Neurophysiological validation of simultaneous intrinsic and reflexive joint impedance estimates

Background: People with brain or neural injuries, such as cerebral palsy or spinal cord injury, commonly have joint hyper-resistance. Diagnosis and treatment of joint hyper-resistance is challenging due to a mix of tonic and phasic contributions. The parallel-cascade (PC) system identifcation technique ofers a potential solution to disentangle
the intrinsic (tonic) and refexive (phasic) contributions to joint impedance, i.e. resistance. However, a simultaneous neurophysiological validation of both intrinsic and refexive joint impedances is lacking. This simultaneous validation is important given the mix of tonic and phasic contributions to joint hyper-resistance. Therefore, the main goal of
this paper is to perform a group-level neurophysiological validation of the PC system identifcation technique using electromyography (EMG) measurements.

Methods: Ten healthy people participated in the study. Perturbations were applied to the ankle joint to elicit refexes and allow for system identifcation. Participants completed 20 hold periods of 60 seconds, assumed to have constant joint impedance, with varying magnitudes of intrinsic and refexive joint impedances across periods. Each hold period
provided a paired data point between the PC-based estimates and neurophysiological measures, i.e. between intrinsic stifness and background EMG, and between refexive gain and refex EMG.

Results: The intrinsic paired data points, with all subjects combined, were strongly correlated, with a range of r = [0.87 0.91] in both ankle plantarfexors and dorsifexors. The refexive paired data points were moderately correlated, with r = [0.64 0.69] in the ankle plantarfexors only.

Conclusion: An agreement with the neurophysiological basis on which PC algorithms are built is necessary to support its clinical application in people with joint hyper-resistance. Our results show this agreement for the PC system identifcation technique on group-level. Consequently, these results show the validity of the use of the technique for the integrated assessment and training of people with joint hyper-resistance in clinical practice.