Dynamic accentuated external resistance (DAER) action refers to loading patterns where the eccentric (ECC) phase of the repetition is followed by the concentric (CON) phase with a lower load (4,7,8). Alteration of the load at different contraction types of the repetition has been shown to contribute to increased maximal strength output (7). Possible explanations for increased maximal concentric force output because of increased ECC load could be increases in neural stimulation, recovery of stored elastic energy, contractile machinery alterations, and increased preload (7).
Some recent studies using this technique have examined the amount of loads to be used for the ECC phase. The loads used in the ECC phase of various high load-low repetition protocols (e.g., 1 repetition maximum [RM]) have varied among 105, 110, 120, and 140% of CON 1RM (1,4,7,8,10,12,13). Based on the data available, no conclusive statement can be given with regard to the optimal ECC load that would maximize maximal concentric force production. In the case of a high load-multiple repetition protocol (e.g., 10RM), used typically in training for muscular hypertrophy, a recent study (15) investigated different DAER loads and concluded that the 90/70% of 1RM (ECC/CON, respectively) loading within a protocol of 4 sets of 10RM might to be beneficial. This conclusion was based on favourable acute metabolic and growth hormone responses induced by the protocol suggesting its possible beneficial use in long-term training for muscle hypertrophy.
In addition to maximum strength (high load-low repetition) and hypertrophic strength (medium to high load-multiple repetitions), muscle power is used to determine the capacity to perform explosive muscle action (2,3,6,11). Optimal loads in pure concentric actions for power output are somewhat lower and seem to be within the range of 30-70% of 1RM, depending on whether the lower- or upper-extremity action was used (11,14). The results obtained in studies investigating stretch-shortening cycle (SSC), elastic component stiffness, or accentuated eccentric resistances suggest the optimal ECC-CON resistance to favour CON force and power production. In a study of Wilson et al. (18), the optimal elastic component stiffness maximized force output during the bench press, and a suitably stimulated ECC phase of the ECC-CON contraction favored work output (17). However, the role of different DAER load levels during the ECC phase of various ECC-CON loading actions for optimal power production has received much less experimental attention. If this kind of protocol can optimize power production during a single training session, it might also be beneficial for training purposes.
This study examined the effects of different dynamic accentuated external resistance (DAER) load levels during the ECC phase of the ECC-CON loading actions on acute neuromuscular, force, and power responses in the bench press exercise. Specific attention was paid to the loads that might be beneficial in the ECC-CON loading strategy to be used for explosive strength and power production in comparison to the loads that might be optimal for maximum strength output.
Experimental Approach to the Problem
Each subject performed first the maximum strength measurements consisting of 4 conditions with different ECC loads (DAER resistance) in the ECC-CON actions. The first one was the traditional 1RM test to find out simply the maximum load each subject could do in the bench press exercise. Then, 3 different load levels of 105, 110, and 120% of concentric 1RM were used in the ECC phase, whereas the load in the CON phase was sustained at 100% of concentric 1RM. Second, each subject performed the explosive strength (power) measurements in the ECC-CON action in the bench press exercise. The first one was the traditional one to find out the maximum power each subject could produce with the 50% load of concentric 1RM. Then, 4 different load levels were used so that the loads for the ECC phase were 60, 70, 80, and 90% of concentric 1RM, whereas the CON resistance was sustained at 50% of concentric 1RM.
Electromyography (EMG) activity of the agonist muscles, ECC, CON, and isometric forces and power were measured before the loading and immediately after each loading condition. We hypothesized that the acute neuromuscular, maximal strength, and power responses would be more favourable at specific optimal loads than at the constant loads used during the traditional ECC-CON actions.
The subject group consisted of 11 healthy, physically active men. Their mean age was 32.4 ± 4.3 years and body mass (BM), height, and body mass index were 86.3 ± 8.8 kg, 177.8 ± 6.2 cm, and 27.4 ± 0.6, respectively. All subjects had previous experience in resistance training. They were selected based on their bench press maximum related to their BM, and they were able to lift a load of 1.2-1.4 times their BM. The subjects were informed in detail of the research project, and they also signed a written consent form before the measurements were performed. The subjects were aware that they were allowed to withdraw from the study at any time if they felt uncomfortable to continue. Before the familiarization period, the subjects were informed that any abuse of anabolic hormones or any other food supplement except regular pure protein or carbohydrates were prohibited throughout the study.
