Effect of Instability and Resistance on Unintentional Squat-Lifting Kinetics

Transcription

1 International Journal of Sports Physiology and Performance, 2007, 2, Human Kinetics, Inc. Effect of Instability and Resistance on Unintentional Squat-Lifting Kinetics Eric J. Drinkwater, Erica J. Pritchett, and David G. Behm Context: Resistance training while using an instability-training device is known to increase activation of stabilizing muscle groups while decreasing the force generated by the prime movers during isometric contractions. Purpose: To investigate differences in squat kinetics during dynamic resistance training in an increasingly unstable training environment. Methods: Fourteen active men participated in this study. In each testing session, each participant performed 3 repetitions of squats with a 10-repetition maximum (10-RM) resistance, 40% of their 10-RM resistance, and kg. The 3 testing session consisted of standing on a stable floor, foam pads, or BOSU balls. All repetitions were recorded with an optical encoder to record barbell kinetics. Results: The transition from stable (floor) to very unstable (BOSU) resulted in high likelihoods (>75%) of clinically meaningful differences ranging from small to large (effect size [ES] ) in factors relating to concentric kinetics, eccentric power, and squat depth, regardless of the resistance used for training. There were also likely differences at the heaviest resistance in peak concentric power (stable to foam: ES 2.06; foam to BOSU: ES 0.38), eccentric power (stable to foam: ES 1.88; foam to BOSU: ES 0.74), and squat depth (stable to foam: ES 0.50; foam to BOSU: ES 0.67). Conclusions: Resistance training in an unstable environment at an intensity sufficient to elicit strength gains of the prime movers results in deleterious effects in concentric squat kinetics and squat technique. Such observations are particularly evident on very unstable platforms. Key Words: resistance training, stability training, power, optical encoder, core stability Instability devices are commonly employed in fitness and rehabilitation centers for their purported benefits in injury prevention, rehabilitation, and improving balance. 1 There is an abundance of training methodologies and exercises implementing various instability-training devices. 2 Proponents of instability resistance training deduce that the greater instability of the interface of the unstable platform and human body will stress the neuromuscular system to a greater extent than traditional resistance-training methods using more stable benches and floors. Instability training has been demonstrated to augment trunk-muscle activation, vertical-jump Drinkwater is with the School of Human Movement Studies, Charles Sturt University, Bathurst, New South Wales, Australia Pritchett and Behm are with the School of Human Kinetics and Recreation, Memorial University of Newfoundland, St John s, Newfoundland, Canada, A1C 5S7. 400

2 Unstable Lifting Kinetics 401 height, and balance and reduce injuries. A number of studies have demonstrated greater trunk activation when comparing similar exercises performed on an unstable surface versus a stable base. 3,4 Six weeks of fixed-foot instability training has also been shown to improve vertical-jump height, possibly as a result of improved balance (decreased body sway), resulting in a more efficient transfer of forces vertically. 5 The rehabilitation literature has reported the successful application of balance training to reduce the incidence of ankle sprains in a group of volleyball players. 6 This decrease in ankle-injury incidence might be related to the improved discrimination of ankle-inversion movements found with wobble-board training. 7 Instability training has also elicited improvement in anteroposterior stability of the knee in young female athletes, potentially leading to a decrease in injuries of the anterior cruciate ligament. 8 Thus, there are a number of positive outcomes associated with instability training. It is common to see individuals performing resistance-training exercises involving dumbbells or barbells while balancing on an instability device. Although training stabilizer muscles might be of value in improving balance and reducing injury, there is unfortunately a reduction in the force generated by the prime movers. Typically, the ability to exert force or power is depressed under conditions of instability. Behm et al 9 found decreases in force output of approximately 70% and 20% when their participants performed leg extensions and plantar-flexor contractions, respectively, while seated on an unstable ball. Similarly, Kornecki and Zschorlich 10 observed 20% to 40% decreases when exerting muscle power against an unstable pendulumlike device. When comparing stable and unstable chest presses, Anderson and Behm 11 showed that isometric chest-press forces were depressed by 60% under unstable conditions. McBride et al 12 found similar decreases in peak force ( 45%) and rate of force development ( 40%) and less EMG activity of the quadriceps muscle (approximately 35%) during unstable isometric squats. In contrast, overload tension on the muscle is essential for fostering strengthtraining adaptations. 13 Recently, a meta-analysis by Rhea et al 14 illustrated that training at 80% of 1-repetition maximum (1-RM) maximizes strength development in well-trained individuals, and a lower intensity of 60% is required in individuals with less training experience. Thus, the decreased force experienced with unstable resistance training might minimize strength-training adaptations. Because force is an integral part of the equation for power (force velocity), power might be similarly depressed with unstable bases. There are no studies reported in the literature to our knowledge that have examined changes in the power profile of resistance exercises to determine the effect of instability training on these actions. The objective of this study was to examine changes in measures related to power, velocity, force, and range of motion with squat movements using various intensities of resistance and degrees of instability. Experimental Design Methods Fourteen participants performed 3 sets of 3 repetitions of squats on 3 different platforms. Each set of squats was performed with a load of a 10-repetition maximum (10-RM) resistance, 40% of the 10-RM resistance, or a kg (hereafter rounded

