Cocontraction measured with short-range stiffness was higher in obstetric brachial plexus lesions patients compared to healthy subjects.

We suggest short range stiffness (SRS) at the elbow joint as an alternative diagnostic for EMG to assess cocontraction. Elbow SRS is compared between obstetric brachial plexus lesion (OBPL) patients and healthy subjects (cross-sectional study design). Seven controls (median 28years) and five patients (median 31years) isometrically flexed and extended the elbow at rest and three additional torques [2.1,4.3,6.4Nm] while a fast stretch stimulus was applied. SRS was estimated in silico using a neuromechanical elbow model simulating the torque response from the imposed elbow angle. SRS was higher in patients (250±36Nm/rad) than in controls (150±21Nm/rad, p=0.014), except for the rest condition. Higher elbow SRS suggested greater cocontraction in patients compared to controls. SRS is a promising mechanical alternative to assess cocontraction, which is a frequently encountered clinical problem in OBPL due to axonal misrouting.
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