Effects of Accentuated Eccentric Loading on Muscle Properties, Strength, Power, and Speed in Resistance-Trained Rugby Players : The Journal of Strength & Conditioning Research

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Original Research

Effects of Accentuated Eccentric Loading on Muscle Properties, Strength, Power, and Speed in Resistance-Trained Rugby Players

Douglas, Jamie1,2; Pearson, Simon1,3; Ross, Angus2; McGuigan, Mike1,4

Author Information
Journal of Strength and Conditioning Research: October 2018 - Volume 32 - Issue 10 - p 2750-2761
doi: 10.1519/JSC.0000000000002772

Abstract

Introduction

Resistance training is an integral component of physical preparation for team sport athletes (35). Physical characteristics improved by resistance training such as strength, power, and speed have been found to be associated with successful match outcomes in Rugby Sevens (40) and performance levels (45) in Rugby Union players. Traditional resistance training (TRT) strategies using the same isoinertial load during both eccentric and concentric phases of a given exercise are therefore widely used in Rugby Union physical preparation programs (27). However, as greater forces may be produced during eccentric vs. concentric contractions (20), the eccentric phase may be insufficiently loaded during TRT programs. Indeed, compelling evidence indicates that accentuated eccentric loading (AEL) of TRT exercises can induce greater enhancements of strength, power, and speed vs. TRT alone (20,39).

Resistance training that incorporates AEL has been demonstrated to elicit a novel adaptive signal within the neuromuscular system (20). Subsequently, superior adaptations in strength, power, and speed have been reported after chronic training with eccentric overload (13,20,30). The performance improvements observed with AEL are proposed to result from increased volitional agonist activation, increased muscle fascicle length, muscle hypertrophy, a shift toward a faster muscle phenotype (e.g., preferential fast-twitch fiber hypertrophy and a possible increase in type IIx fiber composition), and enhancements in stretch-shortening cycle function (20,37). In addition, there is evidence to suggest that the implementation of faster eccentric contraction velocities or tempos with AEL training may elicit greater improvements in fast muscle phenotypic properties, strength, power, reactive strength, and sprinting performance than slower eccentric tempos (30,37).

Rugby players are required to develop several conflicting adaptations simultaneously such as maximal strength and aerobic power (27). Although simultaneous improvements can be made across divergent components of fitness, concurrent training possibly attenuates the magnitude of adaptation (19). Therefore, the inclusion of AEL may be a particularly useful method of further stimulating the neuromuscular system within a concurrent training program for the rugby player. Indeed, previous investigations have identified isoinertial (13) and flywheel (18) eccentric training protocols to be effective in enhancing measures of strength, power, and speed performance in highly trained team sport athletes with a concurrent aerobic training component. However, to date, there have been no investigations comparing isoinertial AEL training using both eccentric and concentric phases of the movement with a control group completing an ecologically valid TRT program in trained rugby players. In the absence of a control group completing a TRT protocol, it remains unclear whether the previously reported findings were a result of AEL per se or simply the inclusion of a resistance training program. Furthermore, no studies have compared the effects of slow and fast tempo AEL protocols with equivalent TRT protocols, and therefore, it remains unclear how eccentric tempo influences the adaptive response to AEL training in resistance-trained athletes.

Thus, the purpose of this investigation was to elucidate the effects of 4 weeks of slow eccentric tempo AEL training, followed by 4 weeks of fast eccentric tempo AEL training, in comparison with a TRT control group on muscle properties, strength, power, and speed in resistance-trained academy rugby players. It was postulated that AEL would elicit superior enhancements in strength, power, and speed compared with TRT. Furthermore, it was proposed that slow AEL would have a larger influence on muscle properties and maximal strength, whereas fast AEL would have a larger influence on power, reactive strength, and sprinting speed.

Methods

Experimental Approach to the Problem

Resistance-trained academy rugby players were recruited to elucidate the effects of AEL resistance training vs. TRT on muscle properties, strength, power, and speed within an ecologically valid setting. Subjects were within the preparatory phase of their representative program and had previously completed 2, 4-week TRT phases preceding the study period. Subjects were pair-matched based on lower-body strength and allocated through block randomization to complete either AEL or TRT protocols within their resistance training program during the study period. The primary difference between groups was the load used during the eccentric phase of selected strength and power exercises. All other elements of the resistance training program such as exercise selection, sets, reps, tempo, and frequency were matched between groups. In addition, all subjects were recruited from the same provincial academy program, therefore the weekly schedule and training load was approximately equivalent across all subjects for the duration of the study. Both AEL and TRT groups completed 2, 4-week training phases (Figure 1).

