Compared to the back, muscles on the front of the torso are relatively well-known even to laypeople. Prominent pectoral muscles and the famed “six-pack” are shown-off in comic books, movies, advertisements etcetera ad nauseam.
However Professor Kos had nothing interesting to say about any of the muscles shown here, except the musculus platysma, which is not on the torso but on the neck.
It is a thin sheet muscle, directly under the skin to which it is connected with fascia. In humans it is a muscle of relatively minor importance, nearly a vestigial organ. But vestigial of what? According to Professor Kos, its function can be really well observed in horses and cattle. These animals use their tails and ears to try to keep flies and mosquitoes at least somewhat at bay, however they cannot effectively reach their necks and parts of torso. However what they can do, and do (I have in fact observed this myself) they can flex their musculus platysma and similar thin muscle sheets directly under the skin, thus giving their skin a mighty shake in some places that can scare some insects off.
It is a nice story, but I doubt that this is original purpose of this muscle and its equivalents somewhere else on the body. More likely it is its repurposing for another function. Who knows?