Influence of Pain on Rotator Cuff Muscle Size and Function: A Case-Control Study with Ultrasound Assessment of Cross-Sectional Area and Tendon Thickness.
Dirk Kokmeyer, Lori Thein Brody, Jace Brown and 2 others
Rotator cuff muscle size was unchanged in people with shoulder pain and intact tendons.
Strength deficits tracked with pain during testing, and endurance deficits persisted beyond pain, consistent with inhibition rather than atrophy as the mechanism of weakness.
Key findings
1Supraspinatus and infraspinatus cross-sectional area and supraspinatus tendon thickness did not differ between painful and asymptomatic shoulders, or between dominant and non-dominant sides (all p > 0.19 for CSA; p > 0.86 for tendon thickness).
2The painful dominant shoulder was significantly weaker than the contralateral side in both external rotation (mean difference 1.7 kg, p = 0.003) and full can (p = 0.003), and weaker than the control dominant side in full can (p = 0.035). Painful dominant shoulders also completed roughly 13 fewer PSET repetitions than the contralateral side and roughly 17 fewer than the control dominant side (both p < 0.05).
3When pain during testing was entered as a covariate, it explained 36% of the variance in external rotation strength and eliminated the dominance effect, and it removed the group-by-dominance interaction for full can strength. For PSET endurance, pain significantly influenced performance (p = 0.038) but between-group differences persisted, indicating additional contributors beyond pain.
Still to come
How it was doneWhat they foundWhat it means for PTs
Read the rest of this summary
You get three full summaries a month, free, and we do not ask for a card. Search, the TL;DRs and your library stay unlimited either way.
What it does not show
The study was not powered for between-group comparisons; the original power analysis was for correlation analyses, so group differences may be underpowered to detect small-to-moderate effects. All participants were right-hand dominant and all pain was right-sided, limiting generalisability to left-sided presentations. The pain group had moderate tissue irritability and retained functional use of the shoulder; more severe or prolonged pain states may produce different findings, including measurable atrophy. Controls were a convenience sample recruited via hospital intranet and clinic patients, which may not represent the general population. Three participants in the pain group could not complete the endurance test, reducing the analyzable sample for that outcome to 9 and potentially underestimating the deficit. Handheld dynamometry measures applied force, not joint torque, and limb length was not measured, so moment-arm differences between participants could affect strength comparisons. The tester was not blinded to group allocation, and participants in the pain group were not specifically screened to exclude cervical radiculopathy or scapular dysfunction as alternative pain sources.
Declared interests
The authors declare no conflicts of interest. The research was partially funded by a mentored research grant from the Maine Medical Center Research Institute.
The easy way to misread this
Do not read the strength and endurance deficits as evidence of muscle atrophy requiring a prolonged hypertrophy programme. Ultrasound showed normal cross-sectional area in every participant, and the between-group strength differences disappeared when pain during testing was entered as a covariate. The authors interpret this as pain-mediated inhibition, which they suggest responds to pain management and motor retraining rather than progressive loading alone. Conversely, do not read the unchanged muscle size as evidence the shoulder is structurally fine; these patients had significant functional deficits and heterogeneous MRI findings.
Summarised by AI from the full paper, without a clinician reviewing it. Check it against the source before it changes what you do. Read it on PubMed →