The Effect of Spinal Manipulation on the Electrophysiological and Metabolic Properties of the Tibialis Anterior Muscle

Abstract

There is growing evidence showing that spinal manipulation increases muscle strength in healthy individuals as well as in people with some musculoskeletal and neurological disorders. However, the underlying mechanism by which spinal manipulation changes muscle strength is less clear. This study aimed to assess the effects of a single spinal manipulation session on the electrophysiological and metabolic properties of the tibialis anterior (TA) muscle. Maximum voluntary contractions (MVC) of the ankle dorsiflexors, high-density electromyography (HDsEMG), intramuscular EMG, and near-infrared spectroscopy (NIRS) were recorded from the TA muscle in 25 participants with low level recurring spinal dysfunction using a randomized controlled crossover design. The following outcomes: motor unit discharge rate (MUDR), strength (force at MVC), muscle conduction velocity (CV), relative changes in oxy- and deoxyhemoglobin were assessed pre and post a spinal manipulation intervention and passive movement control. Repeated measures ANOVA was used to assess within and between-group differences. Following the spinal manipulation intervention, there was a significant increase in MVC (p = 0.02; avg 18.87 ± 28.35%) and a significant increase in CV in both the isometric steady-state (10% of MVC) contractions (p < 0.01; avg 22.11 ± 11.69%) and during the isometric ramp (10% of MVC) contractions (p < 0.01; avg 4.52 ± 4.58%) compared to the control intervention. There were no other significant findings. The observed TA strength and CV increase, without changes in MUDR, suggests that the strength changes observed following spinal manipulation are, in part, due to increased recruitment of larger, higher threshold motor units. Further research needs to investigate the longer term and potential functional effects of spinal manipulation in various patients who may benefit from improved muscle function and greater motor unit recruitment.

The Effects of Spinal Manipulation on Motor Unit Behavior

“…It is interesting to note that this recent study found an increase in conduction velocity at 10% MVC following the spinal manipulation session, whereas, in the current study, using the HD sEMG decomposition methodology, we found a decrease in the conduction velocity at 5% MVC [74]. Both studies recorded EMG during similar "ramp" and "ramp & maintain" conditions at both 5% and 10% MVC [74]. This apparent contradiction in finding may be due to the difference in the way the conduction velocity across the TA was calculated.…”

“…The results revealed a main effect of spinal manipulation on the conduction velocity of the MUs. This supports previous studies that have suggested that spinal manipulation has an impact on sensory and motor neural processing that impacts motor control [3][4][5][6][7]74]. This study aimed to evaluate the influence of spinal manipulation on MU activity and therefore, neuromuscular control.…”

“…The increased MVC in TA strength and increase in conduction velocity, without changes in motor unit discharge rate, suggests that the MVC strength changes observed following spinal manipulation in this study were mostly due to increased recruitment of larger, higher threshold motor units. It is interesting to note that this recent study found an increase in conduction velocity at 10% MVC following the spinal manipulation session, whereas, in the current study, using the HD sEMG decomposition methodology, we found a decrease in the conduction velocity at 5% MVC [74]. Both studies recorded EMG during similar "ramp" and "ramp & maintain" conditions at both 5% and 10% MVC [74].…”

“…A recent study [74] explored the effects of a single spinal manipulation session on maximum voluntary contractions (MVC) of the ankle dorsiflexors, HD EMG, intramuscular EMG, and near-infrared spectroscopy (NIRS) from the TA muscle in 25 participants with low level recurring spinal dysfunction. They found a significant increase in conduction velocity in both the isometric steady-state contractions at 10% of MVC and during the isometric ramp contractions at 10% of MVC compared to the control intervention [74]. The increased MVC in TA strength and increase in conduction velocity, without changes in motor unit discharge rate, suggests that the MVC strength changes observed following spinal manipulation in this study were mostly due to increased recruitment of larger, higher threshold motor units.…”

“…This apparent contradiction in finding may be due to the difference in the way the conduction velocity across the TA was calculated. In a recent study [74], the conduction velocity was calculated using algorithms of cross-correlation and a maximum likelihood multi-channel approach adapted from Farina et al [75]. This method involved iteratively cross-correlating signals in a multi-channel matrix to find the electrodes whose signals had the least mean square error while calculating the similarity between them.…”