F1
Figure 1.:
Study design. 1RM = 1 repetition maximum; AEL = accentuated eccentric loading; IL = inertial load cycle ergometer; TRT = traditional resistance training.

The first phase emphasized a slow eccentric phase tempo and the second phase emphasized a fast eccentric phase tempo. Dependent variables including muscle architectural properties, strength, reactive strength, power, and speed were measured at 3 time points during the study period, including pre-testing at baseline, mid-testing after the first training phase, and post-testing after the second training phase. The effects of AEL and TRT protocols were elucidated through the determination of change scores of dependent variables within and between groups. Effect size (ES) statistics and qualitative inferences were used to determine the magnitude and likelihood of observed effects.

Subjects

Seventeen male resistance trained academy rugby players were initially recruited to participate in this study. After attrition due to contact injury (n = 3), a final sample of 14 subjects (mean ± SD; age: 19.4 ± 0.8 years, range: 18–21 years, height: 1.82 ± 0.05 m, body mass: 97.0 ± 11.6 kg, and relative back squat 1 repetition maximum [1RM]: 1.71 ± 0.24 kg·BM−1) were retained for the initial 4-week training period. One subject was not included in the second, 4-week period because of representative selection. All subjects had at least 1 year of resistance training experience within a supervised program. Subjects were within the preparatory phase of their representative season, although they were also participating in a regional club competition including one ∼80-minute rugby game per week throughout the duration of the study. Subjects were provided with an overview of all study procedures and informed of any potential risks or benefits of participation before data collection. All subjects signed an institutionally approved informed consent document and were older than 18 years. Study procedures were approved by the Auckland University of Technology's Institutional Review Board.

Procedures

Resistance Training Protocols

All subjects completed a combination of strength- and power-based gym sessions, conditioning- and skill-based field sessions, and a club rugby game each week (see Appendix 1, Supplemental Digital Content 1, https://links.lww.com/JSCR/A103). This schedule remained consistent throughout the 12-week study period. Subjects were pair-matched based on lower-body strength and then allocated through block randomization to either an AEL group (n = 7) or a TRT group (n = 7) to be completed within the strength- and power-based gym sessions. Two, 4-week training phases separated by 2 weeks were completed. The first 4-week phase emphasized a slow 3-second eccentric tempo, a lower intensity, and higher repetitions, whereas the second 4-week phase emphasized a fast 1-second eccentric tempo, a higher intensity, and concomitantly lower repetitions, in the back squat exercise (see Appendix 2, Supplemental Digital Content 2, https://links.lww.com/JSCR/A104). The eccentric load for the AEL group was set 18–25% above the TRT load during the strength sessions. Training load during the strength sessions was matched to within ∼10% intensity relative volume (intensity [%1RM as a decimal] × sets × reps) between groups (47). Therefore, concentric intensity was 4–5% lower in the AEL group.

The 2 strength sessions per week completed on Monday and Thursday began with either an AEL or TRT back squat, whereas the power session completed on Tuesday began with AEL or TRT lower-body power movements (see Appendix 3, Supplemental Digital Content 3, https://links.lww.com/JSCR/A105). Those in the AEL group performed back squats in custom-built Smith machine (Goldmine; HPSNZ, Auckland, New Zealand) that provided pneumatic assistance on the lifting, or concentric, portion of the movement to within ±1 kg of the individualized load. The assistance was applied to the barbell through pneumatic cylinders built into the rails of the Smith machine. Subjects were required to descend to a knee angle of approximately 90°, and range of motion was individualized through 2 adjustable switches that were set at the top and bottom of the movement, respectively. The assistance was applied within 0.10 seconds after the barbell triggered the bottom switch at the end of the concentric phase of the movement and was removed 0.25 seconds after the barbell triggered the top switch at the end of the concentric phase of the movement. Tempo was monitored by a linear position transducer sampling at 250 Hz fixed to the bar (Goldmine; HPSNZ).

Those in the TRT group performed a regular back squat with a free barbell in a power rack. Subjects were required to descend to a knee angle of approximately 90° as indicated by a plyometric box set at an individualized height. They were required to perform each repetition at the designated tempo with a verbal count provided by a spotter. Power sessions followed a similar periodization scheme in volume and 3–4 sets of 4–6 reps were completed per exercise while intensity was held constant across each training phase. All subjects included in the final analysis after attrition completed ≥90% of the allocated training program. All training sessions were supervised by 2 experienced strength and conditioning coaches.

