Non-Destructive Determination of Muscle Architectural Variables Through the Use of DiceCT : DETERMINING MUSCLE ARCHITECTURE VIA DICECT

1. The variation of isometric tetanus tension with sarcomere length in single fibres from frog striated muscle has been re-investigated with special precautions to ensure uniformity of sarcomere length within the part of the fibre being studied.2. In most respects the results of Ramsey & Street (1940) were confirmed, but (a) the peak of the curve was found to consist of a plateau between sarcomere lengths of 2.05 and 2.2 mu, (b) the decline of tension above this plateau is steeper than found by Ramsey & Street, and (c) the decline of tension below the plateau becomes suddenly steeper at a sarcomere length of about 1.67 mu.3. Many features of this length-tension relation are simply explained on the sliding-filament theory.4. It is concluded that, in the plateau and at greater lengths, the tension on each thin filament is made up of equal contributions from each bridge which it overlaps on adjacent thick filaments.5. Internal resistance to shortening is negligible in this range but becomes progressively more important with shortening below the plateau.

The maximum tetanic tension (Po) generated by a skeletal muscle is determined by its functional cross-sectional area (CSA) and its specific tension (tension/CSA). Measurements of average fiber length (normalized to a sarcomere length of 2.2 micron), muscle mass, and approximate angle of pinnation of muscle fibers within a muscle were taken from 26 different guinea pig hindlimb muscles and were used to calculate CSA. The specific tension was assumed to be 22.5 N X cm-2 and was used to determine the estimated Po of each muscle studied. In a second group of guinea pigs the in situ Po of 11 selected hindlimb muscles and muscle groups were determined. Estimated and measured Po values were found to have a strong linear relationship (r = 0.99) for muscle and muscle groups tested. The specific tension of the soleus, a homogeneously slow-twitch muscle, was shown to be approximately 15.4 N X cm-2 (P less than 0.01). Therefore, in our hands a specific tension value of 22.5 N X cm-2 appears to be a reasonable value for all mixed muscles studied in the guinea pig hindlimb and can be used to estimate their Po.