An Electromyographical Analysis of Shoulder and Trunk Muscles During Chest Press on Stable and Unstable Platforms




McHardy, Rory D.

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The use of unstable surfaces during resistance training has become increasingly popular. The majority of research examining the effect of replacing a bench with a stability ball during the supine chest press has found that force output capability is compromised without a subsequent decrease in agonist muscle activation. It has been hypothesized that this discrepancy is caused by a co-contraction mechanism at the glenohumeral joint, favoring joint stability over force production. The purpose of this study was to examine the effect of instability on agonist, antagonist, and core musculature during the supine chest press. Twenty-seven healthy male subjects performed isometric chest press at maximal voluntary contraction (MVC), 75% MVC, and 50% MVC while mean and peak electromyographic (EMG) output and mean force output of various muscles were measured. The unstable condition produced a significant decrease in MVC force output and agonist EMG activation at MVC and submaximal intensities. A significant increase in core activation was present on the stability ball during submaximal intensities. No significant changes were present in core EMG activation at MVC or antagonist EMG activation at MVC and submaximal intensities. The only exception was peak EMG activation of the infraspinatus, which showed a significant increase during 75% MVC trials on the stability ball. A significant co-contraction may not be present at the shoulder during the supine chest press, which was the primary hypothesis for the decrease in force output during unstable trials in previous studies. Further, using a stability ball in place of a stable bench during chest press may compromise strength gains expected during high-intensity resistance training without providing any additional benefit to the core musculature.



EMG, Unstable platforms, Resistance training, MVC, Isometric


McHardy, R. D. (2013). An electromyographical analysis of shoulder and trunk muscles during chest press on stable and unstable platforms</i> (Unpublished thesis). Texas State University, San Marcos, Texas.


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