Distinct Neural Information Shared in Spastic Muscle Through Decoding Motoneuron Activity

We aimed to assess whether spinal motoneurons received and sent the same neural information during involuntary and voluntary activation following stroke.

High-density surface electromyography (HD-sEMG) signals of biceps brachii muscle were recorded, while 14 stroke survivors and 10 age-matched controls performed passive stretch and active contraction. Populational motor unit (MU) activity was extracted from HD-sEMG recordings with decomposition algorithms. The MU discharge rate, discharge variability and spatial distribution of MU action potentials (MUAP) were used to detect neural drive to muscles. The cross-correlation analysis between MU discharge timings was performed to detect common synaptic input (CSI) to motoneurons.

In stroke survivors, involuntary activation exhibited higher discharge rate and lower discharge variability than voluntary activation (18.41 ± 2.05 Hz vs. 14.99 ± 1.50 Hz, p = 0.000, d = 1.392; 0.04 ± 0.02 vs. 0.12 ± 0.04, p = 0.000, d = 1.775). The opposite MUAP distribution patterns were observed between involuntary and voluntary activation (lateral-medial: 3.91 ± 1.01 vs. 4.85 ± 0.57, p = 0.000, d = 1.304; distal-proximal: 5.16 ± 0.80 vs. 4.01 ± 0.73, p = 0.001, d = 1.222). CSI was lower in involuntary activation than voluntary activation (0.42 ± 0.07 vs. 0.55 ± 0.08, p = 0.004). The discharge variability was significantly positively correlated with CSI.

Our decoding results demonstrated that information flow between supraspinal and spinal cord was unbalanced following stroke.

The determinants of MU discharge in reflex activity depend more on intrinsic properties of spinal motoneurons than on brain.