The effect of experimental shoulder pain on contralateral muscle force and activation

ABSTRACT

Inhibition of rotator cuff activation and force after local experimental pain has been previously shown. Clinically, strength is often indexed to the uninvolved side in order to quantify deficits during injury and recovery. This study assessed the effect of experimental subacromial pain on contralateral shoulder external rotation (ER) force and activation. We hypothesized that subacromial pain would not affect contralateral shoulder external rotation force and voluntary activation (VA) of the contralateral infraspinatus because we believe that the effects of acute experimental pain will largely exert an ipsilateral, spinal segmental effect. Twenty healthy adults were tested. External rotation force and VA were tested while participants performed maximum voluntary isometric contractions of shoulder external rotation, during which a brief electrical stimulus was applied to the infraspinatus muscle at an intensity that maximized external rotation force. To elicit pain, 1.5 ml of 5% hypertonic saline was injected into the contralateral subacromial space. Mean (SD) pain immediately after injection was 6.3/10 (0.85) resulting in a 7.6% decline in contralateral external rotation force (p < .01) and 3.3% decline in infraspinatus muscle VA (p = .48). A subset of participants (n = 9) showed a more substantial decline in both force (15.4%; p < .000001; d = −3.53) and VA (8.7%; p = .045; d = −0.98). Experimental shoulder pain had a modest effect on contralateral ER force and VA in a subset of participants; therefore, it may be important to index or compare strength bilaterally throughout rehabilitation to capture this bilateral effect as pain resolves.

KEYWORDS:

• Force
• voluntary activation
• experimental pain
• shoulder

Acknowledgments

The authors would like to acknowledge the following people for assistance with recruitment and data collection: Kelly Frederick DPT, Elizabeth Leeman DPT, Meghan Serino DPT, Amanda Sheiber DPT, and Holly Taylor DPT.

Declaration of interest

The authors declare no conflict of interest.

Additional information

Funding

Dr. Stackhouse's time was partially supported by funding from NIGMS 5P20GM103643.