While some gym-goers may perform their sets in front of a mirror to monitor their positioning (or to nab a good selfie), a new study by the Rehabilitation Institute of Chicago claims there's at least one thing mirrors aren't helpful for: avoiding preferential loading (when you put more weight on one leg over the other).
According to a press release, lead Investigator of the study, Monica Rho, MD said, “The goal of most people who perform double leg squats as part of their exercise routine is to make sure their weight is evenly distributed on each leg. Everyone tries to achieve symmetry because they want to work out each leg equally during the squat. However, there is no data out there that demonstrates whether or not a mirror helps the loading symmetry of a squat, and this study was designed to answer that question."
Research was conducted on five men and women between the ages of 18 and 50, who had never undergone hip or pelvic fracture surgery and weren’t currently experiencing lower back pain or hip pain, hip osteoarthritis, or impaired balance. They were asked to stand with their feet on two different force plates, which allowed researchers to determine how much force each person put on their legs during squats.
The participants performed a series of double legged squats: five in 'fixed' position with their feet forward at a fixed distance; five in 'mixed' position with their feet at a fixed distance but in a self-selected position; and five in 'self-selected' position with their feet at a self-selected distance and position. Half began their workout by using the mirror while the other half didn’t, and then they switched.
The results showed that when the participants stood with their feet forward at a fixed distance and squatted in front of a mirror, they tended to shift .56 percent more of their body weight to their dominant leg. When they performed the same workout without a mirror, they shifted one percent more of their body weight to their dominant leg. When given the option to choose their own starting foot position to squat, the participants tended to shift .82 percent of their body weight towards their non-dominant leg with a mirror and .74 percent without—a difference that also proved statistically insignificant. These small differences in percentage, the study authors say, ultimately prove there is “no difference between squatting with or without a mirror.”
Alas, "Our findings indicate that, when it comes to equal weight distribution and symmetry of loading each leg during a squat, the mirror doesn't seem to make a difference," Dr. Rho stated. However, she acknowledged that the study didn’t specify whether doing squats in front of a mirror allows for better form to avoid injury. So, “it is still possible that squatting in front of a mirror is better for people who have difficulty in maintaining good form during their squat,” Rho explained. (You can also still use a mirror for your gym selfies, if you're so inclined, although the study does not explicitly state this.)
As for next steps, Rho suggests it would be beneficial to test a larger population to see if the use of a mirror affects the proper knee-over-ankle positioning during the exercise.