“Functional Training” has become increasingly popular in both rehabilitation and fitness. “Core training” has often been considered a component of functional training because of the transfer of force through the trunk to the extremities. Unstable surfaces such as Thera-Band® Exercise Balls and Stability Trainers are often used with traditional exercise movements to integrate training of the trunk and extremities.
Canadian researchers Anderson and Behm have shown that instability training can provide similar muscle activation levels at lower resistance levels compared to training on stable surfaces. Unfortunately, however, unstable surface training also results in a decrease force output of prime movers.
California State University researchers set out to evaluate the electromyographic (EMG) activity of prime movers and core stabilizers during a seated overhead press exercise. 30 healthy, resistance-trained subjects performed an overhead press with dumbbells and barbells while sitting on a stable surface and on an exercise ball. The investigators were interested in looking for differences in EMG levels between the stable load (barbell) and unstable load (dumbbell), on both stable (bench) and unstable surfaces (exercise ball). Subjects used a 10RM resistance (maximum amount of weight lifted for 10 repetitions) for each of the 4 conditions; thus, each condition had a slightly different absolute load.
As expected, the loads used in each condition decreased with decreasing stability. Most resistance was used with the barbell on the bench (stable load on stable surface), followed by the barbell on the exercise ball (stable on unstable), utilizing 89% of the stable on stable condition. The dumbbell on bench (unstable on stable) utilized 86%, while the dumbbell on the ball (unstable on unstable) used 79% of the stable conditions. The researchers suggested that exercising on unstable surfaces reduces force output approximately 15%.
The upper extremity muscle EMG also decreased as the stability decreased, while the erector spinae muscles had great activation on the exercise ball compared to the stable bench. Interestingly, exercises performed on a stable surface required more abdominal activation, indicating that the abdominals require greater activation with greater loads during an overhead lift.
The authors concluded that exercise balls should not be used during the overhead press exercise when the goal is to increase muscle activation of the arms or abdominals, even with reduced loads. However, performing barbell overhead press on an exercise ball does result in increased activation of the low back muscles.
These results are relatively consistent with other studies that suggest unstable surfaces such as exercise balls should not be used to increase strength of extremities using traditional resistance training exercises. While other studies have shown increased activation of the abdominals with resistance training, these authors did not evaluate the seated overhead press exercise; therefore, preferential activation of certain core muscles may be specific to the exercise performed on the exercise ball. The Thera-Band exercise ball remains a valuable tool for stabilization training, although its role in core strengthening during traditional resistance exercise remains questionable.
REFERENCE: Kohler JM, Flanagan SP, Whiting WC. Muscle activation patterns while lifting stable and unstable loads on stable and unstable surfaces. J Strength Cond Res. 2010 Feb;24(2):313-21.