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Original Research

Quantifying the Movement and the Influence of Load in the Back Squat Exercise

McKean, Mark R; Dunn, Peter K; Burkett, Brendan J

Author Information
Journal of Strength and Conditioning Research: June 2010 - Volume 24 - Issue 6 - p 1671-1679
doi: 10.1519/JSC.0b013e3181d8eb4e
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Abstract

Introduction

The squat exercise is commonly used by strength coaches, health professionals, physical therapists, and fitness trainers in exercise programs. The squat has been generally categorized into 3 groups: partial squats, parallel squats, and full or deep squats (8,31,37). Partial squats represented as a squat less than parallel, parallel squats indicated by thighs being parallel to the floor, and full squats indicated by a deeper squat than parallel thighs, all of which have a broad range of applications across many environments. The most common squat cited in the literature to date is the parallel squat (19,25,34,37), but because of the variety of squat depths cited in the literature, any comparison of joint movements and timing from these studies is difficult. Previous research on squat movements has predominantly focused on the movements of the knees in isolation to the hip for a rehabilitation focus, such as the forces at the knees (4,27,35), knee stability (7,13), and muscle recruitment of the thigh (6,22,33). No previous studies have provided a complete picture of the hip-knee movements and timing in squatting because they have not been examined as part of a coordinated global movement. Knee angles have been shown to reach as much as 40° flexion (9) and hip angles up to 34°(37), yet both knee and hip angles have been reported as maximums only with no mention as to the timing of when these maximum angles occur during the descent or ascent phases of the squat.

Young athletes, people new to strength training, and those recovering from injury are taught to squat as part of the exercise regimen with the aim of increasing the loads used for sport-specific gains (32,36) or developing better muscular control to support the knee after injury (22,26). Coaches, therapists, and trainers who work with these groups of people need to have an understanding of the coordinated pattern of squatting to develop a new pattern of movement or retrain the desired pattern of movement, which may change as a result of injury (21). The coordination of joints and the timing of these movements in squatting need to be quantified to provide a clearer understanding of the squat movement pattern when coaching this exercise.

Multiple loading parameters have also been used in previous squat research ranging from body weight (BW) squats to 1 repetition maximum (1RM). In 2005, Scaglioni-Solano et al. (31) studied movement patterns of squats using BW only, finding that deeper squatting shifted the effort from the knee joint to the hip joint and after 65°knee bend, the hip and knee movements became more similar in behavior. Abelbeck (2) used BW squats to study the biomechanics of linear motion squats reporting segment mass movements and providing feedback on hip and knee positions. Finally, Dionisio et al. (11) applied BW as the squat load to investigate the kinematic, kinetic, and electromyographic pattern in downward squatting. Submaximal loads have been used in research to minimize fatigue (6) and use a repetition range similar to those common in training (15), whereas 1RM loads up to 2.5 times a subject's BW (25) have been used to study behavior at a subject's strength limits. To determine if load influences squat technique, loads that allow a number of repetitions to be performed without fatigue and provide for safe progressions need to be used, so the repetitions captured can be analyzed for consistency and coordination.

Strength training exercises can be broken into the phases of descent and ascent. To the author's knowledge, the differences, if any, in the movements of the hip and knee in each phase is not mentioned in the literature, despite the fact that the squat requires both up and down phases, which subsequently requires different concentric and eccentric muscle innervations. Specific knee angles or the parallel position of the thighs have been used as the finish point for the descent phase of the squat, yet this restriction may limit the user's natural movement pattern of the squat. A more appropriate measure of determining squat depth would be the vertical displacement of the pelvis to set the start and completion of each phase. In exercise prescription, changing training tempo for each phase is commonly used to create a training emphasis (1,20), yet there has been little research to determine if there are changes in joint coordination in the different phases, which may ultimately alter the muscles involved and the influence of adjusting eccentric and concentric speeds. Because of the different contraction types during the decent and ascent phases, each phase should be quantified separately and the vertical displacement was used to set top and bottom positions of the squat.

Forward horizontal movement of the knees over the toes during the squat has been associated with increased forces at the knee joint and suggests that the knees should be restricted to a position behind the vertical line of the toes to protect the knees (30). The study by Fry et al. (19) showed that the technique of restricted forward knee movement adjusted the squat movement and produced more anterior lean and an increased hip angle when compared with the unrestricted squat technique. Forward knee movement in the back squat needs further examination to understand its impact on joint coordination and the relative timing in squatting movements.

