Effects of Growth on Muscle, Tendon, and Aponeurosis Tissues in Rabbit Shank Musculature.

There exist several studies using morphological analyses of skeletal muscles to obtain a better understanding of muscle structure. The structural information obtained are primarily determined from single muscle components using individual animals of discrete ages. Further, little is known about changing dimensions of the aponeurosis, which is an important load-transferring interface in muscle mechanics. Thus, the aim of the present study was to determine how the muscle, tendon, and particularly the aponeurosis geometry of the rabbit shank musculature (M. soleus, M. extensor digitorum longus, and M. plantaris) change during growth. In doing so, morphological studies on muscles of eighty-nine female rabbits aged between 18 and 108 days were conducted. We found an almost linear increase over time in all of the geometrical parameters observed. The aponeurosis of the muscles exhibited lower growth rates in width than in length. The distal and proximal aponeurosis areas were nearly identical. The ratio of aponeurosis area to the physiological cross-sectional area was 2.54, 2.54, and 1.88 for M. soleus, M. extensor digitorum longus, and M. plantaris, respectively. M. extensor digitorum longus and M. soleus exhibited a nearly similar tendon-muscle fascicle length ratio during growth, increasing from 2.86 to 5.30 and 3.48 to 6.16, respectively. Interestingly, the tendon-muscle fascicle length ratio of the M. plantaris started initially with a much higher value (∼8) and increased to ∼18. Taken together, these results provide insight into the structure of the muscle-tendon complex and thus, a general understanding of muscle growth. Anat Rec, 300:1123-1136, 2017.