GriFT: A Device for Quantifying Physiological and Pathological Mirror Movements in Children

Goal: Mirror movements (MM) occur during unilateral actions and manifest as involuntary muscle activity of the passive limb, “mirroring” voluntary actions executed by the contralateral homologous body part. They are a normal motor feature in young children that gradually disappears. In children suffering from neurological disorders, e.g., unilateral cerebral palsy, MMs have been proposed to yield relevant information for diagnosis and therapy. However, in clinical practice, MM are typically assessed using an ordinal rating scale. Here, we introduce the grip force tracking (GriFT) device, a portable system to quantitatively assess MM during repetitive unimanual squeezing while playing a computer game. Methods: The GriFT device consists of two handles, each equipped with two compressive force sensors (range 0-23 kg, Fz 1000 Hz). Children complete three trials of unimanual squeezing, whereby the visual display on the screen determines the squeezing rhythm (0.67 Hz at 15% maximum voluntary contraction, force-level adjusted per hand). MMs are characterized based on frequency, amplitude, and temporal features (synchronization, timing). Results: MM differed significantly between children with different clinical MM scores. MM frequency and amplitude were most discriminative. Categorization of physiological MM proved highly sensitive (89%-97%). Conclusion: We demonstrated feasibility and validity of the GriFT device in a large cohort of typically developing children (N = 174, age 5-15 years), and its clinical applicability in children with unilateral cerebral palsy with various levels of hand function. Significance: The quantification of MM is a promising tool to further investigate and categorize MM in children with unilateral cerebral palsy.

I. Introduction

Mirror movements (MM) are involuntary movements of one body part that mirror the voluntary movements of the contralateral homologous body part [1]. They are mainly observed during hand movements, are symmetrical by nature and their intensity increases with increasing task complexity, fatigue, or decreased attention [2], [3]. Physiological MM are present in typically developing children during early childhood, decrease vastly between ages 5 and 8 years, and disappear after age 10 years [2], [4]. These MM have been attributed to the incomplete maturation of the corpus callosum in young children, which results in less effective interhemispheric inhibition and thus a bilateral activation of motor cortices during unimanual tasks [4]– [6]. Further maturation of the transcallosal pathways with age ensures increasing inhibition of the ipsilateral motor cortex, with a concurrent reduction of the occurrence of MM [5]. Any alteration or dysfunction at the level of the brain due to e.g., neurological disorders may give rise to pathological MM, as seen in unilateral cerebral palsy [7], X-linked Kallmann's syndrome [8] , Parkinson [9] and stroke [10] . Therefore, MM assessments are increasingly performed for diagnosing neurological alterations [10]–[13]. However, previously reported results regarding the value and impact of MM for clinical decision making are inconclusive. These discrepancies might in part be attributed to current ways of assessing MM.