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

BACKGROUND: Cortical processing of proprioceptive-dominant afference arising from the knee joint 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 taps and elicits proprioceptive-dominant afferent input to the cortex in the knee joint. 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 a mechanically simple and MEG-compatible tool that targets proprioceptive-dominant afferents of the knee joint more directly, reduces distal joint involvement. CONCLUSION: The novel stimulator is a feasible and repeatable tool for studying cortical processing of proprioceptive afference from the knee joint using MEG.