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Massage Today
April, 2001, Vol. 01, Issue 04

New Muscles

By Neal Cross, PhD, NCTMB

On several occasions in the past 10 years or so, students and practitioners have asked me about the existence of a new muscle they had recently heard or read about. As an anatomist with over 30 years experience, I immediately questioned such claims.

The human body exhibits a very rich structural variability. As this variation is beyond the scope of most anatomical textbooks, it is, unfortunately, not fully appreciated by many clinicians. On the other hand, experienced gross anatomists and surgeons encounter this variation on a daily basis.

The most recent "new muscle" to be brought to my attention was a muscle that has been called the sphenomandibularis. It was described in a few journals in the mid-to-late 1990's as a heretofore-unknown muscle of mastication. It was also implicated in the etiology of certain types of headaches -- especially trigeminal pain. Ybarra and Bauer recently published a clear, concise rebuttal and explanation of this "new muscle" in the journal Clinical Anatomy.1

The temporalis muscle is a much more complex structurally than textbooks would have us believe. This structural complexity often reflects an underlying functional complexity as well. The first detailed description of the medial portion of the temporalis occurred in the early 1800's. Ybarra and Bauer discuss several other early descriptions of this portion of the temporalis in their article. After dissecting several specimens and giving an exquisitely detailed description of the complex origin and insertion of the medial head of temporalis, these anatomists discuss the possible clinical relevance of its dysfunction. They paid particular attention to the complexity of this portion of the temporalis muscle's attachment to the sphenoid. The authors describe the possible entrapment of the lateral portion of the maxillary division of the trigeminal nerve (V2) in relation to facial pain. They describe the differences in pteryogopalatine fossa anatomy as a possible factor associated with specific pain patterns. These musculoskeletal-based pain patterns may be confused with CNS based pain patterns. Even though the various authors may disagree on the definition of the medial portion of the temporalis (whether should be considered a separate muscle or not), they all agree that it may be involved in certain cases of headache.

The specific muscles associated with headache may be much more complex than we now know. Travell and Simons2 have described many of the muscles commonly (and not so commonly) associated with headache. We also need to consider specific parts of muscles that may be involved in the etiology of headache.

The point is this: the muscular system is quite variable in nature, and some of this variation may be related to complaints of pain. These variants may confuse the practitioner, or worse, may lead to a missed assessment or a clinical mistake. For example, one of the most common muscle variations in the human body is the absence of the palmaris longus. This muscle is absent 10 -15% of the time. Its absence leads to the median nerve being less protected, just proximal to its entering the carpal tunnel. You can easily test to see if you have a palmaris longus by isometrically contracting your wrist flexors against resistance (for example, place your supine hand under the edge of a desk and attempt to flex your wrist). If you have a palmaris longus, it will be seen protruding anteriorly as it passes over the carpal tunnel.

Other common muscle variants, such as the presence or absence of the peroneus (fibularis) tertius, have little or no known (at least to this author) functional or clinical significance. Another type of muscle variation can be considered hypertrophy. In this case, I am referring to the intentional or habitual overdevelopment of part or all of a muscle. One very interesting example of this kind of "functional" variation can be seen in the pronator teres in some fast-pitch softball pitchers. One common method of throwing a drop ball [i.e., a "sinker"] requires strengthening the pronators of the forearm. The resulting hypertrophy of this muscle can put pressure on the median nerve, which travels into the forearm between the two proximal heads of the pronator teres. The resulting complaint can mimic carpal tunnel syndrome, yet have nothing to do with the median nerve at the carpal tunnel. All efforts to correct the problem at the tunnel will result in no diminution of symptoms.

These are but a few examples of muscle variations. This information is definitely something to keep in mind when a patient presents with any very unusual pain pattern. It also points to the need for continuous refreshing of our anatomical knowledge and advanced anatomic study.

References:

  1. Schon Ybarra, M.A., and B. Bauer. 2001. Medial portion of m. temporalis and its potential involvement in facial pain. Clin Anat 14:25-30.
  2. Travell, JG and G. Simons. 1983. Myofascial Pain and Dysfunction: The Trigger Point Manual. Baltimore, Williams and Wilkins.

Click here for previous articles by Neal Cross, PhD, NCTMB.

 

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