The barbell power clean is a resistance exercise typically used to generate and develop high levels of muscular power (3,4,6). For training purposes, there are several commonly used variations that incorporate various portions of the lift (hang cleans, box cleans, clean pulls, etc.) (1,3,4,6,9,20). Power values greater than 6000 W have been recorded for elite weightlifters during the second pull (i.e., top portion of the lift) of the clean during competition (8). Chronic training with the clean exercise and its variations can result in significant improvements for physical activities requiring high power, such as the vertical jump or sprinting (2,12-14,19).
Heavy resistance exercise is generally performed using either free weight or machine modalities. The advantages and disadvantages of each modality have been extensively debated in the strength training literature (4,15,17,18). Although the power clean exercise is almost always performed with an Olympic-style barbell, a weight machine that purportedly replicates the barbell clean exercise has been commercially marketed (Power Trainer, Powernetics, Riverside, Tex.). No data, however, are available concerning how closely this machine replicates the kinetic properties of the free weight power clean exercise.
Resistance exercise with machines has been claimed to be safer than free weights (11), although there are few scientific data to support this. On the other hand, free weights can more readily train large muscle masses and synergistic muscles (18). However, free weights require more precise balance and motor control because of the freedom of movement allowed (18). This movement may be optimal for the experienced lifter, but it could potentially increase the risk of injury for novice lifters. Strength training studies comparing free weight and machine training modalities have often reported no difference in training effect (15-17), although one study has reported greater strength improvements with free weights (19). Equivocal scientific results and unsubstantiated claims have contributed to the controversy surrounding the resistance modality issue. Therefore, to better understand the training stimulus each modality provides, the purpose of this investigation was to compare the kinetic characteristics of free weight and machine modalities for the power clean exercise. These results will provide data concerning the efficacy of the Power Trainer machine as a replacement exercise for the barbell box power clean.
Experimental Approach to the Problem
A simple, repeated-measures design was used to determine the kinetic differences between the free weight and machine power clean exercises. Two familiarization sessions were performed for each exercise modality to ensure that all subjects possessed sufficient technical proficiency for the power clean exercise, regardless of which modality they used. During separate, randomly ordered testing sessions, one-repetition maximum performance (1RM) was established for either the free weight or machine power clean exercise. After 1RM, subjects performed three sets of one repetition at 85% 1RM, during which measures of barbell force, velocity, and power were obtained via a computer-interfaced FiTROdyne dynamometer (Fitronic; Bratislava, Slovakia) attached to the barbell via a tether.
Fourteen healthy men volunteered as subjects for this study (see Table 1 for descriptive data). All subjects were resistance trained and experienced with the free weight power clean exercise, as evident by the inclusion of the power clean in their respective training programs for 4.1 ± 4.3 years (mean ± SD). All subjects signed an informed consent statement approved by the institutional review board at the University of Memphis. Each subject reported to the laboratory on four different occasions, with 7 days separating each of the four sessions.
Free weight barbell power cleans were performed with a York 20-kg Olympic Standard barbell and calibrated rubber bumper and metal plates (York Barbell, York, Pa.). Collars were always used to ensure the secure placement of the plates on the bar. The barbell was lubricated to permit the bar to freely rotate during the exercise. Specially constructed, adjustable-height boxes were used to place the barbell at a midthigh position. The Power Trainer machine is a device designed to mimic the barbell box power clean exercise. The height of the handles is adjustable to accommodate individuals of varying stature, and it is designed to freely rotate, much like the barbell. The resistance is provided at the end of a lever arm that extends from the back of the machine and is attached to the handles through a chain-and-gear mechanism. Because of the mechanical design of the machine, pilot work was performed to determine the actual force exerted at the handles. Although the machine handles move in an arc, it was determined that the resistance throughout the range of motion varied by ≤ 2 kg and was, thus, considered a constant linear load. Figure 1 shows illustrations of both exercise modalities in use.
Subjects were instructed in the proper techniques for the box power clean with free weights and the Power Trainer machine. The box power clean variation of the clean exercise was used to place the barbell at the same height as the machine handles for the start of the lift (i.e., midthigh). Customized wooden boxes were designed for the barbell to set on at the proper height for each subject. For familiarization purposes, each subject performed a standardized warm-up (see Table 2), concluding with 4 × 3 (sets × repetitions) at 50% of their estimated 1RM for both the free weight and machine cleans. The 1RM for the free weight box clean was estimated to be 80% of the 1RM for the power clean when pulled from the floor. Interset rest intervals were 2-3 minutes for all sets.
For further familiarization purposes, each subject performed a standardized warm-up, followed by 4 × 3 at 60% of the estimated 1RM for both exercises (see Table 2). The exercise performed last during session 1 was performed first during session 2 (i.e., counterbalanced design). The net result was that all subjects were thoroughly familiar with the box power clean exercise with both the barbell and the machine modalities.
A standardized warm-up was followed by a 1RM test for either the free weight or the machine clean, the order of testing being randomly assigned. One-repetition maximum was determined in five or fewer lifts for all subjects. After a 10-minute rest, 3 × 1 at 85% 1RM was performed with 2-3 minutes of rest between lifts (see Table 2). This relative intensity was selected because it is commonly used for training purposes. Kinetic characteristics of either the free weight box clean or the machine clean were determined for these three repetitions with a computer-interfaced FiTRODyne dynamometer (Fitronic; Bratislava, Slovakia). This device directly measures barbell or machine velocity at 100 Hz from a light nylon tether attached at the barbell end or at a point in line with the machine handle (10). Free weight or machine acceleration, force, and power are then calculated for the entire range of motion from the known loads and velocities. The repetition with the greatest peak power for each condition was used for statistical analysis. The range of motion was determined from the point of initial pull (i.e., midthigh) to a point at which the vertical velocity was 0.0 m·s−1 at the top of the lift.
