Disrupted Bilateral Coordination of Soleus Motor Units during Early Subacute Stroke Rehabilitation

Stroke can affect sensorimotor control, impairing balance and locomotion. These impairments increase the risk of falls, limit patient independence, and reduce their quality of life. In this study, we investigated how stroke affects the bilateral coordination of soleus motor units during standing, in individuals undergoing subacute rehabilitation. Fourteen participants (n=7 females; time since stroke=19{+/-}8 days; age=60.2{+/-}15.9 years) were recruited after admission for inpatient rehabilitation together with sixteen age- and sex-matched controls (n=8 females; age=61.2{+/-}14.7 years). Both groups attended the laboratory for two sessions separated by one week, during which high-density EMG signals were recorded from soleus muscles during quiet standing on force plates. Patients also performed the Berg Balance Scale in their clinical rehabilitation. To investigate the neural control of soleus muscles during quiet standing, the 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 the center of pressure (COP). The amplitude of motor unit and EMG activity around COP peaks, their directional tuning, and bilateral synchronization were estimated. Individuals post-stroke demonstrated improved scores on the Berg Balance Scale between visits, but the amplitudes of their COP displacement and speed were still greater than controls. In controls, the activity of soleus from both limbs exhibited an anteriorly-oriented tuning. In individuals post-stroke, while the activity of the soleus from the unaffected limb exhibited the same anteriorly-oriented directional tuning, the activity of the soleus from the affected limb was tuned laterally. Synchronization of soleus activities, quantified by computing the amplitude of motor unit and EMG activation in one soleus time-locked to prominent peaks in activity in the contralateral muscle, were also reduced in the affected limb compared to controls. Furthermore, cross correlation demonstrated a greater lag in motor unit and EMG activation between limbs in individuals post-stroke compared to controls. The absolute lag between limbs, measured through peristimulus time histograms, decreased between visits in individuals post-stroke (p=0.008), and this metric was the sole predictor of Berg Balance Scale with forward regression (R2=0.582, P<0.001). These results highlight the importance of bilateral motor unit coordination in balance, which is disrupted in both spatial and temporal domains following stroke.