Some observations upon the electrical responses and shape of the isometric twitch of skeletal muscle (intact)

I. The present communication deals with certain characters of the isometric twitch noted in the course of the experiments recorded in the preceding paper, in which optical records of the electrical and mechanical responses were obtained simultaneously. Accurate optical records of the isometric twitch of muscles with active circulation have lately been made for the first time by Sherrington (1, p. 251). Working upon tibialis anticus of the cat, he found that the twitch regularly exhibited a flat top—a “plateau”—ending more abruptly than it commenced, and this result has been corroborated by Liddell and Sherrington (2) for the quadriceps and semi-tendinosus of the cat. II. In addition to the flat top, which is a noticeable feature of he twitch of the frog’s gastrocnemius as well as of the muscles of the cat, a striking character of the twitch in the frog is the abrupt termination of the plateau (see figs. 1 and 2, Plates 29 and 30). The transition from the plateau tension to subsiding tension is abrupt enough to form on the linear record a clearly defined angle. In a fresh preparation with active circulation, so precise is this angle that it may be used as an exact and convenient point to which to measure the mechanical duration of the twitch. When the electrical changes are recorded, as in these experiments, simultaneously with the mechanical response, one may take as the most accurate interval for measurement the period from the beginning of the electrical change to the angle at the end of the twitch crest. This corner may be appropriately termed the “subsidence angle,” and in the remainder of the paper it will be referred to as the “angle.” With the present method of recording both may be measured significantly to 0⋅001 sec. (1σ).