There is also a contradiction in previous studies regarding differences in the patterns of movement between men and women. Some studies show that there had been a significant difference in these patterns (29,39), and others have shown no significant differences between genders (3,10). To address these issues, the specific aims of the current study were to quantify the timing and movement relationships of the hip and knee joints for the back squat exercise and changes to these patterns because of (a) changes in load, (b) the ascent and descent phases, and (c) gender.

Methods

Experimental Approach to the Problem

A 3-dimensional analysis of the lower limbs and torso was conducted as subjects completed 2 sets of 8 repetitions of the back squat exercise. The independent variables were load, phase, and gender. The dependent variables were hip angle, knee angle, knee forward position, and timing. The 3-dimensional analysis machine allows the timing and movement of the hip and knee joints to be measured simultaneously during both the descent and ascent phases of movement. Subjects performed 8 squats with 2 different loads (BW with no external load, and BW + 50% of BW as an external load); the order for each subject was randomized. For each load, each subject did 8 squats (with 90- to 120-second recovery between each set); data were only collected for 3 of these squats, but the 3 selected for data recording were not made known to the subjects.

In random order, the load for each set was either BW (BW, no external load) or BW plus 50% BW load (BW+50%).

Subjects

Twenty-eight subjects with at least 1-year experience in using the back squat as an exercise in their training routine volunteered for the study (Table 1). Subjects were either in their final year of study to become a personal trainer or already working as an accredited personal trainer. Subjects had been performing squats as an exercise in their strength and conditioning training programs at least twice a week for a minimum of 12 months. Subjects were informed of the experimental risks, and written informed consent was obtained under the guidelines approved by the university human research and ethics committee before any experimental testing. All subjects indicated that they had no existing conditions or history of musculoskeletal injury.

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Table 1:
Subject characteristics (mean and SD).

Procedures

Total body mass was measured to the nearest 0.01 kg with calibrated electronic scales (TI BWB-600P, Digital Personal Scale; Wedderburn Pty Ltd, Willawong, Australia), and standing height of individual subjects was measured to the nearest 1 mm with a portable stadiometer (Telescopic Metal Height Scale [PE063]; Mentone International, Moorabin, Australia) The laboratory is a nationally accredited facility for athletic testing. Real-time kinematic motion of the lower limbs and torso was collected at 120 Hz by a 3-dimensional magnetic tracking device (Motion Monitor, Version 6.50.0.1; Innovative Sports Training, Chicago, IL, USA). Eight magnetic sensors were placed on anatomical segments as described in Table 2. Validation of the system was confirmed against standardized reference measures, and the variation was less than 0.5° and within 0.0034 m. Three-dimensional magnetic tracking has been shown to be more accurate than 2 dimensions and has been previously validated (28).

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Table 2:
Position of sensors for 3-dimensional magnetic tracking device.

The pelvis was digitized using the Bell method, which uses the anterior superior iliac spine (ASIS) width to determine hip joint center and pelvic girdle structure. The knee and ankle joint centers were determined using the centroid method. The hip joint angle is the angle between the alignment of the pelvis and line between the hip and knee joint centers. The knee joint angle being the angle between the lines connecting the joint centers of the hip to knee and knee to ankle as defined in previous studies (2,6,19,31). The most anterior aspect of the patella was digitized with a 3-dimensional landmark referenced to the knee joint center, allowing it to be tracked with regards to forward knee movement in the sagittal plane (19).

The subjects warmed up as per their usual routine for strength training, and then with the sensors attached, the subjects performed a warm-up set of squats. The width of stance was controlled with the inside distance between the subject's heels, the same as the pelvic width measured from right ASIS to left ASIS (15), using skeletal goniometers (TTM Bone Calliper, PE054; Mentone International). The feet were aligned parallel to the sagittal plane, that is, toes pointed straight ahead, and the knees were free to move in any position.

To determine if load influenced movement patterns in a safe and repeatable manner, the loads of BW and a submaximal load of BW + 50% (BW+50%) were chosen. These 2 loads allowed multiple sets of 8 repetitions without fatigue (12), a range of repetitions and loading parameters similar to the training environment they would be exposed to, and the capture of 3 blind repetitions for analysis.

To develop 50% load, an aluminum Olympic bar and associated weights was placed into a standard position across the upper trapezius and superior aspect of the spine of scapula, the hands placed outside shoulder width in a palm forward grip. A standard-sized Olympic bar constructed from aluminum (Australian Barbell Company, Mordialloc, Victoria, Australia) was used because normal ferrous metals in traditional bars cause interference with the magnetic fields of the magnetic tracking device.