Before Experiment Maximum Strength Testing
Before the participation to any of the measurement sessions, the subjects completed a 2-week familiarization training period. The 1RM measurement was performed before participation to the first dynamic maximum strength measurement session A (Table 1). This was performed for the 1RM:BM ratio determination and as a preparation for the dynamic maximum strength measurements. The purpose was to provide familiarization to the specific measurement equipment used during the actual measurements.
Dynamic Maximum Strength Measurements
The maximum strength measurements consisted of 3 conditions with different ECC loads (DAER resistance) in the ECC-CON actions. The loads for the ECC phase were 105 (session A), 110 (session B), and 120% (session C), whereas the load in the CON phase was sustained at 100% RM. Maximum strength sessions A, B, and C were performed in a randomized order (Table 1). Between each effort, a 3-minute recovery was held, and the subjects were instructed to sit on a bench for that time. If the subject failed to lift the resistance, 1 additional attempt was given with that load. After a consecutive failure, the CON load was reduced by the bar weight of 2.5 kg. If the subject succeeded to lift the load, the CON load was increased by 2.5 kg, i.e., 100 to 102.5 kg, and the ECC load was held at 105% of the CON 1RM. In this case, it was increased to 107.625 ~ 107.5 kg. Again, 2 attempts were provided (2 × 1 × 107.5/102.5 kg). This was continued as long as the subject failed to lift the new CON load in 2 attempts. The load in the weight releasers was always chosen such that the percentage ECC resistance was maintained according to the present condition (105, 110, or 120% RM). No cheating from the appropriate bench press technique was accepted.
Dynamic Explosive Strength Measurements
After a recovery period of 5-7 days, the subjects participated in the explosive strength measurement session. This consisted of 4 conditions of different ECC loads (DAER resistance) in the ECC-CON actions (Table 1). In addition to the control condition measurement with the load of 50/50% (ECC/CON, respectively), the ECC (DAER) loads of 60, 70, 80, and 90% were examined, whereas the CON resistance was sustained at 50% RM. The accuracy of 1.5 kg was used in the ECC actions (the smallest discs used in the weight releasers were 0.75 kg), whereas the accuracy in the CON action was 2.5 kg (the smallest discs used in the bar were 1.25 kg). The explosive strength session (D) was organized such that participants performed the different conditions in a randomized order. Two repetitions were performed with each of the loads during the first round, and the same was repeated during the second round. Only 1 out of the total of 4 repetitions in each condition (round 1 and 2 = total of 4 repetitions) was analyzed. The analyzed repetition was selected by the shortest time used for the performance. The CON phase was always performed with the maximum attained velocity for peak power and force production.
Maximum isometric (ISOM) bench press measurements were performed before and after the dynamic bench press measurements (Table 1). Elbow angle was kept at 90 degrees and the arms at 90 degrees abducted and to be maintained on the frontal plane. Maximum elbow flexor muscular activation was measured in ISOM maximum voluntary contraction (MVC) biceps curl for determination of antagonist coactivation after all the sessions A, B, C, and D.
The variable resistance was produced by the help of weight releasers (7,15). The weight releasers have the potential to maximize the resistance in the ECC phase and to reduce it in the CON phase of the movement. The weight releaser is detached from the strength training bar at the end of the ECC phase of the movement, whereas the CON phase can be performed with a lower resistance. The weight releasers detach from the bar and remain at the floor after the performance. Therefore, an assistant returns the weight releasers on the bar after each repetition for consecutive repetitions.
Electromyography activity was measured during both dynamic and ISOM activities. The skin was prepared at the electrode locations by shaving and abrading with antiseptic alcohol, and skin impedance was monitored to be less than 20 kohm. Bipolar surface EMG electrodes were assigned according to the SENIAM (16) guidelines for the medial portion of the triceps brachii (TB), medial deltoid anterior (DA), and pectoralis major (PA). To measure the degree of antagonist muscle activation, an EMG signal was also obtained from the biceps brachii (BB). As the PA electrode locations have varied in previous studies, various electrode locations were tested in a pilot session to determine the most suitable place for recording. PA electrode location was determined according to Cogley et al. (5). The PA electrode was placed at a point one-third of the distance between the anterior aspect of the acromion and the xiphoid process. The determined electrode locations were marked with a small ink tattoo dot after the first visit to the laboratory for consecutive visits (9).
