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Original Research

Changes in Maximal Strength, Velocity, and Power After 8 Weeks of Training With Pneumatic or Free Weight Resistance

Frost, David M.1; Bronson, Stefanie1; Cronin, John B.2,3; Newton, Robert U.3

Author Information
The Journal of Strength & Conditioning Research: April 2016 - Volume 30 - Issue 4 - p 934-944
doi: 10.1519/JSC.0000000000001179
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Abstract

Introduction

The physical preparation of any athlete requires an appreciation for the loads, speeds, and movement patterns fundamental to their sport. As such, there may be value in knowing how or why a particular mode of resistance (e.g., mass, elastic, and pneumatic [PN]) could afford an adaptation that would otherwise be impossible, or at least difficult to achieve with another modality (13). Simply because mass reflects the type of resistance an athlete moves on a day-to-day basis (i.e., their bodyweight), does not imply that it should be the only method of loading used during training. For example, dissimilar inertial properties between free weight (FW) and PN resistance dictate that each resistance will offer a unique stimulus (11) and, therefore, may be more suitable to achieve a particular response or adaptation to training.

As an athlete progresses from phase to phase through a periodized program, coaches will modify repetitions, sets, loads, speeds, rest periods, and/or movement patterns in a linear or nonlinear fashion (4,24,26,27). However, rarely is there consideration given to changing the mode of resistance, despite the fact that transfer of training could be limited by the inertial properties of the loads being used. Free weight resistance, which comprises only mass, exhibits momentum once in motion; hence, the reason we are able to leave the ground when jumping. But this is also the reason why athletes are pulled from the bench when performing a FW bench press as quickly as possible. Once sufficient forces are applied to initiate motion, the mass gains momentum, which reduces the muscular effort required to complete the repetition over the range of movement (11). Furthermore, if the load is not thrown, the performer may need to actively decelerate the barbell during the latter half of the pressing phase (i.e., produce a force in opposition to the direction of travel (25)).

The popularity of band and PN resistance stems largely from the fact that each modality offers a load comprising minimal mass and therefore less inertia and momentum in comparison with that of FW (13). Instead, the external loads are created through band tension or air resistance, respectively, which may afford an opportunity to elicit unique training adaptations. For instance, higher accelerations can be achieved with external loads comprising less mass if the same force is produced (i.e., F = ma), which suggests that either modality could offer unique velocity-specific adaptations to training, or provide a means by which a particular coaching objective could be achieved in a safer simpler fashion. However, there may also be instances where FW resistance does afford an ideal stimulus and perhaps should be viewed as the most appropriate method of external loading (e.g., increasing 1 repetition maximum [1RM] strength). Against this backdrop, our primary objective was to examine the strength, velocity, and power adaptations exhibited by resistance-trained men in response to 2 eight-week periodized exercise programs, differing only in the type of resistance (FW or PN) used to perform heavy and explosive bench press efforts. It was hypothesized that training with PN resistance would elicit larger improvements in peak velocity and power at light loads, whereas training with FW resistance would be more conducive to increasing peak force and maximal strength.

Methods

Experimental Approach to the Problem

Resistance-trained men (3–15 years of experience) completed 2 baseline tests to determine their FW and PN bench press 1RM. During the FW testing session, participants also performed sets of 4 explosive bench press repetitions at loads of 15, 30, 45, 60, 75, and 90% 1RM. A force plate and linear potentiometer were used to measure ground reaction force and barbell displacement, respectively. On completing the baseline tests, participants were assigned to one of the 2 training groups (FW or PN), each matched for 1RM strength. Individuals attended three 90-minute whole-body resistance training sessions each week over a 2-phase 8-week period. Each phase comprised of a strength (80–95% 1RM) and power (30–45% 1RM) component. Both intervention groups completed identical training programs with the exception of the type of resistance used to perform all bench press movements. Once the 8-week training program was completed, the baseline tests were repeated. Participants' FW and PN 1RM, and peak force, velocity, and power at each submaximal load were computed before and after training.

