We used transcranial magnetic stimulation (TMS) to study corticospinal excitability to erector Spinae (ES) muscles during graded voluntary contractions in bilateral trunk extension (BTE) and forced expiratory breath holding (FEBH) in normal individuals. Motor evoked potentials (MEPs) could be produced in all subjects in the absence of voluntary activation. At maximum voluntary contraction, levels of surface electromyographic (EMG) activity were 4 times greater during BTE than FEBH. When EMG was normalized to maximum. MEP amplitudes increased in proportion to contraction in both tasks. MEPs in FEBH were compared with extrapolated values at similar EMG levels in BTE and were found to be larger. EMG and MEPs in left and right ES were symmetrical throughout the range of contractions in both tasks. ES muscles have a facilitation pattern similar to that previously shown in leg muscles, but subtle differences at low levels of EMG suggest that the facilitation is dependent on the task.
A.V. Nowicky is with the Department of Sport Sciences at Brunei University. Osterly Campus, Isleworth. London TW7 5DU, UK. A.H. McGregor is with the Department of Orthopaedic & Trauma Surgery at Imperial College School of Medicine, Charing Cross Hospital. London, W6 8RF, UK. N.J. Davey is with the Division of Neuroscience and Psychological Medicine at Imperial College School of Medicine, Charing Cross Hospital, London, W6 8RF, UK.