title: A frequency and pulse-width co-modulation strategy for transcutaneous neuromuscular electrical stimulation based on sEMG time-domain features.
abstract: OBJECTIVE
  Surface electromyography (sEMG) is often used as a control signal in neuromuscular electrical stimulation (NMES) systems to enhance the voluntary control and proprioceptive sensory feedback of paralyzed patients. Most sEMG-controlled NMES systems use the envelope of the sEMG signal to modulate the stimulation intensity (current amplitude or pulse width) with a constant frequency. The aims of this study were to develop a strategy that co-modulates frequency and pulse width based on features of the sEMG signal and to investigate the torque-reproduction performance and the level of fatigue resistance achieved with our strategy.
  
  
  METHODS
  We examined the relationships between wrist torque and two stimulation parameters (frequency and pulse width) and between wrist torque and two sEMG time-domain features (mean absolute value (MAV) and number of slope sign changes (NSS)) in eight healthy volunteers. By using wrist torque as an intermediate variable, customized and generalized transfer functions were constructed to convert the two features of the sEMG signal into the two stimulation parameters, thereby establishing a MAV/NSS dual-coding (MNDC) algorithm. Wrist torque reproduction performance was assessed by comparing the torque generated by the algorithms with that originally recorded during voluntary contractions. Muscle fatigue was assessed by measuring the decline percentage of the peak torque and by comparing the torque time integral of the response to test stimulation trains before and after fatigue sessions.
  
  
  RESULTS
  The MNDC approach could produce a wrist torque that closely matched the voluntary wrist torque. In addition, a smaller decay in the wrist torque was observed after the MNDC-coded fatigue stimulation was applied than after stimulation using pulse-width modulation alone.
  
  
  CONCLUSIONS
  Compared with pulse-width modulation stimulation strategies that are based on sEMG detection, the MNDC strategy is more effective for both voluntary muscle force reproduction and muscle fatigue reduction.
authors.affiliation: State Key Lab of Bioelectronics, Southeast University, 210096 Nanjing, People's Republic of China.
authors.forename: Yu-Xuan
authors.initials: YX
authors.lastname: Zhou
authors.forename: Hai-Peng
authors.initials: HP
authors.lastname: Wang
authors.forename: Xue-Liang
authors.initials: XL
authors.lastname: Bao
authors.forename: Xiao-Ying
authors.initials: XY
authors.lastname: Lü
authors.forename: Zhi-Gong
authors.initials: ZG
authors.lastname: Wang
mesh_terms: D000328:Adult; D000465:Algorithms; D004576:Electromyography / Q000379:methods*; D025461:Feedback, Physiological / Q000502:physiology; D005260:Female; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology*; D008297:Male; D018482:Muscle, Skeletal / Q000294:innervation / Q000502:physiology*; D010363:Pattern Recognition, Automated; D010525:Peripheral Nerves / Q000502:physiology; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D012815:Signal Processing, Computer-Assisted; D013314:Stress, Mechanical; D004561:Transcutaneous Electric Nerve Stimulation / Q000379:methods*; D014955:Wrist Joint / Q000502:physiology; D055815:Young Adult