Subjects

Eighteen men with at least 3 years of resistance training experience and a maximum bench press greater than their body weight volunteered to participate in this investigation. To detect a 10% difference with 80% power and a significance level of 95%, it was estimated that 16 participants would be needed (8 in each group). At the time of testing, all men reported their resistance training frequency to be a minimum of 2 times per week. Their mean (±SD) age, height, body mass, and resistance training experience were 23.9 (±4.1) years, 1.79 (±0.07) m, 79.8 (±11.8) kg, and 5.0 (±3.8) years, respectively. The University's Office of Research Ethics approved the investigation, and all participants gave their informed consent before data collection began. The study conforms to the Code of Ethics of the World Medical Association (approved by the ethics advisory board of Swansea University) and required players to provide informed consent before participation.

Instrumentation

Free weight testing was performed inside a standard power rack. A bench was secured to the center of a portable force plate (Quattro Jump Model 9290AD, Kistler, Switzerland) using a customized steel bracket. Foot pegs extending horizontally from the end of the bracket were used to accommodate various foot positions; therefore, participants were not obliged to place their feet on the bench or the floor. Before each testing session, the force plate was calibrated with known weights and then zeroed with the weight of the participant and bench. A linear position transducer (PT5A-150, Celesco, Chatsworth, CA, USA) with a signal sensitivity of 0.244 mV·V−1 per millimeter was secured to a wood plank and positioned approximately 1.5 m directly above the center of the barbell. The vertical position of the barbell was zeroed before each repetition, and the initial displacement was recorded as 0.000 m. Displacement and force data were A/D converted using a 16-bit data acquisition board (PCI-6220; National Instruments, Sydney, NSW, Australia) and sampled simultaneously at 2,000 Hz. Labview software (Version 8.1; National Instruments, Austin, TX, USA) was used to acquire, display, and store all data for further analyses.

A squat rack instrumented with PN technology (Keiser, Fresno, CA, USA) was used for all PN testing and training purposes. Resistance was generated through an air compressor (Keiser) and adjusted by pressing foot pedals located at the base of the rack. The rack permitted a traditional bench press to be performed with a PN load while maintaining all 6 permissible movement directions. Resistance was applied by way of cables that extended from a pulley system free to move in the horizontal direction along tracks at the base of the rack. The cables were then attached to a lightweight 2.5 kg barbell (Keiser), designed specifically for use with the PN squat rack. The grip diameter was identical to that of a standard Olympic barbell. A digital screen displayed the PN load (in pounds) as calculated by software within the system. Through pilot testing, it was determined that the PN load could be accurately and reliably set at a predetermined resistance (e.g., 588N equals 60 kg) such that comparisons could be made with a FW load. This was confirmed for all testing trials through force plate data.

Experimental Protocol

Each participant attended 1 familiarization session and 2 testing sessions separated by a minimum of 72 hours. The familiarization protocol consisted of 6 sets of 4 repetitions with PN resistance using loads of 20, 30, 40, 50, 60, and 70% of an estimated FW 1RM, followed by 3 sets of 4 explosive FW efforts using absolute loads of 20, 40, and 60 kg; each separated by 3 minutes. Participants were allowed to self-select their grip and foot width; however, the distances were measured so that they could be used throughout testing. During the FW testing session, participants completed a 1RM test and 6 sets of 4 repetitions at loads equating to 15, 30, 45, 60, 75, and 90% of the previously determined 1RM. Participants were able to complete all repetitions. Ten minutes of rest was given between the 1RM test and commencement of submaximal load testing. Three to seven days after the FW testing protocol, participants returned to determine their PN 1RM.

Each participant's 1RM was determined using a protocol similar to that described by Doan et al. (10). Participants were asked to perform 4 repetitions at 60% of their estimated 1RM, 3 repetitions at 70% 1RM, 2 repetitions at 80% 1RM, and 1 repetition at 90% 1RM. These 4 sets were followed by a maximum of 5 attempts to identify their actual 1RM. Three minutes' rest was given between each set. All 1RM testing was conducted using a stretch shortening cycle (SSC) movement; however, in cases where the barbell contacted the chest or failed to come within 0.05 m of the chest, it was disregarded and repeated after an additional 3 minutes. Subjects were encouraged to move the barbell as quickly as possible but required to keep their hips and back on the bench and their feet on the floor (force plate). The PN 1RM was determined using the same protocol; however, the PN load was recorded to reflect the total resistance as displayed on the digital indicator and the mass of the lightweight barbell and collars (3 kg). Recorded loads were compared with the force plate data to ensure accuracy. All PN loads were set at a rack height of 0.64 or 0.74 m (distance from the pulley to the bottom of the cable attachment) by increasing the resistance, as unpublished work from our laboratory has shown that this setup increases the degree of linearity for progressively heavier loads. After 10 minutes of passive recovery, participants were asked to perform 2 single repetitions with the lightweight barbell (Keiser) to establish a maximum barbell velocity.

