There are two ways the IOPI can be used to measure the strength of the lips. In the first method,  an IOPI bulb is placed directly between the upper and lower lip with the long axis of the IOPI bulb parallel to the lips, and then the patient/subject is asked to compress the lips as hard as possible. If the bulb is placed to the side, rather than in the midline of the lips, one may obtain a comparison of the lip strength on the two sides of the body, a measurement that might be of interest in the case of a patient with known or suspected neurological damage.

The second method involves placing the IOPI bulb under the cheek just lateral to the corner of the mouth and having the patient squeeze the IOPI bulb against the buccal surface of the teeth by pursing the lips as hard as possible.  Although this method has less face validity, I think it is valid, however, because the pressure developed in the bulb depends upon the strength of obicularis oris, the circumferential muscle complex that surrounds the mouth. It is tension in this muscle that allows the lips to be compressed against one another.

Dr. Pamela Mathy at Arizona State University has evaluated both of these methods in a group of subjects and has concluded that the second method, with the bulb in the cheek, gives more reliable measures than the first method. The trouble with the first method is that the bulb tends to roll around between the lips, and feels unstable. Subjects also unconsciously try to "trick" the measurement by folding the lips back over the teeth, which of course lets the jaws exert the pressure on the bulb.

There are currently no normal values for lip strength measured with the IOPI. I have been able to find 16 papers that have measured lip strength in normal subjects and various populations of patients. These papers can all be referenced by using the term "lip strength" to search the Pub Med database at www.NCBI.NLM.NIH.gov They have used a variety of methods, mainly strain gauge systems, but some with dynamometers and such procedures as the "button pull."  All of these methods are legitimate, but my concern is that they all involve rigid mechanical devices that can potentially cause discomfort if the lips are used with their maximal effort, and discomfort may be a limiting factor in producing a person's effort. If that is the case, one could  be measuring pain thresholds rather than strength.  The soft compliant IOPI bulb avoids this potential problem. Nevertheless, anyone interested in the topic of lip strength may find it useful to look at least some of the following references. As you may note, none of these studies has used the IOPI  to measure lip strength.

Goozee JV, Murdoch BE, and Theodoros DG. Interlabial contact pressure exhibited in dysarthria following traumatic brain injury during speech and nonspeech tasks. Folia Phoniatr. Logop. 54:177-189, 2002.
 

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