Stable and unstable surface bench pressing
Research scientists in Norway examined the electromyographic activity of the muscles used in the bench press on both stable and unstable surfaces. They compared 6 repetition maximum loads on three different surfaces. One series on a bench press bench, another on a balance cushion and a third on a Swiss ball. Admittedly, the volunteer numbers were small, at only sixteen; however, the results showed that a more stable platform insured greater EMG activity, which relates to greater strength development.
The EMG probes monitored the biceps brachii, deltoid anterior, erector spinae, oblique external, pectoralis major, and the rectus abdominus muscles.
In relation to using the stable bench, this 6-repetition maximum was approximately 93% greater than when doing it on the balance cushion and approximately 92% greater than for the Swissball. In fact, the contribution of the pectoralis major was approximately 90% using the balance cushion and only 81% using the Swissball, triceps activity was approximate 79% use the balance cushion and only 69% using the Swissball.
In relationship to the balance cushion, the EMG activity of the pectoralis major, triceps, and erector spinae, when using the Swissball was 89% and 80% respectively. However, the activity of the rectus abdominus showed more involvement when using the Swissball when compared to both the cushion and stable bench.
The researchers concluded that the stable bench produced a greater 6 repetition maximum than was achieved with either the cushion or the stability ball.
Unless there is a specific medical reason to be doing bench presses on a cushion or stability ball you are going to get more out of it on a stable bench than if using a cushion or stability ball. If, however, you insist upon using unstable surfaces to bench on, the next best option is the cushion with the stability ball as a last resort.