Closed chain exercises such as weight-bearing through the shoulder in a quadruped position are thought to increase co-contraction around the glenohumeral joint for stabilization. The addition of an unstable surface such as an exercise ball is often prescribed for therapeutic exercises to increase shoulder muscle activation compared to a stable surface.
Researchers in Brazil compared the surface EMG activation of glenohumeral and scapulothoracic muscles in 12 healthy subjects. They performed 3, 6-second contractions in 3-point kneeling with the dominant hand either on a stable surface or on an exercise ball with axial load.
The researchers found no significant difference in EMG levels of the glenohumeral muscles between stable and unstable surfaces, but did note a significant increase in EMG activation in the scapulothoracic muscles. In addition, force output was 23% less on the exercise ball compared to the stable surface. The authors commented that a maintenance or increase in EMG levels despite reduced force output suggests high levels of neuromuscular coordination are used on unstable surfaces such as the Thera-Band exercise ball and mini-ball.
REFERENCE: de Araújo RC, de Andrade R, Tucci HT, Martins J, de Oliveira AS. Shoulder muscular activity during isometric three-point kneeling exercise on stable and unstable surfaces. J Appl Biomech. 2011 Aug;27(3):192-6.