Back Squat One Repetition Maximum

Lower limb muscle strength was determined by the back squat 1RM relative to the subject's body mass in kilograms (kg·BM−1). Subjects were required to descend to a knee angle of approximately 90° and touch a plyometric box with posterior thighs. After 4 warm-up sets of 30% (8–10 repetitions), 50% (4–6 repetitions), 70% (2–4 repetitions), and 90% (one repetition) of the estimated 1RM based on recent testing loads, the load was increased until the resistance could not be overcome, with the intention of attaining the 1RM within 3–4 attempts (34). Subjects were instructed to rest passively for 3–5 minutes between maximum attempts. The back squat 1RM has previously been demonstrated to exhibit high (coefficient of variation [CV] < 5%) absolute interday reliability in a cohort of subjects exhibiting similar strength levels to those recruited in this study (12).

Inertial Load Cycling Power

A custom-built inertial load (IL) cycle ergometer (Goldmine; HPSNZ) was used to assess concentric muscle power of the lower limb (33). The IL assessment involves the determination of torque delivered to an ergometer flywheel across a range of pedaling rates (32). The product of flywheel inertia, angular velocity, and angular acceleration with no frictional resistance applied to the flywheel is used to calculate power (32). A warm-up was completed comprising 3 minutes of submaximal cycling at a self-selected intensity followed by one 6-second effort at 90% of self-selected maximal intensity. Subjects then completed 3 trials separated by 2 minutes. Subjects started from a stationary position with the self-selected dominant foot and accelerated maximally for 4–6 seconds (i.e., 6.5 revolutions) on a verbal command with standardized encouragement (33). Seat and handle heights were self-selected by the subject and remained the same across all testing periods. Instantaneous power and torque data were sampled continuously at every 3° of crank rotation and collected using a custom LabVIEW program (National Instruments Corp., Austin, TX, USA) on a personal laptop, and exported to a custom spreadsheet where the parabolic power-velocity and linear torque-velocity relationships were calculated (33). Peak power (W·kg−1) and the cadence at which peak power occurred (revolutions per minute [RPM]) were determined. This protocol has been previously demonstrated (32) to exhibit acceptable interday reliability (CV < 5%). It has been shown in active subjects without cycling experience that 2 familiarization sessions are necessary for the reliable determination of peak power (32); however, the sample recruited in this study regularly completed maximum cycling power assessments in training, and therefore, only one familiarization session was completed.

0.50-m Drop Jump

The drop jump assessment was completed bilaterally from 0.50 m. Subjects completed one practice jump followed by 3 maximal attempts with 30 seconds of recovery between each trial. Subjects were instructed to perform the drop jumps with hands akimbo and to step forward from the box avoiding stepping down or jumping up. They were explicitly asked to simultaneously attempt to minimize their ground contact time while maximizing their jump height but to prioritize a brief ground contact time (8). Trials in which the technique was notably compromised were excluded and repeated. Drop jumps were performed from a plyometric box onto an AMTI force platform sampling at 1,000 Hz (AMTI, Watertown, MA, USA). A custom-designed LabView (National Instruments; version 8.2) program was used to collect and analyze the data. A fourth-order Butterworth low-pass filter with a cutoff frequency of 200 Hz was used to smooth all force-time data. A vertical force threshold of 30 N was used to establish zero force and remove noise of the unweighted plate. The deviation from zero force was used to demarcate the beginning and end of the ground contact phase, and the end of the flight phase. Contact time (s), flight time (s), and reactive strength index (RSI) (flight time divided by contact time) were determined. Leg spring stiffness (kN·m·kg−1) was calculated for each drop jump using a method described previously (17).

40-m Sprint Profiling

Sprint testing was performed in the same lane of the same indoor Mondotrack across all testing periods. A standardized ∼20-minute warm-up including jogging, dynamic stretching, and submaximal 40-m efforts at 70, 80, and 90%, respectively, of self-selected maximal intensity was completed (16). After the warm-up, subjects completed 2 maximal 40-m sprints separated by approximately 5 minutes. Subjects commenced each sprint from a split stance without a countermovement and instructed to accelerate maximally while avoiding any deceleration before the 40-m mark. A radar device (Stalker ATS II; Applied Concepts, Dallas, TX, USA) set 2 m behind the subject and at a height of 1 m off of the ground was used to capture velocity data at a sampling rate of 46.9 Hz. The radar was operated by a portable laptop using the software supplied by the manufacturer (STATS; Applied Concepts). Velocity-time data were filtered and clipped at the point of deceleration within the STATS program. A rollout distance of 0.50 m was included to enable distance-time data comparable with industry standard timing lights.

