Effect of growth on architecture of rat semimembranosus lateralis muscle

Architecture of semimembranosus lateralis muscle (SMl), an almost parallel-fibered muscle with bony origin and aponeurotic insertion of its muscle fibers, was studied in male Wistar rats of two age groups. Using photographic techniques and maximal stimulation active muscle architecture was determined at muscle optimum length. Muscle length increases were almost similar with fiber length increases (approximately 36%). Aponeurosis length increased (by 31%) as did angle of the aponeurosis with the line of pull (approximately 4 degrees). Angles of the proximal as well as distal fibers with the line of pull were unchanged. Increases in mean fiber diameter were estimated to be approximately 50%. It is likely that increased mean fiber diameter was accommodated at the bony origin by enlargement of its area. At the aponeurosis increased mean fiber diameter was accommodated by increased length of the aponeurosis as well as a change in fiber angle with the aponeurosis. This change in fiber angle proved to be different in the proximal and distal part of the aponeurosis. Under the assumption of a uniform change in fiber diameters, a gradient of length change along the aponeurosis occurs. It is concluded that growth in an almost parallel-fibered muscle with a bony attachment results in different architectural adaptations in the parts close to the bony attachment as compared to those close to the aponeurosis. This difference is due to the fact that adaptation of aponeurosis angle with the line of pull occurred, which was not the case for angle of the line of origin with the line of pull.