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Isometric active length-force characteristics of rat semimembranosus lateralis muscles [rats, n = 6, body mass = 292 (SD 9) g] were recorded. Muscles were photographed during the force plateaus of the tetani to determine lengths of proximal, intermediate and distal fibres. From the mean number of sarcomeres in series in those fibres, mean sarcomere length at different muscle lengths was calculated. The heterogeneity of mean 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. At 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 range 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 showed considerable differences (range 18.7-24.9 mm). The coefficient of correlation between length ranges and mean number of sarcomeres in series in various muscle regions was extremely low (r = -0.14) and not significant. The coefficient of correlation between length ranges and heterogeneity was high (r = 0.88) and significant. These data would suggest that muscles with a similar number of sarcomeres in series can exhibit quite different functional characteristics.

The geometry of rat gastrocnemius medialis muscle (GM) was studied at different muscle lengths. In addition, the number of sarcomeres in series within fibres was estimated. For muscles of immobilized legs (i.e. GM was held at in vivo smallest length) as well as those of the contralateral legs, comparison was made with controls of similar age. Immobilization periods of 4 and 6 weeks were used. For immobilized GM muscles, the number of sarcomeres in series was lower only within distal fibres after 4 weeks of immobilization. Aponeuroses were 25% shorter after both immobilization periods and no differences were found for fibre and aponeurosis angle. For GM of contralateral legs no difference with respect to controls was found regarding the number of sarcomeres in series. Aponeuroses were approximately 15% shorter and the fibre and the aponeurosis angle were also smaller. Based on these geometrical differences, it was expected that both experimental muscles should exert force over a smaller range of muscle length than controls. However, for immobilized muscles a similar range of length was found for which a possible explanation could be the more compliant aponeurosis. For contralateral muscle a 2-mm larger length range as well as a change of distribution of optimal fibre length with respect to optimal muscle length was found. For immobilized muscles indications of a distribution of optimal fibre lengths were also found but these did not differ from those of the controls. The results for contralateral muscles would indicate that the distribution may be changed under the influence of an altered use of the limb and that this may be an important factor determining the length range of active force generation.

Lengths of muscle tendon complexes of the quadriceps femoris muscle and some of its heads, biceps femoris and gastrocnemius muscles, were measured for six limbs of human cadavers as a function of knee and hip-joint angles. Length-angle curves were fitted using second degree polynomials. Using these polynomials the relationships between knee and hip-joint angles and moment arms were calculated. The effect of changing the hip angle on the biceps femoris muscle length is much larger than that of changing the knee angle. For the rectus femoris muscle the reverse was found. The moment arm of the biceps femoris muscle was found to remain constant throughout the whole range of knee flexion as was the case for the medial part of the vastus medialis muscle. Changes in the length of the lateral part of the vastus medialis muscle as well as the medial part of the vastus lateralis muscle are very similar to those of vastus intermedius muscle to which they are adjacent, while those changes in the length of the medial part of the vastus medialis muscle and the lateral part of the vastus lateralis muscle, which are similar to each other, differ substantially from those of the vastus intermedius muscle. Application of the results to jumping showed that bi-articular rectus femoris and biceps femoris muscles, which are antagonists, both contract eccentrically early in the push off phase and concentrically in last part of this phase.

Models are useful when studying how architectural and physiological properties of muscle-tendon complexes are related to function, because they allow for the simulation of the behaviour of such complexes during natural movements. In the construction of these models, evaluation of their accuracy is an important step. In the present study, a model was constructed to calculate the isometric force-length relationship of the rat extensor digitorum longus muscle-tendon complex. The model is based on the assumption that a muscle-tendon complex is a collection of independent units, each consisting of a muscle fibre in series with a tendon fibre. By intention, values for model parameters were derived indirectly, using only the measured maximal isometric tetanic force, the distance between origin and insertion at which it occurred (optimum lOI) and an estimate of muscle fibre optimal length. The accuracy of the calculated force-length relationship was subsequently evaluated by comparing it to the relationship measured in isometric tetanic contractions of a real complex in the rat. When the length of distal muscle fibres, measured during isometric contraction at optimal lOI of the whole complex, was used as an estimate for muscle fibre optimal length of all muscle fibre-tendon fibre units in the model, the calculated relationship was too narrow. That is, both on the ascending limb and on the descending limb the calculated tetanic force was lower than the measured tetanic force.(ABSTRACT TRUNCATED AT 250 WORDS)