Figure 5. Effects of tizanidine on the ulnar-induced modulation of the ongoing rectified FCR EMG.

A, modulation of the ongoing rectified FCR EMG (tonic voluntary contraction of ∼10–20% of MVC, difference conditioned EMG – mean unconditioned EMG as a percentage of the mean unconditioned EMG) after ulnar nerve stimulation at wrist level (2 × MT) in one subject before (continuous line) and 90 min after oral intake (150 μg kg⁻¹) of tizanidine (dashed line). (Calculations for heteronymous monosynaptic latency: median – induced monosynaptic latency, 16 ms (H reflex); distance wrist–elbow, 0.23 m; supplementary afferent conduction time for the ulnar Ia volley, 3.3 (0.23/69) ms; ulnar-induced heteronymous Ia excitation expected at 19.3 (16 + 3.3) ms, corresponding to the latency (19.5 ms) of the early peak in (A). Vertical lines indicate the latency of heteronymous Ia excitation (thick continuous line), and the limits of the windows within which the non-monosynaptic propriospinally mediated group I excitation (dotted lines) and the late high-threshold excitation (dashed lines) were assessed. B and C, group data (5 subjects). Each thin line represents one subject, and the thick lines (and •) represent the mean values for the group. Non-monosynaptic group I (B) and late high-threshold (C) facilitations (difference: conditioned EMG – mean unconditioned EMG as a percentage of the mean unconditioned EMG) assessed within the windows corresponding to the limits between dotted and dashed lines, as in Fig. 5A are compared before (left vertical bar) and after tizanidine (right vertical bar). Mean (±1 s.e.m.) values in the group are shown beneath each corresponding vertical line.