After the FW 1RM test, 4 single repetitions, separated by 1 minute, were performed as explosively as possible at loads of 15, 30, 45, 60, 75, and 90% 1RM. Loads were assigned in an ascending order, each separated by 3 minutes' rest, so that a systematic comparison could be made across participants. Any repetition that contacted the chest or failed to come within 0.05 m of the chest was disregarded and repeated after an additional 1 minute. Participants were required to keep their hips and back on the bench and feet on the floor.

On completion of baseline testing, each participant was assigned to a FW or PN training group, each matched for height, body mass, and FW 1RM. The intervention consisted of an 8-week periodized resistance training program designed to improve whole-body strength and power. An emphasis was placed on whole-body exercise for 2 reasons: (a) to accommodate the needs/interests of the participants so that they did not feel inclined to perform additional training outside of the study setting and (b) to control all resistance training exposures over the 8-week period. Participants attended three 90-minute sessions (including warm-up and cool down) each week and were coached by a National Strength and Conditioning Association–accredited strength and conditioning specialist. With the exception of the resistance used to perform the bench press, both training groups completed identical programs. The 8 weeks were separated into 2 phases, each including a strength (80–95% 1RM) and power (30–45% 1RM) component (Table 1). For example, in phase 1, participants performed a heavy bench press on day 1 (4 × 4 repetitions at approximately 90% 1RM) and an explosive bench press on day 2 (6 × 3 repetitions at approximately 35% 1RM). Participants in the FW group used ballistic repetitions (i.e., the barbell was thrown at the end of the ascent phase) to perform their power efforts. These repetitions were performed inside a standard power rack instrumented with a magnetic brake (Fitness Technology, Adelaide, Australia) that prevented any downward motion of the barbell after the point of release. However, all ballistic bench press repetitions did include an eccentric phase before being thrown (i.e., countermovement) so that all participants could make use of the SSC. Individuals in the PN group used the specialized rack and were instructed to move the load as fast as possible. During weeks 4 and 8, the training volume was reduced to provide participants with a period of active recovery. Details of the training program are outlined in Table 1. Participants were required to attend a minimum of 20/24 training sessions to complete the FW and PN posttests, which began within 1 week of the final training session. The new FW 1RM was used to assign loads for the submaximal posttests so that pre-post comparisons could be used to assess whether exposure to PN resistance altered the force-, velocity-, power-load relationship. However, it must also be noted that several factors such as nutrition, sleep, and time of day may have influenced the determination of participants' posttraining 1RM. For the duration of testing and training, participants were asked to refrain from performing any formal resistance exercise on their own or making changes to their nutritional intake.

Table 1
Table 1:
a The 8-week training program completed by all participants.*
Table 1
Table 1:
b The 8-week training program completed by all participants.*

Data Analyses

The raw displacement data were filtered with a fourth-order, zero-phase low-pass Butterworth filter (10 Hz) and differentiated to calculate barbell velocity. Force data were filtered at 100 Hz to remove any high-frequency noise. Initiation of the descent phase was defined as the first instance of negative displacement, whereas the end of the ascent phase was defined as the point at which the force became zero or maximum barbell displacement. Using peak displacement alone to define the end of the ascent phase will underestimate the mean force and power for submaximal loads if a period of negative work exists (12). Only the ascent phase was analyzed. Peak velocity, force, and power (force × velocity) were computed using the average of 4 repetitions. Pilot testing was performed to determine whether the “best” repetition or average measurement should be included in the analyses. Because the data were highly variable (i.e., the fastest rep was not the same in every instance), the average of 4 trials was thought to offer a much more stable and true representation of the actual change posttraining. The peak barbell velocity and maximum dynamic force were defined as the peak velocity from the two 2.5 kg explosive repetitions and peak force from the FW 1RM, respectively. Data from the ascent phase were also expressed as a percentage of the total barbell displacement so that the pretraining and posttraining power-displacement profiles could be compared.

