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Alteration of hip joint angle, while keeping the knee position constant, changes the orientation of the biarticular rat semimembranosus lateralis muscle with respect to the bony origin at the pelvis. Geometrical and length-force characteristics of the muscle were determined in two experimental groups, each with a different orientation: angles between the muscle and line of origin were set at 68.1 +/- 5.3 degrees (condition 68) and 121.9 +/- 9.1 degrees (condition 122). For muscles of condition 68 and condition 122, the number of sarcomeres in series was similar in both proximal fibers (11,436 +/- 356 and 11,555 +/- 175, respectively) and distal fibers (12,585 +/- 844 and 12,787 +/- 288, respectively). At similar distal fiber lengths, the sarcomere length in proximal fibers was higher in condition 122 than in condition 68, indicating a change in heterogeneity of fiber sarcomere lengths in the muscle. Hence, length-force relationships of proximal and distal sarcomeres were more scattered with respect to muscle length in condition 122 than in condition 68. Range of distal fiber length between optimum and zero muscle force was increased by 2.7 mm (13%) in condition 122. These data indicate that biarticular muscles with bony origins can exhibit different length-force characteristics at similar distal fiber lengths. During hip joint movements and fiber shortening, muscle fibers of such biarticular muscles use shifting fiber length-force relationships with respect to muscle length.

During isometric contractions, muscle length, fibre and aponeurosis length and angles, and force exerted were quantified in order to assess factors contributing to length range of active force exertion. Comparisons of rat gastrocnemius medialis (GM) and extensor digitorum longus (EDL) muscles are of interest as fibre and aponeurosis length as determined at muscle optimum length are similar but fibre and aponeurosis angle are considerably smaller for EDL than for GM. On the basis of this, a wider length range between muscle optimum and active slack length was predicted initially for GM because of the expected higher angular contribution to accumulated length change. However, measurements of length-force characteristics revealed similar values for this length range. Therefore, quantification of length changes of fibre and aponeurosis as well of angular contributions to this length range was performed. Compared to their value at muscle optimum length, changes of fibre length were very similar for the two muscles. Fibre length changes normalized for muscle length changes ranged from approximately 90% near optimum length to approximately 65% near active slack length. Aponeurosis length changes normalized for muscle length changes were similar near optimum length for both muscles (approximately 5%) but deviated substantially near muscle active slack length (approximately 20% for GM and 35% for EDL). Cumulative angular contributions to muscular length changes were maximally 6% for EDL and 14% for GM. The reason for the relatively large length change of EDL aponeurosis could not be explained by different material properties but is likely to be related to yet an additional factor contributing to length range of active force generation: the distribution of mean sarcomere length of fibres with respect to muscle length. A piece of evidence for possible differences in such a distribution between GM and EDL is the difference of muscle length (approximately 1 and 2 mm over muscle optimum length respectively) at which optimum sarcomere length was found for distal fibres of the muscles.

In order to study the effects of unilateral short length immobilization on the contralateral gastrocnemius muscle (GM), length measurements were conducted on photographs taken in the active condition (tetanic plateau). Comparison of geometry of experimental and control muscles was made at optimum muscle length. The results show that a process occurred in the muscle which can be ascribed predominantly in terms of atrophy. This atrophy did not reach a maximum after 4 weeks but gradually increased in time. The altered conditions imposed on the muscle changed its architecture. It was shown that variables of the contralateral GM muscle are not representative of those of normally used muscles and should therefore not be used as control muscles for the determination of immobilization effects.

For static and dynamic conditions muscle geometry of the musculus gastrocnemius medialis of the rat was compared at different muscle lengths. The dynamic conditions differed with respect to isokinetic shortening velocity (25, 50 and 75 mm/s) of the muscle-tendon complex and in constancy of force (isotonic) and velocity (isokinetic) during shortening. Muscle geometry was characterized by fibre length and angle as well as aponeurosis length and angle. At high isokinetic shortening velocities (50 and 75 mm/s) small differences in geometry were found with respect to isometric conditions: aponeurosis lengths differed maximally by -2%, fibre length only showed a significant increase (+3.2%) at the highest shortening velocity. The isotonic condition only yielded significant differences of fibre angle (-4.5%) in comparison with isometric conditions. No significant differences of muscle geometry were found when comparing isotonic with isokinetic conditions of similar shortening velocity. The small differences of geometry between isometric and dynamic conditions are presumably due to the lower muscle force in the dynamic condition and the elastic behaviour of the aponeurosis. It is concluded that, unless very high velocities of shortening are used, the relationship between muscle geometry and muscle length in the isometric condition may be used to describe muscle geometry in the dynamic condition.

Rat gastrocnemius medialis (GM) and semimembranosus (SM) muscles have a very different morphology. GM is a very pennate muscle, combining relatively short muscle fibre length with sizable fibre angles and long muscle and aponeurosis lengths. SM is a more parallel-fibred muscle, combining a relatively long fibre length with a small fibre angle and short aponeurosis length. The mechanisms of fibre shortening as well as angle increase are operational in GM as well as SM. However, as a consequence of isometric contraction, changes of fibre length and angle are greater for GM than for SM at any relative muscle length. These differences are particularly notable at short muscle lengths: at 80% of optimum muscle length, fibre length changes of approximately 30% are coupled to fibre angle changes of 15 degrees in GM, while for SM these changes are 4% and 0.6 degrees, respectively. A considerable difference was found for normalized active slack muscle length (GM approximately 80 and SM approximately 45%). This is explained by differences of degree of pennation as well as factors related to differences found for estimated fibre length-force characteristics. Estimated normalized active fibre slack length was considerably smaller for SM than for GM (approximately 40 and 60%, respectively). The most likely explanation of these findings are differences of distribution of optimum fibre lengths, possibly in combination with differences of myofilament lengths and/or fibre length distributions.