Maximum velocity (Vmax) and time splits at 10, 20, and 40 m (s) were determined. A high-speed video camera recording at 300 Hz was set adjacent to the track at 35 m to capture maximum velocity kinematic variables between 30 and 40 m. Footage was transferred onto a personal computer and analyzed using the video analysis software (Kinovea 0.8.15). Contact time (s), flight time (s), and step rate (Hz) at maximum velocity were determined from each sprint. Vertical stiffness (Kvert) and leg stiffness (Kleg) at maximum velocity was modeled using the methods previously described by Morin et al. (36). Previous research has demonstrated radar assessment to be a valid alternative to photoelectric cells (44), whereas this method of assessment exhibits high (CV < 5%) absolute intraday and interday reliability (43).

Muscle Architecture

In vivo muscle architecture was measured using 2-dimensional (2D) B-mode ultrasonography using an ultrasound transducer (45-mm linear array, 10 MHz; GE Healthcare, Vivid S5, Chicago, IL, USA). Subjects lie supine on an adjustable bench with their right knee fixed at 45° with muscles relaxed. This joint angle was chosen to minimize fascicle curvature (7). The location of the scan was taken at 50% of the femur length, and the vastus lateralis (VL) muscle was scanned (46). Water soluble transducer gel was applied to the probe head between the skin-probe interface to aid acoustic contact and allow for minimal compression of the muscle (46). Scans were performed with the transducer aligned parallel to the muscle fascicles and perpendicular to the skin (6). Images were stored and transferred to a personal computer to be analyzed in the digitizing software (ImageJ, 1.51j8; National Institutes of Health, USA). Vastus lateralis muscle thickness (cm) was taken as the perpendicular distance between the deep and superficial aponeurosis, and fascicle angle (θ) was defined as the angle of the VL muscle fascicles relative to the deep aponeurosis of insertion (2). As the fascicles often extended beyond the recorded image, fascicle length (cm) across the deep and superficial aponeurosis was estimated by the following equation:where MT refers to VL muscle thickness and θ refers to VL fascicle angle (22). The average of 3 scans was taken for each variable.

Statistical Analyses

Data are presented as mean ± SD. Effect size (90% confidence interval [CI]) statistics were then calculated using a statistical spreadsheet (25) and used to determine the magnitude of change within and between the 2 groups (26). The smallest worthwhile change or difference was calculated as 0.2 multiplied by the between-subject SD based on Cohen's ES principle (11). Threshold values for ES were set as: ≤0.19 trivial, 0.20–0.59 small, 0.60–1.19 moderate, 1.20–1.99 large, 2.00–3.99 very large, and ≥4.00 extremely large. Probabilities were calculated to establish whether the true differences were lower, similar, or higher than the smallest worthwhile change or difference. Quantitative chances of higher or lower differences were qualitatively evaluated as follows: ≤0.99% almost certainly not, 1.0–4.9% very unlikely, 5.0–24.9% unlikely, 25.0–74.9% possible, 75.0–94.9% likely, 95.0–98.9% very likely, and ≥99% almost certain. If the chance of higher or lower differences was >5%, the true difference was deemed to be unclear (26).

Results

Pre-testing Differences

After attrition, several small to moderate differences in performance variables were observed between AEL and TRT groups during baseline pre-testing. Inertial load peak power (ES: 0.87; CI: −0.02 to 1.75) and optimal cadence (ES: 1.11; CI: 0.27–1.95) were moderately higher in AEL vs. TRT, respectively. Drop jump RSI (ES: 0.99; CI: 0.14–1.84) and flight time (ES: 0.85; CI: −0.04 to 1.75) were both moderately higher in the AEL group vs. the TRT group. Subjects in the AEL group exhibited moderately faster 10-m (ES: 1.05; CI: 0.18–1.92), 20-m (ES: 1.01; CI: 0.14–1.89), and 40-m (ES: 1.08; CI: 0.24–1.93) times vs. the TRT group in conjunction with a moderate difference in Vmax (ES: 1.13; CI: 0.31–1.94).