Statistical Analyses

Participants' FW and PN 1RM, maximum (any load) dynamic force, barbell velocity and power, and peak velocity, force, and power at each submaximal FW load were computed. A 2-factor mixed-model analysis of variance was used to examine the independent effects of group (FW and PN) and time (pre and post) on each dependent measure. Holm-Sidak post hoc comparisons were used to examine the differences. Statistical significance was set at an alpha level of p ≤ 0.05. Effect sizes (ESs) were also computed to describe the pre-post differences in each dependent measure for both training groups. The strength of the ES was interpreted using the general guidelines offered by Cohen (8), whereby magnitudes of 0.2, 0.5, and 0.8 corresponded to small, moderate, and large changes, respectively. The pre-post changes exhibited by each group were compared using magnitude-based inferences derived from the p-values and effect statistics of an independent t-test (15). Qualitative descriptions of each inference were based on the scale recommended by Hopkins (15): most unlikely, <0.5%; unlikely, 5–25%; possibly, 25–75%; likely, 75–95%; very likely, 95–99.5%; most likely, >99.5%.

Results

Four participants (3 FW, 1 PN) failed to meet the attendance requirements and were removed from the analyses. Given the potential influence of dissimilar 1RMs at baseline, between-group comparisons are described using mechanistic inferences only.

Free-Weight Trained

After 8 weeks of training, participants increased (p ≤ 0.05) their FW and PN 1RM by 11.6 kg (10.4%, ES = 0.61) and 10 kg (9.4%, ES = 0.63), respectively (Table 2). Significant increases were also seen in maximum dynamic force (9.8%, ES = 0.44), peak barbell velocity (11.6%, ES = 0.87), and peak power (22.5%, ES = 0.62).

Table 2
Table 2:
The pretraining and posttraining mean (SD) FW and PN 1 repetition maximum (kg), and maximum dynamic force, velocity, and power for individuals participating in the FW and PN intervention.*†

Although participants improved their FW and PN 1RM, the FW training did not produce an increase in peak force at any submaximal load between 15 and 90% 1RM (Figure 1). Peak velocity significantly increased at 15% and 30% 1RM (ES >1.15); however, at 60% 1RM, it was found to be lower posttraining (ES = 1.65) (Figure 1). A significant increase in peak power was only noted at 45% 1RM (ES = 0.69) (Figure 1).

Figure 1
Figure 1:
The peak force (A), velocity (B), and power (C) exhibited by participants who trained with FW or PN resistance. Data are presented for relative loads of 15, 30, 45, 60, 75, and 90% 1RM before and after the 8-week interventions. The superscript letters above each load denote a significant (p ≤ 0.05) posttraining change (FW; PN). Asterisks (*) reflect the size of the effect (*an effect size between 0.5 and 0.8, whereas **an effect size above 0.8). The qualitative inference listed above each load comparison describes the likelihood of a between-group intervention effect (possibly, 40–75%, likely, >75%). FW = free weight; PN = pneumatic; 1RM = 1 repetition maximum.

Pneumatic Trained

Participants who performed 8 weeks of bench press training with PN resistance exhibited significant increases of 11.8 kg (11.6%, ES = 0.57) and 17.1 kg (17.5%, ES = 0.91) in their FW and PN 1RM, respectively (Table 2). Similar to their FW counterparts, significant increases were also seen in their maximum dynamic force (8.4%, ES = 0.35), peak barbell velocity (13.6%, ES = 0.77), and peak power (33.4%, ES = 1.08).

In comparison with the baseline tests, the peak force–achieved posttraining was significantly higher (ES = 0.92) when using the 15% 1RM FW load (Figure 1). Similar to the FW trained group, peak velocity was significantly higher at 15 and 30% 1RM (ES >0.58) and was also found to decrease (ES = 1.47) at 60% 1RM (Figure 1). Peak power was significantly higher posttraining at each load between 15 and 45% 1RM (ES >0.83) (Figure 1).

