In vivo 3D imaging of human motor units in upper and lower limb muscles.

OBJECTIVE:To assess in-vivo cross-sectional and 3D morphology of human motor units in hand, forearm and lower leg muscles using magnetic resonance imaging (MRI). METHODS:Diffusion weighted MRI was used with in-scanner electrical stimulation in healthy controls to image motor units at a single slice in lower leg, forearm and hand muscles (n = 6) and multiple slices in the lower leg for 3D assessment (n = 7). RESULTS:Motor unit cross-sectional area (CSA) and maximum Feret diameter (FDmax) did not differ between the lower leg (CSA: 22.4 ± 8.4 mm2; FDmax: 8.7 ± 2.4 mm), forearm (CSA: 23.6 ± 14.1 mm2; FDmax: 9.0 ± 3.3 mm) and hand (CSA: 26.8 ± 12.8 mm2 and FDmax: 9.6 ± 2.7 mm) (ANOVA; p = 0.487 and p = 0.587, respectively). Lower leg motor units were 8.0 ± 3.8 cm long with largest CSA in the motor unit's middle section. 3D motor unit imaging revealed a complex structure with several units splitting and re-forming along their length. CONCLUSIONS:Motor unit MRI (MUMRI) can be applied to upper limb muscles, and can reveal the 3D structure of human motor units in-vivo. SIGNIFICANCE:MUMRI provides the first in-vivo 2D images of upper limb motor units and 3D images of lower leg motor units. 3D imaging suggest a more complex human motor unit structure than previously thought.