Short latency stretch reflex in human lumbar paraspinal muscles

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Abstract

The aim of the study was to measure stretch reflex latencies of the lumbar paraspinal muscles. An electromechanical tapping system was constructed enabling an accurate estimation of short latencies by utilizing a new technique combining results for different tapping durations. Latency parameters (onset, peak and zero-crossing of EMG signal) were obtained for the paraspinal muscles at the L3/L4 level for 10 male subjects. Detection of EMG onset, which was determined by a threshold criterion (2.5 S.D. of pre-activity), yielded 7.4 ± 1.4 ms corresponding to a physiological short latency onset of 6.5 ms, which is considerably shorter than previously reported. However, it is shown to be consistent with the expected latency value for a monosynaptic stretch reflex for the paraspinal muscles of the low back.

Introduction

The muscle spindle system and muscle reflex mechanism e.g. short and long latency stretch reflexes are involved in the maintenance of spinal stability. Stretch reflexes in upper and lower limb muscles have been investigated in several studies, but only few studies have examined the characteristics of stretch reflexes in the paraspinal muscles. Dimitrijevic et al. (1980) found a 12 ± 1.6 ms latency of the first response of the tapped lumbar paraspinal muscles (tapping at L5 level and recording at L2/L3 level) using an automatic reflex hammer; Tani et al. (1997) reported latencies measured at different levels from T6 to L5 ranging from 8.8 ± 0.7 ms to 15.9 ± 1.0 ms, respectively, using a manual reflex hammer; the longest latencies was reported by Zedka et al. (1999), 19.3 ± 2.1 ms at the L3 level using an electrodynamic reflex hammer. The latency was measured from the onset of the tapping pulse, however, because of different tapping methods, differences in the duration of the tapping pulses are expected. Zedka reported a pulse rise time of 12.5 ms, while Dimitrijevic and Tani did not report pulse duration. In order to get an accurate estimation of the latency, which is a prerequisite for an appropriate physiological interpretation, the measured latency should not be influenced by the dynamical characteristics of the tapping system. Ideally, this means that the pulse duration should be much less than the above range of reported latencies (9–19 ms). Hence, tapping techniques utilizing pulse duration in the range 5–10 ms, which normally are used for studying stretch reflexes for the muscles of the extremities with considerable longer latencies, are not ideal/appropriate for investigating stretch reflexes of the back.

The above-mentioned studies did not explore the reproducibility of the reflex latency measurement, and according to Toft et al. (1989) results on the inherent variation of the stretch reflex in human beings during fixed experimentally conditions are lacking.

Hence, the aim of the present study was to obtain latency measures characterizing the stretch reflex of the low back muscles independent on the actual dynamical characteristics of the tapping system, and further to evaluate the reproducibility of the parameters.

Section snippets

Subjects

Ten healthy males (mean ± S.D.: 35.3 ± 3.2 years, 77.6 ± 8.1 kg and 182.7 ± 6.1 cm) participated in the study. The subjects had no history of injury or indications of low-back disorders. The local ethics committee of Copenhagen approved the study. All participants gave their informed consent to their inclusion in the study.

Measurement set-up

The electromechanical tapping system consisted of a prodder and a position sensor fixed to the armature of a solenoid (Fig. 1, Fig. 2). A cap 1 cm in diameter was attached to the skin

Results

The results of the first experiment, in which the reproducibility for the three measurements (days 1, 2 and 3) of stretch reflex latency characteristics were tested, are shown in Fig. 5 and Table 1, Table 2. The ANOVA did not demonstrate any difference between days in four of the parameters. However, the zero-crossing parameter showed significant difference between days because it was more consistent on any day. The duration and depth of the taps were not significantly different between days,

Discussion

The zero-crossing parameters demonstrated better reproducibility than the onset parameters since the zero-crossing parameters showed higher ICC values and lower SEM than the onset parameters. However, the zero-crossing showed significant differences between days. Although the tap duration did not differ significantly between days, the mean tap value was a little higher on the third day (4.4 ms) than on the first to days (4.2 and 4.1 ms, respectively). When the zero-crossing parameter was adjusted

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