3 402 Drinkwater, Pritchett, and Behm to 20 kg) Olympic bar on each platform. The platforms were stable floor, foam pads, and BOSU balls. Exercises at all 3 resistance intensities were performed on the same day, with 2 minutes rest between sets. Exercises on the different instability platforms were performed on separate days, with at least 48 hours of rest between. Dependent variables included mean and peak power, rate of force development, and range of motion. Because our subjects were experienced in resistance training but were not experienced in instability training, 10-RM was used as the heaviest resistance because it represents a sufficient load for strength development (ie, 75% 1-RM 15 ) but an amount that could still be safely controlled by participants under the most unstable conditions. Subjects Fourteen subjects participated in the study (mean ± SD; 23.3 ± 3.5 years old, weight 86.5 ± 12.7 kg, and height ± 5.1 cm). All subjects had been active participants in sports for 4.0 ± 2.4 years with training that included resisted squat movements. Their 10-RM squat was 99.4 ± 12.5 kg. Each subject was required to read and sign a consent form before participation. The Human Investigation Committee, Memorial University of Newfoundland, approved this study. Independent Variables Technical criteria for the squat were as follows: The bar was supported on the back in the high-bar position, with the feet shoulder-width apart and knees slightly flexed. The hips and knees were slowly flexed until the tops of the thighs were parallel with the floor. The hips and knees were then extended to return to the starting position. The back remained flat and the heels of the feet remained on the floor. The bar was not permitted to stop at any point throughout the lift. Failing to meet any of these technical criteria constituted an unsuccessful attempt. After an adequate warm-up (at least 2 sets of 10 repetitions that did not elicit failure), a resistance was estimated that would allow each participant to complete fewer than 15 repetitions before failure. Participants 10-RMs were calculated from prediction tables based on the number of repetitions performed. 15 On separate days, squat testing on the various platforms was conducted. On each day, 3 repetitions of 3 intensities of resistance were randomly allocated within a testing session: 10-RM, 40% of 10-RM, and 20-kg Olympic bar. A minimum of 2 minutes recovery was provided between trials at each intensity. Subjects stood on a different platform on each day of the study: a wooden platform covered with high-density rubber matting (Figure 1), two 50-cm length 25-cm width 5-cm height elliptically shaped foam pads (Figure 2), or two 65-cm diameter 30-cm height BOSU balls (Figure 3). The platform testing was randomly allocated over 3 days, with at least 48 hours recovery between testing sessions. The squat action was performed at the desired rhythm of the participant. The bar was placed behind the head and on the shoulders of the individual. Feet were placed shoulder-width apart either on the floor or on the instability devices. The participant descended with the resistance until his thighs were parallel to the floor. There was no appreciable delay between the concentric and eccentric actions of the squat.

