P77. Effects of high resistance muscle training on cortico-spinal output during motor fatigue. A study using transcranial magnetic stimulation

Objective To compare responses to transcranial magnetic brain stimulation during a fatiguing exercise before and after a 3 weeks lasting resistance training, in healthy subjects. Methods The triple stimulation technique (TST, Magistris et al., 1998) was used to quantify a central conduction index (CCI = amplitude ratio of central conduction response and peripheral nerve response, obtained simultaneously by the TST). The CCI removes effects of peripheral fatigue from the quantification of the responses to brain stimulation. It allows a quantification of the percentage of the entire target muscle motor unit pool driven to discharge by a transcranial magnetic stimulus. Subjects ( n  = 15) performed a 3 weeks training regimen (2 min twice per day) of repetitive isometric maximal voluntary contractions (MVC) of abductor digiti minimi (ADM; duration 1 s, frequency 0.5 Hz). Before and after this training, TST recordings were obtained every 15 s during an 2 min exercise, where subjects performed repetitive contractions of the ADM, and repeatedly during a recovery period of 7 min, using stimulation intensities and facilitatory maneuvers sufficient to excite all cortical motor neurons. Results There was a consistent decrease of force to approximately 40% of MVC in all experiments and in all subjects, before and after training. In all subjects, CCI decreased during exercise. While before training, the CCI decreased to 49% (SD 23.7%) after 2 min of exercise, it decreased after training only to 79% (SD 26.4%) ( p Discussion The training regimen increased the proportion of target motor units that could be activated by transcranial magnetic stimulation during a fatiguing exercise. Possible underlying mechanisms at spinal and supraspinal sites are discussed.