Abstract
Background:
We have recently shown, using transcranial magnetic stimulation (TMS) to assess voluntary activation (VA), that neural drive to back muscles is reduced in subjects with chronic low back pain. There is also evidence that central nervous system drive to abdominal muscles is altered in these subjects, however VA has not yet been assessed for these muscles in healthy subjects; this is the purpose of the present study.
Methods:
Twenty one healthy subjects (10M:11F) participated. Electromyographic activity was recorded from back and abdominal muscles and flexor torque was measured using a dynamometer. Subjects performed a series of isometric voluntary contractions (10%–100% MVC) of rectus abdominis during which TMS was applied to the motor cortex. The resulting superimposed twitches (SIT) were measured and VA was derived.
Results:
There was a linear relationship between voluntary torque (50–100% MVC) and SIT amplitude and between voluntary torque (50–100% MVC) and VA. VA at a target torque of 100% MVC was less than maximal (∼86%). Time-to-peak amplitude of SITs displayed a linear relationship with voluntary torque between 10%–100% MVC.
Discussion:
This study has shown that it is possible to assess VA of abdominal muscles using TMS. Further, it appears that VA is submaximal during maximum voluntary contractions, similar to that observed in back muscles. This may reflect the function of trunk muscles in general, which are routinely used for maintenance of posture. Whether imbalances of abdominal and back muscle strength observed in low back pain are reflected in imbalances of neural drive to these muscles remains to be investigated.