This session was identical to session 3, except that the exercise modality not previously tested was performed (random, counterbalanced design; see Table 2).
Dependent variables measured included 1RM strength (kg), peak and mean vertical velocity (m·s−1), peak and mean power (W), velocity and force at peak power, time to peak power (ms), and total lift time (ms). Because of the interrelationship between the dependent variables, a MANOVA was performed with all of the dependent variables included. Post hoc tests were performed using paired t-tests (p ≤ 0.05). All data are expressed as mean ± SD.
The data for both the free weight and the machine clean exercise performed at 85% 1RM are listed in Table 3 and Figure 2. Significant differences were observed for several of the dependent variables (Wilk's Λ (7,7) = 27.77; p = 0.0001). The free weight clean exhibited significantly greater values compared with the machine for 1RM and mean power. On the other hand, the machine clean exhibited significantly greater values for peak and average velocity. No significant differences were observed for peak power, time to peak power, and total lift time. When the force and velocity contributing to peak power were compared for both modalities, subjects exhibited significantly greater force during the free weight condition; during the machine condition, they exhibited significantly greater velocity (see Figure 1). Figure 2 illustrates examples of the start and finish positions for the free weight box power clean and the machine power clean. Although not statistically analyzed, distinctly different trajectories are readily apparent, as evident by the forward body position after completion of the machine power clean.
The most striking dissimilarity between the free weight and the machine clean exercises is the difference in 1RM loads. This load difference is undoubtedly responsible in large part for the different kinetic characteristics of each modality. Assuming that friction between the moving parts on the machine is negligible, the distinctly different trajectory pattern for the handles on the machine (see Figure 1) seems to make it difficult to attain an ideal body position for generating power and force during a clean (3,6,7,20). Proper clean technique with the free weight power clean dictates that the barbell is kept close to the body throughout most of the lift. The fact that the handles on the machine actually move away from the lifter makes it difficult for the lifter to complete the top of the pull and to properly rack the weight at the shoulders. In fact, a number of the subjects commented on this phenomenon and felt that this may have compromised their performance.
The mean power values reported in the present study (W; free weight = 929 ± 190, machine = 770 ± 116) are lower than the values reported by Garhammer (8) for the second pull of elite weightlifters during competition (2206-6077 W). This is most likely attributable to the elite-caliber status of the lifters, the inclusion of body mass in the calculations, and the fact that the barbell was already moving at the start of the second pull in Garhammer's study.
One of the contributing variables to power is velocity, which was also significantly different between the free weight and the machine clean modalities. The mean velocity for both modalities (m·s−1; free weight = 1.24 ± 0.12, machine = 1.37 ± 0.21) are within the range described by Drechsler (3) for the second pull of the clean exercise (1.2-1.6 m·s−1). The peak velocity attained for both the free weight and machine clean exercises (m·s−1; free weight = 2.02 ± 0.18, machine = 2.56 ± 0.21) were greater than the velocities reported by Garhammer (8) for the second pull of weightlifters during competition. Because the velocities observed in the present study are from lifts at 85% 1RM loads, they would be expected to be greater than the velocities reported for lifts at or near 1RM loads as in competition. It is possible that the velocities for the machine are greater because of the lower loads that could be lifted on it compared with the free weight clean.
Although both the free weight and the machine cleans were performed at the same relative intensity (i.e., 85% 1RM), the different velocities suggest that the force-velocity relationships for both modalities are different. Furthermore, peak power for both modalities was not different even though the absolute load was greater for the free weight clean. This may be explained by the differences in the force and velocity at peak power for each modality (see Figure 2). The velocity at peak power is greater when using the machine, and the force at peak power is greater when using free weights. This can be explained in part by the different masses being lifted. It is speculated that these differences are attributable to the mechanical limitations (e.g., trajectory) of the machine modality. Such limitations prohibit the maximization of one's force-producing capabilities. To more completely understand the mechanical limitations of the Power Trainer machine, kinetic properties must be evaluated across the entire spectrum of relative intensities for both modalities.
In summary, on the basis of this study, when compared with the free weight barbell box power clean, the Power Trainer machine seems to be a somewhat different lift and provides a lower force stimulus than the power box clean. Maximal lifting capabilities on the Power Trainer machine were only 75% of the 1RM values for the free weight barbell box power clean. Furthermore, the different velocities, forces, and powers suggest that the force-velocity relationship for the Power Trainer may be different when compared with free weights. It is likely that the lift trajectory required for the Power Trainer machine is a primary cause of these differences.
Although further study of these different lifting modalities is warranted, these data suggest that this machine provides a different exercise stimulus compared with the free weight barbell box power clean. These relative differences include 1) lower 1RM loads (i.e., 75% of free weights), 2) a possibly different force-velocity relationship, and 3) a different lift trajectory. In the opinion of the authors, the machine power clean does not satisfactorily simulate the free weight power clean. It could be argued that by using similar relative percentages of 1RM, training with this machine allows for faster movements, which would decrease the time required to perform the motion and might lead to improved performance (5,14). On the other hand, training with the barbell box power clean requires greater force production at the same relative intensity (i.e., 85% of 1RM), which would also lead to improved power production (14). Training for increased power production is best achieved by combining speed of movement with the resistance that maximizes power production (14). It may be that to use this machine effectively, a different relative intensity needs to be used compared with the free weight power clean. However, if speed of movement needs to be emphasized, the simple solution is to use a lighter load during the free weight power clean.
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Keywords:© 2008 National Strength and Conditioning Association
free weights; machines; force; power; velocity