The EMG electrodes were connected directly to the small preamplifiers located approximately 10 cm from the recording site. The signals were led through shielded wires to the telemetric Noraxon Telemyo 2400T V2 transmitter (Noraxon U.S.A. Inc, Scottsdale, Calif.), with a bandwidth of 10-10,000 Hz and a common mode rejection ratio >100 dB. EMG signal was then captured by the Noraxon 2400R receiver from where it was transferred to the signal processing analogue to digital converter and to the actual recording unit. EMG signal was rectified and band pass filtered according to the recommendations of SENIAM (16).
Joint Angle Measurement
To determine joint angle displacement and velocity, an electronic goniometer (Department of Biology of Physical Activity, Jyväskylä, Finland) was used. The goniometer was adjusted at the elbow joint, and it was connected to the computer through the signal processing analogue to digital converter. The goniometer was located such that the other shaft of it was in line with the humerus and another was in line with radius. The midflipping point of the goniometer was located on top of the lateral epicondyle of the humerus. The goniometer signal was low-pass filtered at a frequency of 20 Hz.
Bar Displacement and Velocity
Bar displacement and velocity were determined using the ultrasonographic Axon device. The Axon was located perpendicularly under the weight bar at the starting position of the movement. The recording was started from the starting point where the subject was supine laying and holding the bar with the hands with extended elbow joints.
Bench Press Technique
All the actions were performed lying supine while maintaining both legs lifted up to the 90-degree hip angle. An arched back or bouncing from the chest was not allowed. Also, the grip was controlled such that the thump was required to go around the bar (thump lock was not allowed). The subjects were instructed to choose the grip width on the bar that was wider than shoulder width. The final adjustment to the grip width was performed by the subject, and then the chosen width was marked on the bar and kept for the consecutive testing sessions. The grip width was marked on the bar by tape marks. The location of the subject's head on the bench was standardized and remained the same. A repetition started from a position where the subject held the bar on his hands with straight elbows. The bar was expected to be lowered to a vertical distance of 0.5-1.0 cm on top of the tip of a sternum (xiphoid process) at the lowest bar position.
The results were provided as group mean ± SEM. Four-factorial repeated-measures analysis of variance (ANOVA) (p ≤ 0.05) was applied to all absolute, absolute change, and percentage change of force, velocity, and EMG data for statistical significances within and between the group means. Also, a two-tailed pair wise comparisons t-test (p ≤ 0.05) was applied for significances between the control and experiment repetitions in maximum dynamic strength measurements.
Maximum Strength Sessions
One Repetition Maximum and Force
No significant differences were observed between the loading conditions within the control and experimental measurements in CON 1RM (Figure 1). However, in all experimental conditions, CON 1RMs reduced significantly compared with the control condition, with the greatest decrease recorded in the 120/100% condition from 108.4 ± 3.6 to 104.5 ± 3.3 kg (p < 0.01).
No significant differences were observed within the conditions in mean CON force, but reductions between the control and the experimental conditions occurred in the 110/100 and 120/100% conditions (p < 0.01) (Figure 2). No differences were observed within the loading conditions in the control and experimental measurements in mean ECC force, but increases from the control to the experimental condition occurred in the 110/100 and 120/100% conditions (p < 0.001).
No significant differences were observed between the loading conditions in the control and experimental measurements in peak ISOM force. However, significant reductions from before to after the measurement were observed in the loading conditions of 105/100 (p < 0.001), 110/100 (p < 0.001), and 120/100% (p < 0.001).
No significant differences were observed in any of the muscles within the conditions or between the control and experimental conditions in ECC EMG activity (Figure 3). No significant differences were observed in muscle activities between the conditions or between any of the control and experiment conditions in CON EMG activity (Figure 4). No significant differences were observed in ISOM antagonist coactivation of BB during the exertions between the 3 conditions or between the control and experimental conditions.
No significant differences were observed in mean bar velocity or in mean elbow angle velocity within or between the conditions. ECC mean (±SEM) bar velocities ranged from −0.29 ± 0.02 to −0.27 ± 0.02 m·s−1 in the control conditions and in the experimental conditions from −0.27 ± 0.03 to −0.22 ± 0.04 m·s−1. In the CON (±SEM) phase, mean bar velocities ranged from 0.11 ± 0.00 to 0.12 ± 0.14 m·s−1 during the control conditions and during the experimental conditions from 0.10 ± 0.00 to 0.11 ± 0.01 m·s−1.
