Basic science
Experimental pain inhibits infraspinatus activation during isometric external rotation

https://doi.org/10.1016/j.jse.2012.05.037Get rights and content

Background

The effect of pain on muscle activation is poorly understood. This study examined the effects of acute experimental pain on rotator cuff muscle force and voluntary activation (VA). We hypothesized that acute subacromial pain would cause inhibition of infraspinatus VA with a corresponding decrease in external rotation force.

Materials and methods

Seventeen healthy adults with no known shoulder pathology were tested. Isolated external rotation force was tested on a dynamometer. Participants performed 2 baseline maximum voluntary isometric contractions of external rotation, during which maximal electrical stimulation was used to assess VA. To elicit pain, 1.5 mL 5% hypertonic saline was injected into the subacromial space, and testing of maximum voluntary isometric contractions force and VA was repeated 3 times at 5-minute intervals.

Results

Mean ± standard deviation initial pain from the injection was 6.6 ± 1.3 points of 10 possible and produced a 32.8% decline in force and a 22.7% decline in VA (P < .05). Pain diminished over a 10-minute period. As pain resolved, force and VA improved (P < .0125). There was a strong relationship between force and VA (r2 = 0.78, P < .05) and a moderate relationship between pain and VA (r2 = 0.31, P < .05).

Conclusions

Experimental subacromial pain elicits a decline in force and VA of the infraspinatus. Although this study only examines acute experimental pain, it supports the concept that pain affects rotator cuff muscle recruitment and function, which may contribute to abnormal shoulder mechanics in patients with rotator cuff pathology.

Section snippets

Materials and methods

All participants in this study reviewed and signed informed consent forms that were approved by Arcadia University's Institutional Review Board.

Results

All injections produced dull or diffuse pain in the anterior and lateral regions of the upper arm. The pain in 4 of 17 participants was also perceived into the forearm (Fig. 2, A). Complete data were obtained on all but 1 participant. This person had a syncopal episode approximately 30 seconds after the injection, which resolved quickly after immediate removal from the chair and lying down. Upon further questioning, we found that this person had a history of syncope and claustrophobia, which we

Discussion

In this study, acute experimental subacromial pain decreased VA of the infraspinatus and subsequently reduced maximum voluntary external rotation force. We recognize that this model of acute pain may be different than the clinical pain related to rotator cuff pathologies. Studies examining clinical populations with shoulder pain associated with rotator cuff pathology indicate that the average pain intensity experienced was rated 4 of 10 on a NPRS16, 20 and approximately 7 of 10 with movement.16

Conclusion

This study supports the concept that acute subacromial pain affects rotator cuff muscle recruitment and function, which may contribute to abnormal shoulder mechanics in patients with rotator cuff pathology. External rotation force and VA of the infraspinatus are closely related, and both are inhibited by subacromial shoulder pain.

Disclaimer

Material support for data collections was provided, in part, by the Arcadia University Stacy Anne Vitetta '82 Professorship to Scott K. Stackhouse.

The authors, their immediate families, and any research foundations with which they are affiliated have not received any financial payments or other benefits from any commercial entity related to the subject of this article.

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    This study was approved on January 18, 2010, by Arcadia University's Committee on the Protection of Research Subjects, Protocol No.: 10-01-02.

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