The squat, which measures lower body and trunk strength, is the first of three lifts in powerlifting competition. The starting and ending position for the powerlifting squat is when the lifter is in the upright position with the knees and hips near full extension. After the “squat” command is given by the head judge, the lifter descends until an imaginary line from the top of the knees to the hips moves below a parallel position relative to the ground, and in a continuous motion the lifter ascends back to the upright position. According to the American Drug Free Powerlifting Association (ADFPA) rules at the time of this study, causes for disqualification include failure to wait for the referee’s “squat” command at the beginning of the lift, not descending to the below parallel position, positioning the bar on the back greater than 5 cm below the acromion, any stopping or downward movement of the bar once the ascent begins, any shifting of the feet throughout the lift, and failure to wait for the “rack” command by the head judge at the completion of the lift. All squat trials analyzed in the current study were in accordance with these rules.
Strength athletes, such as American football players and powerlifters, often employ the barbell squat in their training or rehabilitation regimens. These athletes use the squat to enhance hip, thigh, and back strength. Although varying squat stance widths and foot angles are employed in training according to an athlete’s goals and preferences, the efficacy of one stance over another is unclear. Only a few studies have quantified stance widths or foot angles during the squat (8,10,20,25,29). Escamilla et al. (8) reported a preferred mean stance (inside heel to inside heel distance) of 40 ± 8 cm and a preferred mean forefoot abduction of 22 ± 11° from 10 male powerlifters and bodybuilders performing the squat. In a follow-up study, Escamilla et al. (10) examined the effects of defined narrow and wide stance widths on knee biomechanics. McCaw and Melrose (20) examined the effects of defined narrow and wide stance widths on lower extremity muscle activity. Both Signorile et al. (29) and Ninos et al. (25) examined the effects of turning the feet in or out on lower extremity muscle activity. However, none of these aforementioned studies have examined how varying stance widths affects joint and segment angles and joint moments and moment arms.
Because the squat is considered a closed kinetic chain exercise (31,37), it can also be employed in knee rehabilitation programs, such as after anterior cruciate ligament (ACL) reconstruction. Studies have shown that the squat is an effective exercise during ACL rehabilitation (31,37). The moderate to high hamstring activity that has been reported during the squat (8,31) may help protect the ACL during knee rehabilitation. However, the role of varying stance widths during the squat is unclear in knee rehabilitation. It is hypothesized that knee and hip moments will increase as stance width increases.
Although there are several studies that have quantified joint moments during the barbell squat (2,8,12,16,17,22,26,28,34,35), none of these studies examined the effects of stance width on joint moments. Similarly, there are only a few studies that have quantified select joint and segment angles during the barbell squat (16,17,21,28), and none of these studies examined the effects of stance width on joint and segment angles. In addition, one limitation to most barbell squat studies that quantified joint and segments angles and joint moments is that a two-dimensional (2-D) analysis was employed using a single camera to record a sagittal view of the lifter. Although trunk movements through spinal and hip flexion and extension occur primarily in the sagittal plane, flexion and extension movements at the ankle and knee occur in the sagittal plane only if the feet are positioned in that plane (i.e., pointing straight ahead). This is because the ankles and knees primarily function as hinge joints during the squat, and thus move in the direction the feet point. Therefore, the lower extremities will move out of a sagittal plane as the feet turn outwards and the stance widens. These will cause erroneous measurements of lower extremity joint and segment angles and ankle and knee moments and moment arms (9). These errors are minimal when the feet are pointing straight ahead, but considerable errors occur as the feet turn out to a greater extent and the stance widens (9). Therefore, the purpose of this study was to compare joint and segment angles and ankle, knee, and hip moments and moment arms between 2-D and 3-D analyses while performing the squat with varying stance widths. It was hypothesized that the number of significant differences in joint kinematic and kinetics between 2-D and 3-D analyses will increase as stance width widens and that 3-D joint kinematics and kinetics will be significantly different among varying stance widths.
MATERIALS AND METHODS
Thirty-nine male powerlifters served as subjects, with a mean mass of 91.0 ± 25.2 kg, a mean height of 174.9 ± 7.0 cm, a mean age of 45.7 ± 5.2 yr, and a mean load lifted of 225.4 ± 58.0 kg. All subjects wore a one piece lifting suit. All subjects participated in a national powerlifting masters’ championship that was sanctioned by the ADFPA. To participate in masters’ level powerlifting competition, all lifters had to be at least 40 yr old. All subjects signed a human consent form giving their approval to be videotaped and participate in this study.
Two synchronized Sony HVM 200 video cameras were used to collect 60-Hz video data. One camera faced the subject’s left side while the other camera faced the subject’s right side, with each camera’s optical axis forming a 45° angle to the sagittal plane of the lifter. The cameras were positioned approximately 14 m apart and faced perpendicular to each other, with each camera approximately10 m from the subject. To minimize the effects of digitizing error, the cameras were positioned so that the lifter-barbell system was as large as possible within the viewing area of the cameras.