4 Unstable Lifting Kinetics 403 Figure 1 Full squat on stable floor. Figure 2 Full squat on foam blocks. Figure 3 Full squat on BOSU balls. Dependent Variables Barbell kinematics of each repetition were measured using a Gymaware optical encoder (Kinetic Performance Technology, Canberra, Australia). 16 The Gymaware consists of a spring-powered retractable cord that passes one end around a pulley mechanically coupled to an optical encoder. The Gymaware is positioned on the floor perpendicular to the movement of the barbell. The other end of the cord is attached to the barbell, thus enabling it to measure velocity and displacement of the barbell. Velocity and displacement information is stored on a personal digital assistant (Tungsten-e, Palm, Milpitas, CA) where it is integrated with mass to calculate power. One data point is time-stamped every 0.3 mm of barbell displacement to a maximum rate of approximately 25 Hz. Statistical Analysis Each variable was analyzed by repeated-measures ANOVA (3 surfaces 3 intensities) with a Fisher least-significant-difference post hoc assessment in SPSS 14.0 for Windows (Chicago, IL). In graphical format, we have expressed mean differences between surfaces, and uncertainty in the estimate is expressed with 95% confidence intervals. To interpret the magnitude of differences between conditions, we modified Cohen s guidelines for qualitative interpretation of effect size. 17 Cohen 17 regarded.1,.3, and.5 as the thresholds for small, moderate, and large correlations, respectively. These thresholds correspond to effect sizes of 0.20, 0.63, and 1.16, not 0.2, 0.5, and 0.8 as suggested by Cohen. We did not regard Cohen s thresholds as definitive, so we used effect sizes of 0.2, 0.6, and 1.2 as thresholds for small, moderate, and large effects. 18

5 404 Drinkwater, Pritchett, and Behm We were interested in practical differences between surfaces so we based the smallest worthwhile change on a small effect size (>0.2). The likelihood that the observed effect size was larger than the smallest worthwhile change (ie, was clinically meaningful) was calculated based on previous methods. 19 Briefly, chances of clinically meaningful difference were estimated with a spreadsheet (see and interpreted qualitatively as follows: <1%, almost certainly not; <5%, very unlikely; <25%, unlikely; 25 75%, possible; >75%, likely; >95%, very likely; and >99% almost certain. Therefore, results are expressed by effect size and percent likelihood that the observed difference was greater than a small effect size. Peak Concentric Power Results Results for peak concentric power are shown in Figure 4, and comparisons of different surfaces at different resistances are shown in Table 1. The likelihood that the transition in moving from stable to foam only reached very likely at 100% 10-RM but was unlikely at 40% 10-RM and almost certainly not at 20 kg. In moving from stable to BOSU, the likelihood of clinical impact progressively increased at heavier resistances, being unlikely at 20 kg but very likely at 40% 10-RM and almost certain at 100% 10-RM. Finally, differences between foam and BOSU conditions did not exceed possible at lighter resistances, although differences were likely at 100% 10-RM. Figure 4 Differences in peak concentric power at 3 different resistances under stable to foam (dots), stable to BOSU (diagonal stripes), and foam to BOSU (horizontal stripes) conditions. Bars represent mean differences between conditions, and error bars are 95% confidence limits.

6 Table 1 Effect Sizes and Likelihoods of Clinically Meaningful Differences When Comparing Different Surfaces at Different Resistances Property Surface Resistance Cohen s d Likelihood of difference being clinically meaningful (%) Peak concentric power stable to foam 20 kg 0.01 <1 stable to BOSU 20 kg foam to BOSU 20 kg 0.11 <1 stable to foam 40% 10-RM stable to BOSU 40% 10-RM foam to BOSU 40% 10-RM stable to foam 100% 10-RM stable to BOSU 100% 10-RM 1.30 >99 foam to BOSU 100% 10-RM Peak concentric force stable to foam 20 kg stable to BOSU 20 kg foam to BOSU 20 kg 0.07 <1 stable to foam 40% 10-RM stable to BOSU 40% 10-RM foam to BOSU 40% 10-RM stable to foam 100% 10-RM stable to BOSU 100% 10-RM 0.82 >99 foam to BOSU 100% 10-RM Peak concentric velocity stable to foam 20 kg stable to BOSU 20 kg foam to BOSU 20 kg 0.83 >99 (continued) 405

