Beyond Psychophysics: A Physiological Marker of Proprioceptive Function After Stroke.

Proprioceptive input is essential for motor control, yet clinical assessment of proprioceptive deficits relies largely on psychophysical tests that depend on self-report, are vulnerable to cognitive interference and bias, and probe conscious aspects of sensation. In this proof-of-concept study, we propose the automatic muscular response to passive limb displacement-the placing response-as a complementary, more objective marker of proprioceptive adequacy. Using surface electromyography (EMG) from the anterior and posterior deltoid during shoulder movements, we first confirm that placing is a genuine response in young and older healthy controls and in persons with stroke (PwS). We then use these data to derive and test a simple biomechanical model that predicts temporal synergy between these muscle heads under intact versus impaired proprioception. Group-level analyses show that an EMG-based metric differentiates between paretic and non-paretic arms in PwS and between PwS and controls. At the individual level, a classifier based on this physiological metric performs comparably to a psychophysical classifier but appears less susceptible to extreme "guessing-like" behavior. These findings suggest that automatic motor reactions to passive limb displacement offer a promising avenue for more comprehensive assessment of proprioceptive adequacy after neurological injury.