Feasibility of a novel MEG-compatible proprioceptive stimulator of the knee joint

Abstract

Introduction: Proprioceptive afference refers to the transmission of information about body’s position and movements from proprioceptors to the brain. This information is crucial for sensorimotor control, with muscle stretch reflexes, such as the patellar tendon reflex, playing essential roles. Magnetoencephalography (MEG) provides a non-invasive means to examine cortical processing of the proprioceptive afference to peripheral stimuli. While evoked cortical responses have been investigated for most sensory domains, a gap exists in lower-limb proprioception research, particularly at the knee joint. In addition, the MEG environment requires special attention from the stimulation devices used, since stimulators must be designed to avoid generating interference with magnetic signals. Therefore, the objective for this study was to test the feasibility of a MEG-compatible proprioceptive stimulator (reflex hammer) of the knee joint and to provide preliminary insights into the cortical processing of proprioceptive afference of the patellar tendon reflex.

Methods: 15 healthy volunteers participated in the cross-sectional study (28.1 ± 5.3 years, 6 females). The patellar tendon of the dominant leg (Waterloo footedness questionnaire, 14 right-footed) was stimulated at two different intensities, adjusted by altering the height from which the reflex hammer was dropped (high and low measurements). The kinematics of the stimuli were examined with an accelerometer attached to the stimulator. Brain responses following the stimulations were imaged with MEG. Muscle activity resulting from the patellar tendon reflex was recorded using surface electromyography (EMG), and force responses of the knee extensor muscles (vastus lateralis and vastus medialis) were measured with a force transducer.

Results: Four participants were excluded from the final analyses due to disturbances in the MEG or EMG signals. Thus, clear reflex and brain responses were successfully recorded from 11 participants (73%). Cortical responses were localized to the MEG sensors over the somatosensory cortex, as hypothesized. The peak magnitudes of the stimuli remained consistent throughout the measurements, as assessed by the group-level coefficient of variation (high: 11% and low: 10%). When comparing the results of high and low measurements, a significant change was only observed in the amplitudes of MEG responses (high: 203 ± 57 fT/cm and low: 174 ± 39 fT/cm, p=0.02). There was less group-level variation in the MEG responses (high: 28% and low: 22%) compared to the reflex responses (≥68%). The MEG responses did not correlate with the EMG or force responses (p>0.05).

Conclusions: This study is the first to examine the proprioceptive afference of the patellar tendon reflex in the cortex. The proprioceptive stimulator appears to be feasible for research purposes, as it evoked measurable responses at both the muscle and brain levels. The MEG responses exhibited less variability than muscle-level responses, and the effect of stimulus intensity was observed only in the MEG responses. Although the study provides preliminary results on the topic, MEG appears to be a suitable method for investigating the proprioceptive afference of stretch reflexes.