Between-Group Comparisons

No between-group differences were noted in the pre-post improvement in FW 1RM or peak barbell velocity (possibly trivial, >37%). Training with PN resistance may have afforded a superior opportunity to improve PN 1RM (likely, 87%) and peak power (possibly, 40%), whereas the FW program may have provided a better stimulus to increase maximum dynamic force (possibly, 68%). However, with the light loads, the PN intervention may have been a superior training stimulus to improve peak force (e.g., 85% likely to see superior benefit of PN resistance with a load of 15% 1RM; Figure 1). Eight weeks of PN training may have also provided a better opportunity to improve peak power, although the advantage seems limited to the lightest loads tests. Above 60% 1RM, the FW intervention may have been better suited to improve peak power (Figure 1). A group effect was also seen with velocity—PN training may have provided an advantage with the light loads, but the FW program seems to have been abler to improve peak velocity with loads above 60% 1RM.

Representative power-displacement profiles for the PN and FW participants are presented in Figures 2 and 3. Because subtle differences in the shape of participants' profiles would mask the effect of training (i.e., the maximums and minimums did not align when normalized by barbell displacement), the pretraining and posttraining data for the strongest participant from the FW (pre and post FW 1RM of 140 and 150 kg) and PN (pre and post FW 1RM of 132.5 and 155 kg) groups are presented. The PN trained individual exhibited substantial changes during the first half of the ascent phase with loads of 15, 30, and 45% 1RM but failed to display similar improvements when the load was further increased. In contrast, the most substantial adaptations exhibited by the FW trained participant appeared during the latter half of the ascent phase, and to a much greater extent than his PN counterpart when performing with the 3 highest loads.

Figure 2
Figure 2:
Pretraining and posttraining power-displacement profiles for the strongest pneumatic (left side) and free weight (right side) trained participant at loads of 15, 30, and 45% 1RM. The data presented reflect the average of 4 repetitions performed at that load. 1RM = 1 repetition maximum.
Figure 3
Figure 3:
Pretraining and posttraining power-displacement profiles for the strongest pneumatic (left side) and free weight (right side) trained participant at loads of 60, 75, and 90% 1RM. The data presented reflect the average of 4 repetitions performed at that load. 1RM = 1 repetition maximum.

Discussion

The primary objective of this longitudinal training study was to examine the strength, velocity, and power adaptations exhibited by resistance-trained men in response to 8 weeks of exercise using either FW or PN resistance. Interestingly, despite performing all bench press movements with one type of resistance throughout training, participants in both groups significantly increased their FW (10.4 and 11.6% for FW and PN groups) and PN 1RM (9.4 and 17.5%) posttraining. Limiting participants' exposure to a particular type of resistance did not seem to impede the potential strength-oriented adaptations that could be achieved using either training modality, as group mean improvements were of a similar magnitude to those reported previously (22–24,27). In fact, the largest posttraining FW 1RM improvement was exhibited by a PN-trained participant (132.5–155 kg), which supports the notion that the type of resistance used while training may not be needed to mimic that used during testing to maximize strength-oriented adaptations (21). As was suggested by Anderson et al. (1), it is also possible that using a combination of the 2 resistances would have provided an even more favorable strength-oriented stimulus in comparison with using either resistance alone if the potential disadvantages of one resistance type could be accommodated by the advantages of the other. For example, during a FW bench press sufficient force must be produced to overcome inertia and accelerate the barbell upwards; however, this also serves to increase the barbell's momentum and reduce the muscular effort required through the midrange of the motion (20). The use of pneumatics (or bands) will negate the influence of momentum and thus provide an opportunity to engrain a coordination strategy that is better suited to maintaining a consistent muscular effort throughout the range of motion (13).

Given participants' posttraining force, velocity, and power adaptations to each submaximal load, and the unique physical demands of most sports, there may also be instances when a specific response would be better achieved by using a particular resistance. After 8 weeks of training, both groups improved their 15 and 30% 1RM peak barbell velocity, but only the PN-trained participants exhibited a corresponding increase in peak force. Since a similar increase in force was not exhibited at loads of 45–90% 1RM, this implies that individuals who trained with PN resistance were able to produce higher relative forces (% 1RM) posttraining when using the lighter loads. Furthermore, because the highest forces during a bench press are typically produced in-between or immediately after the transition from the descent to ascent phase when barbell acceleration is highest (7), the observed increases in peak force could also reflect an improved rate of force development (RFD). Stevenson et al. (29) proposed that strength and conditioning professionals should consider using elastic resistance to improve RFD, which like PN resistance imposes a demand that is not influenced by inertia and momentum. Having an opportunity to train for 8 weeks with pneumatics, whereby higher velocities can be achieved consistently in a comparison with an equivalent load with FW (11), may have been more conducive to improving participants' RFD. However, given evidence to suggest that performers' intent to move quickly may be more important than their actual movement speed (5,19), additional research is needed to substantiate this contention.