Just before a subject initiated their lift, an external light source was activated in both camera views to help match video frames when viewing the videotapes. Before and just after the subjects were videotaped, a 2 × 1.5 × 1 m 3-D calibration frame (Peak Performance Technologies, Inc., Englewood, CO), surveyed with a measurement tolerance of 0.5 cm, was positioned and videotaped in the same volume occupied by the lifter-barbell system. The calibration frame was comprised of 24 spherical balls of known spatial coordinates, with the x- and z-axes positioned parallel to the ground, and the y-axis pointing vertical.
In powerlifting competition, lifters are given three attempts during the squat to maximize the amount of weight they can lift. A lifter’s first attempt is usually submaximal, whereas their second and third attempts are near the maximal weight they are capable of lifting. Therefore, only second and third attempts that were successfully completed (i.e., ruled a “good lift” by a panel of three judges) were analyzed. Twenty-two of the 39 lifts analyzed were third attempts. The 17 second-attempt lifts were used because the third attempts were unsuccessful due to the lifter attempting a weight that was beyond their one repetition maximum (1 RM). Therefore, it was thought that all lifts analyzed were very near each lifter’s 1 RM.
Previously, the squat has been divided into three descent and three ascent phases (16,17,21) : 1) starting position to 45° knee flexion angle (KF); 2) 45°KF to 90°KF; 3) 90°KF to maximum KF; 4) maximum KF to 90°KF; 5) 90°KF to 45°KF; and 6) 45°KF to lift completion. The end of each of the first five phases was chosen for kinematic and kinetic analyses: 1) 45°KF during descent; 2) 90°KF during descent; 3) maximum KF; 4) 90°KF during ascent; and 5) 45°KF during ascent. Kinematic and kinetic analyses were also performed at minimum bar velocity, which always occurred between 90° and 45°KF during the ascent. Because the starting and ending positions of the squat are similar with the knees and hips near full extension, kinematic and kinetic analyses were not performed at these positions.
A 3-D video analysis system (Peak Performance Technologies, Inc.) was used to manually digitize data for all 39 subjects. A 15-point spatial model was created, comprised of the top of the head and centers of the left and right mid-toes, ankles, knees, hips, shoulders, hands, and end of bar. All points were seen in each camera view. Each of these 15 points was digitized in every video field (60 Hz), which was adequate due to the slow movement of the lift (3,17). Digitizing began five video fields (0.17 s) before the start of the descent and ended five video fields after the end of the ascent.
A fourth-order, zero-lag Butterworth digital filter was used to smooth the raw data with a cutoff frequency of 5 Hz. A cutoff frequency between 3 and 5 Hz has been demonstrated to be adequate during lifting 1-RM loads involving slow movements (9). By using the direct linear transformation method (33), 3-D coordinate data were derived from the 2-D digitized images from each camera view. An average resultant mean square calibration error of 0.3 cm produced an average volume error of 0.121%.
The origin of the 3-D orthogonal axis system was first translated to the right ankle joint and rotated so that the positive x-axis pointed to the left ankle joint, the positive z-axis pointed anteriorly in the direction the lifter was facing, and the y-axis pointed in the vertical direction (9). The vertical positions of the digitized left and right ankles were within 1 cm of each other. This axis system was initially used to calculate all joint moments, moment arms, and angles. Muscle moment arms were not quantified in this study. Because hip flexion and extension during the squat occur primarily in the y-z sagittal plane about the x-axis, hip moments were calculated about the x-axis and hip moment arms were calculated in the z-axis direction. Ankle and knee moment arms were also calculated in the z-axis direction, which equates to a 2-D analysis using one camera to record a sagittal view of the lifter. These 2-D data were compared with 3-D data from the 3-D analysis. To calculate the actual ankle and knee moment arms from a 3-D analysis, the axes system was translated to each ankle joint center and rotated so that the positive z-axis pointed from the ankle joint center to the mid-toes, the y-axis pointed vertical, and the x-axis was orthogonal to the y- and z-axes (9). Hence, for both sides of the body, ankle and knee moments were calculated about the x-axis, and ankle and knee moment arms were calculated in the z-axis direction. In addition, movement of knees relative to the ankles were measured in the z-axis (i.e., in the direction of the longitudinal axis of the foot) to determine how far forward the knees translated over the feet during the squat, with left and right side measurements averaged.
Linear and angular displacements and velocities were calculated for both the left and right sides of the body, and then averaged (9). Relative knee and hip angles and absolute trunk, thigh, and shank angles were defined in accordance with previous lifting studies (3,9). Trunk, thigh, and shank angles were measured relative to the x-z horizontal plane (i.e., from a right horizontal relative to a sagittal view of the lifter’s right side). Knee angles were measured relative to thigh and leg segments, whereas hip angles were measured relative to trunk and thigh segments. As long as the trunk is rigid and straight, this relative angle approximated the true hip angle. From qualitative analyses of the squat, unlike the deadlift, the trunk typically remains rigid and straight. Knee and hip angle measurements were expressed as 0° at full knee and hip extension by subtracting relative angle measurements from 180°. To compare joint and segment angle differences between 2-D and 3-D analyses, hip, knee, thigh, and shank angles from 3-D analyses were projected onto a 2-D sagittal plane (9). Foot angle was defined as the angle formed between the foot segment and the y-z sagittal plane. Stance width was defined as the linear distance between the left and right ankle joint centers, whereas hand width was defined as the linear distance between the left and right hand centers.
