Unstable surfaces such as foam pads and air-filled disks have historically been used to increase muscle activation of the legs while performing a squat. Some believe that standing on an unstable surface requires more activation of muscle to maintain stability. Research featured on the Academy Blog has shown, however, that standing on unstable surfaces actually decreases the force output and EMG activation levels of the extremities. Unfortunately, some studies show conflicting results due to different methodologies, leading to confusion in daily practice.
Dr. Jeffery McBride and his colleagues at Appalachian State University in North Carolina performed a study on the EMG levels of the thigh and lower back muscles while subjects squatted on a stable and unstable surface using an inflatable disc under each foot. The subjects performed these squats at various loads to look at the effects of different levels of resistance as well as instability.
The stable squat condition resulted in similar or higher EMG activation of the vastus lateralis, hamstrings, and lumbar erector spinae compared to the unstable condition at all resistance levels. The researchers concluded that unstable squatting results in significantly less muscle activation and should not be used if the goal of the exercise is to increase strength. While Thera-Band stability discs may be appropriate for balance training, they are not recommended for use with a loaded squat when the goal is to increase strength.
REFERENCE: McBride JM, Larkin TR, Dayne AM, Haines TL, Kirby TJ. Effect of absolute and relative loading on muscle activity during stable and unstable squatting.Int J Sports Physiol Perform. 2010 Jun;5(2):177-83.