The back squat exercise is used in a wide variety of environments ranging from clinical rehabilitation to formal strength and conditioning programs to everyday gym environments. The squat exercise is popular because of its similarity and applicability to both activities of daily living and many athletic movements, and it is a multijoint movement that requires many large muscle groups to function together (18,23,32). Many health professionals whose focus is on performance enhancement often recommend the squat exercise to increase ligament, tendon, and bone strength; develop strength, speed, and power of the lower-back, hip, and knee musculature; and improve neuromuscular efficiency (6,31). However, other health professionals whose primary focus is rehabilitation after knee injury list the deep squat as a contraindicated exercise (28) and recommend performing the squat exercise with limited knee flexion because of the commonly held belief that the squat exercise may elicit dangerously high forces on the knee (14,29). Despite the large body of work performed studying the squat exercise (4,5,8,10,18,25,29,30,32,33,35,36,38,41,42), it remains difficult to reconcile the differences in recommendations between these 2 schools of thought due to the lack of a comprehensive study comparing squats as recommended by both (10,29).
The American College of Sports Medicine recommends that when engaging in resistance training, the apparently healthy adult should “perform every exercise through a full range of motion” (15). Working through a full range of motion allows for strength adaptations to occur at every angle the joint moves through, which may reduce injury potential in those ranges. In addition, using full range of motion maintains flexibility for joint integrity. Some coaches even use the deep squat as an assessment tool for lower-body flexibility and symmetry of movement (7). In addition, squatting-type activities are widespread in many cultures where squatting below parallel is commonplace when toileting or resting (27). Even though many health professionals agree that working through full range of motion is important, the knee joint and, specifically, the squat are often seen as exceptions to the rule.
Many health professionals often recommend limiting squat depth to parallel or higher under the assumption that doing so will reduce the potential of knee laxity and minimize harmful forces at the knee joint (20); yet, some studies have called into question the assumptions regarding muscle activation and knee laxity. In an electromyography study, Caterisano et al. (4) found that increasing squat depth only resulted in increased relative contribution of the gluteus maximus. In a study of instrumented laxity, Chandler et al. (5) reported no difference in knee laxity after an 8-week training program including either above-parallel or below-parallel squats. Chandler et al. (5) also found that both powerlifters and weightlifters had less laxity, that is, “tighter” knees, than controls. The concern for harmful forces at the knee centers on peak patellofemoral joint reaction force (pPFJRF), which may lead to patellofemoral pain syndrome by overloading the cartilage, potentially leading to cartilage degeneration and subsequent subchondral bone degeneration (22). Patellofemoral pain syndrome is one of the most common disorders of the knee, accounting for nearly 30% of all knee injuries treated in sports medicine clinics and influencing both athletes and nonathletes alike (9,12,22). If the relationship between pPFJRF and patellofemoral pain syndrome is correct, it would be beneficial to understand how pPFJRF is influenced by variations in the squat exercise.
Previous biomechanical analyses have indicated that increased pPFJRF is coupled with increased knee flexion angles (8,33). In squats up to 90° of knee flexion without an external load and with a load of 35% of subject’s body weight, pPFJRF was found to increase linearly with peak external knee flexion moments (pEKFM) in untrained men (38). Dahlkvist et al. (8) estimated increases in pPFJRF while squatting to full depth in an unconventional squat with no external load where the heels do not remain on the ground. In contrast, when comparing squats with knee flexion angles of 70°, 90°, and 110° in trained women, Salem and Powers (35) found no differences in pEKFM or pPFJRF with an 85% 1RM load. It is difficult to interpret these conflicting reports on knee loading because of the large variations between studies in load and technique. In addition, when basing loads off of 1RM values, 1RM values for each squat depth are not often independently tested. In studies using a conventional squat movement, pEKFM was found to be the major contributor of both pPFJRF and patellofemoral stresses (35,38). Given the strong agreement between pEKFM and pPFJRF at flexion angles where both have been experimentally observed and the lack of data on pPFJRF above 110° of knee flexion, for this study we examined pEKFM as a surrogate for pPFJRF.
Our aim was to compare pEKFM during the squat with loads and depths that are often prescribed in both rehabilitation and strength training programs, including depth-specific 1RM measurements. To achieve our aim, we tested the hypotheses that back squat 1RM will decrease with increased depth (i.e., above-parallel 1RM > parallel 1RM > below-parallel 1RM) and that this decrease in 1RM would allow pEKFM to remain constant for load- and depth-matched squats (i.e., pEKFM for above-parallel squat with 85% above-parallel 1RM ≈ pEKFM for below-parallel squat with 85% below-parallel 1RM).
