Introduction
Back squat exercise is often used to develop and test maximal lower-body strength and forms the basis of many strength and conditioning programs and powerlifting competition (2,3). Because relatively heavy loads can be used during back squat exercise, support equipment is often worn (3,7). Knee wraps are worn to both support the knee joint and gain mechanical advantage during back squat exercise, with anecdotal evidence suggesting that wearing knee wraps enables athletes to lift greater loads or perform more repetitions with a given load (3–7). It is thought that this is because elastic energy is generated as knee wraps stretch during the lowering phase, returning this energy during the lifting phase (3,7).
Knee wraps are typically constructed from thick canvas that is often interwoven with rubber filaments (3). To gain the perception of support around the knee joint or mechanical advantage, knee wraps must be applied as tightly as possible and are often applied by another person (3,5,6). The tight fit is known to cause considerable discomfort and can create a “wedge”-like physical barrier at the back of the knee joint that can cause changes in back squat technique, tipping the athlete forward (3,7). However, little is currently known about the effect that wearing knee wraps has on mechanical output and performance characteristics of back squat exercise. It is therefore critical that research is performed to provide data that will enable strength and conditioning practitioners to make informed decisions about knee wrap use during back squat exercise.
Harman and Frykman (3) found that when knee wraps were worn during simulated back squat exercise, significantly greater forces, recorded by digital scale, were applied to the center of mass at the conclusion of the lowering phase. They concluded that increased force was a reflection of elastic energy generated, and stored, as the wraps were stretched across the knee joint during the descent phase (3). However, although these findings offer some insight into the mechanisms that underpin the mechanical advantage that can be gained from wearing knee wraps, their methodology was crude, and did not afford detailed study of changes in the mechanical output and performance characteristics of back squat exercise.
Mechanical output and performance characteristics of ground-based resistance exercise can be obtained by manipulating ground reaction forces (GRFs) recorded in 3 orthogonal axes at the foot-floor interface, using Newton's second law of motion. Impulse applied to the center of mass describes the application of force in a given direction over a given time period and is obtained by summing the area under the force-time curve (1). Mechanical advantage gained from wearing knee wraps would be reflected by an increase in vertical impulse applied to the center of mass. Power applied to the center of mass describes the rate at which mechanical work is performed (1) and is calculated by multiplying force applied to, by velocity of the center of mass, where velocity is obtained by integrating the product of dividing net force (force − weight) by barbell and lifter mass (1). Assuming back squat exercise is controlled, increases in vertical impulse applied to the center of mass would lead to an increase in the rate at which mechanical work is performed. However, if knee wraps create a physical barrier at the back of the knee joint, it is likely that the lifter would be tipped forward. This would be reflected by increased horizontal displacement of the barbell, obtained from digitized sagittal plane motion of the barbell and would likely affect horizontal impulse applied to the center of mass, which would increase because of excessive forward motion. Therefore, the aim of this study was to investigate the effect that wearing knee wraps had on mechanical output and performance characteristics of back squat exercise.
Methods
Experimental Approach to the Problem
A counterbalanced design was used to test the hypotheses that wearing knee wraps during back squat exercise would significantly affect mechanical output and performance characteristics. These were quantified by the dependent variables of vertical and horizontal impulse and peak power applied to the center of mass, horizontal displacement of the barbell, and absolute and relative lowering and lifting phase duration. Resistance trained men with experience of wearing knee wraps during back squat exercise took part in the study, half wearing knee wraps first, the other half wearing them after performing back squats without wraps. Mechanical output and performance characteristics data were obtained from GRF and barbell motion recorded during back squat exercise with 80% of their 1-repetition maximum (1RM). Data from back squats performed without knee wraps were included as a control, to which dependent variables recorded during back squat exercise wearing knee wraps were compared using paired sample t-tests. The magnitude of knee wrap effect was quantified using effect sizes (ESs).