Lending further support for the notion that the type of resistance used while training could impact the extent to which a specific adaptation is achieved at a submaximal load, only the PN-trained participants exhibited significant increases in peak power at loads of 15–45% 1RM. Although the small sample size of both groups may have biased these findings, viewed in combination with the dissimilar power-displacement profiles of the strongest FW- and PN-trained participants, each resistance may provide a unique training advantage for specific populations and/or objectives. For example, the training load(s) needed to maximize power output for the bench press and squat have been studied extensively in an attempt to identify a training stimulus that could elicit results outside of the weight room environment (i.e., transfer to sport performance) (2,3,9,18). However, based on the findings of this investigation, and those of others e.g., (6,11,14,17,29), both the magnitude of performers' power output and the manner in which it is produced (i.e., coordination) is likely influenced by the type of resistance used during training. As such, it may be possible to change the load(s) at which an individual produces their highest power outputs (16,28), which could be of particular benefit for athletes who participate in sports characterized by high speeds and light to modest external loads (e.g., baseball, basketball). In these instances, it may also be advantageous to first identify the loads and/or speeds that best characterize the desired adaptations, so that an appropriate training stimulus (load, speed, and resistance) can be selected, rather than computing or choosing an “optimal” load for each individual based on their current abilities. In this study, the strongest PN-trained individual (pre and post) exhibited posttraining FW adaptations that resembled what would be expected from someone who had trained with a resistance characterized by little momentum and inertia; substantial changes in power output were seen during the first half of the ascent phase with the light loads (i.e., 15–45% 1RM). In contrast, the strongest FW-trained individual showed marked improvements during the second half of the ascent phase with loads of 60–90% 1RM, which highlights the potential benefit of strategically choosing the resistance type that best suits the intended adaptation.

The utility of block and undulating periodization models, whereby the volume and intensity of training are progressed in a linear and nonlinear manner, respectively, are often compared when investigating the most favorable training stimulus to improve strength and power in highly trained athletes (4,24,26,27). However, the results of this investigation lend support to the notion that perhaps the type of resistance used while training should be considered as well. Pneumatic, band, and FW resistance are each characterized by unique mechanical properties that will influence the training stimulus and thus performers' adaptations. As such, for the purpose of improving an individual's speed, strength, power, endurance, and/or whole-body coordination and control, there is likely merit in exploring the potential benefit of acute and long-term training strategies that exploit the advantages of specific resistance(s).

Practical Applications

When designing a periodized program to improve athletes' strength, speed, power, and or whole-body coordination and control, consideration should be given to the type of resistance(s) that will be used while training. In addition to changing the reps, sets, loads, speeds, rest periods, and/or movement patterns to elicit a particular adaptation, exploiting the mechanical properties of the resistance(s) (e.g., FW, elastic, and PN) could provide an opportunity to impose a stimulus and/or achieve an objective that would otherwise be challenging to accomplish. The results of this study are that while both FW and PN resistance can be used to increase the maximal strength, velocity, and power of experienced performers, training with PN resistance may have helped to facilitate unique force, velocity, and power adaptations when performing with the lowest relative loads. Because PN resistance is characterized by minimal mass and momentum, performers can achieve higher velocities while performing with an equivalent load and will be forced to maintain a consistent muscular effort throughout the range of motion. However, the absence of mass and momentum could also make FW (or a combination of the 2 resistances) a better option to elicit high force–related adaptations, as was seen by among the free weight trained participants above loads of 60% 1RM.

In summary, having an appreciation for the potential advantages and limitations of each resistance type will provide coaches and researchers with an opportunity to develop mixed-method training strategies to suit the specific needs of athletes who participate in sports with different force-, velocity-, and movement-related demands.

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Keywords:

air resistance; athlete; bench press; force; periodization

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