Experimental Approach to the Problem
To investigate the effect of commonly prescribed squat loads and depths on pEKFM, 9 combinations of differing loads and depths were analyzed. The 3 predetermined depths consisted of the thigh being above parallel, parallel, and below parallel to the ground. These depths were defined as follows: the above-parallel squat, to 90° of knee flexion; the parallel squat, to where the inguinal crease falls just below the proximal patella; and the below-parallel squat or deep squat, to when the backs of the thighs come into contact with the calves. Normalized loading conditions were calculated based on the 1RM loads determined at each of the 3 depths. The 3 loads chosen for each depth were unloaded, 50% 1RM, and 85% 1RM. An unloaded squat may be prescribed during rehabilitation from injury or during the initial periods of learning. Loads of 50% 1RM and 85% 1RM are frequently used for power and strength training, respectively. The order of the 9 trials was randomly chosen for each subject. Peak external knee flexion moments were determined for each trial.
Sixteen healthy recreationally trained men (n = 16; age, 22.7 ± 1.1 years; body mass, 85.4 ± 2.1 kg; height, 177.6 ± 0.96 cm; mean ± SEM) with no previous lower-limb surgeries or other orthopedic issues and at least 1 year of consistent resistance training experience while using the squat exercise participated in the study. All subjects were visually assessed for the ability to squat at full depth without pain and with appropriate technique including the maintenance of having both heels in contact with the ground and proper lumbar lordosis throughout the whole movement. Experimental protocols were approved by the Institutional Review Board at the Ohio State University, and written informed consent was obtained from each subject after a full presentation of procedures.
Data were collected during 2 separate sessions with >24 hours and <1 week between sessions. All testing was conducted during the months of January to March. Subjects were instructed to maintain their typical daily routine in regards to hydration, nutrition, and sleep and to not engage in any heavy physical activity several days before testing. The 1RM testing was performed during the first session following recommendations set forth by the National Strength and Conditioning Association (3). Subjects performed their normal warm-up typically done before squatting. The above-parallel squat depth was determined by measuring 90° of knee flexion using a goniometer placed at the knee (Figure 1A). The parallel squat depth was determined when the inguinal crease fell just below the proximal patella (16) (Figure 1B). The below-parallel squat depth was determined when the subject sat fully in a squat position where the hamstrings come into contact with the calves (Figure 1C). Subjects were instructed to take a natural stance. Order of the 1RM testing was not randomized and started with the below-parallel squat and ended with the above-parallel squat because generally the 1RM is lower for the below-parallel squat. Subjects were given as much time as they desired between attempts but took no less than 1 minute for each rest interval. Previous research has found that rest intervals as short as 1 minute do not affect 1RM back squat performance when a repeated 1RM attempt is made (24). Test-retest reliability for 1RM back squat testing has resulted in an intraclass correlation coefficient of r = 0.94 (p < 0.05) (24). After 1RM testing, the second session was scheduled between 24 hours and 7 days later.
A specially designed “light curtain” device (Banner Engineering Corp., Minneapolis, MN, USA), and a visible signal light and a programmable logic controller (Allen-Bradley Corp., Milwaukee, WI, USA) were used to provide feedback to the subject to control squat depth and to limit any bouncing or inconsistencies in the amount of time in the bottom squat position for both 1RM and motion capture testing (Figure 2). The light curtain is a device that senses when an object passes through a rectangular “curtain” of infrared light between the 2 parts of the device. The position of the curtain was adjusted to the barbell height for each subject corresponding to each of the squat depths. The visible signal light indicated to the subject when to descend, when to hold in the bottom position, and when to ascend again. This technique ensured that each subject consistently descended to the appropriate depth for each trial and stayed at that depth for 1 second before ascending back to standing.