7 Table 1 (continued) Property Surface Resistance Cohen s d Likelihood of difference being clinically meaningful (%) stable to foam 40% 10-RM stable to BOSU 40% 10-RM foam to BOSU 40% 10-RM stable to foam 100% 10-RM stable to BOSU 100% 10-RM 0.80 >99 foam to BOSU 100% 10-RM 1.32 >99 Squat depth stable to foam 20 kg stable to BOSU 20 kg foam to BOSU 20 kg stable to foam 40% 10-RM stable to BOSU 40% 10-RM foam to BOSU 40% 10-RM stable to foam 100% 10-RM stable to BOSU 100% 10-RM 1.73 >99 foam to BOSU 100% 10-RM Peak eccentric power stable to foam 20 kg stable to BOSU 20 kg foam to BOSU 20 kg stable to foam 40% 10-RM stable to BOSU 40% 10-RM foam to BOSU 40% 10-RM stable to foam 100% 10-RM stable to BOSU 100% 10-RM 1.42 >99 foam to BOSU 100% 10-RM

8 Unstable Lifting Kinetics 407 Peak Concentric Force Results for peak concentric force are shown in Figure 5. Although the loss of peak force in the transition from stable to foam at 20 kg was possible, the clinical implications increased to probable at 40% 10-RM and 100% 10-RM. In the transition from stable to BOSU, the likelihood of clinically meaningful differences was probable at 20 kg and 40% 10-RM and was almost certain at 100% 10-RM. All differences between foam and BOSU at any resistances were, at the least, unlikely. Peak Concentric Velocity Results for peak concentric velocity are shown in Figure 6. All differences between stable and foam at any resistances were, at the least, unlikely. Conversely, all resistances in the transition from stable to BOSU can be seen as very likely or almost certain. All resistances in the transition from foam to BOSU can also be seen as very likely or almost certain. Squat Depth Results for squat depth are shown in Figure 7. Moving from stable to foam, there was a very likely decrease in squat depth when at 20 kg and at 100% 10-RM, although it was unlikely to have played a meaningful role at 40% 10-RM. Differences were also very likely in the transition from stable to BOSU at 20 kg and 40% 10-RM but were almost certain at 100% 10-RM. Trends reverse when moving from foam to BOSU, with differences being unlikely at 20 kg and 40% 10-RM but very likely at 100% 10-RM. There were no practical effects of increasing resistance on squat depth when all surfaces were combined. Figure 5 Differences in peak concentric force at 3 different resistances under stable to foam (dots), stable to BOSU (diagonal stripes), and foam to BOSU (horizontal stripes) conditions. Bars represent mean differences between conditions, and error bars are 95% confidence limits.

9 Figure 6 Differences in peak concentric velocity at 3 different resistances under stable to foam (dots), stable to BOSU (diagonal stripes), and foam to BOSU (horizontal stripes) conditions. Bars represent mean differences between conditions, and error bars are 95% confidence limits. Figure 7 Differences in squat depth at 3 different resistances under stable to foam (dots), stable to BOSU (diagonal stripes), and foam to BOSU (horizontal stripes) conditions. Bars represent mean differences between conditions, and error bars are 95% confidence limits. 408

10 Unstable Lifting Kinetics 409 Figure 8 Differences in peak eccentric power at 3 different resistances under stable to foam (dots), stable to BOSU (diagonal stripes), and foam to BOSU (horizontal stripes) conditions. Bars represent mean differences between conditions, and error bars are 95% confidence limits. Peak Eccentric Power Results for peak eccentric power are shown in Figure 8. In moving from stable to foam, the magnitudes and likelihoods of clinical differences increase as resistance increases. At 20 kg, the difference is unlikely but is possible at 40% 10 RM and is likely at 100% 10-RM. Differences in moving from stable to BOSU range from likely at 20 kg and 40% 10-RM to almost certain at 100% 10-RM. Differences in moving from foam to BOSU are very unlikely at 20 kg and unlikely at 40% 10-RM but are probable at 100% 10-RM. Discussion The most unique findings of this study were the significant instability-induced decrements in peak concentric power, force, and velocity, as well as squat depth and peak eccentric power. Differences were observed primarily in the transition from the stable (floor) to the very unstable (BOSU) surface. Differences in transitions involving the moderately unstable surface (ie, floor to foam or foam to BOSU) were primarily seen at the heaviest resistance. These findings suggest that squats performed under increasingly unstable conditions might not provide an optimal environment for strength and power training. The instability-induced kinetic impairments in the current study are similar to previously reported instability-induced decrements in force. Behm et al 9 found decreases in force output of approximately 70% and 20% when their participants

