Upward gaze direction increases center of pressure displacement and decreases squat stability

Introduction

The squat is a common, everyday movement pattern seen in simple tasks such as sitting down and standing up, as well as more complex tasks typical of sports and physical activity. The squat is also considered one of the most effective methods of performance enhancement. Squatting trains the musculature responsible for many of the movements that are crucial to athletics (i.e. running, jumping, and throwing). Muscles activated during the squat include, but are not limited to, the gastrocnemius, quadriceps, hamstrings, and gluteus maximus, as well as core musculature in the back and abdominal regions. ¹

Due to its role in not only athletic performance, but also injury prevention, the squat has been utilized in exercise/coaching environments in many different forms. ² One of the most common forms of the squat, as it relates to strength training, is the barbell back squat. The barbell back squat is characterized by an athlete or lifter standing upright, with feet flat on the floor approximately shoulder width apart. The lifter then flexes at the hips, knees, and ankles, allowing their trunk to incline forward until their thighs are at least parallel to the floor. After a brief pause the lifter forcefully extends the hips knees and ankles to return to an upright standing position. ³

While application of the barbell back squat is generally similar throughout the strength and conditioning profession, the techniques used during implementation of this exercise are not. ² A controversial technique consideration while coaching or performing the barbell back squat is head position. Head position is often manipulated by making drastic changes to a lifter’s gaze direction through differing instructions as to where to look at on a wall in front of them. Specifically, the lifter can be asked to look UP (i.e., high spot on the wall), STRAIGHT (i.e., spot near eye-level) or DOWN (i.e., low spot on the wall) while performing the squat repetition. Some believe that looking UP will keep the lifter’s head tilted back, preventing them from falling forward and creating a more stable squat. ³ Alternatively, it has been suggested that looking DOWN during a squat could potentially increase injury due to improper technique and loading of joints. ⁴ The STRAIGHT direction is a compromise between UP and DOWN.

Despite these lines of reasoning, there is a dearth of objective data available comparing UP, STRAIGHT and DOWN gaze directions during the barbell back squat. Previous research has only utilized subjective methods for measuring squat stability in different gaze conditions. ⁴ An objective and sensitive means of measuring stability during squatting can be achieved by measuring postural control with a force plate. Postural control is defined as the amount of movement a person’s center of pressure (COP) experiences over time, as quantified by a force plate device. In the case of back barbell squatting, individuals with large displacements in COP are thought to have less postural control and, therefore, worse balance.^5,6

The purpose of the present study, therefore, was to objectively quantify the stability of barbell back squat as a function of gaze direction (i.e., UP, STRAIGHT and DOWN instructions), using a low-cost force plate. Based on more subjective assertions in the literature it was expected that less COP displacement (i.e., greater stability) would be seen for the UP versus DOWN gaze direction. The results of this study provide objective information that will assist in the instruction of barbell back squatting, allowing athletes to both increase performance and avoid injury by developing a more stable and balanced squat movement.

Methods

Procedures

The setup for this study is shown in Figure 1. A squat rack (Rogers Athletic Co, Farwell, MI) was used along with an Olympic barbell (Texas Strength Systems, San Antonio, TX) and Olympic weightlifting plates (Pendlay, McPherson, KS). Postural control in the form of COP was measured using the BTrackS Balance Plate (Balance Tracking Systems, San Diego, CA), which has been validated for gathering COP accurate and reliable data.^7,8

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Figure 1.

Test set-up, featuring the balance board and the three large dot placements on the wall signifying the UP, STRAIGHT, and DOWN gaze focal points during the squat.

The BTrackS Balance Plate measured 60 × 40 cm with four integrated sensors, one located at each corner. In order to accommodate the stance width of participants, the surface of the BTrackS Balance Plate was modified using a sheet of plywood (100 × 50 × 2cm) placed on top of the plate with a thin cardboard barrier of the same dimensions placed between the plywood and the plate. The plywood and cardboard were affixed using two steel “U” clamps along the front of the plate/plywood. Data generated during the test was collected and saved to a laptop computer running the BTrackS Explore Balance data acquisition software. Tearing (i.e., zero force subtraction) of the BTrackS Balance Plate was done with the plywood on it, in a fashion that was consistent with that which has been previously validated. ⁹

Statistical analysis

The dependent variable measured for this study was COP displacement. COP displacement was determined by first quantifying the point-to-point COP Path Length between successive time points according to the following formula:

COP Path Length = ( ( COP x2 − COP x1 ) 2 + ( COP y2 − COP y1 ) 2 ) 0.5

Where, COP_x2 and COP_x1 are adjacent time points in the COP_x (medial/lateral) time series and COP_y2 and COP_y1 are adjacent time points in the COP_y (anterior/posterior) time series. The sum of all COP Path Lengths were then added together. Following this, an Upper and Lower fences outlier analysis was conducted and nine of the 288 total reps were removed as extreme scores. Paired, one-tailed t-tests with an alpha of p < 0.05 were utilized to evaluate the effect of gaze direction. Analyses were conducted in SPSS (IBM, Armonk, NY).