During the 1 RM squat (21), the barbell initially accelerates at the beginning of the ascent to a first peak velocity, then decelerates to a minimum velocity, accelerates again to a second peak velocity, and finally decelerates until the end of the ascent. Therefore, the squat ascent was divided into four lifting phases (9) : a) acceleration phase → maximum KF to first peak bar velocity; b) sticking region → first peak bar velocity to minimum bar velocity; c) maximum strength region → minimum bar velocity to second peak bar velocity; and d) deceleration phase → second peak bar velocity to lift completion.
Because segment and barbell accelerations are very small while lifting maximum or near maximum loads, joint moments can accurately be calculated using quasi-static models (16,17,21,22,26). Lander et al. (17) found that joint moments varied less than 1% between quasi-static and dynamic analyzes during the squat exercise with near maximum loads. Hip, knee, and ankle moments and moment arms were calculated for left and right sides and then averaged (9). Body segment center of masses and weights were calculated by using appropriate anthropometric data (7), and each lifter’s known mass and segment lengths. The geometric center of the barbell represented the center of mass of the barbell (COMbar). Position coordinates for x, y, and z were calculated for both COMbar and the center of mass of the system (COMsystem), which included both barbell and body masses. Joint moments and moment arms were calculated relative to both barbell weight and system weight (9). The system weight used to calculate joint moments was the sum of the barbell weight and the weight of body segments above the joint in which the moments were calculated (9). Ankle moment arms (MAankle) were calculated as the distance in the z-axis direction from the ankle joints to COMbar or COMsystem, with the system comprising barbell and body masses minus the mass of the feet. Ankle moments were calculated as the product of MAankle and barbell weight and the product of MAankle and system weight. Knee moment arms (MAknee) were calculated as the distance in the z-axis direction from the knee joints to COMbar or COMsystem, with the system comprising barbell and body masses minus the masses of the legs and feet. Knee moments were calculated as the product of MAknee and barbell weight, and the product of MAknee and system weight. Hip moment arms (MAhip) were calculated as the distance in the z-axis direction from the hip joints to COMbar or COMsystem, with the system comprising barbell and body masses minus the masses of the thighs, legs and feet. Hip moments were calculated as the product of MAhip and barbell weight, and the product of MAhip and system weight.
Because bar motion primarily occurred in the vertical direction, vertical bar displacement was calculated from maximum KF to lift completion and normalized by body height. Mechanical work, which was calculated relative to both barbell weight and system weight, was the product of system or barbell weight and total vertical displacement of COMbar or COMsystem. The system weight used to calculate mechanical work was the sum of barbell weight and body weight minus the weight of the feet.
To compare stance widths relative to individual size differences, stance width was normalized and expressed as a percent of each subject’s shoulder width (distance between digitized shoulder joint centers). The 39 normalized stances were ranked from lowest to highest (87–196% shoulder width) and divided into defined narrow, medium, and wide stance groups. The 13 lowest normalized stances (87–118% shoulder width) were assigned to the narrow stance (NS) group; the middle 13 normalized stances (121–153% shoulder width) were assigned to the medium stance (MS) group; and the highest 13 normalized stances (158–196% shoulder width) were assigned to the wide stance (WS) group. To assess kinematic and kinetic differences among the three stance groups, a three-way, mixed-factor multiple analysis of variance (MANOVA) was employed (P < 0.05). The repeated factors in the MANOVA consisted of 2-D versus 3-D comparisons and descent versus ascent comparisons. Stance was the between subjects factor in the MANOVA, which consisted of narrow, medium, and wide stances. A simple linear regression was used to assess the relationship (P < 0.05) between normalized stance width and joint and segment angles.
A representative graph of joint and segment angles as a function of time is shown in Figure 1. During the squat, the knees and hips flex and extend together with similar magnitudes and shapes. Thigh angles follow the same general pattern as knee and hip angles. Shank angles have similar shapes and magnitudes as trunk angles. The minimum bar velocity shown in Figure 1 corresponds to the minimum bar velocity in Figure 2 (occurring at approximately 2.15 s), which shows a representative graph of vertical bar velocity during the squat. The acceleration and deceleration phases were the shortest in duration, whereas the sticking and maximum strength regions were the longest in duration. Typical differences between 2-D and 3-D joint angle analyses for the NS and WS are shown in Figure 3. Representative graphs for hip, knee, and ankle angular velocities as functions of knee and hip angles are shown in Figures 4 and 5. Shapes and magnitudes of hip and knee angular velocities as functions of hip and knee angles were similar during the squat motion. In addition, hip, knee, and ankle angular velocities as a function of knee angles were very similar to hip, knee, and ankle angular velocities as a function of hip angles. Ankle angular velocities remained low and fairly constant throughout the descent and ascent phases. Employing different stance widths did not affect the general shapes and magnitudes seen in the representative graphs from Figures 1, 2, 4, and 5.
