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Research Note

The Effects of Two Different Arm Positions and Weight Status on Select Kinematic Variables During the Bodyweight Squat

Glave, A. Page; Olson, Jacilyn M.; Applegate, Danika K.; Brezzo, Ro Di

Author Information
Journal of Strength and Conditioning Research: November 2012 - Volume 26 - Issue 11 - p 3148-3154
doi: 10.1519/JSC.0b013e318243fefb
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Abstract

Introduction

The squat exercise is ideal for developing lower body strength because it engages both the quadriceps and hamstrings (16) and is generally regarded as safe and effective (1,8). It is commonly used in exercise programs for the general population (27), in rehabilitation (1,8,27), and for athletic development (1,7,27). The squat may also improve quality of life because of the muscle recruitment involved and similarity to many functional tasks (18). Additionally, the squat may be performed with or without external loads and may be performed anywhere. This combination of factors makes the squat exercise an imporant part of most exercise programs.

The squat is used with various populations and maximizing its benefit is critical considering the limited amount of time dedicated to exercise. Therefore, understanding the value and benefit of the squat for a given population is vital. Most of the literature has focused on examining the forces associated with the movement and how these forces affect the safety of the movement (2,10,11,13,16,19,21,22,26). In establishing the safety of the movement, few studies have examined how variations in technique affect the range of motion through which the exercise is performed. Adjusting the squat technique to increase the range of motion has important implications for improving strength throughout the movement and maximizing benefit to the individual. By examining how alterations in technique are beneficial or detrimental, it will allow practitioners to prescribe the type of squat that will most benefit the client. Examining how technique variations affect different populations is also important because technique variations may affect groups differently.

Several studies have found that alterations in squat technique have a significant impact on the range of motion. Body position (9,16), direction of gaze (5), squatting to a target depth (12), and equipment alterations (15,22) have all been shown to alter joint range of motion. Escamilla et al. (9) found that squatting with a narrow stance, compared with a medium or wide stance, resulted in decreased hip flexion and increased forward movement of the knee. Fry et al. (16) found that restricting anterior knee movement resulted in decreased knee and ankle flexion and increased trunk flexion, putting the spine in a more compromised position. A study examining the effects of direction of gaze found that a downward gaze led to a significant increase in hip flexion and a nonsignificant increase in trunk flexion compared with looking straight ahead or upward (5). Knee range of motion was not affected. Squatting to a targeted depth (a chair) resulted in increased hip flexion placing a greater demand on the hip extensors, whereas squatting to a self-selected depth resulted in greater knee and ankle flexion placing a greater demand on the knee and ankle extensors (12). Lander et al. (22) found that using a U-shaped bar instead of a standard weight bar resulted in increased trunk extension with no change in thigh or knee angle. Squatting on a declined platform resulted in an increased range of motion at the hip, knee, and ankle (15). This research supports the importance of investigating how alterations in technique affect the range of motion and the importance of considering the impact on trunk and knee flexion.

Most studies have focused on young, healthy adults (2,5,9,10,13,15,16,19,21,22,26); a single study examined a healthy, older group (70–85 years old) (12). No research has examined the squat in an overweight (OW) population. Understanding how obesity affects biomechanics is important because the prevalence of obesity (defined as a body mass index [BMI] >30) and OW (defined as a BMI of 25.0–29.9) continues to rise. As of 2008, the prevalence for obesity was 33.8% of adults, and the prevalence for OW and obese combined was 68.0% of adults (14). The obese population has also been shown to have decreased lower body strength relative to body weight (24). This is problematic because decreased lower body strength is associated with decreased physical function (18). These factors highlight the importance of resistance training for the obese individual. Resistance training is encouraged for the OW individual as a means to improve health, aid fat loss, and help preserve lean muscle mass (6). The squat is a very popular exercise and should be easily considered a common component of an exercise prescription for an OW individual.

Although the squat has not been examined, several studies have found differences in a similar movement, the sit-to-stand, in an OW population. Prepubertal OW boys were found to have increased sit-to-stand time and decreased force production compared with normal-weight (NW) boys (4). The OW boys also experienced greater center of gravity sway velocity indicative of difficulty controlling forward trunk motion. Overweight adults with chronic lower back pain used a sit-to-stand strategy involving decreased trunk flexion, which limited low back torque, on the first of 10 sit-to-stand movements (17). However, on the final sit-to-stand movement, the OW group adopted a strategy similar to that of the NW control group, with increased trunk flexion and a greater burden on the lower back. Sibella et al. (29) also found decreased trunk flexion during the sit-to-stand movement and additionally found that OW participants moved the feet backward during the sit-to-stand movement. These studies indicate real differences in how the OW population executes functional movements.

