Effects of transcranial magnetic stimulation on the H reflex and F wave in the hand muscles

Objective: In 14 healthy subjects, we studied the effects of transcranial magnetic stimulation (TMS) on the excitability of spinal motoneurons in the abductor pollicis brevis muscle (ABP), by testing the F wave and H reflex. Methods: TMS pulses were delivered with the subjects at rest and at various motor threshold (Mth) intensities. Electrical stimuli were delivered to the median nerve at the wrist at two different intensities. High-intensity pulse was used to evoke an F wave and low-intensity paired pulse to evoke an H reflex in the ABP muscle. The effects of TMS were studied using a conditioning–test paradigm. The tests F wave and H reflex were conditioned by TMS (120% Mth) at various interstimulus intervals (ISIs) (30–100 ms) and intensities (90–200% Mth). Results: At 30 ms but not at ISIs from 40 to 100 ms, conditioning TMS (120% Mth) significantly increased the F-wave area. At the 30 ms ISI, conditioning TMS at 120% Mth intensity significantly increased the F-wave area whereas higher intensities (140–180% Mth) did not. At 200% Mth intensity, the F-wave area decreased significantly. At 30 and 40 ms ISIs, conditioning TMS at 120% Mth significantly reduced the H-reflex area. At 50–100 ms ISIs, the H-reflex area almost matched the control value. At the 30 ms ISI, conditioning TMS at ≥100% Mth intensity significantly decreased the H-reflex area. Conclusions: In conclusion, our findings suggest that the distinct changes in the TMS-conditioned F wave and H reflex reflect changing excitability in the motoneuronal populations activated by the cortical input.