Alterations in H reflex magnitude by variations in baseline EMG excitability

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.