In addition to considering the impact of excess bodyweight, the impact of fat distribution should also be noted. Little is known about how fat distribution affects movement. The few studies that took fat distribution into account did not find alterations in balance (23) or external knee adduction moment (28). Both studies did find that body weight affected the variables of interest. Although the information regarding fat distribution is limited, there are definite differences in how men and women tend to store excess body fat (20). Men tend to store excess body fat around the abdomen (android type), and women tend to store excess body fat around the hips and thighs (gynoid type). These differences could play a role in how obesity alters biomechanics. For this reason, this study has been restricted to women to minimize the potential influence of fat distribution.

Because the OW or obese individual's bodyweight is likely to provide sufficient resistance for a beginner, examining technique alterations in the bodyweight squat is a logical starting point for addressing the needs of this population. Varying arm position during a bodyweight squat is common. Performing the movement with the arms extended at the shoulders is one such variation. Knowing how technique variations and obesity interact is a crucial piece of information for the practitioner tasked with creating an effective exercise program. Therefore, the purpose of this study was to examine the effect of 2 different arm positions, the arms held at the sides with the elbows flexed to approximately 90° (Figure 1) and the arms held with the shoulders flexed to 90° and slightly horizontally abducted (Figure 2) and weight status on the maximum knee and trunk flexion attained in the bodyweight squat. It was hypothesized that both arm position and weight status would significantly affect range of motion.

Figure 1
Figure 1:
A). Arms at the sides position: sagittal view. B). Arms at the sides position: frontal view.
Figure 2
Figure 2:
A). Arms-up position: sagittal view. B). Arms-up position: frontal view.

Methods

Experimental Approach to the Problem

Technique variations have been shown to alter squat mechanics (5,9,12,15,16,22) making it imperative to have a purpose for varying technique. For the bodyweight squat, changing arm position is a common and easy to perform variation. The focus of this study has been limited to 2 arm positions because of the early state of research involving the mechanics of obesity and to isolate the effects of the arm position. The arms at the side position was chosen to serve as a neutral condition, similar to sitting down. Keeping the arms down at the sides also minimizes the effects of the arms on the moment of inertia of the trunk and the center of gravity of the body. The arms held at shoulder level position was chosen because balance has been shown to be a problem in the obese population (23). By holding the arms up and extended anteriorly, the anterior moment of inertia of the trunk is increased and the center of gravity is shifted forward. It was thought that these 2 factors may serve to offset the posterior shift of the center of gravity associated with the squatting motion to allow the individual to move through a greater range of motion before feeling unbalanced.

Kinematics of the knee and hip are especially important in performing the bodyweight squat safely and effectively. Knee flexion and trunk flexion were selected as key variables for this investigation. Knee flexion was chosen because it is an indication of the depth of the squat, with greater knee flexion meaning the person moved through a greater range of motion. Trunk flexion was also examined because of the tendency for novice lifters (16) to have increased anterior lean. Increased forward lean is problematic because of the detrimental effects on the spine, and this should particularly be avoided in the obese population because of an increased incidence of lower back pain (17).

Subjects

Although the focus of this investigation was on the effects of obesity, it was important to include a group of NW participants. The effect of arm position during a bodyweight squat has not been previously studied, so an NW group was needed to examine whether arm position worked in isolation or in conjunction with obesity. BMI was used to group the participants as NW or OW because it is the most commonly used technique. Data collection occurred during the fall semester during normal working hours (8:00 AM to 5:00 PM). The subjects were allowed to eat and drink ad libitum before testing. To secure a larger sample, training experience was not controlled, and no strength measures were taken. This kept the focus on the effects of obesity and arm position. All the participants were instructed on proper squatting technique and allowed to practice until they indicated they were comfortable to minimize the impact of not controlling for training experience.