Significant positive correlations (r = 0.5–0.85) were found between normalized stance width and joint and segment angles, with joint angles increasing as normalized stance width increased. The highest correlations were found between normalized stance width and thigh angles (r = 0.75–0.85). A representative graph between normalized stance width and thigh angles is shown in Figure 6. Low to moderate correlations (r = 0.5–0.7) were observed between normalized stance width and hip and shank angles, whereas nonsignificant correlations (r = 0.15–0.35) were observed between normalized stance width and trunk angle.
There were no significant differences in kinematic and kinetic measurements between left and right sides of the body. Subject characteristics among the NS, MS, and WS are shown in Table 1. Among the three stance groups there were no significant differences in age, body height, body weight, barbell load, and hand width. Joint and segment angles among the three stance groups are shown in Table 2, with significant differences found in hip, thigh, and shank measurements. Most significant differences occurred between NS and WS groups, with no differences observed between MS and WS groups. In the MS and WS groups at 45°, 90°, and maximum KF, the hips flexed approximately 10° more compared with the NS. Compared with the NS, the shanks were approximately 8° more vertical, the thighs were approximately 10° more horizontal, and the feet were turned out approximately 6° more in the WS. There were no significant differences in trunk positions among the stance groups. At 45°KF, there was significantly less hip flexion and forward trunk tilt in the descent phase compared with the ascent phase. There was significantly greater hip flexion (12–14°) and forward trunk tilt (7–10°) at minimum bar velocity during the ascent (Table 2) compared with hip and trunk angles at corresponding KF angles during the descent, in which the hips flexed 67 ± 7°, 68 ± 10°, and 69 ± 12°, respectively, and trunk angles were 65 ± 5°, 64 ± 4°, and 66 ± 5°, respectively, during the NS, MS, and WS. There were no significant differences found among the three stance groups at minimum bar velocity. Comparing joint and segment angles between 2-D and 3-D analyses, the NS showed the fewest number of significant differences, whereas the WS showed the greatest number of significant differences (Table 3). Hip and thigh angles were 3–13° less in 2-D analyses compared with 3-D analyses.
There were no significant differences in temporal, work, and bar velocity comparisons among the three stance groups (Tables 4–6). On the average, it took 4.60 ± 1.20 s to complete the 1 RM squat, with a descent time of 1.92 ± 0.51 s and an ascent time of 2.68 ± 1.04 s. Significant differences among the three stance groups in peak hip, knee, and ankle angular velocities, and corresponding knee and hip angles, are shown in Table 7. The only significant differences found among the three stance comparisons were greater hip angles in the MS and WS compared with the NS at peak hip, knee, and ankle angular velocities. Relative to the ankle joints the knees translated forward over the feet 21.7 ± 4.4 cm during the NS, 18.0 ± 2.6 cm during the MS, and 16.0 ± 4.6 cm during the WS. There was significantly greater forward knee translation over the feet during the NS compared with the MS and WS.
The joint moments and moment arms expressed in Tables 8–10 are relative to barbell or system loads. Positive moment arms are anterior to joint, producing positive hip flexor, knee extensor, and ankle dorsiflexor system moments. Hip extensor, knee flexor, and ankle plantar flexor resultant muscle moments are needed to counteract these system moments. Negative moment arms are posterior to joint, producing negative hip extensor, knee flexor, and ankle plantar flexor system moments. Hip flexor, knee extensor, and ankle dorsiflexor resultant muscle moments are needed to counteract these system moments. Joint moments and moment arms were significantly different among the three stance groups (Table 8), with most differences occurring between the NS and WS. From Table 8, most significant differences in moments and moment arms involved the ankle, whereas significant differences in knee and hip moments and moment arms occurred only at 45°KF. Ankle plantar flexor resultant muscle moments were generated exclusively during the NS, whereas ankle dorsiflexor resultant muscle moments were generated exclusively during the MS and WS. Peak ankle moments and moment arms occurred at maximum KF during the NS and at 45°KF during the MS and WS. Peak knee moments and moment arms occurred at maximum KF, whereas peak hip moments and moment arms occurred at minimum bar velocity. The only significant differences between the squat descent and ascent occurred at 45°KF, in which hip moments and moment arms were significantly greater during the ascent compared with the descent. Ankle, knee, and hip moment arms and ankle and hip moments were not significantly different between COMbar and COMsystem. However, knee moments were significantly greater in system load compared with barbell load for all three stance groups (Table 9). Significant differences in ankle and knee moments and moment arms between 2-D and 3-D analyses are shown in Table 10 for the NS and WS. During the NS, ankle moment arms were 7–8 cm less in a 3-D analysis compared with a 2-D analysis, whereas knee moment arms were 9–10 cm greater in a 3-D analysis compared with a 2-D analysis. This same tread in ankle and knee moment arms occurred during the MS and WS, with a 12–14 cm difference between 2-D and 3-D analyses during the MS and a 16–18 cm difference between 2-D and 3-D analyses during the WS. During the NS, ankle plantar flexor resultant muscle moments were significantly greater in 2-D analyses compared with 3-D analyses. During the WS, ankle resultant muscle moments changed from plantar flexor in 2-D analyses to dorsiflexor in 3-D analyses. In 2-D analyses knee moments were greater with an NS compared with a WS. Conversely, in 3-D analyses knee moments were greater with a WS compared with a NS. All data in Tables 1–10 and Figures 1–6 are from 3-D analyses unless specified otherwise.
