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.).
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.
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.
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).
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.
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.
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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Keywords:Copyright © 2012 by the National Strength & Conditioning Association.
force; impulse; power; horizontal displacement; ergogenic