Motion capture analysis was conducted during the second session. Retroreflective markers (B&L Engineering, Santa Ana, CA, USA) were attached to the legs of each subject with 2-sided latex-free adhesive tape using a modified point cluster technique (1,11) (Figure 3). This technique has shown comparable results to those obtained with intracortical pins placed into the femur and tibia (1) and has been used to study knee kinematics during walking and seated leg extension (11), run-to-cut maneuvers (19), and with anterior cruciate ligament-reconstructed and uninjured contralateral knees (37). The modifications included a marker placed at the manubrium, and a ring of 8 markers placed on the iliac spine including the anterior and posterior superior iliac spines, iliac crests, and the midpoints between iliac crest and posterior superior iliac spine markers. Nine total trials were randomly assigned, each containing a unique combination of squat depth (above parallel, parallel, or below parallel) and load (unloaded, 50% 1RM, and 85% 1RM of the squat depth being performed). Unloaded squat trials consisted of the subject squatting with a polyvinyl chloride pipe held in the same position as a barbell to best mimic the normal squatting style for each subject. Loaded squats used a standard Olympic barbell and weight plates. Subjects were encouraged to use the same squatting style that they typically use in training. All subjects used a high-bar technique with heels remaining in contact with the floor, a natural stance (43.91 ± 6.7 cm between markers on the posterior aspect of the heel; across all trials), a forward or upward gaze, and maintained lumbar lordosis throughout the squat. Marker motion and ground reaction force data were recorded using an 8-camera Vicon MX-F40 system (Oxford Metrics, Oxford, United Kingdom), 2 Bertec 4060-10 force plates (Bertec Corporation, Columbus, OH, USA), and Vicon Nexus software (Oxford Metrics). Three-dimensional marker data were filtered using a generalized cross-validation Woltring filter (40) within Vicon Nexus. Custom scripts in MATLAB (MathWorks, Natick, MA, USA) and Vicon Bodybuilder (Oxford Metrics) were then used to calculate knee kinematics and kinetics, following previously described methods for processing point cluster data (1) and calculating net external joint reaction moments (2), using conventions for tibiofemoral kinematics described by Grood and Suntay (17). Peak external knee flexion moments (newton meters) were normalized by body weight (BW; newtons) and height (ht; meters) to obtain pEKFM (%BW × ht) used for analysis (26). Slopes of the pEKFM versus %1RM curves were also calculated for analysis.
A 1-way repeated measures analysis of variance (ANOVA) was used to compare 1RM at each squat depth. Two separate 2-way repeated measures ANOVAs (depth × load and depth × load progression) were used to compare pEKFM and slopes. The t-tests were used as a post-hoc analysis when a significant F ratio was observed from the ANOVA. A Bonferroni correction was used for multiple comparisons. All data analyses were performed using SPSS version 19.0 (IBM SPSS Statistics, Inc., Chicago, IL, USA). Significance levels were set at p < 0.05. All data are presented as mean ± SEM.
Significant differences were found for squat 1RM at each depth (F 1.50, 21.88 = 61.21, p < 0.001). The largest 1RM was found for the above-parallel squat at 150.42 ± 4.92 kg decreasing to 129.97 ± 4.82 kg and 125.28 ± 4.34 kg (p < 0.05) for the parallel and below-parallel squats, respectively (Figure 4).
Significant main effects were found for load (F 1.35, 20.29 = 191.36, p < 0.001) and depth (F 1.23, 18.38 = 45.69, p < 0.001) in addition to a significant interaction effect for depth × load (F 1.93, 28.87 = 10.18, p = 0.001) for pEKFM (Figure 5). Increases in either load or depth for the back squat exercise significantly increased pEKFM. Within each %1RM or squat depth, each point was significantly different from the other two (p ≤ 0.02; Figure 5).
To check the consistency of knee flexion angle at different depths, peak knee flexion angles were calculated for each of the 9 conditions (Table 1). On average, subjects squatted to a knee flexion angle of 98.73 ± 1.54° for the above-parallel squat, 123.70 ± 2.03° for the parallel squat, and 140.51 ± 2.00° for the below-parallel squat. Significant differences were found in the knee flexion angle for the above-parallel and parallel squats as load increased. During the above-parallel squats, knee flexion angle decreased as the load increased from unloaded to 50% 1RM (p = 0.001) and again to 85% 1RM (p = 0.014). The subjects also squatted with a knee flexion angle of ∼5° less during the parallel squats with an 85% 1RM load compared with the unloaded and 50% 1RM trials (p <0.05). No differences were seen in knee flexion angles for the below-parallel squats.