The subject assumed the setup position, and the width of feet was established using a steel measuring ruler and marks placed on the floor. Starting in an upright position, the subject descended to the lowest point they felt in control and comfortable, with no limit placed on the depth of the squat. The squat was performed according to the National Strength and Conditioning Association (NSCA) position guidelines on squats and monitored by the main researcher who is an NSCA Certified Strength Conditioning Specialist coach at the elite level. A digital of a typical man squatting with a BW+50% load is shown in Figure 1.

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Figure 1:
A digital of a man squatting with a warm-up load.

One squat was complete when the subject returned to the original upright starting position. If there was a safety concern, or the subject wished to cease the action, 2 spotters were positioned to support the subject. The subject exhaled on the ascent and inhaled on the descent, and each subject was given 90- to 120-second recovery between each set. The middle 3 repetitions of each set were collected for analysis with the subject being blind to which repetitions were collected for the project. All angles are orthopedic angles of the lines connecting the digitized joint centers of the hip, knee, and ankle. The hip angle was defined as the anterior angle between the trunk and the thigh; the knee angle was defined as the posterior angle between the thigh and the lower shank (Figure 2). Forward knee movement (measured in the sagittal plane) was defined as the horizontal distance the anterior aspect of the knee moved with respect to the front of the shoes. If the knees remained behind the toes, this was reported as a negative score. If the knee moved anterior to that of the vertical line drawn from the front of the shoe, the score was positive.

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Figure 2:
Angle conventions used for analysis.

The time for each phase (decent and ascent) was normalized, with the starting point at the top of the squat (highest vertical displacement of the sacrum) represented as 0%, whereas the deepest part of the squat (lowest vertical displacement of the sacrum) represented as 100%, regardless of the phase of the movement.

Statistical Analyses

A total of 6 different responses were analyzed (maximum hip angle, the time at which the maximum hip angle occurred, maximum knee angle, the time at which the maximum knee angle occurred, the maximum knee forward position relative to the toes, and the time at which the maximum knee forward position relative to the toes occurred). Each of these 6 responses was analyzed separately for differences among gender (men or women), phase (ascent or decent), and load (BW or BW+50%). The results (Tables 3-5) are presented as mean and SE analyzed for gender (men or women), load (BW or BW+50%), and phase (ascent or decent). Intraclass correlation coefficients (ICCs) were calculated to assess the reliability of the repeated measures using the Bland and Altman method (5). Intraclass correlation coefficient values less than 0.4 represented poor reliability, 0.4-0.7 fair reliability, 0.70-0.90 good reliability, and greater than 0.9 excellent reliability (16). Sixty-six percent of ICC values were in the excellent category (above 0.9) and 25% in the good category (above 0.7), giving a total of 91% of ICC values above 0.7, showing good reliability of the data. Statistical interpretation focused on the main effects, and the threshold for statistical significance was set to p ≤ 0.05. Linear mixed models were used to model the data using subjects as random effects. Models were fitted individually with one response and one explanatory variable (gender, phase, or load) as a fixed effect. Differences were then detected using the p values from the analysis of variance F tests on the corresponding fixed effect.

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Table 3:
The maximum angle for the hip joint and normalized time for when this occurred (mean and SE) during the descent and ascent phases of the back squat, for body weight load and body weight + 50% body weight.
T4-33
Table 4:
The maximum angle for the knee joint and normalized time for when this occurred (mean and standard error) during the descent and ascent phases of the back squat, for body weight load and body weight + 50% body weight.
T5-33
Table 5:
The maximum knee forward movement distance and normalized time for when this occurred (mean and standard error) during the descent and ascent phases of the back squat, for body weight load and body weight +50% body weight.

The ICC values for the dependent variables are included in Table 6.

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Table 6:
ICC values for dependent variables.*

Results

The maximum hip joint angle and normalized time for when the maximum occurred during the descent and ascent phases for both men and women are presented in Table 3.

Load was significantly associated with the squat technique in men with the BW+50% squat reducing the hip and the knee angles, when compared with the BW squat. There was no significant change in the time for when the maximum angles was reached because of different loads. There was no statistically significant difference in the maximum angles achieved in the BW squat at the hip joint for men and women, with differences of 0.1° between phases and 0.3° between genders. However, with an increased load of BW+50%, men achieved a maximum hip angle 3.9° deeper, and the ascent hip angle was 3.8° deeper than women.

The maximum knee joint angle and normalized time for when the maximum occurred during the descent and ascent phases for both men and women are presented in Table 4.

