Introduction
Survivors of Polio demonstrate collateral reinnervation, with motor units up to ten times normal amplitude as measured using electromyography (EMG). Previous investigations using scanning EMG have shown normal motor unit transect length but with “silent regions” suggesting increased motor unit complexity.
Motor Unit Magnetic Resonance Imaging (MUMRI) pairs MR imaging with electrical stimulation to visualise individual motor units. We aimed to image the motor units of patients with previous Polio to assess their dimensions and shape complexity.

Methods
Patients were recruited from the British Polio Fellowship with a self-identified history of Polio. They attended for medical history & examination of lower limb power, as well as measures of fatigue (Fatigue Severity Score). MUMRI was performed in Tibialis Anterior. The motor units were identified on imaging and the average motor unit profile was measured by way of maximum Feret diameter, cross-sectional area, and complexity as computed using the Hausdorff box-counting method. The results were compared to healthy age-matched controls.

Results
Eight participants were included for analysis and compared to 50 age-matched controls. The mean motor unit maximum Feret diameter was 11.18mm compared to 12.4mm in healthy age-matched controls (p=0.338). The mean motor unit shape complexity measure was 0.64 (SD 0.11) compared to 0.52 (SD 0.127) (p=0.015).

Discussion
Survivors of Polio demonstrate motor units with normal dimensions but increased shape complexity, suggesting incomplete collateral reinnervation. This technique provides a non-invasive method of imaging motor units in neurogenic diseases such as Polio and provides insights into motor unit remodelling which could be applied in studies of post-Polio syndrome.