Explosive Strength Session
Power and Force
No significant differences were observed between the loading conditions in peak CON power, although the highest peak power was observed in the 70/50% condition (Figure 5). Figure 6 illustrates power levels in the control 50/50% condition compared to the condition (77.3 ± 3.2/50%) that produced the greatest power for each individual. Peak power of 1,060 ± 55 W in the individually highest condition was greater (p < 0.001) compared to that of 946 ± 59 W recorded in the control condition. Mean power also differed (p < 0.05) between these two conditions. Significant differences in ECC force were observed in the 80/50% (891 ± 50 N) and 90/50% (961 ± 46 N) conditions compared to the 50/50% (640 ± 46 N) condition, between the 70/50% and 80/50% conditions, and between the 80/50% and 90/50% conditions. No significant differences were observed in CON force. No significant differences were observed between the first and last condition in ISOM force.
Eccentric deltoid mean EMG activity was higher in the 80/50% (p < 0.05) and 90/50% (p < 0.01) conditions compared to the control 50/50% condition (Figure 7). No significant differences were observed in mean ECC PA or in TB EMG activity between the loading conditions. No significant differences were observed between the loading conditions in CON mean EMG activity in any of the muscles (Figure 8). However, a significant difference was observed between the control 50/50% condition compared to the condition (77.3 ± 3.2/50%) that produced the greatest power for each individual in ECC-agonist EMG activity but not in CON-agonist EMG activity (Figure 9). No significant differences were observed in ECC- or in CON-antagonist coactivity of BB, which ranged in ECC from 22 ± 5% (50/50% condition) to 39 ± 11% (90/50%) and in CON from 14 ± 3% (70/50%) to 17 ± 4% (50/50%).
Vertical bar velocity in the ECC phase was significantly lower (p < 0.001) compared with bar velocity in the CON phase in all of the loading conditions. No significant differences were observed between the conditions in vertical bar velocity because ECC bar velocity ranged from 0.45 ± 0.04 m·s−1 (50/50% condition) to 0.38 ± 0.05 m·s−1 (70/50%) and CON mean bar velocity from 0.80 ± 0.02 m·s−1 (70/50%) to 0.83 ± 0.02 m·s−1 (60/50%). No significant differences were observed between the loading conditions in mean ECC elbow angle velocity. However, CON elbow angle velocity decreased in all experiment conditions of 60/50, 70/50, 80/50, and 90/50% (17.88 ± 0.40, 17.80 ± 0.47, 17.72 ± 0.57, and 17.91 ± 0.57 degrees·s−1, respectively) compared with the control condition of 50/50% (31.4 ± 0.5 degrees·s−1) (p < 0.001).
The data of the present study showed that the different heavy DAER loads during the ECC phase of the ECC-CON bench press action did not enhance maximum concentric strength production. During the explosive actions, concentric power production did not change systematically with the change of the DAER load, but interestingly, power with the optimal load for each individual was larger compared with power created during the traditional control condition.
Previous studies have demonstrated that increased concentric force output could be enhanced because of an increased ECC load and that possible explanations could be increases in neural stimulation, recovery of stored elastic energy, contractile machinery alterations, and increased preload (e.g., 7). However, the present maximum strength sessions showed that maximal concentric force production in the bench press action did not benefit from the application of the different DAER loads. Actually, CON 1RM and CON force reduced in all of the experimental conditions of 105/100, 110/100, and 120/100% compared to the traditional control condition of 100/100%. Possible reasons for these conflicting findings may result, in part, from differences in the experimental design, equipment used, range of loads used (e.g., maybe a need for somewhat smaller loads for the ECC actions), possible fatigue produced by the present protocol, and trained status of the subjects. In addition to the magnitude of stretch, elastic energy can be affected also by time between ECC and CON actions, as well as velocity of stretch (7), but the latter 2 variables were not specifically manipulated in this study as possible contributors to maximal concentric force production.
Nevertheless, as expected, ECC force increased in the present study from the control to the experimental conditions systematically such that greater DAER loads were associated with greater increases in ECC force. This simply suggests that the greater ECC loads required greater force generation. However, the increased ECC force in the present DAER loadings failed to provide additional stimulus for the neuromuscular system that would have contributed to the increase in maximal concentric force production of the bench press action (7). ISOM force reduced significantly from pre- to postmeasurement in all of the conditions. This suggests that fatigue progressed at a similar rate in each condition despite the greater ECC forces required with the greater DAER loads. Our data further showed that in the maximum strength bench press action, ECC agonist muscle activity remained the same throughout the control and the DAER loading conditions. Similarly, no significant differences were observed in any of the muscles between the conditions or between any of the control and experimental conditions in CON EMG activity. These findings suggest that none of the present DAER loads led to systematic changes in muscular activity. The present data further showed no significant differences in antagonist coactivity during the experimental and control loading conditions. Nevertheless, the finding that no significant differences were observed either in ECC or CON bar velocity was in agreement with the force and muscle activity observations of the present study.