The participants were 28 college-aged women who volunteered for the study. When divided into groups by weight status, with a BMI <25 being considered NW and a BMI ≥25 being considered OW, there were 17 NW participants and 11 OW participants. The groups were nearly identical in age (NW: 20.94 ± 1.39 years; OW: 20.73 ± 1.27 years) and height (NW: 1.67 ± 0.06 m; OW: 1.68 ± 0.06 m). However, as expected, there were large differences in both bodyweight (NW: 61.20 ± 7.11 kg; OW: 86.46 ± 17.94 kg) and BMI (NW: 21.79 ± 1.63; OW: 30.97 ± 6.17). All the participants were students at a state university and provided written informed consent before participating in the study. The informed consent provided information about the procedures involved in the study, a description of benefits and risks, provided the opportunity to ask questions, indicated the person was able to withdraw from the study at any time without penalty, and an explanation of how the person's information would be kept confidential. The project was approved by the university institutional review board. Participant characteristics may be found in Table 1.

Table 1
Table 1:
Participant characteristics.*†

Procedures

Upon arriving at the testing facility, the participants were presented with the informed consent and allowed to read and ask questions about the project before granting consent. Height (in meters) and weight (in kilograms) were measured using a standard balance scale (Detecto, Webb City, MO, USA). The participants were then escorted to the motion analysis area. Reflective markers were attached on the right side of the body over the approximate center of rotation at the shoulder, hip, and knee. Markers were also placed at the base of the fifth toe and posterior aspect of the heel. For both squatting conditions, the participant was instructed to place the feet approximately shoulder width apart in a comfortable stance and to look straight ahead. For the arms at the side condition, the participant was instructed to keep the elbows bent to approximately 90° and keep the elbows close to the body. The participant was allowed to practice until she indicated she was comfortable. Data were then collected for the arms at the side condition. The arms at shoulder level position was then introduced with instructions to hold the arms at shoulder height slightly wider than shoulder width with the elbows extended. The participant was allowed to practice this condition until she indicated she was comfortable and data were collected. All the participants completed the arms at the sides condition followed by the arms at shoulder level position. A single trial of each position was collected.

Data were recorded at 60 Hz because of the slow movement speed. A Canon ZR50 camcorder (Canon USA Inc., Lake Success, NY, USA) and Peak 9 motion analysis software (Vicon Inc., Centennial, CO, USA) were used to collect data. Peak 9 motion analysis software was also used to process and analyze the data. Points were digitized using the automatic digitization function; any points not identified automatically were manually digitized. Each frame was digitized, and the default filtering settings were used. Knee angle was defined as the anatomical 180° angle between the hip, knee, and ankle markers. Positive values indicated knee flexion. Trunk angle was defined as the anatomical 180° angle between the shoulder, hip, and knee markers. Negative values indicated trunk flexion. Data analysis was restricted to the frame before movement initiation through the frame after movement completion. Before testing, all the researchers participated in pilot testing to ensure consistency of marker placement. For consistency, a single member of the research team was responsible for processing all of the data.

Statistical Analyses

To test the hypothesis that both arm position and weight status would affect range of motion, it was necessary to use 2-way repeated measures analyses of variance using the methods described in O'Rourke et al. (25). There was one within-subject factors (arm position) and one between-subjects factor (weight status). All model assumptions were tenable. SAS 9.2 (SAS Inc., Cary, NC, USA) and SPSS 17.0 (SPSS Inc., Chicago, IL, USA) were used to analyze the data. Statistical significance was set at the p ≤ 0.05 level. There was no significant Pearson correlation (p = 0.38 for the arms at sides position; p = 0.44 for the arms-up position) between trunk and knee flexion; therefore, significance was not adjusted for the number of tests. Partial eta squared (partial η2) was used to determine effect size. Retrospective power was also calculated.

Results

There was no significant interaction effect for either knee flexion (F1,26 = 3.82, p = 0.06) or trunk flexion (F1, 26 = 1.59, p = 0.22), so the main effects were examined. The hypothesis that arm position would affect range of motion was supported for knee flexion. The results for knee flexion indicated a significant difference for arm position (F1, 26 = 8.32, p < 0.01). The overall mean for knee flexion for the elbows at 90° condition was 80.81 ± 15.17°, whereas the overall mean for knee flexion for the shoulders at 90° condition was 86.31 ± 15.21°, an increase of nearly 6°. The means for all the groups on each squat condition may be found in Tables 2 and 3. There was also a significant difference for OW vs. NW knee flexion (F1, 26 = 4.33, p < 0.05). The mean for knee flexion for the NW group was 87.60 ± 13.20°; the mean for knee flexion for the OW group was 76.30 ± 16.47°. The analysis of variance results for knee flexion are found in Table 4. The results for trunk flexion indicated a significant effect for weight status (F1, 26 = 4.19, p = 0.05; Table 5). The mean for trunk flexion for the NW group was –93.15 ± 20.79°, whereas the mean for trunk flexion for the OW group was –77.51 ± 18.80° (Table 5). There was no significant effect of arm position on trunk flexion (F1, 26 = 0.53, p = 0.47).

