The Effects of Spinal Manipulation on Central Nervous System Activity Measured by Reflexive Calf Muscle Recruitment

Overview

This study investigated the effects of spinal manipulation on central nervous system activity. The presence of postactivation potentiation, an increase in muscular force production following prior muscular contractions, was measured with electromyography and the muscular force production during electrically-induced calf muscle reflexes. It was hypothesized that significantly greater potentiation would be stimulated by a calf muscle contraction with spinal manipulation delivered immediately beforehand than the potentiation arising from the contraction only.

A randomized, controlled, single-blind crossover study design was utilized, and the three independent variables were spinal manipulative therapy (SMT), a 10 second plantar flexion maximal voluntary isometric contraction (MVIC) or SMT immediately preceding the MVIC (SMT + MVIC). The treatment order was randomized for each of the three sessions before the tibial nerve Hmax/Mmax stimulation protocol. Each of the four dependent variables were evoked during the tibial nerve H-reflex stimulation protocol at the conclusion of each session, and included the Hmax/Mmax ratio (%) of the gastrocnemius and soleus muscles and the isometric twitch torque occurring at Hmax and at Mmax. Hmax, the highest H-reflex amplitude, is an indication of the greatest possible reflex activation; as such, it is an estimate of the number of motor neurons a subject is capable of activating in a given state. Further increases in the stimulation intensity cause the subsequent M-wave to reach its highest amplitude, Mmax. Mmax is a compound muscle action potential (CMAP) which represents full muscle activation. Specific to the current investigation, Mmax indicated activation of the total volume of the gastrocnemius/soleus motor neuron (MN) pool. Given that Hmax is an inference of the number of MNs being recruited, and Mmax constitutes the entire motor neuron pool, the proportion of the entire MN pool capable of being recruited can be estimated with the Hmax/Mmax ratio. The Hmax/Mmax ratio was determined by division of the EMG peak-to-peak amplitudes (mV) evoked at Hmax by the preceding Mmax EMG peak-to-peak amplitudes. Differences in each of the four dependent variables (Hmax/Mmax ratios of the gastrocnemius and soleus and the peak twitch torques evoked at Hmax and Mmax) following each treatment form of SMT, MVIC or SMT+MVIC delivered during the three data collection sessions on three separate days were determined with a two-way (treatment × time point) repeated measures ANOVA. Percent changes from baseline were also calculated for each of the dependent variables, and the same type of ANOVA was used to determine differences in the within-subjects effects of each treatment.