Determination of excitability in human proprioceptive reflexes: analysis and characteristics of EMG thresholds of postural muscle.

In order to derive indirect indices of reflex excitability, human soleus (Sol) H reflex and direct M response threshold intensities (HT, MT) were determined by probit analysis of quantal responses elicited in the surface electromyogram (EMG). Within sessions, HT and MT co-varied with respect to time along a logarithmic time-course, both attaining effectively stable levels after 40-60 min of recording. The pattern of this co-variation was imposed by non-reflex factors, primarily the electrode system employed. Sampling distributions of stable thresholds were positively skewed (median values: HT, 5.20 mA, MT, 6.83 mA), and concurrently determined values were strongly correlated (r = 0.965). The Sol threshold ratio (HT/MT) was largely unaffected by non-reflex factors and was therefore stable within subjects, both within sessions (with respect to time) and between sessions. This ratio was normally distributed (mean = 0.753 +/- 0.079 S.D.). Criteria of indirect indices of reflex excitability are proposed: HT satisfies the criteria applicable to a relative index. However, the characteristics of the threshold ratio permit its use as a powerful absolute index of proprioceptive reflex excitability.