The data from our explosive strength session showed no significant differences between the experimental and control conditions in the group comparisons of peak power and force. Furthermore, no significant differences were observed in CON force output, but, as one could expect, ECC peak force increased from the 50/50% condition to the experimental conditions in which higher ECC loads were used. These differences in ECC peak force can be simply explained by the changes in ECC loads in the different conditions.
Interestingly, our further analyses showed that the condition of 77.3 ± 3.2/50% that produced the individually highest power values differed significantly from power recorded in the control condition of 50/50%. The difference was significant both with regard to individual peak and mean power production. In the light of the present power and force data, the differences observed only in the individual comparison clearly suggest that a selection of the load for the DAER actions in the bench press seems be most effective when the load is selected individually instead of selecting the load based on the group mean.
The present data further showed that a significant increase occurred from the 50/50% condition to the 80/50% and 90/50% conditions in ECC mean deltoid EMG activity. No significant differences were observed between the conditions either in ECC BB, PA, TB, or in any of the examined muscles during the CON phase in EMG activity. The ECC deltoid activity differences can be explained by the changes in the ECC load in the different conditions. The increase in DA during the ECC contractions may, in part, relate to the increase in power output during the DAER loading conditions. When the individual EMG data were used, the significant difference occurred in ECC EMG activity of the 3 agonists between the condition of 77.3 ± 3.2/50% that produced the individually highest power compared with the control 50/50% condition but not in the CON agonist EMG activity. The present increase in ECC EMG activity concurrently with the increased power output from the control to the experimental condition suggest that use of the elastic component may have been higher during the present experimental conditions, contributing to increased power production. The changes in force and power in the present study could be related to optimal stiffness of the elastic components of the muscles (18). In addition, the differences between the individual results in the explosive DAER loading conditions could be related to differences in the training adaptations in the elastic elements based on the type of exercises the subjects had been accustomed to in the past. Hence, it could be expected that the individuals with more trained elastic elements with a larger recovery of stored elastic energy could have produced more power with the greater DAER loads compared to individuals with lower levels of trained status. It could be suggested that each individual should “seek” for the optimal threshold level in the elastic properties of the muscles leading to the greatest possible power production in various explosive DAER exercises.
Mean CON elbow angle velocity decreased significantly from the control (50/50%) condition to all of the experimental conditions (60-90/50%). The great reduction in mean elbow angle velocity could have contributed to the significant differences observed in individual power responses between the control and experiment conditions. Despite this significant difference noted in mean elbow angle velocity, no significant differences were observed in mean bar vertical velocity. This could be an indication of minor alterations occurring in the bench press technique between the control and experimental conditions despite the fact that the grip width and position in the bench were carefully controlled throughout the experiment. Possible alterations in the bench press technique could be related to the distance of the elbows respective to the medial line of the body, and the upper arms could have been more abducted during the experimental conditions, although no data were collected to verify this.
To summarize, no benefits for maximal CON strength production were observed in the present bench press action caused by the application of the different DAER loads of 105/100, 110/100, or 120/100% of 1RM. Actually, CON 1RM and CON force reduced during the DAER conditions, as compared to the traditional control condition of 100/100%. The data from the present explosive session showed that concentric power production during the explosive actions did not change systematically with the increase of the DAER load, but power was significantly larger in the loading condition of 77.3 ± 3.2/50% that produced the highest power for each individual compared with power produced during the control condition of 50/50%.
The primary findings showed that during the present explosive actions, CON power was significantly larger in the loading condition of 77.3 ± 3.2/50% that produced the highest power for each individual compared with power produced during the traditional control condition of 50/50%. Thus, the present data suggest that a proper load for the ECC phase of the DAER explosive loading conditions for optimal CON power production should be based on the individualized selection. The levels of maximal strength and power, individual neuromuscular performance characteristics, as well as specific training history may contribute to the magnitude of the optimal load. A longitudinal training study is required to investigate possible benefits of explosive strength training using individually based optimal loads for the ECC phase of the DAER loading protocol.
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Keywords:© 2009 National Strength and Conditioning Association
DAER loading; maximum strength; muscle activation; power production