The effect of experimental shoulder pain on contralateral muscle force and activation.
Scott K Stackhouse, Brett A Sweitzer, Philip W McClure
PMID 31696775WHAT IT FOUND
Injecting pain into one shoulder slightly reduced strength in the opposite shoulder but did not significantly reduce muscle activation.
However, nearly half the participants showed a meaningful strength drop on the pain-free side. Clinicians should measure both shoulders, not just the injured one, when assessing progress.
Key findings
01Experimental shoulder pain caused a modest decrease in contralateral external rotation force but did not significantly reduce voluntary activation.
02A subset of participants (9 out of 20) experienced a clinically relevant decline in contralateral force of 10% or more.
03The reduction in contralateral strength was not primarily explained by a reduction in central drive (voluntary activation).
STILL TO COME
How it was doneWhat they foundWhat it means for PTsWhat it means for OTs
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What it does not show
The study used healthy volunteers with experimentally induced acute pain, which may not reflect the chronic pain or altered pain processing seen in clinical patients. There was no control group receiving a saline injection, so it is unclear if the mechanical volume of the injection contributed to the force changes. The study was underpowered to detect small changes in voluntary activation, though it did detect force changes. Only the infraspinatus muscle was assessed; other shoulder muscles may respond differently. The sample size was small (n=20), limiting the generalizability of the subset findings.
Declared interests
The authors declared no conflicts of interest. The study was supported by internal university funds.
The easy way to misread this
Do not conclude that pain-free side strength is always reliable for comparison in acute pain. While the average group effect was small, 45% of participants had a significant drop in strength on the pain-free side, meaning a single baseline measurement could be misleadingly low.