Abduction of one arm preferentially activates erector spinae muscles on the other side to stabilise the body. We hypothesise that the corticospinal drive to the arm abductors and the erector spinae may originate from the same hemisphere. In 18 subjects, transcranial magnetic stimulation (TMS) was applied using an angle double-cone coil placed symmetrically over the vertex. Motor evoked potentials (MEP) could not be evoked systematically seated at rest but could be evoked bilaterally in erector spinae muscles during unilateral arm abduction. TMS was applied at 110% and 120% motor threshold (MT) for the contralateral erector spinae muscle when an arm was abducted against resistance. The electromyographic (EMG) activity in the erector spinae at L4 vertebral level during contralateral arm abduction was significantly higher (P<0.05) than in the ipsilateral erector spinae. The mean (+/-S.E.M.) latencies of MEPs in the contralateral muscle to TMS at 120%MT (left 16.0+/-0.8 ms; right 17.0+/-0.8 ms) were significantly (P<0.05) longer than in the ipsilateral erector spinae (13.9+/-1.0 ms; 16.6+/-0.4 ms). In two of six subjects from the same group, it was possible to elicit MEPs by TMS applied selectively to one hemisphere using a figure-of-eight coil. MEPs ipsilateral to the TMS had longer latencies than contralateral MEPs. The study revealed an unexpectedly longer rather than shorter latency of the MEP recorded from the lumbar erector spinae muscles when co-activated during abduction of the opposite arm. A speculative explanation is that TMS might activate back muscles contralateral to arm abduction via an uncrossed, ipsilateral corticospinal tract that is slower conducting than the conventional crossed corticospinal tract. The study has implications for the design of measures to promote recovery and rehabilitation of motor function in disorders such as stroke and spinal cord injury.