Many athletes and coaches believe that technique variations occur in the squat as different stance widths are employed. There are currently no known studies that have quantified joint angles, moments, and moment arms while performing the squat with varying stance widths. Therefore, the objective of this study was to compare squat kinematics and kinetics between 2-D and 3-D analyses among three defined stance groups. The results from the current study demonstrate that kinematic and kinetic differences do occur among the three stance groups and that 2-D kinematic and kinetic analyses produce erroneous results compared with 3-D analyses, especially during the WS.
In the current study, linear and angular displacements and velocities, as well as joint moments and moment arms, were averaged from the left and right sides of the body. There were no significant differences between bilateral measurements, which demonstrate the symmetrical nature of the squat exercise. This implies that during the squat analyzing only one side of the body may be adequate in calculating joint and segment angles, joint moments, and joint moment arms. The symmetrical nature of the deadlift, which is similar to the squat, has previously been demonstrated by Escamilla et al. (9), who found no significant differences in kinematic and kinetic measurements between left and right sides of the body.
Joint and segmental angles.
Although trunk angle was not significantly different among the three stance groups at 45°, 90°, and maximum KF, greater hip flexion, a more horizontal thigh position, and a more vertical shank position were observed in the WS and MS compared with the NS. These changes occurred in part because the NS had approximately 4–6 cm greater forward knee movement in the direction of the toes compared with the MS and WS. Greater forward knee movements during the squat have been shown to increase knee shear forces (2), which implies that employing a MS or WS may be more effective than an NS in minimizing knee shear forces.
Trunk, knee, hip, and thigh angle patterns and magnitudes from Figure 1 are similar to data from several other studies which quantified joint and segment angles during the squat (1,12,16,17,21,26,35). Averaged among all three stance groups, minimal bar velocity occurred at 62 ± 10°KF, which is slightly lower than the 75 ± 6° KF reported by McLaughlin et al. (21). Averaged among all three stance groups, hip, trunk, thigh, and shank angles at minimum bar velocity were 81 ± 9°, 57 ± 6°, 139 ± 6°, and 70 ± 5°, respectively, in the current study, and 110 ± 11°, 39 ± 11°, 150 ± 2°, and 74 ± 5°, respectively, from McLaughlin et al. (21). Although subjects from the current study and subjects from McLaughlin et al. (21) all participated in a national powerlifting competition involving 1-RM lifting, subjects from McLaughlin et al. (21) had 13° more knee flexion, 29° more hip flexion, and 18° more forward trunk tilt at minimum bar velocity. This implies that, compared with the subjects in McLaughlin et al. (21), the subjects in the current study obtained minimum bar velocity later in the ascent and maintained a more upright trunk position. Some of these joint position differences between the current study and McLaughlin et al. (21) are probably due to methodological differences, because McLaughlin et al. employed a 2-D analysis whereas the current study employed a 3-D analysis. However, because trunk positions between 2-D and 3-D analyses should be similar due to the trunk moving in a sagittal plane, the observed joint position differences between these two studies are also probably due to technique differences.
All previous studies that quantified joint and segment angles employed a 2-D sagittal plane analysis (16,17,21,26,35). It was hypothesized that hip, thigh, and shank angles would show a greater number of significant differences between 2-D and 3-D analyses during a WS compared with an NS. During the WS, the feet are typically turned out to a greater degree compared with an NS. The more the feet turn out, the greater the lower extremities deviate from sagittal plane movements, and the greater the differences will be between 2-D and 3-D analyses. The significantly greater foot angles in the WS compared with the NS contributed to several significant differences between 2-D and 3-D analyses in the WS, but fewer significant differences were observed between 2-D and 3-D analyses in the NS. This is supported by Figure 3, which shows typical differences in joint angles between 2-D and 3-D analyses for the NS and WS. During the NS, 2-D and 3-D joint angle analyses were nearly identical, whereas larger differences between 2-D and 3-D analyses were observed during the WS. However, the relatively small 6° difference in foot angle between the NS and WS may have contributed to several WS comparisons between 2-D and 3-D analyses not being significantly different (Table 3). In examining 2-D and 3-D joint and segment angle comparisons during sumo (wide stance) and conventional (narrow stance) style deadlifts, Escamilla et al. (9) generally found no significant differences between 2-D and 3-D analyses during the conventional deadlift, whereas all 2-D versus 3-D comparisons were significantly different in the sumo deadlift. However, mean foot angles were 14 ± 6° for the conventional deadlift and 42 ± 8° for the sumo deadlift. Furthermore, a comparison between deadlift stance widths (9) and squat stance widths from the current study revealed that the NS had a 33% greater stance width than the conventional deadlift, whereas the sumo deadlift had an 11% greater stance width than the WS. These data imply that a 2-D analysis may be adequate to calculate joint and segment angles when the foot angle is relatively small (i.e., 0–15°) and a narrow stance is employed, but significant errors can occur from 2-D analyses as foot angles and stance widths increase.
