Introduction
Muscle power production substantially affects performance in dynamic movements, such as throwing and jumping, in many sport activities. As a result, power assessments are used in research, athletics, and rehabilitation settings to evaluate or describe current training status, to monitor training or rehabilitation progression, or to modify exercise prescription. Upper body muscle power is often assessed using maximal effort bench press throws (2,4,6,7,14). Accurate measures of bench press throw performance (force, velocity, and power of movement) can be obtained using free weight equipment, but performance of free weight bench press throws has an inherent risk of injury. In addition, free weight bench press throws involve a curvilinear barbell path that is not always compatible with linear accelerometers and transducers. Instead, smith machines that allow for only vertical movement of the barbell are commonly used when assessing bench press throw performance (2-6,8,9,12-14).
Smith machines often use a counterbalance system that consists of counterweights attached to the barbell carriage via wires passing over pulleys located at the top of each side of the equipment. The smith machine barbell carriage comprises a barbell plus the carriage so that the mass of the barbell carriage is generally >20 kg. The purpose of the counterweights on a smith machine is to reduce the net weight of the barbell, thus allowing users to exercise with a resistance that is less than the actual weight of the barbell carriage. Publications for studies involving smith machines rarely specify if a counterbalance weight system was used (e.g., [2-6,8,9,12-14]). Of the 31 studies that involved a smith machine and were published in the 13 issues of JSCR from January 2010 through January 2011, only 3 articles specified if a counterbalance system was used. However, based on the type of equipment used and other information in the articles, it appears that up to approximately 50% of the studies did in fact involve a counterbalance system.
The counterweights and the barbell carriage are positioned on the either side of the pulley system and thereby exhibit reciprocal movements with respect to each other. For slow movements, the magnitude of the acceleration is the same (uniform) for the barbell carriage and the counterweight, but the movements occur in opposite directions (up vs. down). During explosive movements, such as a maximal bench press throw, this uniform acceleration can disappear. For a counterbalance system to be effective, the acceleration of the barbell and counterbalance weight throughout the movement must be uniform in magnitude (1). During rapid movements, such as the concentric phase of bench press throws, the upward acceleration of the bar can exceed 9.82 m·s−2 (12), which is the downward acceleration of the counterweight due to gravity (g). When this happens, the wire connecting the counterbalance weight and the bar becomes slack; thus, the counterbalance weight system becomes ineffective and no longer reduces the weight of the bar (Figure 1). Consequently, the external resistance for the movement is increased by an amount equal to the weight of the counterbalance.
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Figure 1: Mechanics of a counterbalance weight system during a maximal bench press throw. A 1: Barbell acceleration during eccentric phase; a 1 counterweight acceleration during eccentric phase; A 2: barbell acceleration during concentric phase; a 2 counterweight acceleration during the concentric phase.
Linear accelerometers and transducers are used to quantify performance measures for a movement (e.g., force and power) and rely on manual input of the mass (to determine external resistance) to accurately calculate these performance measures. Thus, when the external resistance during a movement changes and is different from what was inputted the measures of peak force and peak power obtained by linear accelerometers or transducers could become inaccurate. In addition, when the total external resistance is increased, the relative intensity (i.e., percentage of 1 repetition maximum [1RM]) used for the exercise is also increased. The relative intensity has a substantial effect on force and power output (3-6,12,14), and thus, changes in the relative intensity could further influence these performance measures.
Although equipment with a counterbalance weight system is commonly used for the assessment of performance characteristics for rapid movements, and despite the potential drawbacks to such systems, to the authors' knowledge, it has not been demonstrated if the use of a counterbalance weight system affects performance measures. Therefore, the purpose of this study was to determine if using a counterbalance weight system affects measures of bench press throw performance.
Methods
Experimental Approach to the Problem
This study used a 2 × 2 (counterbalance × load) repeated measures design with the order of conditions assigned using randomization to minimize possible confounding effects of order. Each participant completed 3 consecutive maximal effort bench press throws without pause under 4 different conditions using a smith machine: (a) with a counterbalance weight system engaged and using a ‘light’ net barbell load, (b) without a counterbalance weight system engaged and using a ‘light’ net barbell load, (c) with a counterbalance weight system engaged and using a ‘moderate’ net barbell load, and (d) with a counterbalance weight system engaged and using a ‘moderate’ net barbell load. Both light and moderate loads were used to investigate the possibility that the effect of a counterbalance weight system might differ at different relative intensities. Performance variables (peak force, peak velocity, and peak power) for each throw were measured using a linear accelerometer attached to the barbell.
