates a greater power output and force-producing ability across the concentric phase (20,26,42). Linked to this concept, previous investigators have speculated that applying additional external load throughout the eccentric phase of the jumping movement could amplify the SSC mechanism and modulate jumping performance and jump exercise intensity (2,15,24,27,30,32,38,39).

In an attempt to regulate the exercise intensity in jumping movements and to enhance jumping performance (e.g., jump height), previous investigators have recommended either manipulating the body mass (BM) using weight vests or performing drop jumps (DJs) from various heights (4,6,15,24,26,30–32,39). In contrast to these recommendations, some studies have reported that using a constant load of weight vest may not be an effective method for improving the DJ exercise intensity (24). This lack of effect of a weighted vest is thought to be because of the fact that such jumps are usually performed from 0.2 to 0.6 m drop heights, where the additional constant load may create negligible gravitational acceleration and consequently cause minimal changes in the magnitude of ground reaction forces (GRFs) and rate of force development (RFD) (24). An additional reason for lack of effect of weighted vests may be that the external load was applied throughout the entire jump (eccentric and concentric phases). This may, in fact, hinder the ability of the subject to increase concentric peak power and jump height (30). In the case of using increased drop height to enhance DJ performance, Bobbert et al. (6) have reported a limit to which the height can be raised, beyond which safety becomes an issue. They have warned about situations in which subjects hit the ground hard with their heels and are incapable of tolerating the sharp rise in GRF, resulting in injury. As a result, there is a limit to which this approach can be used to improve performance.

To increase the intensity of the DJ and to overcome the above limitations, elastic bands could be used to increase the load on the subject during the eccentric phase of DJs. Cronin et al. (9) have established that the recoil force of an elastic device could be an effective means of increasing landing velocity in countermovement jumps (CMJ) and consequently improving jumping performance through taking advantage of the SSC mechanism. Aboodarda et al. (2) suggested a modification of this technique in which they released the elastic load just before the concentric phase of the jump. In doing so, the authors eliminated any possible negative effects of the additional external load at take-off. They demonstrated that using the downward recoil force of the elastic material during the eccentric phase of a CMJ enhanced jump performance and take-off velocity. The results were attributed to increased eccentric GRF. They speculated that a greater eccentric GRF would subsequently apply greater elongation of the series elastic components of the knee and hip extensors and reinforce the contribution of the muscle-tendon interaction effect in producing significantly greater stretch-induced muscle function gains during loaded CMJs. It would be of interest to investigate if a similar mechanism can contribute to modulation of jumping performance and muscle activation levels during loaded and nonloaded DJs as well.

The aims of this study, therefore, were to determine the effect of increased eccentric phase loading, as delivered using an elastic device, on DJs performed from different drop heights. Of specific interest were changes in (a) the kinetics; eccentric and concentric impulse, RFD, concentric velocity and (b) the electromyographic (EMG) activity of leg muscles.

Methods

Experimental Approach to the Problem

The investigation used a randomized repeated-measure design to study the effect of using elastic bands to create additional eccentric load on DJ intensity, jumping performance, and muscle activation. Subjects performed DJs from 3 heights (20, 35, and 50 cm) under 3 different conditions: (a) body weight only (free DJ), (b) with elastic bands providing downward force equivalent to 20% of BM (+20% DJ), and (c) with elastic bands providing a downward force equivalent to 30% BM (+30% DJ). Three trials were performed for each type of jump, and the order of the measurements was randomized across the 9 exercise conditions (3 heights × 3 loads). The GRF, EMG activity of 4 leg muscles, and body positions of subjects were recorded during each DJ. Subjects were verbally encouraged to reach maximal jump height through all 9 types of DJ.

