The Kinematics of 3D Arm Movements in Sub-Acute Stroke: Impaired Inter-Joint Coordination is Attributable to Both Weakness and Flexor Synergy Intrusion.
Inbar Avni, Ahmet Arac, Reut Binyamin-Netser and 3 others
PMID 39113590WHAT IT FOUND
In sub-acute stroke, impaired arm coordination is driven by both weakness and flexor synergy intrusion.
Weakness alone can lower Fugl-Meyer scores without synergy presence, meaning clinical scales may reflect distinct underlying deficits depending on the patient.
Key findings
01Flexor synergy intrusion was detected during reaching tasks requiring elbow extension, but not during cup-to-mouth tasks requiring elbow flexion.
02Regression analysis showed that both weakness and synergy intrusion significantly predicted kinematic performance and clinical scores (FMA and ARAT).
03Participants with severe FMA impairment all had significant weakness, but only most had synergy intrusion, indicating weakness can drive low scores independently of synergies.
STILL TO COME
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What it does not show
The cup-to-mouth task required less shoulder flexion than the reaching task, which may have influenced the detection of synergy differences. The markerless motion capture system was not validated specifically on stroke populations, though it was validated on healthy adults. Spasticity was measured using the Modified Ashworth Scale, which may not capture subtle spastic contributions; more refined measures like tonic stretch reflex threshold were not used. The study did not use EMG, so the neural origin of the observed kinematic synergies could not be confirmed. The sample size was moderate (33 stroke participants), and the cross-sectional design limits causal inference about recovery trajectories.
Declared interests
The authors declared no potential conflicts of interest. The work was funded by the United States-Israel Binational Science Foundation.
The easy way to misread this
Do not interpret low Fugl-Meyer scores as definitive evidence of flexor synergy intrusion. The study shows that weakness alone can produce low scores in the absence of synergy, so clinical assessment must consider both factors separately.