Disrupted bilateral co-ordination of soleus motor units during early subacute stroke rehabilitation

Stroke can affect sensorimotor control, impairing balance and locomotion. These impairments increase fall risk, limit patient independence and reduce quality of life. Here, we investigated how stroke affects bilateral co-ordination of soleus motor units during standing in individuals undergoing subacute rehabilitation. Fourteen individuals were recruited after admission for inpatient rehabilitation with sixteen age- and sex-matched controls. Both groups attended two sessions separated by 1 week, during which high-density electromyography (EMG) signals were recorded from soleus muscles when standing on force plates. Patients also performed the Berg Balance Scale (BBS). EMG signals were decomposed into motor unit spike trains, from which peristimulus time histograms and EMG waveform averages were computed in relation to peaks in centre of pressure (COP). The amplitude of motor unit and EMG activity around COP peaks, directional tuning and bilateral synchronization were estimated. Individuals post-stroke improved their BBS score between visits, but COP displacement and speed were still greater than controls. Soleus activity was tuned anteriorly in both control and non-paretic limbs, whereas it was tuned laterally in paretic limbs. Synchronization of soleus activity was also reduced in the paretic limb compared to controls. Furthermore, cross-correlation lag between limbs was greater in individuals post-stroke than in controls at visit 1, whereas it decreased to control levels at visit 2 (P = 0.008). The decrease in lags was strongly correlated with improvement in BBS score (R = -0.94, P < 0.001). These results highlight the importance of bilateral motor unit co-ordination in balance, which is disrupted in both spatial and temporal domains following stroke. KEY POINTS: We examined bilateral motor unit co-ordination during quiet standing in individuals in the early subacute phase following unilateral stroke, whereas previous works have focused on the chronic phase post-stroke. Although the non-paretic limb of patients exhibited a spatial tuning of its activity oriented in the same direction as those of neurologically intact controls, the spatial tuning of the activity of the paretic limb was altered. Bilateral synchronization between soleus motor units was reduced in individuals post-stroke in comparison with neurologically intact controls. The level of bilateral synchronization between soleus motor units was associated with balance function and improved after one week of inpatient rehabilitation.