Relationship between muscle activity of the frontalis and the associated brow displacement

Although the relationship between regional facial muscle activity and resultant displacement of the face is important to the refinement of many of the treatments of facial paralysis and palsy, this relationship has not been adequately explored. We analyzed the relationship between frontalis muscle activity as measured with surface electromyography and the associated eyebrow displacement as measured with the maximal static response assay in 16 human volunteers. We hypothesized that graded, sustained muscle activity would correlate with graded, sustained elevation of the eyebrow. We found that (1) the relationship between the muscle activity and the corresponding displacements was best described by activity versus displacement squared (r2 = 0.993); (2) there was a wide variation between individual brows for the relationship between muscle activity and displacement; (3) there was a normal amount of asymmetry of the relationship between muscle activity and displacement between the two brows of a subject; (4) left and right frontalis generated the same amount of muscle activity, but left brows elevated significantly higher; (5) the method of simultaneous measurement of muscle activity and displacement used in this study yielded results similar to those previously reported; and (6) the methods used in this study are useful to further investigate the relationship between muscle activity and displacement.