Predictability of ventilatory muscle optimal length based on excised dimensions

The purpose of the present study was to assess the relationship between excised length (unstressed length of excised muscle; L(ex)) and optimal force-generating length (L(o)) in a variety of respiratory muscles, with the goal of establishing a reliable method whereby L(o) could be rapidly and easily estimated with a high level of accuracy. Experiments were conducted on 111 muscle bundles obtained from 18 mongrel dogs. Segments of costal diaphragm, parasternal intercostal, scalene, sternomastoid, triangularis sterni, rectus abdominis, external oblique, and transversus abdominis muscles were studied. We noted a linear relationship between the distance measured between two fixed points in excised bundles (L(ex)) and at the muscles' L(o). Correlation coefficients ranged from 0.83 (P < 0.01) for the transversus abdominis to 0.92 (P < 0.01) for the triangularis sterni and external oblique muscles. Pooled L(ex) for all muscles averaged 61.4 ± 6.3% (SD) L(o), with specific values ranging from 55.5 ± 3.9% L(o) for triangularis sterni bundles to 63.0 ± 5.1% L(o) for external oblique bundles. In three additional dogs, we verified the usefulness of this relationship and prospectively estimated L(o) from excised length in 10 costal diaphragm bundles and 10 transversus abdominis bundles and then measured L(o) directly. Predicted L(o) averaged 100.0 ± 6.0% L(o) for diaphragm and 97.6 ± 5.9% L(o) for transversus abdominis muscle. We conclude that L(o) can be conveniently and accurately estimated from excised dimensions. This rapid estimation technique should prove valuable for future studies in respiratory muscle physiology.