Although there were no significant differences in vertical bar distance and mechanical work among the three stance groups (Table 4), the overall mechanical work on the system (1444 ± 366 J) was significantly greater than the total mechanical work on the bar (1107 ± 278 J). This difference between bar mechanical work and system mechanical work implies that the total energy expenditure during the squat is underestimated if only the bar mechanical work is calculated, because energy expenditure during the squat increases linearly as mechanical work increases (5). Bar and system mechanical work values in the current study were approximately 10% greater than bar and system mechanical work values reported by Escamilla et al. (9) during the conventional style deadlift, which is performed similar to the squat. Because the subjects in Escamilla et al. (9) lifted a mean load of 222 ± 34 kg, which is nearly identical to the mean load lifted in the current study, the 10% difference in mechanical work between these two studies is from a 10% greater vertical bar distance in the current study. High energy expenditures have been reported during both the deadlift (4,9) and squat (5), which suggests that these types of multi-muscle, multi-joint exercises are more effective in energy expenditure and muscle development compared with single-joint, single-muscle exercises. Several studies have shown moderate to high muscle activity during the squat from the quadriceps, hamstrings, gluteus maximus, thigh adductors, abdominals, obliques, and erector spinae (6,8,14,17,20,25,29–31,34–36). These are the largest and most powerful muscles in the body and generate a high force production and energy expenditure when active.
Selected events and lifting phases.
Mean peak vertical bar velocity during the squat descent (0.531 ± 0.132 m·s-1) was slightly greater than mean peak bar velocity during the squat ascent (0.452 ± 0.117 m·s-1) and occurred at approximately 27% of both the descent time and the descent vertical bar distance. Mean peak vertical bar velocity during the descent was nearly identical to the high skilled squat group from McLaughlin et al. (21) but approximately 15% lower than the less skilled squat group from McLaughlin et al. (21). This implies that higher-skilled lifters lower the bar at a slower rate compared with lesser-skilled lifters. This is important because several studies have reported significantly greater tibiofemoral shear and compressive forces during a fast squat cadence compared with a slow squat cadence (1,6,11). This occurs because faster descent rates require greater deceleration forces from the knee and hip extensors in order to slow down and stop the weight at the end of the descent.
The vertical bar velocity curve shown in Figure 2 is the same pattern reported by McLaughlin et al. (21). Of the 39 lifters in the current study, 28 lifters achieved maximum vertical bar velocity at their second peak vertical bar velocity, whereas the remaining 11 lifters achieved maximum vertical bar velocity at their first peak vertical bar velocity. However, for all stance comparisons, the mean second peak bar velocity was not significantly different from the mean first peak bar velocity (Table 5). These data are similar to the 1 RM squat data reported by McLaughlin et al. (21), in which most lifters also reached their maximum vertical bar velocity at their second peak vertical bar velocity. Both the acceleration and deceleration phases of the squat comprised 15–25% of the ascent time, whereas the sticking region and maximum strength region comprised 30–40% of the ascent time. These values are similar to data reported by McLaughlin et al. (21). These results imply that during the squat approximately twice as much time is spent in the sticking region and maximum strength region compared with the acceleration and deceleration phases.
The end of the sticking region (i.e., minimum bar velocity) has previously been reported as the “sticking point”(21), which occurred at approximately 60–65°KF and 80–85° hip angle (Table 2). The “sticking point” appears to be the most difficult part of the lift, and is often where powerlifters fail in their attempt for a successful lift. Because knee and hip moments and moment arms generated by the system weight generally decrease during the ascent as the knees and hips extend (16,22,35), a mechanical disadvantage is believed to occur among knee and hip muscle extensor moments during the sticking region, being greatest near the sticking point. The sticking point phenomena may in part be due to mechanical principles of skeletal muscle, such as to the length-force relationship and muscle moment arm lengths. It is well known that as a muscle contracts concentrically and shortens its ability to generate force diminishes. Because the product of muscle force and muscle moment arm determines the net muscle moment generated at a joint, a decrease in both of these variables or a disproportionately decrease in one variable with respect to the other will cause a decrease in the net muscle moment. The net hip extensor moment generated by the gluteus maximus, hamstrings, and ischial fibers of adductor magnus has been shown to be maximum at 90° hip angle, decreasing progressively as the hips extend (23). This decrease in the net hip extensor moment with hip extension contributes to the sticking point. Interestingly, in contrast to hip extensor moments, the hip extensor moment arms for both the hamstrings and gluteus maximus have their smallest values at 90° hip angle (24), increasing progressively as the hips extend. This implies that the length-force relationship and muscle forces from the gluteus maximus and hamstrings have a much greater influence on hip extensor moments compared with these muscle’s respective moment arms. It has been previously demonstrated that changes in muscle forces affect muscle moments to a much greater extent than changes in muscle moment arms (18). All moment arms from the hip extensors have been shown to initially increase as the hips extend from 90° hip angle (24). The moment arms of the adductor magnus increase until approximately 75° hip angle, and then progressively decrease with further hip extension (24). Because the adductor magnus is a uni-articular muscle, a decrease in this muscle’s ability to generate force