The Impact of Rectification on the Electrically Evoked Long-Latency Reflex of the Biceps Brachii Muscle

Introduction: For long latency reflex (LLR) recordings full wave rectification (FWR) is recommended [1], although its usefulness has not been proven.

Objectives: (1) To investigate the impact of FWR on amplitude values of synthetic signals and original bioelectric signals (LLR). (2) To find out whether FWR is a necessary step before averaging LLR responses.

Materials & Methods: LLR were elicited electrically and recorded simultaneously with FWR and without in 10 healthy subjects. After single or train stimuli (sensory radial nerve; interstimulus interval 3 ms), amplitude values were measured over the bent biceps brachii (BB) muscle, either without or with 1.5 kg weight load. Furthermore, average amplitude values of rectified and non-rectified curves were studied using 11 sine waves or damped sine waves with equal phase intervals that were varied from 0 – 35 °.

Results: After FWR, mean LLR amplitude values made up 30% of the non rectified data, independent from the stimulus strength and weight load. In the non rectified data, a significant gain in amplitude resulted from train stimuli compared with single stimuli, and from weight load compared with no weight load (Fig. 1). No such significant difference was detected when rectified data were analysed. In studies using simulated rectified sine and damped sine curves, phase shifts ranging from 10 – 25 ° resulted in excess amplitude decline as compared with the half of the non rectified amplitude values (Fig. 2).

Conclusion: The long and polysynaptic course that LLR information takes is associated with considerable overlap of responses to subsequent stimuli. This overlap of motor unit potentials forming the LLR obviously results in excess amplitude cancellation after rectification as shown for sine and damped sine waves. Rectification causes an increase in the frequency content of the data that renders it prone to phase cancellation. In the present study, this cancellation was harmful as it prevented detection of important factors of influence such as stimulus strength and motor unit recruitment [3]. A similarly harmful influence of rectification on the studied signals as in the present investigation has previously been shown for the interference with background EMG activity [2]. Taken together, our results do not suggest that FWR is a necessary step when LLR amplitude data is to be investigated.

Fig. 1: The average amplitude data derived for the different phase shift conditions.

Fig. 2: Box plots of LLR2-amplitudes obtained with rectification and without for different stimulus and weight load conditions.

References:

[1] Deuschl G and Eisen A. Long latency reflexes following electrical nerve stimulation. Electroencephalogr Clin Neurophysiol Suppl, 1999; 52: 263 – 268

[2] Keenan KG, Farina D, Merletti R, Enoka RM. Amplitude cancellation reduces the size of motor unit potentials averaged from the surface EMG. J Appl Physiol, 2006; 100: 1928 – 37

[3] Alaid S, Zawierucha A, Kornhuber M. The electrically evoked long latency reflex of the biceps brachii muscle: The impact of train stimuli, preceding stimuli, and voluntary muscle contraction. Neurosci Lett, 2012; 526: 91 – 95