Subjects

Based on a priori statistical power analysis of related articles (2,6,9,24) to achieve an alpha of 0.05 and a power of 0.8, 15 highly resistance trained male subjects (age, 24.7 ± 5.7 years; height, 179.4 ± 8.9 cm; mass, 87.8 ± 10.5 kg; and BMI = 27.2) participated in this study. All participants were able to squat over 2 times their BM and were competitors in rugby and crossFit sports. They had been participating in regular weight training and plyometric exercises 4–5 days a week for at least 6 months before participation in the study. None of the participants had a history of taking medications, and there were no reports of musculoskeletal injuries or metabolic disease. All participants were requested to continue their training protocols but refrain from vigorous physical activity for a period of 1 week before their involvement in the study, and thus avoiding confounders such as fatigue and delayed onset of muscle soreness because of strenuous training programs. The institutional review board for research involving the use of human subjects approved the study. After explanation of the possible risks and discomfort and subsequently completing a medical screening questionnaire, subjects gave their written informed consent.

Testing Procedure

Participants attended a single testing session commencing with a warm-up consisting of 5-minute cycling (50 rpm) followed by 2 sets of 6 CMJs performed with maximal effort. During the 5-minute recovery, self-adhesive Ag-AgCl electrodes (MeditraceTM 130 ECG conductive adhesive electrodes, Mansfield, MA, USA) were positioned 2 cm apart (center to center) parallel to the direction of the vastus lateralis (VL), vastus medialis (VM), biceps femoris (BF), and soleus (SOL) muscle fibers on the centre of the muscle belly of the dominant leg (17). The ground electrode was placed on the lateral femoral epicondyle of the same leg. Before the placement of electrodes, the area of skin was shaved and abraded to remove dead skin with sandpaper and cleansed with an isopropyl alcohol swab to decrease skin resistance. An interelectrode impedance of <5 kΩ was obtained before recording to ensure an adequate signal-to-noise ratio.

Participants then performed 3 maximal DJ trials for each condition on a force platform (400 × 600 sandpaper × 83 mm, model BP400600 HF-2000; AMTI, Watertown, MA, USA) connected to an amplifier (AMTI Miniamp MSA-6–Gain 2000). All force plate data were collected at a rate of 200 Hz. To minimize the possibility of fatigue, a 2-minute rest period was provided between the trials.

Free Drop Jump (Body Weight Only)

Participants dropped off a platform (located 20 cm from the back edge of the force plate) landing on the force plate with both feet simultaneously and performed a vertical jump immediately after landing. They were instructed to keep their feet parallel and their hands on their hips for the entire jump and give a maximal effort for all jumps. Participants were also instructed to minimize lateral and anterior displacements by jumping vertically and landing directly on the force plate.

Loaded Drop Jumps (With Additional 20 and 30% of Body Mass Provided by Elastic Bands)

The process of performing the loaded DJs was similar to that described above for the free DJ; however, elastics bands were attached to either sides of a harness worn by participant at the hip level (Figure 1). The bands were manually stretched (at 20° with sagittal plane) and fixed (beneath the feet of 2 research assistants at either side of the participant) in line with the center of force plate. The bands were released once the subject landed and reached full flexion of hip and knee (at the beginning of the DJ concentric phase). Thus, participants experienced additional loading because of the elastic bands during the airborne and landing phase, whereas they were free to reach a maximal jump height without restriction of the bands in the concentric phase. The same 2 research assistants, who had undergone considerable practice in the timing of release of the bands, were used throughout the study to reduce the possibility of nonsynchronized release of the bands. The resting length of the elastic bands (Hygienic Corporation, Akron, OH, USA) for each subject was half of the distance from the harness to the ground. On the basis of the method recommended by Page and Ellenbecker (34), the elastic bands at each side of the participant were stretched 100% to provide downward tensile force (1). Different colors of elastic material were either added or removed to meet the tensile force equivalent to 20 and 30% BM (2). The external force of the bands (20 and 30% BM) was determined while subjects were standing on a weight scale placed on top of the jumping platforms (20, 35, and 50 cm). To achieve this, subjects stood on the platform with fully extended hip and knee joints, and the elastic bands were stretched until they reached the ground. Pilot testing was conducted revealing that the bands still provided a downward force equivalent to 5–7% subject's BM at the initial stage of landing phase.

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Figure 1:
Elastics bands were attached to either sides of a harness worn by the participant at the hip level.

