Reflexes evoked in human erector spinae muscles by tapping during voluntary activity

We studied the stretch reflexes, an early R1 and a late R2, by tapping the voluntarily contracted erector spinae muscles and recording from the same spinal level with the subject in the prone position. The onset latencies increased progressively towards the caudal level from 8.8 +/- 0.7 ms at T5-6 to 15.9 +/- 1.1 ms at L4-5 for R1, and from 33.3 +/- 2.7 ms at T5-6 to 49.1 +/- 2.8 ms at L4-5 for R2. The latency changed significantly (P < 0.05) between two adjacent segments from T5-6/T6-7 through L1-2/L2-3 for R1 and T5-6/T6-7 through L3-4/L4-5 for R2. When recorded remote from the stimulus site, R1, considered segmental in origin, showed, as expected, only a small latency change consistent with the time required for the mechanical event to propagate to the recording site. In contrast R2 was shorter in latency with more rostral stimulation irrespective of the distance to the recording sites. This finding implies a centripetal propagation of the afferent impulse along the central pathway, which shortens with more rostral site of stimulation. Of the two components, the more reproducible R1 has a potential diagnostic value for segmental evaluation of thoracic nerve root compression and truncal neuropathies.