11 410 Drinkwater, Pritchett, and Behm performed leg extensions and plantar-flexor contractions, respectively, while seated on an unstable ball. Similarly, Kornecki and Zschorlich 10 observed 20% to 40% decreases when their participants exerted muscle power against an unstable pendulumlike device. When comparing stable and unstable chest presses, Anderson and Behm 11 showed that isometric chest-press forces were depressed by 60% under unstable conditions. Similar to these previous studies, the current research cannot account for any loss in force measured because of compression of the air-filled instability devices. The decrease in peak concentric force in the current study was generally considered trivial to small as described with effect-size calculations. There was a moderate effect-size change in peak velocity when comparing stable with BOSU squats at 100% 10-RM and clinically meaningful differences with all resistances when comparing foam with BOSU squats. Similarly, there were meaningful effect sizes when evaluating squat-depth changes moving from stable (floor) to unstable (foam and BOSU) conditions at most resistances. These findings would imply that velocity and range of motion might be affected to an even greater degree than force by an unstable platform. The findings would have important implications for training in sports that emphasize speed and power and call into question the training specificity of instability devices for power and speed. According to the concept of training specifically, 13 because not all kinetic variables (ie, force and power) are produced under stable conditions (eg, shooting a puck while balancing on a single skate blade in ice hockey, performing a routine on a balance beam in gymnastics, changing direction rapidly by pivoting on 1 foot on uneven natural turf in football) training must attempt to closely mimic the demands of the sport or occupation. For instance, Behm et al 20 showed significant correlations (P <.005) between hockey skating performance and static-balance tests, with the highest correlation between balance and the skating ability of hockey players under the age of 19 years (r =.65), although this relationship was not as strong in players older than 19 years (r =.28). In addition, college baseball pitchers with weaker vestibular input were reported to have higher levels of pitching errors. 21 Nonetheless, attempting to accomplish training adaptations for both power and balance simultaneously might not provide the overload stress necessary for power adaptations. The instability-induced decreases in force, velocity, power, and range of motion might be hypothesized to result from changes in muscle-activation patterns. An increased focus on muscle stabilization could hamper coordination by increasing the stiffness of the joints performing the action. Carpenter et al 22 indicated that a stiffening strategy was adopted when individuals were presented with a threat of instability. Similarly, Adkin et al 23 reported that when subjects received a postural threat (fear of falling), the magnitude and rate of voluntary movements were reduced. Kornecki et al 24 found that contributions of stabilizing muscles increased on average by 40% when the handle changed from stable to unstable during pushing movements. Similar to the current study, they showed that the process of muscle stabilization caused drops in force, velocity, and power. Although Anderson and Behm 11 found a 60% drop in isometric chest-press force when performed on an unstable ball, there was no significant change in upper limb EMG activity between stable and unstable conditions. They suggested that the limb muscles would be forced to play a greater stabilizing role under unstable conditions.

