A Comparison of Rat Extensor digitorum longus and Gastrocnemius medialis Muscle Architecture and Length-Force Characteristics

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Abstract
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.

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