Gender had a significant influence on knee angle achieved with men reaching greater knee flexion angles for both phases, when compared with women. However, the timing of when maximum knee angles were achieved was not significantly associated with gender. Knee movements also produced significant results in regards to changes brought about by the increased load of BW+50%. The descent knee maximum angle was 5.5° deeper, and the ascent knee angle was 4.2° deeper for BW+50% squats. Further, for the descent and ascent phases, and for BW and BW+50% load, men achieved significantly deeper angles of knee flexion compared with women. Finally, when comparing the difference between the maximum angles at the hip and the maximum angles at the knee, women differed by a maximum of 4.6°, whereas men differed by a minimum of 12.9°.

The maximum knee forward movement and normalized time for when the maximum occurred during the descent and ascent phases for both men and women are presented in Table 5.

Both men and women showed a forward horizontal movement of the knee. This distance was significantly different for gender in both phases. There was also a significant difference in the time of when the maximum forward horizontal movement of the knee was achieved because of gender, with men achieving the maximum distance at less depth in the squat in both phases. Men significantly increased the forward movement of the knees during the BW+50% squats compared with BW, whereas women significantly reduced the forward knee movement during the BW+50% squats compared with BW.

When subjects squatted with no restriction on knee angles, forward knee movement, or depth of the squat, all subjects reached their maximum hip and knee angles within 2% of the deepest part of the squat, in both descent and ascent phases, regardless of load and gender. A screen shot of a typical hip and knee movement comparison is shown in Figure 3.

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Figure 3:
Sample male hip and knee movement in a body weight plus 50% body weight load squat.

Discussion

The specific aims of this study was to first quantify the timing and movement relationships of the hip and knee during the back squat exercise, then to consider any changes to these movements with respect to changes in load, the ascent and descent phases, and gender.

Squatting depth has long been a contentious issue in the practical training environments, and the safe depth of squatting, commonly reported as being a parallel squat, is usually indicated by the thigh being parallel to the floor (14). Deep squats are cited less often as a recommended technique. The freely selected squat depth within the current study would categorize the squats performed in this study as deep, and this greater range of movement provides the participant with the greatest freedom to move naturally.

The significant differences of both the hip and the knee angles achieved by men with respect to changes in load suggest that men differ in the manner in which their lower extremity absorbs and transfers load through their muscles and joints when performing squats. These results show that as the loads increase, men's hip and knee angles increase to accommodate the loads, thus altering the relative angles of these 2 key joints yet maintaining the timing of when these maximum angles occur. The low standard of error in the timing of when the maximal angles were reached reinforces the consistency of the timing and the patterns of movement achieved across both loads and gender and suggests that this is a dominant overriding component of the squatting pattern of movement, which is somehow maintained at all times.

Women on the other hand produced almost identical hip and knee angles for BW squats and BW+50% squats, and the descent and ascent phases of the squat, suggesting a more coordinated squatting movement. No similar studies were found in the literature that compared these variables. Stature may be considered as an influence on this changed behavior. Men are taller, and this may account for the altered hip-knee movements when compared with women. Research has shown that a subject's height, tibial length, and femur length account for 77.8% of the explained variances in a subject being able to keep their heels flat when squatting (18). To keep the heels flat, a subject would need to alter the hip-knee movements to maintain balance. This alteration in movements may be similar to the manner in which men in the current study altered their hip-knee actions.

The similarities in angles achieved by women at the hip and knee joints also suggest that women tend to be more synchronized for the hip-knee interjoint-coordinated movement of squatting. This is demonstrated as the hip and knee joints differ by 4.6° or less in both loads and phases in women, compared with men whose hip and knee joint angles differed by as much as 14.5°. This finding is also supported by Lindbeck and Kjellberg (24) who suggest that women have better hip-knee joint coordination for lifting tasks, when compared with men. Although women maintained similar angles at the hip and knees, the pattern of squatting for men shows that they increase the angles at the hip and knee joints to perform the back squat when loads are increased to BW+50%. These increased angles at the hip and knee suggest that the heavier loads allow men to squat deeper, which may reflect the increased strength levels of men or the fact that the 50% load was a lower percentage of the men's 1RM.

When squatting with BW, only there was no difference in the timing of the hip and knee joints between men and women. There was, however, a significant difference in the knee flexion angle between genders, which highlights a difference in movement patterns across genders and supports other studies (17,23,24,38). As the load increased, only men changed their degree of flexion at each joint, which further reinforces this finding.