12 Unstable Lifting Kinetics 411 In addition to the possible increased stabilization functions of the prime movers, there could also be greater cocontractile activity during performance of unstable squats. Increased antagonist activity might occur to increase joint stiffness to promote stability. 25 Behm et al 9 demonstrated that unstable plantar-flexor and leg-extensor conditions experienced 30.7% and 40.2% greater antagonist activity than the stable conditions, respectively. Antagonist activity has been reported to be greater when there is uncertainty in the required task. 26 Although increased antagonist activity could be used to improve motor control and balance, it could also contribute to a greater decrement in force, power, and velocity with the unstable conditions by providing greater resistance to the intended motion. Continued training, though, can result in lower coactivation levels in certain types of work. 27 Carolan and Cafarelli 28 demonstrated a decrease in coactivation associated with a resistance-training program of the leg extensors. Whereas the use of instability resistance training to improve balance and stability and decrease movement uncertainty might be hypothesized to decrease cocontractions, which in terms of energy conservation would improve movement efficiency, the use of instability devices might not provide an adequate stimulus for power adaptations. If balance training is necessary, our results would suggest that it should be pursued separately from power training. For example, balance and power could be trained in the same session but should be employed during different exercises or sets. On the contrary, Willardson 29 states that the optimal method to promote increases in balance and proprioception for any given sport is to practice the skill itself on the same surface on which the skill is performed in competition. (p 71) Conversely, there are other reports of instability-device training s providing sport-related benefits. Recently, the rehabilitation literature has reported the successful application of balance training to reduce the incidence of ankle sprains in a group of volleyball players. 6 This decrease in ankle-injury incidence might be related to the improved discrimination of ankle-inversion movements found with wobble-board training. 11 Kean et al 5 reported an increase in vertical-jump height after 5 weeks of wobbleboard training. They postulated that improvements to balance-training-induced decreases in postural sway resulted in a more optimal or vertical takeoff during the vertical jump. Although the findings of the current study argue against the use of instability platforms for strength and power training, instability devices might still play a role in overall balance or trunk-stability training. This could be especially true for the recreational fitness population, who might not use closed-kinetic-chain free-weight exercises such as free-weight squats. Practical Applications Research has been done establishing the benefits of resistance training on an unstable platform for the purposes of injury prevention and improving core stability and balance. Despite these benefits there are deleterious effects on power output, lifting velocity, and squat technique during instability training. It is not uncommon for a person to have a need to develop different components of fitness whose training methods conflict with each other, such as cardiovascular endurance and strength training. 30 Such individuals require an appropriate periodized program that meets the different fitness goals by cycling blocks of training that meet each

13 412 Drinkwater, Pritchett, and Behm fitness goal individually. Strength and conditioning coaches should not attempt to combine instability training with training designed to improve maximal strength or power output. Conclusion The use of unstable platforms during a resisted free-weight squat movement decreased measures of concentric force, velocity, and power in addition to range of motion and eccentric power. The current emphasis on instability devices with resistance training might be misguided for those who seek training gains associated with strength and power. Whereas previous research has espoused the beneficial effects of various instability devices for balance and trunk-muscle activation, the pursuit of both power and balance adaptations might necessitate separate protocols. Acknowledgments This research was financially supported by the Natural Science and Engineering Research Council (NSERC) of Canada. References 1. Akuthota V, Nadler SF. Core strengthening. Arch Phys Med Rehabil. 2004;85:S86- S Goldenberg L, Twist P. Strength Ball Training. Champaign, IL: Human Kinetics; Anderson K, Behm DG. Trunk muscle activity increases with unstable squat movements. Can J Appl Physiol. 2005;30: Behm D, Leonard A, Young W, Bonsey W, MacKinnon S. Trunk muscle electromyographic activity with unstable and unilateral exercises. J Strength Cond Res. 2005;19: Kean CO, Behm DG, Young WB. Fixed foot balance training increases rectus femoris activation during landing and jump height in recreationally active women. J Sports Sci Med. 2006;5: Verhagen EA, van Tulder M, van der Beek AJ, Bouter LM, van Mechelen W. An economic evaluation of a proprioceptive balance board training programme for the prevention of ankle sprains in volleyball. Br J Sports Med. 2005;39: Waddington G, Seward H, Wrigley T, Lacey N, Adams R. Comparing wobble board and jump-landing training effects on knee and ankle movement discrimination. J Sci Med Sport. 2000;3: Paterno MV, Myer GD, Ford KR, Hewett TE. Neuromuscular training improves single-limb stability in young female athletes. J Orthop Sports Phys Ther. 2004;34: Behm DG, Anderson K, Curnew RS. Muscle force and activation under stable and unstable conditions. J Strength Cond Res. 2002;16: Kornecki S, Zschorlich V. The nature of the stabilizing functions of skeletal muscles. J Biomech. 1994;27: Anderson K, Behm D. Maintenance of EMG activity and loss of force output with instability. J Strength Cond Res. 2004;18: McBride JM, Cormie P, Deane R. Isometric squat force output and muscle activity in stable and unstable conditions. J Strength Cond Res. 2006;20:

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