Analysis of the pEKFM slopes (the 6 lines in Figure 5) demonstrated significant main effects for increases in load (unloaded to 50 vs. 50–85, F 1, 15 = 26.42, p < 0.001) and squat depth (F 2, 30 = 11.09, p < 0.001). A significant load × depth interaction effect was also observed (F 2, 30 = 5.88, p = 0.007; Figure 6). The average slope of pEKFM for all 3 depths from unloaded to 50% 1RM was 70% higher than the average slope from 50% 1RM to 85% 1RM. Specifically, above-parallel (p = 0.007), parallel (p = 0.001), and below-parallel (p < 0.001) squats show significantly steeper slopes of pEKFM while progressing from unloaded to 50% 1RM than from 50% 1RM to 85% 1RM (Figure 6, marked with ‡‡, ‡‡‡). Additionally, the below-parallel squat has a significantly steeper slope of pEKFM while progressing from unloaded to 50% 1RM compared with the parallel and above-parallel squats (p ≤ 0.004; Figure 6, marked with ††).
This study demonstrates that the typical decreases in squat load seen with increasing squat depth were not enough to offset the increases in pEKFM seen with increasing knee flexion. As either load or depth is increased, pEKFM also increased. Therefore, our hypotheses were not supported. Moreover, increases in load on the bar from unloaded to 50% 1RM exhibit higher slopes of increased pEKFM than when increasing load from 50% 1RM to 85% 1RM. Although it is currently unknown what threshold of peak or repetitive forces would be detrimental to the knee joint, the data reported here can be used to help design and implement programs using the back squat exercise.
Our findings are somewhat contrary to previous research involving the squat exercise conducted by Wallace et al. (38) who found no difference in external knee flexion moments (reported as internal knee extension moments) at 70°, 90°, and 110° of knee flexion using a load of 85% 1RM. The authors did not specify whether depth-specific 1RM values were found, but if they only determined a single 1RM for the whole study, that alone would account for the differences between their data and ours. Other potential reasons for these contradictory results may be that the previous study had a small sample size of 5 subjects and used female collegiate athletes.
Comparing our data with previously reported data for squatting is challenging because of the wide range of squatting styles, experience levels of subjects, loads used, and methodological differences. A summary of comparison data on studies that have reported pEKFM can be found in Table 2. The largest values for pEKFM were found by Escamilla et al. (13) in a study of master’s-level powerlifters. They reported values as high as 756 Nm for a 1RM squat with a wide stance. Although the loads were nearly double that of the above-parallel back squat with an 85% 1RM load in our study, the pEKFM were over 4 times as high. Some of the lowest values were reported by Wallace et al. (38). Bodyweight squats with the hands behind the head and barbell squats using 35% of the subject’s body weight (24.85 ± 5.6 kg) to 90° of knee flexion gave pEKFM of 0.62 and 0.88 Nm·kg−1, respectively. Comparatively, we observed pEKFM values that were nearly double, although our knee flexion angles were ∼10° larger and our loaded trial was ∼3 times greater (75.21 ± 9.83 kg), accounting for the differences.
The present study showed that the slope of pEKFM was greater when progressing from an unloaded squat to squatting with a 50% 1RM than when progressing from 50% 1RM to 85% 1RM, with the greatest slope seen for the below-parallel squat. This important finding suggests that as beginners or individuals in a rehabilitation program make initial progressions in load and depth, caution should be expressed because increases in pEKFM occur at greater rates during these times. Further research should examine the relationship between pEKFM and load across the entire range of weight on the bar from unloaded to 100% 1RM.
Currently, the data in this study can be a useful guide to those involved in prescribing the squat exercise. Progression can be maximized by incorporating the principle of “progressive overload” or the gradual increase of stress placed upon the body (21), which should be considered when prescribing the squat exercise. To control for pEKFM while still working toward full range of motion movement, these results suggest that a squat progression should start with unloaded above-parallel squats and progress to deeper squats before the addition of load. Squat progression should again proceed from above-parallel squats to deeper squats once a load has been added.
As noted in Table 1, subjects only squatted to a consistent depth for the below-parallel squat. This has important safety implications for the squat exercise. Considering that increased squat depth requires reduced loads, going below an intended depth may lead to a failed repetition and a potential increased risk for injury. When loads were increased to 85% 1RM, subjects squatted ∼5–10° less during the above-parallel and parallel squats. This may be done to decrease the difficulty of the exercise as loads become heavier. With below-parallel squats where the thighs come into contact with the calves, there is a natural stopping point because a lower depth cannot be achieved. This eliminates the possibility of unintentionally going below the intended depth where there is an increased probability of a failed repetition.
