Mechanical characteristics and functional length of canine expiratory muscles

The expiratory muscles of the abdominal region actively contribute to breathing. In dogs, the transversus abdominis appears to be the main abdominal muscle of expiration. The in vitro mechanical properties of the transversus abdominis have not been reported to date, and formed the basis of the present investigation. Moreover, in order to understand better the nature of the mechanical interplay between the various abdominal muscle groups, we also evaluated the effects of posture on the operational length of canine transversus abdominis and external oblique muscles and related their in situ length to optimal length. The experiments were performed on twelve mongrel dogs, anesthetized with pentobarbital sodium. Contractile properties of excised transversus abdominis muscle strips were evaluated at 37°C and revealed similar twitch, force-frequency and length-tension properties as previously reported in canine external oblique (Farkas and Rochester, J. Appl. Physiol. 65: 2427-2433, 1988). In the supine posture, we noted that external oblique was operating at 88% Lo, while the transversus abdominis was operating at a significantly shorter length of 74% Lo. Thus, in the supine posture, the external oblique is better situated than the transversus abdominis to generate tension. In the prone posture, however, we noted that both abdominal muscles were located at similar positions along their length-tension curve, operating at a length of roughly 77% Lo. Since both muscles share common length-tension characteristics, the present results indicate that the tension generating potential of both muscles in prone dogs is equal for a given neural input. However, we conclude that the preferential recruitment of the transversus abdominis in prone animals must be related to factors other than simple tension generation.