Results

Figure 2 shows the mean +/− standard error COP displacements for each gaze direction. On average, the highest stability (i.e., lowest COP displacement) was found in the DOWN vs STRAIGHT vs UP conditions. Based on the t-test results, it was found that the DOWN condition was significantly more stable than UP condition (p = 0.02). Neither UP or DOWN conditions were significantly different than STRAIGHT (p = 0.22 and p = 0.28 respectively).

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Figure 2.

Average (+/− Standard Error) COP Displacement across the UP, STRAIGHT and DOWN gaze direction conditions. Asterisk (*) denotes significant difference between UP compared to DOWN condition.

Discussion

This study sought to objectively quantify barbell back squat stability as a function of gaze direction in the UP, STRAIGHT and DOWN directions. This was accomplished using a low-cost force plate that objectively measured COP displacement during multiple repetitions of each gaze condition. Contrary to the hypothesis based on previous subjective research, it was found that stability was greatest (i.e., COP displacement was lowest) in the DOWN direction compared to UP. This result suggests that a DOWN gaze direction instruction is the most stable barbell back squatting technique.

Significantly reduced COP displacement for the DOWN gaze direction could be the result of a more natural spinal alignment and body position while descending into the bottom of the squat position. During the back squat, flexion at the hips, knees, and ankles, which allow the lifter to lower into the bottom of the squat position, will also cause a change in the angle of the trunk. As the lifter’s trunk begins to angle forward as if taking a bow, the head will follow. If the lifter is looking down, the angle of the head and cervical spine will remain constant relative to the trunk and remainder of the spine. However, if the lifter were to look up as their trunk begins to angle forward, the head will be tilted back relative to the trunk and the cervical spine will no longer remain constant relative to the remainder of the spine. This would explain why significance was found when comparing DOWN to UP, but not while comparing DOWN to STRAIGHT or STRAIGHT to UP. This breakdown of spinal alignment across conditions mirrors the breakdown of proper squat technique, which could potentially be the cause of increased COP displacement while looking UP compared to DOWN.

During squatting with an UP-gaze direction, there is a natural tendency to tilt the head backward into neck extension. This posture of the head and neck is notable, as it has been shown to significantly impact performance of the vestibular apparatus in the inner ear, and subsequently increase COP displacement through poorer postural control. ¹⁰ In this case, any biomechanical benefit related to offsetting forward head positions during squatting in the UP direction could be nullified. It is also possible that the present findings are related to the so-called “pelvo-occular reflex.” This term has been utilized primarily in the strength and conditioning field to describe changes in the orientation of the pelvis in response to head position with little to no support of its existence in the realm of sensorimotor neuroscience. ¹¹

Further, while the present results favor the use of a DOWN direction for barbell back squat training, previous research has suggested that this direction might increase injury due to improper technique and loading of joints. ⁴ Indeed, there is an apparent cost/benefit tradeoff for use of the DOWN direction whereby enhanced squat stability may increase the likelihood of musculoskeletal damage. To what extent use of the DOWN direction and a barbell front squat technique may provide a better alternative for squat training is unclear. However, recent work suggests that use of barbell front squats can both enhance extensor muscle strength development and prevent possible lumbar injuries during maximum loading. ¹²

Recent research suggests that STRAIGHT or UP gaze during squatting reduces forward lean of the trunk. ¹³ This contrasts evidence that a retracted neck posture during heavy lifting can be beneficial to maintaining more balanced muscle activation and therefore has the potential to reduce pain. ¹⁴ Specifically, the retracted neck posture was defined as upper cervical flexion, lower cervical extension, and extension of the upper thoracic vertebrae. These neck posture results, however, were measured during a stoop lift rather than a barbell back squat lift.

There were several limitations worthy of note in this study that warrant further investigation. First, to reduce the number of potential variables, only female participants were tested. While there no known evidence to suggest sex differences during squatting based gaze direction, a follow up study including both males and females could provide additional insight. Second, while significant results were observed in our collegiate athlete participants, we believe that the result of this study could have an even greater impact on novice, non-athlete lifters. Due to their superior coaching, increased experience level, and increased athleticism, college athletes may have the ability to better stabilize themselves while squatting, no matter the direction of gaze.

Practical applications

The results of this study have direct applications for teaching and coaching the barbell back squat. If an athlete or client is struggling to maintain balance during the squat movement, instructing the lifter to look DOWN at a fixed point on the floor may increase squat stability by increasing postural control (i.e., balance).