Knee position, relative to the toes, is also a contentious topic when performing squats. Very little scientific data have been recorded regarding the distance the knees move forward, with respect to the feet during squats. The current study found that subject's knees did naturally move forward of the front of the foot by a considerable margin. Only 1 reference was found measuring forward knee movement in squatting (11), and forward knee movement was measured from the start of the squat and not with respect to the position of the front of the foot. Any comparison with the current study is difficult because the previous study allowed subjects to only half squat and restrictions were placed on the position of the trunk relative to a wall in front of the subjects.

Any restriction placed on the knee movement in the squat exercise will result in mechanical changes at the hip and knee (19). The results from the current study could indicate a truer relationship of the hip-knee simply because the subjects were not restricted in knee movement or squat depth and performed the squat in a way that was most comfortable for them as individuals, resulting in a forward knee movement appropriate to them as individuals. The timing of the maximum knee forward position was before that of the maximal angles of the hip and knee. This mechanism has not been found in the literature and indicates that the knees may in fact become more static once this maximum forward position was reached. In all cases except men with BW+50%, the maximum forward knee position was greater in the descent than for the ascent. Under BW+50% load, men achieved a greater maximum forward position in the descent than in the ascent phase, which may reflect the increased angles of flexion achieved at the hip and knee for the increased load.

The knees of both men and women moved forward horizontally past the vertical line of the front of the foot; however, women produced a greater forward movement than men when squatting. It should also be noted that for all subjects, the distance the knees moved forward during the squat produced the greatest variation of all the data reported. This shows that the forward knee movement in squatting produces a range of very individual results and suggests that this is a secondary function in squatting that occurs because of some other movement. Added to these differences, the current results also found that the time for when these maximum forward positions were reached differed significantly. In the BW+50% squats, men reached their maximum forward knee position 6.6% sooner in the time of the descent and 13.5% later in time of the ascent compared with women. The current research has shown differences in lower limb kinematics across gender, with men reaching their maximum forward knee position much sooner than their hip and knee angles reach their maximum angle of flexion. Conversely, women on the other hand reach their maximum forward knee position much closer to the time at which their hip and knee angles also reach their maximum, confirming that women tend to be more synchronized in the squatting movement.

Practical Applications

This research showed that there were 2 key aspects to squatting: (a) the timing of the hip and knee maximum angles and (b) the forward knee movement past the toes.

This research found that performing the squat with BW and submaximal loads establishes a very definite timing of joint maximum angles and coordination of the lower extremities, that is, hip joint, knee joint, and forward knee movement. The timing of when the hip and knee reach their maximum angles with BW and +50% BW loads shows that experienced squat performers with submaximal loads coordinate the hip and knee joints to reach maximum angles of flexion within 2% of the maximum descent position, which may be a controlling factor in performing the squat. This study may provide additional cues and timing knowledge for improving teaching and monitoring of squat techniques as by establishing key guidelines on movement coordination of the hip and knee that could be used as a standard for squatting.

Care should be taken when prescribing exercises to men and women because significant differences in the manner in which men and women perform the squat and how they adjust these squat movements with an additional load of BW+50% were found. As the load increased, men squat deeper achieving increased hip and knee angles. This result suggests that under the heavier load of BW+50%, men still controlled joint coordination timing but adjusted the rate at which the movement occurred at each joint to do so. Women on the other hand produced almost identical hip-knee movements for BW squats and BW+50% squats, suggesting a more controlled and coordinated squatting movements. This joint coordination timing appears to be maintained regardless of load and gender and suggests that it is a key aspect to squat movement pattern.

In the practical setting, coaches and trainers should be aware of the differences between men and women in adjusting hip and knee movements to handle additional loads. Allowing for the adjustments made by men, the timing element of these maximal angles still appears to be maintained, and this could be used to visually assess the coordination of the squat movement. If the timing of when maximum angles are reached appears to be significantly different, this research suggests that they may have an altered movement pattern and may need to have further coaching on squat technique to ensure that maximum angles are reached almost simultaneously near the bottom of the squat.

The current study also showed that subjects with at least 1-year squatting experience tended to squat deep when not limited by instruction and allowed their knees to move forward over the front of the feet during the performance of the back squat. This research shows that to perform a deep squatting action, subjects will not restrict the forward movement of the knees over the toes, instead allowing them to move forward as required as if the forward knee movement is a secondary element of squatting, which relies on other movements to control its action. In developing athletes, those with little strength training experience, and people performing squats as part of a rehabilitation process, this study suggests that coaches and trainers should allow the knees to move to a position in squatting where the hip and knee coordination is maintained rather than restricting the knee any specific alignment and causing a change to the hip-knee coordination.

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Keywords:

technique; hip; knee; knee movement; pattern

Copyright © 2010 by the National Strength & Conditioning Association.