Alterations in H reflex magnitude by variations in baseline EMG excitability

Electroencephalogr Clin Neurophysiol. 1985 Jun;60(6):492-9. doi: 10.1016/0013-4694(85)91109-5.


The effect of systematic variations in the baseline level of electromyographic activity of the flexor carpi radialis muscle on the magnitude of its H reflex was studied in 4 normal subjects. H reflex magnitude increases with variations in the baseline EMG activity corresponding to a range of 0.1-7.4% of the motoneuron pool as estimated by the maximum M response (Mmax) to peripheral nerve stimulation. Variations in H reflex magnitude with changes in baseline EMG activity were evident for all median nerve stimulus voltages examined ranging from 29% to 72% of that necessary to evoke an Mmax. H reflex magnitude increased with increases in baseline EMG activity, or net tension which provided an alternate estimate of pre-existing motoneuron activity. The greatest increase occurred when the H/M ratio was less than 50%, where the H reflex magnitude was found to increase by as much as 400%. The research establishes that baseline EMG activity and/or the net force being exerted by the muscles must be controlled and measured at different H/M ratios in order to interpret unambiguously studies using single stimuli to construct H-M recruitment curves or paired stimuli to evaluate changes in reflex excitability.

Publication types

  • Clinical Trial
  • Randomized Controlled Trial
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • Electromyography*
  • Extremities / physiology
  • H-Reflex*
  • Hemiplegia / physiopathology
  • Humans
  • Muscle Contraction*
  • Reflex, Monosynaptic*