Subjects
Ten men and 14 women (mean ± SD: age, 25 ± 4 years; height, 173 ± 10 cm; weight, 77.7 ± 18.3 kg), ranging in training status from untrained to highly resistance trained, were recruited from the university and surrounding community. All the participants were informed of the risks and benefits of participation in the study and subsequently provided written informed consent. The study was approved by the University of North Texas Institutional Review Board. The participants completed medical health questionnaires designed to screen for pre-existing medical conditions that might put them at risk during the exercise protocol or affect the outcomes of this study. Exclusion criteria included any orthopedic limitations that would prevent the completion of a full range of motion bench press throw and not being able to perform a bench press throw with a 24-kg total load (the mass of the barbell carriage).
Procedures
The test session began with anthropometric measurements (height and weight) followed by a warm-up consisting of dynamic upper body stretches. Then, the participants completed 4 bench throw tests (1 test for each condition) with 5 minutes of rest between the tests. Each test consisted of 3 consecutive maximal rebound bench press throws performed on a smith machine. There was no pause between the throws, other than that needed for the participant to grip the barbell again. The participants gripped the barbell with hands shoulder width apart, and the grip width was standardized, so it was constant for every throw for a particular participant. The participants were instructed to begin the movement with the elbows fully extended and then to lower the barbell rapidly (without pulling it down) during the eccentric phase to just above their midchest and then immediately to push the barbell upward for the concentric phase; no pause was allowed between the eccentric and concentric phases of the movement. The barbell was not allowed to touch the chest but had to be <5 cm above the chest between the eccentric and concentric phases. Throughout the exercise, the participants lay on their back with head, shoulders, and buttocks in contact with the flat bench surface and with feet flat the floor. The participants were instructed to push (“throw”) the barbell as fast and as high as possible. The bar was caught by research personnel after each attempt and immediately returned to the participant's outstretched arms for the subsequent throw. For the light load condition, the initial net weight was 48 ± 13% of 1 repetition maximum (1RM) and for the moderate load condition the initial net weight was 61 ± 13% of 1RM. The counterbalance weight had a mass of 13.6 kg. Bench press 1RM was estimated using the procedures of Epley (10). Before the test session, the participants refrained from upper body exercise for 48 hours and from eating for 2 hours.
Equipment
The smith machine (New York Barbells, Elmira, NY, USA) had a barbell fixed to vertical steel shafts on either sides of a frame (Figure 2). Linear bearings attached to either end of the barbell (carriage) allowed it to slide up and down the 2 steel shafts (only vertical movement) with minimum friction. The smith machine had detachable counterbalance weights attached to the barbell carriage via a wire and pulley system. The counterbalance system was modified so that weight plates were used instead of the counterbalance weights supplied by the manufacturer. The modification was done to eliminate the friction between the equipment counterweight and the side rail. The counterweights were used to reduce the resultant weight of the barbell carriage itself. To assure that the initial total resultant weight was identical for both the counterbalance and the no-counterbalance condition, weight equal to that of the counterbalance weight (13.6 kg) was added to the barbell for the counterbalance conditions.
Figure 2: Smith machine with the detachable counterbalance weight system. (A) Starting position for the eccentric phase. (B) Midposition (start of the concentric phase).
For each throw, peak force, peak velocity, and peak power measures were obtained using a linear accelerometer and associated software (Myotest® Pro, Acceltec SA, Sion, Switzerland). The accelerometer was attached to the barbell and kept within the vertical plane. The Myotest accelerometer has been validated for use in bench press throw testing (11).
Statistical Analyses
For each condition, data from the throw (out of 3) that produced the highest peak power were used for further analysis. Data for absolute peak force, peak velocity, and peak power were analyzed using 2-way analyses of variance (counterbalance × load) with repeated measures on both factors. When significant differences were found, Fisher's least significant difference post hoc tests were used to determine pairwise differences. For each load condition, the relative difference (Δ score) between counterbalance and no-counterbalance conditions was calculated for each variable. For each variable, the Δ scores for the 2 load condition were compared using paired t-tests. The alpha level for significance was set at p ≤ 0.05. All statistical procedures were performed using the Statistica software package (StatSoft, Tulsa, OK, USA). Data are presented as mean difference ± SD unless otherwise specified.
Results
The use of a counterbalance weight system resulted in significantly lower (p < 0.001) measures of peak force (Figure 3), peak velocity (Figure 4), and peak power (Figure 5) for both the light and moderate load conditions. There were no differences in the absolute or relative reductions from the counterbalance weight system for any variable for either of the 2 load conditions.
Figure 3: Peak force. Absolute and relative (Δ) difference between the no-counterbalance and counterbalance conditions for light and moderate loads. Mean ± SE. *Significant (p ≤ 0.05) difference between counterbalance conditions.
Figure 4: Peak velocity. Absolute and relative (Δ) difference between the no-counterbalance and counterbalance conditions for light and moderate loads. Mean ± SE. *significant (p ≤ 0.05) difference between counterbalance conditions.
