Mechanics of acutely shortened canine diaphragm muscle in vitro.

Acute changes in lung volume, as might be seen in an airway-obstructed patient, may lead to different amounts of shortening of the various inspiratory muscles, of which the diaphragm is the most important. The purpose of this study is to investigate the twitch and contractile characteristics of acutely shortened canine diaphragm muscle in vitro during isometric contraction. We found that with diaphragmatic muscle shortening from Lo (optimal length, defined as being that length at which peak twitch tension was generated) to 70% Lo, the time-to-peak tension, half relaxation time and twitch tension were significantly reduced. The force-frequency curves, obtained from different muscle lengths, disclosed that shorter muscles generate disproportionately less force during low-frequency stimulation. These factors may, at least in part, contribute to the vulnerability to fatigue of those with shorter diaphragmatic muscle length.