Moving the arm away from the body alters the centre of gravity and calls for truncal muscle activity to maintain stability. Corticospinal drive to the erector spinae (ES) muscles increases during contralateral arm abduction and led to speculation that activation of an arm and the opposite back muscle could originate from the same cortical hemisphere (Davey et al., 2002). Individuals use one arm in preference to the other for single handed activities and we now ask whether symmetry of cortical drive to ES muscles is related to handedness. With local ethical approval six right handed and four left handed healthy individuals were recruited. Motor evoked potentials (MEPs) to transcranial magnetic stimulation (TMS) of the motor cortex were recorded bilaterally from the ES muscles at L4 spinal level. TMS was applied using a MagStim 200 stimulator connected to a double cone coil with its cross-over located over the vertex. The subjects were seated and asked to maintain 90° arm abduction against weak variable resistances of 2N, 4N, 6N and 8N applied at the wrist. Threshold (T) TMS intensity for MEPs in ES muscles was determined at 2N abduction. Six stimuli at 1.1T were delivered at each of the four force levels when the right arm was abducted. This procedure was repeated with the left arm abducted. The area of the averaged rectified MEPs in each trial was measured. In order to assess any bias to one or other ES muscle while excluding the effects of muscle bulk, a ratio was calculated in each trial; the MEP area in the ES muscle during contralateral arm abduction (Carm) was divided by the MEP area during ipsilateral arm abduction (Iarm). In right handed individuals, the mean (± SEM) Carm/Iarm ratio was larger (Student’s paired t-test; P 0.05) Carm/Iarm ratio in the right ES (1.60 ± 0.37) than in the left ES (1.00 ± 0.11). Furthermore, the mean Carm/Iarm ratio in the left ES was significantly larger (P 0.05) between right and left handed subjects. We conclude that the drive from the motor cortex to the ES muscles involved in stabilising the body during arm abduction is stronger during movement of the dominant arm. This raises the possibility that continued use of one arm in preference to the other could lead to asymmetry in the activation of paraspinal muscles which is a known link with low back pain.