Distinct Neural Information Shared in Spastic Muscle through Decoding Motoneuron Activity.

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

Methods: 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.

Results: 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.

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

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