Schmidt, K.L., VanSwearingen, J.M., and Levenstein, R.M.(2005).
Speed, amplitude, and asymmetry of lip movement in voluntary puckering and blowing expressions: Implications for facial assessment. Motor Control 9 (3): 270-280.

Abstract
The context of voluntary movement during facial assessment has significant effects on the activity of facial muscles. Using automated facial analysis, we found that healthy subjects instructed to blow produced lip movements that were longer in duration and larger in amplitude than when subjects were instructed to pucker. We also determined that lip movement for puckering expressions was more asymmetric than lip movement in blowing. Differences in characteristics of lip movement were noted using facial movement analysis and were associated with the context of the movement. The impact of the instructions given for voluntary movement on the characteristics of facial movement might have important implications for assessing the capabilities and deficits of movement control in individuals with facial movement disorders. If results generalize to the clinical context, assessment of generally focused voluntary facial expressions might inadequately demonstrate the full range of facial movement capability of an individual patient.

Key Words: facial expression, Orbicularis oris, voluntary movement