Figure 5: Peak power. Absolute and relative (Δ) difference between the no-counterbalance and counterbalance conditions for light and moderate loads. Mean ± SE. *Significant (p ≤ 0.05) difference between counterbalance conditions.
Discussion
This study appears to be the first to investigate the effect of using a counterbalance weight system on the calculation of performance measures in ballistic resistance exercise movements. The primary finding of this study was that the common use of a counterbalance weight system reduced accelerometer-based measures of bench press throw performance.
Bench press throw peak force that was calculated based on measures obtained by the linear accelerometer was significantly reduced by using the counterbalance weight system. Linear accelerometers rely on manual input of the external resistance to accurately calculate force from the measured acceleration of a movement. The reduced measure of force was likely because of an increase in the external resistance during the concentric phase of the explosive movement as the counterbalance weight became ineffective. An increase in the external resistance would reduce the barbell acceleration based on the inverse relationship between maximal force requirement and velocity (5,12). A reduced acceleration combined with an underrepresentation of the actual external load, and thus, the underestimation of the force requirement from the manual input of the initial load would result in a reduced calculated peak force. If as we propose the external load increased during the movement, then the actual peak force should have increased for the counterbalance weight condition, because force output increases with increased external resistance (5,12). Thus, it appears that the use of a counterbalance system results in a greater underestimation of peak force than simply the difference between the 2 measures obtained in this study.
Peak velocity for the bench press throw was reduced with the use of a counterbalance system. Because a manually inputted load does not affect the determination of velocity by an accelerometer, the effect of the counterbalance weight system on peak velocity must be caused by a direct effect of that system on the actual velocity of movement. Previous studies (5,12) have found that, for ballistic bench press throws, increasing the external resistance reduces the peak velocity. The reduction in peak velocity found with the use of the counterbalance weight system is, therefore, also likely because of the increase in external resistance during the movement.
Because power is a product of force applied and velocity of movement produced, the reduction in peak power found with the counterbalance system is most likely a result of the reductions in calculated force and velocity and thus can be explained by the mechanism of increasing external resistance during the explosive movement. This effect on power via increased force requirement and resultant reduced velocity is supported by previous studies (3,5,12,14) involving ballistic bench press throws that found that, with increasing external resistance (>30% of 1RM), the peak power exerted on the barbell is reduced.
In this study, no difference in the effects of using a counterbalance weight system was found between load conditions. It appears that a counterbalance weight system reduces accelerometer-based measures of performance in explosive movements similarly for both light and moderate loads. The 2 load conditions examined were only separated by 13% of 1RM and represent the midrange of relative intensities generally used for measurement of explosive movement performance. It is possible that, at heavy loads (e.g., 80-100% of 1RM), the counterbalance system will have little or no effect on performance measures. The upward acceleration at heavy loads will be substantially lower than at the loads examined in this study. If the peak upward acceleration is less than −g (∼9.82 m·s−2), then the effectiveness of the counterbalance weight system is likely maintained. At loads that are substantially lighter than the 48% of 1RM used in this study (e.g., 30% of 1RM), the counterbalance system would remain ineffective and reduce performance measures. Indeed, it seems likely that, at very light loads, the effect of the counterbalance system might be greater than that found in this study because the relative increase in external load would be much greater. The actual magnitude of the effect of a counterbalance weight system at very light and heavy loads remains to be determined.
In conclusion, the use of a counterbalance weight system reduces accelerometer-based performance measures for the bench press throw exercise performed using both light and moderate loads. We suggest that this underestimation in measures is likely because of 2 distinct effects: (a) an increase in the external resistance during the movement which results in a discrepancy between the manually input and the actual value for external load and (b) a reduced velocity because of the increased force requirement. Future studies should investigate if it is an increase in the external resistance or some other mechanism that is responsible for the reduction in the performance measures found with a counterbalance weight system. Future studies might also investigate whether using a counterbalance system affects measures obtained using other types of testing devices, such as a force plates. Finally, in future publications, authors should include a description of any counterbalance weight system used.
Practical Applications
Equipment that uses a counterbalance weight system (e.g., a counterbalanced smith machine) is commonly used during explosive performance (power) testing, especially in research and athletic settings. Based on the current findings, the use of counterbalanced equipment combined with linear accelerometers will result in an underestimation of performance (peak power, peak force, and peak velocity) capabilities. Hence, a counterbalance weight system should not be used when measuring peak power, peak force, and peak velocity of movement for a resistance exercise using data from a linear accelerometer.
Counterbalanced equipment is often used for training purposes, in addition to testing. It should also be noted that if the external resistance increases during the explosive movement when counterbalanced equipment is used, this will change the relative intensity involved and thus the desired intensity (e.g., 30% of 1RM) is no longer maintained, changing the force-velocity relationship for the movement.
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