Subjects
Ten resistance trained men, who had been free of lower-body pathology for at least 6 months, volunteered to participate. They had a mean (SD) of 4.4 (1.4) years of experience with the back squat exercise and had all squatted with knee wraps. However, none of the subjects used knee wraps regularly so undertook a familiarization session with the wrapping style used by Harman and Frykman (3). This occurred approximately 15 minutes after 1RM testing and involved performing several single repetitions with 60–80% of their 1RM. Subject physical and performance characteristics were age: 21.9 (2.2) years; mass: 93.3 (10) kg, stature: 171.8 (34.8) cm, and back squat 1RM: 160.5 (18.4) kg. All testing was performed during February, 3 weeks into a 6-week post-Christmas adaptation mesocycle. Ethical approval for this study was gained from the ethical review panel at the University of Chichester, Chichester, United Kingdom, before data collection. After a thorough explanation of the experimental aims, procedures, and potential risks, the subjects provided written informed consent.
Procedures
A schematic of the experimental procedures is presented in Figure 1. All the subjects attended the laboratory at the same time of the day for each testing session, approximately 2 hours after a light lunch and 48 hours after their last lower-body resistance training session. During the first session, maximal back squat strength (1RM) was established using a protocol similar to the one used by Wallace et al. (8) (Figure 1). Seven days later, the subjects returned to the laboratory to perform submaximal back squat testing, where after warming up they performed 6 sets of maximal effort single back squats with 80% 1RM; 3 wearing wraps, 3 without. This load was selected because it represented a typical training load, was heavy enough to overcome resistance presented by wearing knee wraps, but allowed for repeat performance necessary to obtain a set of representative data. Knee wrap use order was counterbalanced; with half of the subjects performing with knee wraps first, the other half performing without knee wraps first. A rest period of 3 minutes was enforced between each lift. The descent phase of back squat performance continued until the tops of the thighs were parallel to the ground, after which the subjects were instructed to perform the ascent phase as quickly as possible. Two sets of Hercules knee wraps (Strength Shop Ltd., Edinburgh, United Kingdom) that were 0.02 × 0.08 × 2.00 m and composed of a heavy cotton fabric with interwoven elastic rubber filaments, similar to those used in previous research (3), were worn during back squat exercise. The “figure of eight” wrapping technique, described and used by Harman and Frykman (3), was used to fit the knee wraps and is illustrated in Figure 2. The same experimenter applied the wraps as tightly as possible immediately before each trial, standardizing the number of wrap revolutions to 9 revolutions per subject. Pilot testing demonstrated that each wrap reached full “stretch” when a force of 446 N was applied to the wrap by suspending it from one end of the wrap while the other end was attached to an immovable object. During a simulation of the procedure used during the experiment, the experimenter applied a mean force of 343 (20.9) N to the wrap over 9 wraps. This was recorded by a hanging scale (HCB200K500 Kern and Sohn GmbH, Balingen, Germany).
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Figure 1: Schematic of experimental protocol.
Figure 2: Application of knee wraps using the “Figure of eight” method used by Harman and Frykman (
3).
Before data collection, a spherical, retroreflective marker illuminated by a spotlight positioned behind a high-speed digital camera (Basler A602fc-2, Ahrensburg, Germany) was affixed to the end of an Olympic barbell. The camera, positioned 8 m from and perpendicular to the right side of the subject, recorded back squat exercise at 100 Hz after first recording a 1-m-long calibration pole. Simultaneously, horizontal (anterior-posterior) and vertical GRF of back squat exercise were recorded at 500 Hz from both feet with two 0.4 × 0.6 m in ground force platforms (model: 9281E, Kistler Instruments Ltd., Hook, United Kingdom) using BioWare 3.21 software (Kistler Instruments Ltd.).
