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.