The Effects of Slow Accentuated Eccentric Loading and Traditional Resistance Training Protocols

At mid-testing after the first 4-week training phase, a small improvement was found in back squat strength for those completing slow AEL (Table 1); this improvement was likely superior (+0.12 kg·BM−1; ES: 0.48; and CI: 0.14–0.82) to slow TRT (Figure 2). Slow AEL resulted in small improvements in 20- and 40-m times. The improvement in 40-m time with slow AEL was possibly superior compared with slow TRT (−0.07 seconds; ES: 0.28; and CI: 0.01–0.55). Slow AEL training also elicited likely small improvements in Vmax, contact time, step rate, and leg stiffness. Alternatively, flight time increased with slow TRT in conjunction with a reduction in step rate (Table 2). The reduction in step rate did not seem to impair 40-m performance or the attainment of Vmax in the slow TRT group. Improvements in Vmax (+0.20 m·s−1; ES: 0.52; and CI: 0.18–0.86), contact time (−0.01 seconds; ES: −0.45; CI: −0.78 to −0.12), step rate (+0.20 Hz; ES: 0.83; CI: 0.27–1.39), and vertical stiffness (+0.05 kN·m·kg−1; ES: 0.50; CI: 0.19–0.81) were likely greater with slow AEL vs. slow TRT training. Leg stiffness did not exhibit a clear change with either slow AEL or TRT protocols.

T1
Table 1.:
Performance data (mean ± SD) for subjects (n = 7) in the intervention group completing an accentuated eccentric loading (AEL) resistance training program.*†
F2
Figure 2.:
The standardized (Cohen) difference for subjects completing slow AEL (n = 7) vs. subjects completing slow TRT (n = 7). Differences are for the change in selected performance variables. Negative values indicate a larger effect with TRT, and positive values indicate a larger effect with AEL. Qualitative inferences indicate a positive or negative effect of AEL vs. TRT. Error bars indicate uncertainty in the true mean changes with 90% confidence intervals. The shaded area represents the smallest worthwhile change. 1RM = 1 repetition maximum; AEL = accentuated eccentric loading; ES = effect size; IL = inertial load cycle ergometer; RSI = reactive strength index; TRT = traditional resistance training; Vmax = maximum sprinting velocity.
T2
Table 2.:
Performance data (mean ± SD) for subjects (n = 7) in the control group completing a traditional resistance training (TRT) program.*†

In contrast to the changes in strength and speed after slow AEL at mid-testing, there was a moderate increase in IL optimal cadence with slow TRT (Table 2), which was likely greater compared with slow AEL (+3.6 RPM; ES: 0.52; and CI: 0.12–1.16). There was a possible small increase in drop jump flight time with slow TRT; however, this had no effect on the RSI performance measure. Neither slow AEL nor slow TRT protocols influenced muscle architectural variables (Table 3).

T3
Table 3.:
Vastus lateralis muscle architectural data (mean ± SD) for subjects completing slow and fast accentuated eccentric loading (AEL; n = 7) and traditional resistance training (TRT; n = 7) programs.*†

The Effects of Fast Accentuated Eccentric Loading and Traditional Resistance Training Protocols

At post-testing after the second 4-week phase of training, a likely reduction in drop jump contact time was observed in the fast AEL group resulting in a possible small increase in RSI and likely moderate increase in leg stiffness (Table 1). The reduction in contact time with fast AEL was likely greater (−0.02 seconds; ES: 0.66; and CI: −0.12 to 1.43) than fast TRT (Figure 3). However, there was no clear difference in RSI. In contrast to the slow phase of training, there were moderate reductions in 10-, 20-, and 40-m times with fast AEL. Furthermore, there were small reductions in Vmax in both fast AEL (Table 1) and fast TRT (Table 2) groups. No differences were observed between fast AEL and fast TRT for any sprint performance variable (Figure 3).

F3
Figure 3.:
The standardized (Cohen) difference for subjects completing fast AEL (n = 6) vs. subjects completing fast TRT (n = 7). Differences are for the change in selected performance variables. Negative values indicate a larger effect with TRT, and positive values indicate a larger effect with AEL. Qualitative inferences indicate a positive or negative effect of AEL vs. TRT. Error bars indicate uncertainty in the true mean changes with 90% confidence intervals. The shaded area represents the smallest worthwhile change. 1RM = 1 repetition maximum; AEL = accentuated eccentric loading; ES = effect size; IL = inertial load cycle ergometer; RSI = reactive strength index; TRT = traditional resistance training; Vmax = maximum sprinting velocity.