Statistical Analyses
Back squat exercise with and without knee wraps was the independent variable, mechanical output and performance characteristics of back squat exercise the dependent variables. Impulse applied to the center of mass were calculated as the sum of the area under the vertical and anterior-posterior force-time curve during the lowering and lifting phases, respectively. The lowering and lifting phase was identified from vertical displacement of the barbell. Peak power was defined as the highest instantaneous vertical power applied to the center of mass during the lifting phase and was calculated as the product of vertical GRF and vertical velocity of the center of mass. Vertical velocity of the center of mass was obtained by integrating the result of dividing net vertical force (vertical GRF—barbell and lifter weight) by barbell and lifter mass. Horizontal displacement of the barbell consisted of the total horizontal displacement recorded from a vertical reference line that began at the end of the barbell in the top position of the lift, immediately before the start of the lowering phase and was calculated for both lowering and lifting phases. All horizontal displacement data were rectified to avoid negative displacements canceling out positive displacements, by calculating the square root of squared horizontal displacement. Absolute lowering and lifting phase duration was determined from changes in barbell displacement. Relative lowering and lifting phase duration was calculated by dividing absolute phase duration by the sum of absolute lowering and lifting phase duration respectively and multiplying these by 100, expressing these as a percentage.
Differences between dependent variables recorded with and without knee wraps were analyzed using paired sample t-tests. The magnitude of the effect that wearing knee wraps had on dependent variables was quantified using ES, which was calculated by dividing the differences between back squat exercise with and without knee wraps by their pooled SDs. The magnitude of ES was quantified using the scale recently presented by Hopkins et al. (4) where an ES of 0.20, 0.60, 1.20, 2.0, and 4.0 represented small, moderate, large, very large, and extremely large effects, respectively. Within- and between-session reliability of the dependent variables was examined using intraclass correlations (ICC). Statistical analysis was performed in SPSS (version 18, SPSS Inc., Chicago, IL, USA) and Microsoft Excel (Microsoft Ltd., Reading, United Kingdom) and an alpha value of p ≤ 0.05 used to indicate statistical significance. Statistical power for the sample size used was between 1 − β = 0.576 and 0.833.
Results
Within- and between-session reliability of the dependent variables was high, with ICC R values between 0.93 and 0.99. Descriptive statistics, results from the paired t-tests, and ES are presented in Table 1, whereas representative barbell trajectories from back squat exercise with and without knee wraps are presented in Figures 3 and 4.
Table 1: Mean (SD) mechanical output and performance data from back squat exercise, mean percentage differences, p value results from paired t-test comparison, and ESs.*
Figure 3: Typical horizontal displacement of the barbell during unwrapped back squat performance. The phase between points “a” and “b” is the lowering phase, “b” to “c,” then “c” to “d” the lifting phase.
Figure 4: Typical horizontal displacement of the barbell during wrapped back squat performance. The phase between points “a” and “b” is the lowering phase, “b” to “c,” then “c” to “d” the lifting phase.
Wearing knee wraps during back squat exercise significantly reduced horizontal displacement during the lowering phase (p = 0.037) but not the lifting phase (p = 0.407, Table 1). Although lowering phase vertical (p = 0.366) and horizontal (p = 0.409) impulse applied to the center of mass were not affected by wearing knee wraps, the lifting phase equivalents were, with both demonstrating a moderate to large ES (vertical ES: 1.12, horizontal ES: 0.81). Wearing knee wraps significantly reduced the absolute lowering phase duration (p = 0.006) but not absolute lifting (p = 0.391), relative lowering (p = 0.083), or relative lifting (p = 0.083) phase duration. However, wearing knee wraps significantly increased peak power (p = 0.019, Table 1).
Discussion
Knee wraps are often worn during back squat exercise to improve the load that can be lifted or the amount of repetitions that can be performed with a given load (3). However, this mechanical advantage has not been quantified. Therefore, this study set out to establish whether wearing knee wraps provided a mechanical advantage during single repetition back squat performance with 80% 1RM and to establish the effects on performance characteristics.
The results showed that knee wraps did provide a mechanical advantage. Wearing knee wraps during back squat exercise increased vertical impulse, decreased lowering and lifting phase duration indicating that vertical force applied to the center of mass increased, particularly during the lowering phase. This finding validates the results presented by Harman and Frykman (3), who recorded the weight of lifters who performed simulated back squat exercise with and without knee wraps, which involved subjects being lowered on digital scales. They found that lifters were significantly heavier when knee wraps were worn. However, the results of this study progresses the work of Harman and Frykman (3), establishing mechanisms underpinning their findings.
