Target Precision Shapes Trunk Compensation After Stroke: Neuromechanical Consequences for Upper Limb Rehabilitation
- 1Univ Rennes, M2S, F-35000 Rennes, France
Abstract
BackgroundParetic-arm nonuse and trunk compensation after stroke are often interpreted as impairment or learned behavior but may also reflect task-dependent optimization. We previously showed that gravitational shoulder effort and force reserve shape these behaviors. Here, we tested whether endpoint precision is an additional constraint.
MethodsTwenty-two individuals with stroke and 22 matched controls performed a seated Proximal Arm Nonuse reaching task toward small (2 cm) and large (12 cm) targets. Stroke participants were tested at normal arm weight and with the paretic arm mechanically lightened; controls were tested at normal arm weight and while holding a dumbbell corresponding to 85% of maximal voluntary shoulder force.
ResultsIn stroke participants, trunk compensation was 15% lower for the large target than for the small target, independently of arm weight condition (F(1,13)=8.166, p=.013, partial {superscript 2}=.39). Target size did not significantly affect trunk compensation in controls or anterior deltoid activation in either group. In stroke participants, movement time was 24% longer for the small target than for the large target (F(1,13)=8.533, p=.012, partial {superscript 2}=.40). In controls, the small target increased movement time only when the arm was weighted (F(1,18)=14.9, p=.001, partial {superscript 2}=.45), not at normal arm weight (F(1,18)=0.158, p=.695, partial {superscript 2}=.01).
ConclusionsEndpoint precision altered reaching after stroke partly independently of gravitational shoulder effort. Trunk compensation after stroke was therefore modulated by task demands rather than fixed by impairment, a pattern consistent with, although not a direct demonstration of, an optimal-control account. Rehabilitation should evaluate compensation by its functional role and the cost of suppressing it, not only by its deviation from typical kinematics, and should treat target size as an adjustable parameter of the task.
Clinical Trial: NCT04747587. https://clinicaltrials.gov/study/NCT04747587
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BibTeX
@misc{faity2026target,
title = {{Target Precision Shapes Trunk Compensation After Stroke: Neuromechanical Consequences for Upper Limb Rehabilitation}},
author = {G. Faity and D. Mottet and J. Froger and M. Delorme},
howpublished = {medRxiv},
year = {2026},
month = oct,
doi = {10.64898/2026.10.01.26364468},
url = {https://www.medrxiv.org/content/10.64898/2026.10.01.26364468v1},
note = {CC BY-NC},
}
RIS
TY - UNPB
TI - Target Precision Shapes Trunk Compensation After Stroke: Neuromechanical Consequences for Upper Limb Rehabilitation
AU - G. Faity
AU - D. Mottet
AU - J. Froger
AU - M. Delorme
PY - 2026
DA - 2026/10/01
PB - medRxiv
DO - 10.64898/2026.10.01.26364468
UR - https://www.medrxiv.org/content/10.64898/2026.10.01.26364468v1
AB - BackgroundParetic-arm nonuse and trunk compensation after stroke are often interpreted as impairment or learned behavior but may also reflect task-dependent optimization. We previously showed that gravitational shoulder effort and force reserve shape these behaviors. Here, we tested whether endpoint precision is an additional constraint. MethodsTwenty-two individuals with stroke and 22 matched controls performed a seated Proximal Arm Nonuse reaching task toward small (2 cm) and large (12 cm) targets. Stroke participants were tested at normal arm weight and with the paretic arm mechanically lightened; controls were tested at normal arm weight and while holding a dumbbell corresponding to 85% of maximal voluntary shoulder force. ResultsIn stroke participants, trunk compensation was 15% lower for the large target than for the small target, independently of arm weight condition (F(1,13)=8.166, p=.013, partial {superscript 2}=.39). Target size did not significantly affect trunk compensation in controls or anterior deltoid activation in either group. In stroke participants, movement time was 24% longer for the small target than for the large target (F(1,13)=8.533, p=.012, partial {superscript 2}=.40). In controls, the small target increased movement time only when the arm was weighted (F(1,18)=14.9, p=.001, partial {superscript 2}=.45), not at normal arm weight (F(1,18)=0.158, p=.695, partial {superscript 2}=.01). ConclusionsEndpoint precision altered reaching after stroke partly independently of gravitational shoulder effort. Trunk compensation after stroke was therefore modulated by task demands rather than fixed by impairment, a pattern consistent with, although not a direct demonstration of, an optimal-control account. Rehabilitation should evaluate compensation by its functional role and the cost of suppressing it, not only by its deviation from typical kinematics, and should treat target size as an adjustable parameter of the task. Clinical Trial: NCT04747587. https://clinicaltrials.gov/study/NCT04747587
ER -