Although this study adds important insight into how load and depth affect forces about the knee, understanding the limitations of this study will allow for proper interpretation of the results. The retroreflective marker movement that occurred with the deeper squats may have affected the values reported in this study. As depth increased, the cluster of markers on the thigh became visibly deformed, potentially causing error in the estimated knee flexion angle. Because of the fact that inverse dynamics calculations proceed upward from the foot, error as a result of thigh deformation should have had minimal effect on the estimation of pEKFM, but deformations of the foot or shank are a potential source of error in pEKFM. Additionally, cocontraction from the hamstrings and gastrocnemius with the quadriceps was not considered. Cocontraction would cause pPFJRF to increase without a change in the pEKFM, lessening the value of pEKFM as a surrogate. Optimal placement of markers on the thigh and pelvis and including the effects of cocontraction should be considered for future studies.
All data were presented in response to 1 repetition for each given load and depth. Subjects were not in a fatigued state, and therefore, application of the results should be made accordingly. Peak external knee flexion moments will likely change as subjects become fatigued and their squat technique suffers.
Thigh-calf contact generally occurs during deeper squat depths of ∼130° of knee flexion. Thigh-calf contact produces a moment in the same direction as that of the quadriceps muscle group, thereby decreasing the actual net flexion moment at the knee, causing an overestimation of pEKFM in this study. Recent research has shown that tibiofemoral compressive force, shear force, and patellar tendon force decreased while squatting to full depth where thigh-calf contact occurred (42). Future studies should consider thigh-calf contact using pressure sensors or the like when examining knee forces during high-flexion activities.
Generalizing the present results beyond young healthy recreationally trained men should be done with care. A number of technique differences between elite powerlifters classified as low-skill or high-skill have been previously found (25). It remains unknown whether larger differences exist between elite and recreational lifters or whether differences between men and women exist in squatting mechanics. Moreover, the recreationally trained men in this study varied in their performance of the squat. Four of the subjects were not able to increase 1RM load as squat depth decreased from a below-parallel to a parallel squat. Advanced lifters may show greater differences between 1RM loads at varying depths, which could result in altered pEKFM. Subjects differing in age, gender, height, weight, experience level, and health status may exhibit altered biomechanics in comparison with the presented results.
The main finding of this study is that the 1RM decreases seen with increasing depth are not enough to offset the increases in pEKFM at loads consisting of unloaded, 50% 1RM, and 85% 1RM. While progressing from unloaded to 50% 1RM, slopes of pEKFM are 70% greater than when progressing from 50% 1RM to 85% 1RM loads. To our knowledge, this is the first study investigating pEKFM using a range of typical loads and depths found in rehabilitation and performance training settings. Health and fitness professionals are advised to use the information presented here in addition to individual assessments to determine how to best incorporate the squat exercise into rehabilitation and training programs.
The back squat is a commonly prescribed exercise because of its similarity to activities of daily living and many sporting activities and also being a multijoint movement incorporating large muscle groups. The hypotheses were that back squat 1RM would decrease with increased depth and that this decreased 1RM would allow pEKFM to remain constant for load- and depth-matched squats. However, reducing the load based on the decrease in 1RM was not enough to allow pEKFM to remain constant, that is, a 50% 1RM squat to parallel still resulted in greater pEKFM than a 50% 1RM squat to above parallel when using the 1RM for each depth. Based on these findings, the professional designing a progression for a client should take into account the large increase in knee loading from an unloaded squat to a load consisting of a 50% 1RM because forces increase at the greatest rate during these loads. The largest forces about the knee were found with the 85% 1RM load at the below-parallel depth. Proper progressions should be designed to allow the client to adapt to the increasing forces at the knee. Close supervision and correct spotting techniques should be incorporated, especially at depths in which the thighs do not come into contact with the calves, because going below the intended depth may result in failed attempts. Given the age, gender, height, weight, experience level, and health status of the subjects involved in this study, extrapolating the results to those with differing characteristics should be done cautiously. The information here should be used as a reference point for program design involving the squat exercise after a comprehensive individualized assessment has been completed.
The authors thank all the subjects for graciously volunteering their time and effort. Additionally, the authors are grateful for the volunteer students and their tremendous help in completing this study.
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Keywords:Copyright © 2013 by the National Strength & Conditioning Association.
biomechanics; deep flexion; back; moment; tibiofemoral