The lowering phase was performed faster when knee wraps were worn, and elastic energy generated and stored within the knee wraps was released, increasing vertical force applied to the center of mass. In turn, this reduced the time in which the mechanical work performed by vertically displacing the barbell and body system center of mass through a standardized range of motion could be performed; this was reflected by an increase in peak power. Indeed, vertical impulse and peak power were the only dependent variables that wearing knee wraps had an almost large effect on.
An unlikely result of this study was the relatively large reduction (lowering phase: 39%, lifting phase: 99%) in horizontal displacement of the barbell when knee wraps were worn. It was postulated that wearing knee wraps would create a physical barrier at the back of the knee joint that would tip the lifter forward. This result clearly demonstrates that this was not the case and is supported by horizontal impulse data. It was hypothesized that if the lifter tipped forward, changes in anterior and posterior forces would reflect this increasing to counterexcursion of the center of mass. Instead, horizontal impulse decreased by 7% during the lowering phase and increased by 5% during the lifting phase. This finding raises concerns about the effect that wearing knee wraps can have on back squat technique, both in terms of training specificity and injury potential.
Wearing knee wraps appears to significantly affect traditional movement patterns of back squat exercise, by forcing the lifter to use different techniques. Unfortunately, analysis of performance motion was limited to horizontal displacement of the barbell. However, the results of this study suggest that wearing knee wraps restricted motion around the hip joint, which caused (a) a more upright posture and more importantly (b) forced greater flexion at the knee joint. In terms of training specificity, this would suggest that contribution of the powerful hip flexors and extensors is restricted when knee wraps are worn, stored elastic energy within the wrap compensating. In terms of injury potential, this raises 2 concerns: (a) continued use of knee wraps would restrict development of hip extensor and flexor musculature and (b) continued flexion around the physical barrier created by knee wraps may compromise the integrity of the knee joint. Harman and Frykman (3) and Totten (7) described the physical barrier at the back of the knee joint caused by the use of knee wraps as a pivot that can “unhinge” the knee joint. Although further research would be needed to corroborate these claims, changes to technique cannot be denied, and it is for this reason that we feel that knee wraps should not be used during the strength and conditioning process.
Although this is the first study to demonstrate a mechanical advantage from wearing knee wraps during back squat exercise, several experimental limitations must be considered. The main limitation is that although the amount of “wraps” applied to each subject was standardized, and back squat performance was standardized so that subjects squatted until thighs were parallel to the ground, the range of motion was not controlled relative to subject anthropometry. Therefore, some subjects may have performed back squat exercise through a greater rage of motion, which may have resulted in the generation of more elastic energy. This may explain the high SDs that were reported in the results. Further, although wearing knee wraps during back squat exercise provided a mechanical advantage, it is important to remember that it was found during single repetition performance with 80% 1RM. Typically, knee wraps are worn during either maximal strength testing, whether in a competition environment or not or with submaximal loads with the aim of performing maximal repetitions. Although it is reasonable to assume that the results of this study could be replicated in either of the above scenarios, further research would be needed to clarify this. The rationale for using this load was that it is often used in training and demands proper technique but enables repeat performances for statistical purposes.
Practical Applications
The results of this study demonstrate that, when worn during single repetition back squat performance with 80% 1RM, knee wraps create a mechanical advantage that occurs when elastic energy, generated during the lowering phase, is released. Wearing knee wraps alters the back squat technique in a way that leads us to believe that (a) development of balanced lower-body musculature may be compromised and (b) that the combination of the modified body position observed when knee wraps were worn and the physical barrier at the back of the knee joint may compromise the integrity of the knee joint. We therefore propose that knee wraps should not be worn during the strength and conditioning process and that if an athlete feels that additional support is needed for the knee, the integrity of the joint is thoroughly assessed and treated rather than relying on artificial aid that could exacerbate any underlying issues.
Acknowledgments
The authors thank Strength Shop Ltd., Edinburgh, United Kingdom, who donated the knee wraps used in this investigation. However, the results of this study do not constitute endorsement by the authors or the National Strength and Conditioning Association.
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