HETEROGENEITY OF MEAN SARCOMERE-LENGTH IN DIFFERENT FIBERS - EFFECTS ON LENGTH RANGE OF ACTIVE FORCE PRODUCTION IN RAT MUSCLE

Isometric active length-force characteristics of rat semimembranosus l ateralis muscles [rats, n=6, body mass=292 (SD 9) g] were recorded. Mu scles were photographed during the force plateaus of the tetani to det ermine lengths of proximal, intermediate and distal fibres. From the m ean number of sarcomeres in series in those fibres, mean sarcomere len gth at different muscle lengths was calculated. The heterogeneity of m ean sarcomere lengths for each muscle was quantified according to the coefficient of variation of sarcomere lengths at muscle optimal length . Absolute muscle length changes were equal to fibre length changes. A t all muscle lengths, mean sarcomere lengths in the distal fibres were significantly greater than those in proximal and intermediate fibres. The heterogeneity of mean sarcomere lengths augmented the length rang e of active force production between muscle optimal length and active slack length by about 38% to 43%. We concluded that in a muscle with a low degree of pennation, the heterogeneity of mean sarcomere lengths should be considered as a substantial contributor to the length range over which active force can be produced. In our experiment, the length ranges of active force production between optimal and slack length sh owed considerable differences (range 18.7-24.9 mm). The coefficient of correlation between length ranges and mean number of sarcomeres in se ries in various muscle regions was extremely low (r=-0.14) and not sig nificant. The coefficient of correlation between length ranges and het erogeneity was high (r=0.88) and significant. These data would suggest that muscles with a similar number of sarcomeres in series can exhibi t quite different functional characteristics.