Feasibility and repeatability of MEG-compatible patellar-tendon stimulator for eliciting knee-joint proprioceptive cortical responses

Background Cortical processing of the knee-joint proprioception is largely unknown. Magnetoencephalography (MEG) can be used to quantify the cortical processing of the proprioceptive afference, but MEG-compatible and well-controlled stimulation of the knee joint is technically challenging, and thus has received less attention. New method We introduced a novel MEG-compatible stimulator that delivers controlled patellar tendon stretches to activate muscle afferent of the knee extensors. The stimulus intensity is adjustable, allowing graded activation of proprioceptive input and, when required, elicitation of the patellar-tendon reflex. Results The novel stimulator elicited clear muscular and cortical responses in both intensity conditions. Cortical responses demonstrated moderate to excellent intersession reliability for peak evoked field amplitude (ICC: 0.69--0.96), beta suppression (0.89--0.90) and beta rebound (0.96--0.97). Notably, beta suppression peaked more laterally than expected in both hemispheres. Peak EMG amplitudes in VL and VM muscles were reliable for both intensity conditions (ICC: 0.66--0.89), and stimulus kinematics remained consistent throughout measurements. Comparison with existing methods Previous robotic or motor-driven devices have been used to evoke cortical responses to knee-joint proprioceptive stimulation, but mechanical coupling across adjacent joints may limit knee-specific input. The present stimulator provides mechanically simple and MEG-compatible alternative that targets knee extensor afferents more directly, reduces distal joint involvement. Conclusion The novel stimulator is a feasible and repeatable tool to study cortical processing of proprioceptive afference from the knee-joint using MEG. The spatially unexpected beta rhythm suppression suggests that knee-joint proprioceptive afference may involve more unique sensorimotor cortical neuronal network than previously recognized.