A small increase in IL peak power was observed with fast TRT at post-testing, which was likely greater (+0.72 W·kg−1; ES: 0.40; and CI: 0.00–0.79) than fast AEL (Figure 3). There was a possible small increase in VL pennation angle with fast TRT; however, no clear difference vs. fast AEL was observed (Table 3).

Discussion

This study compared the effects of slow and fast tempo AEL resistance training with a control TRT program on muscle architectural properties, strength, power, and speed performance. The main finding was that 4 weeks of slow AEL resistance training was superior to slow TRT resistance training in improving lower-body strength and sprinting speed in resistance-trained rugby players when integrated within a concurrent training program. By contrast, besides a possible increase in reactive strength, a second 4-week training phase of fast AEL did not seem to elicit any further improvements in strength or speed, and may have compromised the previously observed enhancements in sprint performance. These results partly support our hypothesis of the superiority of AEL training vs. TRT in team sport athletes completing a concurrent training program. However, the pattern of adaptation differed from what was initially hypothesized. It was also identified that team sport athletes may be less responsive to either fast eccentric stimuli, or susceptible to eccentric-related fatigue and impairments in performance with this periodization approach.

It has been well established that eccentric training can lead to greater increases in total strength, that is, combined eccentric, isometric, and concentric strength, than concentric training (39). The superiority of slow AEL in increasing lower-body strength in this study is arguably underpinned by neural mechanisms, as no clear changes in VL muscle thickness, fascicle angle, or fascicle length were observed (20). Although program volume was closely matched between AEL and TRT groups, those completing slow AEL were exposed to absolute loads 18–25% higher than those completing slow TRT, which may have elicited a disinhibition of mechanisms constraining volitional agonist drive and therefore force production (1). It seems that the TRT protocol provided an insufficient stimulus to increase strength in 4 weeks in a resistance-trained cohort undergoing concurrent aerobic training. Indeed, it has previously been identified the difficulty in increasing strength and power in academy-aged rugby players with a high training load (4). This may be explained by the interference phenomenon whereby concurrent aerobic training attenuates the magnitude of adaptation to a given strength program because of divergent phenotypic signals (19). Concurrent aerobic training possibly attenuated myofibrillar protein synthetic rates in response to both protocols (10); however, the additional neural stimulation with slow AEL elicited a small increase in strength independent of changes in muscle cross-sectional area.

Slow AEL induced a superior improvement in 40-m sprint performance vs. slow TRT, with no clear differences observed between the groups for the shorter distances. The improvement in 40-m sprint performance with slow AEL was accompanied by an increase in maximum velocity and underlying kinematic variables. The ability to apply greater mass-specific vertical forces and possibly maintain a stiffer leg spring seem to underpin the attainment of a faster maximum velocity (48). Indeed, it has been consistently demonstrated that improvements in back squat strength positively transfer to sprinting speed (42). The reduced contact time and concomitant increases in step rate and vertical stiffness indicate improvements in mass-specific vertical force production and lower limb stiffness after 4 weeks of slow AEL training. The storage and return of energy within elastic structures of the lower limb play an increasingly important role at higher sprinting speeds up to maximum velocity (9). As previous research has identified the efficacy of eccentric training protocols in increasing tendon stiffness (31) and upregulating muscle collagen synthesis rates (24), the improvement in maximum velocity with slow AEL may have been partly related to modulated stiffness properties of tendon and fascial elements within the lower limb (9,38). Nonetheless, muscle-tendon unit tissue stiffness was not directly measured in this study, and therefore, any contribution remains speculative.

No changes in IL peak power were observed in either group after the slow phase of training. Perhaps, in contrast to expectations, those completing slow TRT, however, did exhibit an increase in the cadence at which peak power occurred. A higher optimal cadence is generally believed to reflect a larger proportion of fast-twitch muscle fibers comprising the lower limb musculature (23). However, there is little corroborating evidence such as coinciding improvements in speed and power to indicate an increase in fast-twitch fiber composition with slow TRT. The training protocol in this study may have been a sufficient stimulus to improve velocity-specific neural activation at higher cadences independent of changes in muscle morphological or architectural properties (41), and indeed, concentric power. There were no changes in drop jump RSI for either group after the slow training phase. Neither protocol included specific reactive strength exercises in this phase of training, and this strength quality is considered to be largely influenced by neuromuscular qualities such as muscle preactivation, reflex excitability, and rapid force application, which may be best developed through exposure to the task (3).