Kinematic Data

All DJs were recorded using a camera (Sony HDR-XR100, SONY Inc. San Diego, CA, USA, 30 Hz) and the displacement of the hip was analyzed using Kinovea software (Kinovea 0.8.15). Video and force plate data were synchronized with force plate and EMG data through the use of a light, placed in the cameras field of view. At the initiation of force and EMG data, the light illuminated. The frame number at which the light came on was used to align force, EMG, and video data. Based on the video data, each jump was divided into eccentric and concentric phases, defined as (a) eccentric phase (from the moment the toes touched the force platform (GRF showed positive value) until the hip reached minimum vertical displacement) and (b) concentric phase (from the minimum negative vertical displacement of hip until the moment the GRF dropped to near zero).

The variables measured during concentric and eccentric phases included the peak vertical GRF, time to peak GRF, rate of force developments (RFD = peak eccentric or concentric GRF divided by time to peak GRF of corresponding phase), and duration of concentric and eccentric phases. Net force (N) was calculated by subtracting BM from GRF. In addition, impulse was determined finding the area under the net GRF curve, using rectangular rule. The relative eccentric and concentric impulses (25,29) were calculated as follows:

Jump Height

Jump height was determined for each DJ through calculation of take-off velocity (11) using the following formula:

Electromyographic Activity

Electromyographic activity of the 4 leg muscles (VL, VM, BF, and SOL) was recorded using an EMG system (DA USA 100: analog-digital converter MP150WSW; Biopac System, Inc., Holliston, MA, USA). All EMG signals were recorded with sampling rate of 2000 Hz using a commercially designed software program (AcqKnowledge III; Biopac System, Inc.). Electromyographic signals were filtered with a Blackman −61 dB band-pass filter between 10 and 500 Hz, amplified (bipolar differential amplifier, input impedance = 2 MΩ, common mode rejection ratio >110 dB minute (50/60 Hz), gain × 1,000, noise <5 µV), and analog-to-digitally converted (12 bit) and stored on personal computer for further analysis. Integrated EMG (iEMG) of each muscle was calculated across concentric and eccentric phases. Then, the iEMG was normalized based on the duration of corresponding phases (e.g., eccentric iEMG = iEMG/duration of eccentric phase). The iEMG of the quadriceps muscle was calculated as the average of VL and VM iEMG. To investigate the effect of various loading conditions on modulation of prelanding EMG activity pattern, the onsets of EMG for the 4 muscles were assessed in relation to the contact time during 9 types of DJs. The full-wave rectified EMG signals were filtered with a Blackman −61 dB and low-pass filter at 10 Hz. The onset of EMG activity was identified visually for each EMG trace as instant before impact where muscle EMG activity began to continuously increase.

Statistical Analyses

Statistical analyses were computed using SPSS software (version 16.0; SPSS, Inc., Chicago, IL, USA). Intraclass correlation coefficient (ICC) was computed for eccentric and concentric GRF and concentric velocity based on the 3 trials of 9 types of DJs. A 2-way repeated-measures analysis of variance (ANOVA) (3 × 3) was used to identify the effect of 3 loads (free, +20% BM, and +30% BM) and 3 heights (20, 35, and 50 cm) on kinetics variables of DJs, jump height, EMG amplitude, and EMG onset. If a significant result was obtained from the interaction effects of loads × heights, a series of paired t-tests with Holm-Bonferroni corrections were used to compare different conditions (21). In addition, if results showed significant main effect for load or height, Bonferroni post hoc test was used to identify differences between conditions. Additionally, Cohen effect size (ES) statistics were conducted to evaluate the magnitude of the changes in various DJs according to criterion of > 0.8 large; 0.5–0.8 medium, <0.2–0.5 small (7). Significance was defined as p ≤ 0.05.

Results

All participants were able to successfully complete all jump conditions. None of the participants reported any evidence of muscle fatigue or muscle injury during the entire testing session. The ICCs for eccentric GRF, concentric GRF, and concentric velocity for the 3 trials of the 9 DJ conditions were above 0.89, 0.91, and 0.87, respectively. In addition, the coefficient of variance calculated for 9 DJ conditions was 22.9% for eccentric GRF, 22.3% for concentric GRF, and 9.1% for concentric velocity.

