Different ways to balance the spine: subtle changes in sagittal spinal curves affect regional muscle activity.

STUDY DESIGN Exploratory study of regional muscle activity in different postures. OBJECTIVE To detail the relationship between spinal curves and regional muscle activity. SUMMARY OF BACKGROUND DATA Sagittal balanced spinal posture (C7 above S1 in the sagittal plane) is a goal for spinal surgery and conservative ergonomics. Three combinations of thoracolumbar and lumbar spinal curves can be considered sagittal balanced postures: (i) flat-at both regions, (ii) long lordosis-lordotic at both regions, and (iii) short lordosis-thoracic kyphosis and lumbar lordosis. This study compares regional muscle activity between these 3 sagittal balanced postures in sitting, as well as a slump posture. METHODS Fine-wire electromyography (EMG) electrodes were inserted into the lumbar multifidus (deep and superficial), iliocostalis (lateral and medial), longissimus thoracis, and transversus abdominis in 14 healthy male volunteers. Fine-wire or surface EMG electrodes were also used to record activity of the obliquus internus, obliquus externus, and rectus abdominis muscles. Root mean square EMG amplitude in the flat, long lordosis, short lordosis, and slump sitting postures were normalized to maximal voluntary contraction, and also to the peak activity across the sitting postures. Muscle activity was compared between postures with a linear mixed model analysis. RESULTS Of the extensor muscles, it was most notable that activity of the deep and superficial fibers of lumbar multifidus increased incrementally in the 3 sagittal balanced postures; flat, long lordosis, and short lordosis (P < 0.05). Of the abdominal muscles, obliquus internus was more active in short lordosis than the other postures (P < 0.05). Comparing the sagittal balanced postures, the flat posture showed the least muscle activity (similar to the slump posture at most muscles examined). CONCLUSION Discrete combinations of muscle activity supported the 3 different sagittal balanced postures in sitting, providing new detail for surgeons, researchers, and therapists to distinguish between different sagittal balanced postures.