INTRODUCTION: IN THIS STUDY WE COMPARED TRANSCRANIAL MAGNETIC STIMULATION (TMS) ELICITED MOTOR EVOKED POTENTIALS (MEPS) IN A POSTURAL (BILATERAL LOW BACK EXTENSION: BLBE) and a respiratory (forced expiration during breath holding: FEBH) task. METHODS: Using TMS of the left motor cortex, simultaneous patterns of corticospinal facilitation of the contralateral erector spinae (ES) and rectus abdominis (RA) muscles during graded voluntary activation were compared in seven healthy subjects. RESULT: The facilitation pattern demonstrated task dependency by showing that MEP amplitudes in the ES muscle tended to be smaller at any given contraction level in the FEBH task than in the BLBE task. DISCUSSION: The results suggested a linear-type relationship between the size of MEPs with increasing background contraction of ES and RA in the BLBE task. However, both muscles showed a plateau effect with higher background contractions (>50% of maximum) during the FEBH task. The varied response of ES and RA across these two tasks reinforces the importance of task specific training in clinical settings.