Comparison of spinal myotatic reflexes in human adults investigated with cross-correlation and signal averaging methods

A cross-correlation method for recording spinal myotatic reflexes has been developed to meet the need for brief test periods in babies and children an d subjects with central neurological pathology. In normal adult subjects th e method has been validated by comparing excitatory and inhibitory reflexes obtained with cross-correlation with those obtained with conventional sign al averaging. In the cross-correlation method a pseudo-random binary sequen ce of 64 brief tendon taps was delivered in <1.5 s, and in the averaging me thod 20-150 taps at one per second. The reflexes were expressed as unit imp ulse responses to enable direct, quantitative comparisons to be made. With cross-correlation the responses were slightly expanded in time, had lower p eak amplitudes, and onset latencies advanced by 10 ms, the clock period of the pseudo-random binary sequence. The amplitude of biceps phasic stretch r eflex increased with muscle contraction in a similar manner with both metho ds. In tests for stationarity the amplitude of biceps phasic stretch reflex varied <10% in the first six repeats of the pseudo-random binary sequence. The tap force required at threshold for cross-correlation was approximatel y half that for averaging, but with both methods the magnitude of biceps ph asic stretch reflex varied linearly with tap force over the range of one to two times threshold. The validity of responses obtained with cross-correla tion was assessed by a statistical procedure. In conclusion, the cross-corr elation method is robust and gives similar results to those obtained with a veraging. (C) 2001 Elsevier Science B.V. All rights reserved.