due to muscle shortening, combined with its decreasing moment arms as the sticking point is approached, produce a decrease in the net hip extensor moment near the sticking point. Although the moment arms of the uni-articular gluteus maximus increase progressively with hip extension (24), a disproportional decrease in muscle force as this muscles shortens may produce a decrease in the net hip extensor moment near the sticking point. Because the hamstrings are bi-articular, during the ascent these muscles lengthen at the knees and shorten at the hips, which helps maintain a more optimal force-length relationship during the squat. However, hip extension causes the hamstrings to lengthen to a greater extend than knee flexion causes the hamstrings to shorten (19,32). Therefore, it is common near the sticking point for lifters to slightly increase forward trunk tilt and hip flexion, as shown in Figure 1 at minimum bar velocity, so as to increase the length of the hamstrings and other hip extensors, thus increasing these muscle’s ability to generate force. The inclined forward trunk tilt also allows a greater contribution from the powerful back muscles. Although hip moment arms from the hamstrings increase as the hips extend from 90° to 35°(24), knee moment arms from the hamstrings peak near 50–60°KF (15,27), which increase knee flexor moments near the sticking point. The gastrocnemius also generates knee flexor moments during the squat as they contract to cause ankle plantar flexion during the ascent. These increased knee flexor moments generated by the hamstrings and gastrocnemius, which oppose the knee extensor moments generated by the quadriceps, contribute to the sticking point. In addition, knee extensor moments generated by the quadriceps decrease as the knees extend toward the sticking point. This decrease in knee extensor moments as the knees extend is primarily due to a decrease in quadriceps force, because patellar tendon moment arms change only a few millimeters from full knee flexion to full knee extension (13,15). Interestingly, the 12–14° greater hip flexion and 7–10° greater forward trunk tilt observed at the sticking point compared with the same KF position during the squat descent implies that the sticking region causes an asymmetrical pattern to occur between the squat descent and ascent. This explains why at 45°KF there was significantly greater hip flexion and forward trunk tilt during the ascent compared with the descent (Table 2).
Joint moments and moment arms.
Although several studies have quantified joint moments during the squat (2,8,12,16,17,22,26,28,31,34,35), there are no known studies that have quantified hip, knee, and ankle moment arms during the squat. Positive ankle moment arms during the NS produced dorsiflexor system moments (Table 8) that must be counterbalanced by plantar flexor resultant muscle moments. In contrast, negative ankle moment arms during the MS and WS generated ankle plantar flexor system moments (Table 8) that must be counterbalanced by dorsiflexor resultant muscle moments. This implies that the ankle plantar flexors may be recruited to a greater extent during the NS compared with the MS and WS, whereas the ankle dorsiflexors may be recruited to a greater extent during the MS and WS compared with the NS. Escamilla et al. (8), whose subjects had a stance width similar to the NS group, reported low to moderate gastrocnemius activity during the 12 RM squat. In addition, gastrocnemius activity has been reported to be 10–15% greater in the NS squat compared with the WS squat (10). Ankle moment arms peaked at maximum KF, which is consistent with squat data from Escamilla et al. (8) and Isear et al. (14), which show peak gastrocnemius activity near maximum KF.
Several studies have shown moderate to high activity from the quadriceps during the squat (8,20,25,29–31,34,35), with peak activity occurring near maximum KF (8,14,25,30,31,35). Moderate to high knee extensor muscle moments imply overall knee extensor activity, especially during the MS and WS, which produced significantly greater knee moments compared with the NS. However, two studies have reported no significant differences in quadriceps activity between the NS and WS (10,20).
Moderate to high hip extensor muscle moments suggest overall hip extensor activity, especially in the MS and WS groups, which at 45°KF produced significantly greater hip moments compared with the NS. At minimum bar velocity (approximately 60°KF) peak hip extensor muscle moments were generated in all stance groups. Escamilla et al. (8) reported peak hamstring activity (approximately 50% of a maximum voluntary isometric contraction) near 50°KF, whereas other studies have reported peak hamstring activity during the squat between 10° and 60°KF (14,25,31). The hamstrings, gluteus maximus, and ischial fibers of the adductor magnus have all shown significantly greater activity in the WS squat compared with the NS stance (10,20). Compared with the squat descent at corresponding KF angles, the significantly greater hip flexion and forward trunk tilt observed during the squat ascent at minimum bar velocity and 45°KF caused significantly greater hip moments and moment arms.
It is difficult to compare ankle, knee, and hip moments among squat studies in the literature because methodologies and loads lifted (20–270 kg range) varied greatly. In quantifying ankle, knee, and hip moments, some studies used a single camera (2-D) and no force platform (2,12,22,26), some studies used a single camera and one force platform (16,17,28,34,35), whereas some studies used multiple cameras (3-D) and one force platform (8,31). In addition, some studies had subjects squat with one foot on a force platform (8,31), some studies had subjects squat with both feet on a force platform (16,17,34,35), some studies quantified joint moments relative to barbell weight (2,26), and other studies quantified joint moments relative to system weight (8,12,16,17,22,28,31,34,35). Peak ankle moments have been reported between 50 and 300 N·m (16,17,22,26), peak knee moments between 100 and 500 N·m (2,8,12,16,17,22,26,28,31,34,35), and peak hip moments between 150 and 600 N·m (16,17,22,34,35). Hence, ankle, knee, and hip moment magnitudes in the current study are similar to corresponding moment magnitudes in the squat literature.
