I was wondering about the number of single muscles in the human body: that is those not occurring in bilateral pairs. I would consider the following: occipitofrontalis (incl.frontalis,occipitalis) m. uvulae platysma diaphragm (thoracic) I wonder about bulbospongiosus sphincter ani externus puborectalis pubovaginalis (sphincter vaginae) puboprostaticus (levator prostatae) although I understand the latter three to be parts of Levator ani and that muscle to be paired bilaterally. I wonder if in some cases for medial muscles that join up and essentially function as a single unit, that the muscle name becomes so associated with the two together (e.g., puborectalis, sphincter ani externus)that more for conventional reasons the muscle name is never considered as a name for two bilaterally paired muscles, although morphologically there is no reason for regarding them as a single muscle. Still I do understand that there are single muscles in the body and would like to know if you agree with my first list and what other muscles you would include.
I agree with your list of medial muscles, but have some additions, which are noted below. Human beings, like all vertebrates, are bilaterally symmetrical. Those muscles that you list as median unpaired muscles also developmentally begin as bilaterally symmetrical paired structures that ultimately fuse, and so function as single, median muscles. So while embryologically these structures are paired, or derive from paired elements, in their postnatal roles they function as single muscles. I would argue that developmentally there is no reason to consider them a single muscle, but functionally and morphologically the muscles you list do appear to be a single unit.
Some, in fact, have more complex embryological origins than being simply bilateral structures fusing in the midline. For example, the diaphragm derives from four different sources: the septum transversum gives rise to the central tendon, while the pleuroperitoneal folds, the esophageal mesentery, and body wall musculature all give rise to the muscular parts of the diaphragm.
Occipitofrontalis is a good example of paired structures which become fused. Occipitofrontalis usually consists of fairly clearly distinguished left and right pairs of muscle bellies joined in the midline with one another via fascial connections. I would consider all the various sphincters and the musculature which makes up the pelvic diaphragm to also be cases muscles which develop bilaterally and fuse to form a morphological and functional unpaired unit. In addition to those that you listed, I would also include the sphincter urethrae, as well as the pharyngeal constrictors.
While your question appears to only consider skeletal muscles, there are other muscles in the body, which could be considered unpaired muscles. These are particularly associated with the gut and structures derived from it. The best cases of unpaired muscles may be among those associated with the thoracic viscera. The heart and the esophagus, for example, are unpaired muscles which develop as unpaired structures in situ.
In short, I think there are two answers to your question. On the developmental and embryological level, the number of unpaired muscles is very restricted. On the functional and morphological level, there are a larger number of unpaired muscles