Eccentric Phase

There was a significant interaction effect of height × load for eccentric impulse (F(4,56) = 2.66, p = 0.042). Subsequently, paired t-tests revealed that at all drop heights, using 30% elastic load evoked 13 to 46% greater eccentric impulse compared with free DJs (all p ≤ 0.05, 0.71 > ES > 0.34) (Figure 2). In addition, paired t-tests showed that at all 3 loading conditions, the 50-cm drop height elicited 41–81% greater eccentric impulse compared with 20 cm (all p < 0.001, 1.53 > ES > 1.27) and 15–19% greater eccentric impulse compared with 35-cm drop height (all p < 0.005, 0.55 > ES > 0.49). Similarly, the 35-cm drop height showed 18 to 54% greater eccentric impulse compared with 20 cm (all p < 0.007, 0.77 > ES > 0.3) (Figure 2).

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Figure 2:
A graphical representation of the eccentric impulse (mean ± SD) generated during DJs performed from 3 drop heights and 3 external loading conditions. “*” denotes that 30% elastic load evoked more eccentric impulse compared with free DJs. “§” denotes that 50-cm drop height elicited greater eccentric impulse compared with 20 and 35 cm in all loading conditions. “†” denotes that 35-cm drop height elicited greater eccentric impulse compared with 20 cm.

The interaction effect of height × load was also significant for time to peak eccentric impulse (F(4,56) = 4.24, p = 0.005). Paired t-tests showed that at 20-cm drop height, using 30% elastic load reduced the time to peak eccentric impulse compared with 20% (p = 0.046, ES = 0.50) and free DJs (p = 0.007, ES = 0.99) (Figure 3). In addition, paired t-tests revealed significant decreases in time to peak eccentric impulse when DJs were performed from 35 cm (all p < 0.017, 0.98 > ES > 0.34, −20 to −23%) and 50 cm (all p < 0.02, 1.35 > ES > 0.42, −15 to −32%) compared with 20 cm.

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Figure 3:
A graphical representation of the time to peak eccentric impulse (mean ± SD) during DJs performed from 3 drop heights and 3 external loading conditions. “*” denotes that 30% BM elastic load reduced time to peak eccentric impulse at 20-cm DJ. “§” denotes that time to peak eccentric impulse was shorter for free and 20% BM DJs executed from 35 and 50 cm compared with 20 cm.

There was also a main effect of external load (F(2,28) = 15.95, p < 0.001) and drop height (F(2,28) = 52.79, p < 0.001) on the magnitude of eccentric RFD. The interaction effect of height × load, however, did not reach a significant level (F(4,56) = 1.37, p = 0.255) (Figure 4). Bonferroni post hoc test demonstrated that using greater external load (30% BM compared with 20% BM and free DJs; and 20% BM compared with free DJ) significantly increased eccentric RFD (all p < 0.031). Similarly, performing DJs from higher drop heights (50 cm compared with 35 and 20 cm; and 35 cm compared with 20 cm) significantly increased eccentric RFD (all p < 0.001). In addition, a significant load effect was observed for eccentric duration (F(2,28) = 7.95, p = 0.002) (Table 1). Bonferroni post hoc test demonstrated that using 20 and 30% external load significantly reduced eccentric duration (p ≤ 0.05) compared with free DJs. The main effect for drop height (F(2,28) = 1.36, p = 0.267) and interaction effect of load × height (F(4,56) = 0.71, p = 0.588) did not show any change in eccentric duration.

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Figure 4:
A graphical representation of the eccentric rate of force development (mean ± SD) during DJs performed from 3 drop heights and 3 external loading conditions. “§” denotes that the main effect for increasing drop height showed a significant result. “*” denotes that the main effect for increasing external load demonstrated a significant result.
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Table 1:
Kinetic and kinematic measurements of DJs performed from 3 drop heights and 3 external loading conditions.*

Concentric Phase

A significant interaction effect of height × load was observed for concentric impulse (F(4,56) = 3.73, p = 0.009). Paired t-tests revealed that at free and +20% loading conditions, concentric impulse showed 10–11% decreases at 50 cm compared with 35-cm DJs (all p < 0.013, 1.21 > ES > 0.5). This decline, however, was not observed for DJs executed with 30% additional elastic load (Figure 5). In addition, 50-cm DJs with 30% elastic load evoked 10% more concentric impulse compared with free DJ from the same height (all p = 0.013, ES = 0.47). The main effect of external load also showed a significant value for concentric duration (F(2,28) = 3.43, p = 0.046) (Table 1). Bonferroni post hoc test demonstrated that using 20% external load significantly increased concentric duration (p < 0.044) compared with free DJs. No statistical difference was observed for other variable including time to peak concentric impulse, concentric RFD, take-off velocity, and jump height.