One of the most important findings from the current study is the numerous significant differences observed in moments and moment arms between 2-D and 3-D analyses. Most of the above squat studies that quantified joint moments employed a single camera to record a sagittal view of the lifter, thus performing a 2-D analysis. Although a 2-D analysis may be appropriate in calculating hip and spinal moments and moment arms during squat, because the trunk moves primarily in the sagittal plane, it is only appropriate in calculating ankle and knee moments and moment arms if lower extremity movements occur primarily in the sagittal plane. However, as the stance widens and the feet turn out, greater errors in 2-D moment and moment arm calculations will occur, because lower extremity movements move out of the sagittal plane. Escamilla et al. (9) reported only a few cm differences in ankle and knee moment arm calculations between 2-D and 3-D analyses during the conventional deadlift exercise, in which a very narrow stance was employed (32 ± 8 cm, 80 ± 16% shoulder width) with the feet only slightly turned out (14 ± 6°). However, these authors reported 20–25 cm differences in ankle and knee moment arms between 2-D and 3-D analyses during the sumo deadlift, in which a very wide stance was employed (70 ± 11 cm, 188 ± 37% shoulder width) with the feet turned out 42 ± 8°. Although 2-D analyses in the current study were significantly different than 3-D analyses for all three stance groups, errors in moment arm magnitudes were twice as great in the WS compared with the NS. This occurred because the WS had a significantly greater stance width and foot angle compared with the NS.
When circumstances do not permit multiple cameras to be employed during the squat, a 3-D analysis can still be performed on some kinematic and kinetic variables by inputting 2-D data from a single camera into appropriate derived mathematical equations. For example, ankle moment arms derived from a 3-D analysis can be calculated given ankle moment arms from a 2-D analysis by employing the following equation: MA3D = cos θ(MA2D) − (0.5Swidth)sin θ, where MA2D is the ankle moment arm measured from a 2-D analysis, MA3D is the predicted ankle moment arm from a 3-D analysis, θ is the angle measured between the sagittal plane and the longitudinal axis of the foot, and Swidth is the stance width measured between ankles. This equation is easily derived by utilizing elementary trigonometry. The values predicted for MA3D by employing this equation were nearly identical to the actual measured values of MA3D from the 3-D analysis. A 2-D analysis from a single camera will yield correct ankle moments and moment arms only when the feet are pointing straight ahead (i.e., θ = 0°) and will produce only small errors when the feet are slightly turned out with an NS employed. For example, if during the NS squat MA2D measured 9 cm, stance width was 35 cm, and the foot angle was 10°, MA3D would yield a value of approximately 6 cm, which is only a few cm different than MA2D. However, if during the WS squat MA2D measured the same 9 cm, but stance width was 70 cm and foot angle was 45°, MA3D would yield a value of approximately −18 cm, a difference of 27 cm. Also, the difference in the moment arm sign implies a change from a plantar flexor net muscle moment to a dorsiflexor net muscle moment. The employment of a 3-D analysis is clearly more paramount during the WS squat with a large foot angle compared with the NS squat with a small foot angle.
When peak mean joint moments from the squat literature involving 2-D analyses are normalized by body height and system weight, their normalized peak mean values are very similar to the normalized mean peak 2-D moments calculated in the current study. For example, from Table 10 the peak mean 2-D knee moment calculated for the NS was 297 N·m. When this peak knee moment is normalized by the product of body height (1.77 m, Table 1) and system load (2852 N, Table 1) and expressed as a percentage, the normalized mean peak knee moment for the NS is between 5.5 and 6%. Compared with other squat studies that reported peak mean knee moments using a 2-D analysis, Ariel (2) found a normalized peak mean knee moment of approximately 5.5%. Similarly, Lander et al. (17) and Wretenberg et al. (35) found peak mean knee moments of approximately 6–6.5%, which are similar to the normalized peak mean knee moment in the current study. As seen in Table 10, knee moments and moment arms are significantly underestimated in 2-D analyses compared with 3-D analyses.
The knee moment significant differences observed between barbell load and system load implies that knee moment contributions from body segments should not be discounted when calculating the actual joint moments that occur during lifting. It is interesting that hip and ankle moments were not significantly different between barbell load and system load. There are several reasons why this occurred. First, knee moment arms were generally greater and hip and ankle moment arms were generally less for the system loads compared with the barbell loads. Therefore, these smaller system hip and ankle moment arms would produce a relative decrease in hip and ankle system moments. Second, the system load was greater for knee moments than hip moments, because there is greater body mass above the knee joints compared with the hip joints. Third, the relative small ankle moment arms compared with the knee and hip moment arms produced relatively large standard deviations for both ankle moments and moment arms (Table 8).
The authors extend a special thanks to Andy Demonia and Christian Welch for all their help in collecting the data and Abidemi Bolu Ajiboye, Herbert Bohnet, and Brian Pullin for all their assistance in manually digitizing the data. Also, we would like to extend a special thanks to Tom and Ellen Trevorah, powerlifting meet directors, for all their support throughout this project.
Address for correspondence: Rafael Escamilla, Ph.D., C.S.C.S., Duke University Medical Center, P.O. Box 3435, Durham, NC 27710; E-mail: [email protected]
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