Unstable surfaces have become more popular for balance and stabilization training. Thera-Band® Exercise Balls and Stability Discs offer multiple options for unstable surface training. These unstable surfaces are thought to increase muscle activation and co-contraction, particularly for core stabilization training. Japanese researchers investigated the effect of unstable surface training on trunk muscle activity during common lumbar stabilization exercises in 19 healthy subjects. Their findings were published in the Journal of Orthopedic and Sports Physical Therapy.
The researchers used surface electromyography (EMG) to assess the “global” muscles: rectus abdominus, erector spinae, and external obliques, and fine-wire EMG for the deeper “local” muscles: transverse abdominus and multifidus. The subjects performed 5 lumbar stabilization exercises for 3-second holds, both on stable and unstable surfaces. The exercises were: prone plank on elbows, supine bridge, quadruped arm-and-leg, side bridge, and curl-up. EMG levels were normalized to maximal contraction.
The prone plank exercise on unstable surfaces (including an exercise ball and stability disk) produced significantly more muscle activity of all core muscles. High levels of oblique and rectus muscles were noted in both conditions for the prone plank exercise. The supine bridge exercise performed on an unstable surface did not significantly increase the activation of any muscle; the levels remained below 40% in both conditions.
For the remaining 3 exercises (the quadruped arm-and-leg, side bridge, and curl up), the superficial global muscles (obliques, rectus abdominus, and erector spinae) all increased in activation on the unstable surfaces, while the deeper stabilizing muscles (transverse abdominus and multifidus) did not. The researchers suggested that this increase in superficial muscles is needed to control trunk rotation and extension, particularly from the external oblique.
These findings are somewhat contradictory to traditional belief that exercising on unstable surfaces increases activation of deep stabilizing muscles. There were several limitations to this study in regards to clinical application: there were only 9 healthy subjects, the onset of activation was not investigated, nominal values were not provided, and there was no statistical comparison between exercises. Despite these limitations, the researchers concluded that lumbar stabilization exercises performed on unstable surfaces including and exercise ball and stability disk can increase the activation of trunk muscles.
REFERENCE: Imai A, et al. Trunk muscle activity during lumbar stabilization exercises on both a stable and unstable surface. J Orthop Sports Phys Ther. 2010 Jun;40(6):369-75.