title: Contributions to muscle force and EMG by combined neural excitation and electrical stimulation.
abstract: OBJECTIVE
  Stimulation of muscle for research or clinical interventions is often superimposed on ongoing physiological activity without a quantitative understanding of the impact of the stimulation on the net muscle activity and the physiological response. Experimental studies show that total force during stimulation is less than the sum of the isolated voluntary and stimulated forces, but the occlusion mechanism is not understood.
  
  
  METHODS
  We develop a model of efferent motor activity elicited by superimposing stimulation during a physiologically activated contraction. The model combines action potential interactions due to collision block, source resetting, and refractory periods with previously published models of physiological motor unit recruitment, rate modulation, force production, and EMG generation in human first dorsal interosseous muscle to investigate the mechanisms and effectiveness of stimulation on the net muscle force and EMG.
  
  
  RESULTS
  Stimulation during a physiological contraction demonstrates partial occlusion of force and the neural component of the EMG, due to action potential interactions in motor units activated by both sources. Depending on neural and stimulation firing rates as well as on force-frequency properties, individual motor unit forces can be greater, smaller, or unchanged by the stimulation. In contrast, voluntary motor unit EMG potentials in simultaneously stimulated motor units show progressive occlusion with increasing stimulus rate. The simulations predict that occlusion would be decreased by a reverse stimulation recruitment order.
  
  
  CONCLUSIONS
  The results are consistent with and provide a mechanistic interpretation of previously published experimental evidence of force occlusion. The models also predict two effects that have not been reported previously--voluntary EMG occlusion and the advantages of a proximal stimulation site. This study provides a basis for the rational design of both future experiments and clinical neuroprosthetic interventions involving either motor or sensory stimulation.
authors.affiliation: Department of Biomedical Engineering, Case Western Reserve University, Cleveland, OH 44106, USA. Cleveland Functional Electrical Stimulation (FES) Center, Cleveland, OH 44106 USA.
authors.forename: Patrick E
authors.initials: PE
authors.lastname: Crago
authors.forename: Nathaniel S
authors.initials: NS
authors.lastname: Makowski
authors.forename: Natalie M
authors.initials: NM
authors.lastname: Cole
mesh_terms: D000200:Action Potentials / Q000502:physiology*; D003198:Computer Simulation; D004558:Electric Stimulation / Q000379:methods*; D004576:Electromyography / Q000379:methods*; D006801:Humans; D008954:Models, Biological; D009119:Muscle Contraction / Q000502:physiology*; D053580:Muscle Strength / Q000502:physiology*; D018482:Muscle, Skeletal / Q000294:innervation / Q000502:physiology*; D010525:Peripheral Nerves / Q000502:physiology*; D013314:Stress, Mechanical