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Figure 5:
A graphical representation of the concentric impulse (mean ± SD) during DJs performed from 3 drop heights and 3 external loading conditions. “§” denotes that the concentric impulse decreased at 50 cm compared with 35 cm (for both the free and 20% BM loads). “*” denotes that 30% elastic load elicited greater concentric impulse compared with free and 20% at 50-cm DJs.

Electromyographic Activity

The ANOVA showed a significant main effect for load for SOL iEMG during the eccentric phase (F(2,28) = 5.76, p = 0.011) (Table 2). Bonferroni post hoc test demonstrated that using 20% external load significantly decreased SOL iEMG (p < 0.031) compared with free DJs (Table 2). In addition, despite the fact that statistical differences between loaded and unloaded DJs for iEMG of quadriceps muscle did not reach to a significant level (0.153 < p < 0.198), using 20 and 30% external load at various drop heights resulted in small to moderate ES increases in quadriceps iEMG across the eccentric phase (0.23 > ES > 0.51) (Figure 6). The other muscle groups did not show any significant change (all p > 0.05, ES < 0.2) in the iEMG between different DJ conditions during the eccentric and concentric phases (Table 2).

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Table 2:
EMG activity (mV) measured during DJs performed from 3 drop heights and 3 external loading conditions.*
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Figure 6:
A graphical representation of the eccentric quadriceps iEMG (mean ± SD) from 3 drop heights and 3 external loading conditions. “§” denotes that the 30% BM DJ condition elicited greater iEMG than free DJ (moderate ES). “†” denotes that the 30% BM DJ condition elicited greater iEMG than free DJ (small ES). “*” denotes that the 20% BM DJ condition elicited greater iEMG than free DJ (small ES).

In addition, further analysis on modulation of the prelanding EMG onset demonstrated significant main effects for drop height (p = 0.003, 0.001, 0.001 for VL, VM, SOL, respectively) and load (p = 0.005, 0.023 for VM, SOL, respectively) (Figure 7). Bonferroni post hoc demonstrated significantly earlier onsets of EMG activities when DJs performed from 50 cm compared with 20 cm (p = 0.003, 0.003, 0.001 for VL, VM, SOL, respectively) and when 30% BM load was used compared with free DJs (p = 0.016, 0.004 for VM, SOL, respectively). In addition, significant interaction of load × height was observed for prelanding EMG onset of BF (p = 0.031). Paired sample t-test demonstrated significantly earlier BF EMG onset during 30% loaded DJ compared with free DJ from 20-cm drop height (p = 0.039).

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Figure 7:
A graphical representation of the prelanding EMG onset (mean ± SD) for 4 muscles during performing various DJ conditions. “*” denotes that significantly earlier onsets of EMG activities occurred when DJs performed from 50 cm compared with 20 cm. “†” denotes that significantly earlier onsets of EMG activities occurred when 30% additional load was used compared with free DJs. “#” denotes that significantly earlier BF EMG onset occurred during 30% loaded DJ compared with free DJ from 20-cm drop height.

Discussion

This study sought to determine the effect of using additional eccentric loading on modulation of jumping performance and muscle activation in highly trained athletes. The results indicated that, in addition to the conventional technique of increasing drop height, using a tensile load during the airborne and eccentric phases of the DJ could further improve eccentric impulse, eccentric RFD, and quadriceps iEMG (with small to moderate effect). The greater eccentric loading, however, did not immediately alter concentric phase kinetics and jump height nor did it alter muscle activation levels during concentric phase. However, earlier EMG onsets were observed for all muscles during loaded DJs (+30% BM) from all drop heights, the effect that may suggest an anticipatory adjustment mechanism to counteract against greater eccentric loading. As it has been suggested that GRF and RFD are indicators of exercise intensity (24,30), these findings suggest the loaded DJs, using additional elastic load, may be an effective technique for improving DJ exercise intensity without acute effects on the jumping performance and neuromuscular activation level in highly trained athletes.

