The effects of PNF on muscle electrical activity and correlation between sEMG signal and isometric muscle strength in post-stroke patients

Background Objective assessment of post-stroke muscle strength remains lacking. Objective This study explored whether Proprioceptive Neuromuscular Facilitation (PNF) improves the correlation between surface electromyography (sEMG) and muscle strength to facilitate sEMG-based evaluation in stroke patients. Methods A crossover-design experiment enrolled 10 post-stroke patients. Baseline isokinetic testing (60°/s knee flexion/extension) determined the affected side's maximum isokinetic extension torque (MET) and angle of peak torque. Two isometric knee extension tasks (separated by a 1-day washout) were performed targeting 40%, 60%, and 80% MET. PNF was randomly applied in only one trial (either the first or second), with the alternate trial serving as control. Rectus femoris sEMG and isometric maximum knee extension torque (ISMET) were recorded. Results At 40% MET, no significant correlation was observed. Without PNF, sEMG root mean square (RMS) and ISMET/body weight (ISMET/BW) showed positive correlations at 60% ( r = 0.38, P = 0.095) and negative correlation at 80% MET ( r = −0.85, P = 0.003). With PNF, correlations shifted to positive and strengthened with intensity, with significant associations at 60% ( r = 0.5, P = 0.014) and 80% MET ( r = 0.43, P = 0.044). Conclusion PNF strengthens the correlation between sEMG-derived electrical activity and torque output in post-stroke individuals (notably at moderate-to-high intensities), suggesting its potential for optimizing clinical muscle strength evaluations after central nervous system injury.