Measurement of voluntary activation of abdominal flexors using transcranial magnetic stimulation.

Background: Transcranial magnetic stimulation (TMS) has been used to assess voluntary activation (VA) of limb and back muscles, however its ability to assess abdominal muscle VA is unknown. The assessment of abdominal muscle VA using TMS could be applied to patients with trunk dysfunction to enable further understanding of the neurophysiology of trunk control, inform practice and enable the development and monitoring of rehabilitation programmes.

Aim: The aim of this study was to investigate use of TMS and the twitch interpolation technique to measure voluntary activation of abdominal muscles.

Methods: Twenty healthy participants performed sets of isometric abdominal contractions of varying levels, during which TMS was applied to the primary motor cortex. The evoked twitches were measured as torque, while simultaneous surface electromyographic (EMG) activity was recorded bilaterally from rectus abdominis, erector spinae, tensor fasciae latae, and rectus femoris. VA was calculated as: (1 - superimposed twitch amplitude/estimated resting twitch amplitude) x 100. Estimated resting twitch amplitude was calculated by extrapolation using linear regression of superimposed twitch amplitude against torque for contraction strengths 50-100 % maximum voluntary contraction (MVC).

Results: There was a strong linear relationship between voluntary torque of 50-100 % MVC and TMS-evoked twitch amplitude (r = 0.994, p = 0.035), and voluntary torque between 50-100 % MVC and VA (r = 0.997, p = 0.025). VA at a target torque of 100 % MVC was less than 100 % (86.20 ± 2.29 %).

Conclusions: VA of abdominal muscles can be assessed with twitch interpolation using TMS. VA has been shown to be submaximal during maximum voluntary contractions, and it has been demonstrated that superimposed twitch amplitude decreases in a linear fashion with increasing contraction intensity. Using this technique to explore trunk muscle function could help to improve understanding of the neurophysiology of trunk control, including the sites on any deficit in drive and also improve monitoring of the efficacy of treatment regimes for clinical conditions associated with dysfunctions in trunk control e.g. low back pain.