Observing significantly greater eccentric impulse and RFD during the loaded DJs supported our hypothesis that, using an elastic band could effectively modulate the DJ exercise intensity. For many years, increasing drop height has been the conventional technique for developing DJ exercise intensity (5,6,26,30,31,40). Bobbert et al. (6) indicated that high drop heights could result in intolerable increases in GRF leading to possible lower-limb injuries. The present results suggest that loaded DJs from lower drop heights (e.g., 35 cm) can evoke a similar exercise intensity (eccentric impulse and RFD) as unloaded DJ from higher drop heights (e.g., 50 cm) (Figures 1 and 3). In support of this idea, the results depicted in Figure 4 illustrate that performing 35-cm loaded DJs could avoid the decline of concentric impulse that occurred during 50-cm DJs. As corroborative evidence, previous studies (2,3,9) have demonstrated that using the recoil force of elastic material during the eccentric phase of CMJs increased the eccentric velocity of the body during the lowering phase of CMJs. This increased velocity was hypothesized to be because of the extensibility and stiffness of the elastic bands. Unfortunately, the approach used in this study would not let us measure an eccentric velocity. Participant landed on the force plate with slight changes in the knee or ankle joints angle during different conditions of DJs; therefore, it was difficult to define a systematic way to measure the airborne velocity and consequently the eccentric velocity. However, further video analysis demonstrated that the airborne time was shorter (∼15–20%) during loaded DJs (+20 and +30% BM) from 35 and 50 cm compared with free DJs from the same drop height. Nonetheless, observing greater eccentric impulse and RFD (Figures 2 and 4) with the presence of more resistive elastic bands (i.e., in +30% BM condition) indicates that magnitude of external load provided by elastic material could be a key factor in the development of the loaded DJ exercise intensity.

The literature is fraught with evidence indicating that sequencing of a fast eccentric (stretching) and concentric (shortening) actions improves jumping performance (2,6,9,15,24,30,37,39). The myogenic and neurogenic mechanisms of this performance facilitation has been manifested as (a) the effect of faster eccentric loading on the preparatory active state of actin-myosin cross-bridge attachment before the concentric phase, (b) stretch of the series and parallel elastic components and recovery of stored elastic energy during the concentric phase of the jump, and (c) further stretching of the intrafusal muscle fiber and provoking the extrafusal muscle fibers that may increase neural stimulation. However, a question that needs to be addressed is “why in the present study, did greater and faster eccentric loading not improve the immediate concentric jumping performance?” The exact mechanism for this paradoxical finding is unclear; however, previous literature has proposed several plausible explanations.

In previous investigations that studied loaded jumps, the external load was kept attached to the subjects during the entire jumping task (8–10,39). It was speculated that the reduction of the concentric performance (e.g., peak power and jump height) was possibly because of a reduction of concentric velocity. Accordingly, we released the elastic resistance just before the concentric phase to eliminate the restriction of external load and prevent reduction of concentric velocity (2). However, our results demonstrated no change in take-off velocity (Table 1). This finding suggest that although releasing external load was an effective technique in preventing reduction of take-off velocity, it did not enhance this parameter either. This finding is in line with those reported by Bobbert et al. (5). These authors reported that despite considerable increase in eccentric GRF and eccentric duration during DJs compared with CMJ, no enhancement was observed in vertical take-off velocity or jump height. Therefore, it could be concluded that unaltered take-off velocity might have been a contributing factor in observing no change in concentric performance despite achieving a faster eccentric loading in loaded DJs.