The second 4-week phase of training with faster contraction speeds did not induce any further improvements in strength in either group. There was a small increase in IL peak power with fast TRT, which may reflect the efficacy of the strength and power training (15), or indeed, the incurrence of less fatigue than the fast AEL protocol. A small increase in VL pennation angle with fast TRT, which possibly represents a small increase in physiological cross-sectional area of the muscle (28), may also have contributed to the observed improvements in IL peak power (15). The apparent detrimental effect of fast AEL on IL peak power compared with fast TRT may have been a result of residual fatigue. Chronic eccentric exercise in combination with the concurrent training load possibly suppressed a positive training effect (29). Indeed, the lack of improvement in strength, moderate impairments in 10-, 20-, and 40-m sprint performance, and a small impairment in maximum velocity after fast AEL all suggest a fatigue-induced suppression of performance. Previous research has found that improvements in concentric power reached their peak 8 weeks after the cessation of an eccentric training intervention (29), whereas improvements in concentric force production have been shown to peak after 6 weeks of detraining following eccentric training (14).

A longer recovery period is likely necessary to allow for improvements in performance to materialize after an extended (e.g., 8-week) period of chronic eccentric training (20,29). This may be especially relevant to rugby players undergoing a concurrent training program whereby residual fatigue is likely to be even greater than in previous reports. In contrast to the impairments in strength and speed, there was an improvement in drop jump RSI with fast AEL, which was underpinned by a reduction in ground contact time. This finding suggests that reactive strength is either less susceptible to fatigue incurred by chronic eccentric training than other measures, or alternatively, may have exhibited a more substantial improvement had an extended recovery period been available. It is possible that the specific nature of the fast AEL protocol with the inclusion of overloaded drop jumps underpinned these performance responses. It has previously been demonstrated that contact time during a drop jump from 0.50 m is determined primarily by braking, or eccentric, force production (21). The inclusion of AEL drop jumps likely had a marked effect on eccentric force production capabilities, which in turn lead to an improvement in contact time during unloaded drop jumps.

There were several methodological limitations that may affect the interpretation of these data. We were restricted in the number of subjects available in the academy training squad and therefore sample size. After attrition (n = 3) due to injury or absence, baseline differences between groups were magnified and subjects within the AEL intervention group were moderately more powerful and faster than the TRT control group. This may therefore have confounded the training responses to several variables. Although it should be noted that it is more difficult to elicit adaptation in individuals with a higher baseline or more training experience (5), these differences plausibly attenuated the efficacy of the AEL protocol. We tested one periodization model of slow followed by fast AEL and TRT, and it is not clear whether the performance effects observed were due to tempo per se or the order of the training blocks. Further research should therefore investigate the effects of fast tempo AEL training without a preceding slow AEL phase. Finally, the inclusion of concurrent conditioning training units may be considered a limitation; however, we believe that this improved the ecological validity of the study.

These findings are proposed to be relevant to the practitioner seeking to implement eccentric training with trained rugby players or team sport athletes undergoing a broader physical preparation program. The short-term, 4-week incorporation of slow AEL seems to be superior to commonly implemented TRT in improving lower limb strength and maximum velocity sprinting speed in rugby players undertaking a concurrent preparatory program. Aside from a possible improvement in reactive strength, a second 4-week phase of fast AEL did not lead to additional improvements in strength, power, or speed. Indeed, previously realized improvements in speed may have been suppressed because of residual fatigue.

Practical Applications

The additional eccentric load afforded by slow AEL may provide a superior stimulus to the neuromuscular system, a stimulus that could be especially relevant to trained athletes simultaneously attempting to increase strength, power, speed, and aerobic fitness. Although the improvements were generally of a small magnitude, these findings are nonetheless of interest to sport scientists and strength and conditioning practitioners given the short duration of the intervention, the training status of the subjects, and the inclusion of conflicting modalities that likely interfered with neuromuscular adaptation. As this method of training is highly taxing to the neuromuscular system, 8 weeks of AEL training may be inappropriate within a team sport setting, unless a sufficient recovery period is available to realize performance responses.

Acknowledgments

The authors have no conflicts of interest to disclose.

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Keywords:

resistance training; neuromuscular performance; athletes

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