Shortening-length diagram: a new method of modeling skeletal muscle performance.

Muscle shortening is usually characterized in terms of force-velocity and length-active tension properties. We have examined a third relationship, that between shortening and initial length to further characterize muscle performance. This analysis shows that optimal shortening is achieved at a length (Ls) different from the length (Lo) of optimal tension development. Moreover, the analysis reveals discrepancies between results expected (using classical modeling) and experimental findings and these discrepancies increase with age. This effect of age on shortening has not been previously identified. Diaphragm muscle strips were dissected from male Fischer 344 rats of 3 and 24 months of age. Muscles were subjected, in vitro, to isotonic loads of either 30% or 50% of peak isometric tension (Po*) and shortened from initial lengths ranging from 80% to 130%Lo. The relationship between shortening and initial length was fitted by a second order polynomial using linear regression analysis. Results at 30% P/Po* indicate a difference in ability to shorten of 7% at Lo between muscles of young and elderly rats. The age-related change was enhanced under a load of 50% P/Po*, with a 46% difference in ability to shorten between young and old rats. The results show an age-related decrease in shortening of the diaphragm that is not predicted using classical muscle mechanics. Factors other than the length-tension relation and load must therefore be explored to explain decrements of shortening with age.