Elongation of the transition time between the eccentric to concentric phase has been suggested as one of the mechanisms that can hinder concentric performance when excessive eccentric loading is applied (31,43). Athletes in this study, however, showed no change or slightly shorter eccentric duration (Table 1) and time to peak eccentric impulse in response to an additional load (Figure 3). These findings suggest that they were able to adapt a landing technique that avoids elongation of transition time from eccentric to concentric phase. In support of this idea, the temporal analysis of EMG onset in our study demonstrated significantly earlier EMG activity for all 4 recorded muscle groups during loaded DJs (+30% BM) compared with free DJs (Figure 7). The evidence suggests that athletes increased leg stiffness (14,18,19) in response to additional eccentric loading (Figure 7 and Table 1). This modulation of muscle activation pattern could be interpreted as an anticipatory mechanism (feedforward motor control strategy) to decelerate and eventually terminate the joint rotations and to provide adequate muscle tension for absorption of the impact force (12,28). Accordingly, the theory of dissipation of stored elastic energy because of elongation of the transition period seems unlikely for our data.

The iEMG of the knee extensors and ankle plantar flexors in this study demonstrated no significant difference between loaded and unloaded DJs across concentric phase. This result is not in agreement with the known hypothesis that in response to faster eccentric loading elicited during loaded jumps, a contraction-induced increase in muscle stiffness might occur, which could be accompanied by increased stretch reflexes and ultimately increase motor output to active muscles in response to muscle spindle activation (23,26,27). The rationale for unchanged iEMG activity across the concentric phase probably is that participants in our study were highly motivated to show their best performance during all DJ conditions. Unchanged muscle activation level may indicate minimal contribution of stretch reflex excitation in activation of additional muscle unit pools on top of a system, which is already under maximal drive. Thus, the lack of change in muscle activation level during different conditions of loaded and unloaded DJs could be a potential explanation for unchanged DJ performance.

Another plausible explanation why no change in concentric performance and iEMG was observed could be the contribution of inhibitory mechanisms of Golgi-tendon responses, which may ultimately nullify any benefits associated with SSC mechanism (27,43). In line with this concept, several studies have shown that increased eccentric loading (increasing DJ heights higher than the CMJ maximal height) could generate reflex inhibition, which in turn would reduce muscle activation and subsequently decrease concentric performance (16,27,35,42). The reduction that we observed in SOL iEMG across the eccentric phase of 20% BM condition seems to support this hypothesis. However, the slight increases in both quadriceps and SOL EMG during the 30% BM condition (Table 2) are counter to this argument. Clearly, these results suggest that further research is required to elucidate the effects of different magnitude of external load on activation level of muscle groups participating in loaded DJ performance.

However, we should not rule out limitations such as iEMG sensitivity in detecting further changes in the activation level of different compartments of a single muscle group. In line with this assumption, there are some research studies (5) that failed to show higher EMG in knee extensors and plantar flexors despite observing considerably larger moments about the knee and ankle in DJs with faster eccentric loading. These findings point to the necessity of more studies to elucidate the role of facilitatory and inhibitory mechanisms contributing in loaded DJ performance.

Practical Applications

The use of high DJ heights or holding weights while jumping could have injury implications. An alternative means of accelerating the body from a DJ could be the use of additional tensile load with elastic bands to adjust DJ exercise intensity. Plyometric exercises using this technique could be applied to increase the tolerance of athletes to repetitive exposure to high eccentric loading during landing. In this context, previous investigators have recommended high-intensity eccentric loading jumps as a useful technique for prevention of lower-extremity injuries in sports such as gymnastic, track and field, and alpine skiing in which athletes expose themselves to higher GRFs. The earlier onsets of muscle activity observed during loaded DJs in the study indicate that loaded DJs may provide an anticipatory mechanism leading to a greater level of muscle stiffness acting to protect the ligaments and joints from injury. However, these comments remain speculative until further investigations determine the effect of greater eccentric GRF, power, and velocity with loaded DJs on jumping performance or prevention of musculoskeletal injuries. In addition, these techniques should be implemented into a training plan to ascertain their effectiveness and appropriate volume of training load. Although 2 researchers controlled the elastic bands in this study, other alternatives such as quick release belts could be used to improve the practicality of the exercise, allowing the athlete to execute the activity without additional personnel.

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Keywords:

stretch-shortening cycle; plyometrics; elastic band; ground reaction force; electromyography

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