Coupling of shoulder joint torques in individuals with chronic stroke mirrors controls, with additional non-load-dependent negative effects in a combined-torque task.
Joseph V Kopke, Levi J Hargrove, Michael D Ellis
PMID 34496876WHAT IT FOUND
No stroke-specific difference appeared in how shoulder rotation changed with load.
Stroke arms were weaker and produced less torque overall, possibly because of background muscle tone rather than abnormal synergy.
Key findings
01The effect of load on shoulder rotation strength did not differ between stroke arms and control arms.
02Paretic arms generated less torque after each arm's maximum was accounted for than control and non-paretic arms in all four combined-torque tasks.
03Combined average paretic external rotation torque was 7.5 Nm and internal rotation torque was 10.1 Nm, compared with 28.8 Nm and 25.1 Nm in controls.
STILL TO COME
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What it does not show
Only 12 stroke participants were analysed after 2 exclusions, and the sample was limited to chronic stroke with moderate to severe upper-extremity impairment. The study did not test loads above 50% of maximal abduction or adduction strength, which the authors note may maximize synergy expression. The statistical models had large within-group variance and may have been underpowered to detect a group-by-load interaction. The OpenSim model was built from male anatomy and assumes intact motor control, cognition, and vision, so it may not fully represent stroke performance. Raw strength was averaged by sex because muscle mass was not measured, and the model did not adjust bone size or moment arms.
Declared interests
Funding came from the National Institute of Biomedical Imaging and Bioengineering and the National Institute of Child Health and Human Development. The supplied text lists only these funders and no other conflict statements.
The easy way to misread this
Do not conclude that abnormal shoulder synergy explains the stroke-arm weakness. The change with load was not different between stroke and control arms, so the study does not show a stroke-specific synergy effect.