Table 2
Table 2:
Means table for maximum knee flexion.*
Table 3
Table 3:
Means table for maximum trunk flexion.*
Table 4
Table 4:
Analysis of variance for squat condition and weight status for knee flexion.*†
Table 5
Table 5:
Analysis of variance for squat condition and weight status for trunk flexion.*†

Effect size and observed power can be found in Tables 6 and 7. Effect sizes were interpreted based on Cohen's (3) guidelines. Partial η2 indicated large effect sizes for the effects of weight status for both knee (partial η2 = 0.143) and trunk (partial η2 = 0.139) flexion. Arm position had a large effect size for knee flexion (partial η2 = 0.243) and a small effect size for trunk flexion (partial η2 = 0.20). The interaction of arm position and weight status had a medium effect size for both knee (partial η2 = 0.128) and trunk (partial η2 = 0.058) flexion. For knee flexion, power was 0.52 for weight status, 0.79 for the effect of arm position, and 0.47 for the interaction of arm position and weight status. For trunk flexion, power was 0.51 for weight status, 0.11 for arm position, and 0.23 for the interaction of weight status and arm position.

Table 6
Table 6:
Effect size: partial eta squared.*†
Table 7
Table 7:
Post hoc power analysis.*†

Discussion

The results of this study provided limited support for the hypotheses that arm position and weight status would affect knee and trunk flexion during the bodyweight squat. Similar to other studies examining technique variations (5,12), altering technique resulted in a greater range of motion at the knee. With the arms held at shoulder height, knee flexion was increased indicating that the participant moved through a more complete range of motion. Trunk flexion was not affected by arm position. This could be seen as positive because increased trunk flexion places the spine in a compromised position (27). It is possible that the arms-up position did increase the anterior moment of inertia of the trunk and shift the center of gravity forward to allow greater knee flexion before a loss of balance. The main implication of this finding is that technique alterations should be explored because of the potential to increase the benefit of the squat by allowing the individual to move through an increased range of motion. Squatting with the arms extended at shoulder height would be a viable technique variation when an individual is unable to perform the bodyweight squat to a satisfactory depth.

The results also indicated that weight status plays a significant role in the range of motion during the bodyweight squat. Both knee and trunk flexion values were reduced in the OW group compared with that of the NW group. The reduction in trunk flexion is similar to the reduction found in sit-to-stand research (4,18,29) and may be related to protecting the spine (16). Reduced knee flexion may be related to strength deficiencies (24) and serve to minimize forces on the knee by restricting anterior knee movement (16). The finding that the arms-up position resulted in greater knee flexion is particularly important for the OW population because it may allow the individual to exercise through a greater range of motion to help address lower body strength deficiencies. The differences in squatting kinematics between OW and NW individuals also highlight the importance of examining different variations of the squat and other exercises in varied populations.

Practical Applications

This study found that OW participants squatted to a lesser depth than did NW participants, which will limit the strengthening benefits of the movement. Overweight participants also maintained a more upright trunk position. Raising the arms to shoulder height allowed all the participants to squat to an increased depth. Therefore, using an arms-up position, especially with OW clients, can improve the muscular development of the squat by allowing a greater range of motion. Technique variations should have a specific intent and need to be carefully considered because of the potential impact on movement mechanics. Particular care should be taken when choosing technique variations for OW individuals to address possible movement and strength deficiencies.

Acknowledgments

The authors have no conflicts of interest to report. This project was not supported by outside funding. The results of this study do not constitute endorsement of the equipment used by the authors or the National Strength and Conditioning Association.

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Keywords:

biomechanics; non-weighted; obesity; resistance training

Copyright © 2012 by the National Strength & Conditioning Association.