External perturbation of the trunk via sudden loading and unloading is an established method to study control of spinal stability and postural equilibrium. As differential control of the deep and superficial lumbar multifidus occurs during predictable sagittal loading, we hypothesized that the deep and superficial components of the thoracic paraspinal muscles would also be differentially active during loading and unloading of the trunk. Variation in sagittal mobility between regions of the thorax and previous data of differences in control of the thoracic paraspinal muscles between regions in other tasks supported a hypothesis that there would be region-specific differences in responses to loading and unloading. This study used fine-wire electrodes to record electromyographic (EMG) activity from the right deep (multifidus/rotatores) and superficial (longissimus) muscles at T5, T8, and T11 in ten healthy subjects during predictable and unpredictable sudden loading and unloading of the trunk. EMG amplitude was calculated during 10 ms epochs for 50 ms before the onset of trunk perturbation and 150 ms after the perturbation. Contrary to our hypotheses, deep and superficial thoracic paraspinal muscles were similarly active (loading: p=0.470; unloading: p=0.137) and similarly affected by the degree of predictability at all levels. Thus, deep and superficial thoracic paraspinal muscles are recruited en bloc during sagittal plane trunk perturbations. This contrasts previous findings of differential control between the deep and superficial thoracic paraspinal muscles during rotational tasks, and provides evidence that discrete control of thoracic paraspinal muscle fascicles is specific to the direction of forces applied to the trunk.
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