Three dimensional architecture of the whole human soleus muscle in vivo
Background Most data on the architecture of the human soleus muscle have been obtained from cadaveric dissection or two dimensional ultrasound imaging. We present the first comprehensive, quantitative study on the three dimensional anatomy of the human soleus muscle in vivo using diffusion tensor imaging (DTI) techniques. Methods We report three dimensional fascicle lengths, pennation angles, fascicle curvatures, physiological cross sectional areas and volumes in four compartments of the soleus at ankle joint angles of 69 12deg (plantarflexion, short muscle length; average SD across subjects) and 108 7deg (dorsiflexion, long muscle length) of six healthy young adults. Microdissection and three dimensional digitisation on two cadaveric muscles corroborated the compartmentalised structure of the soleus, and confirmed the validity of DTI based muscle fascicle reconstructions. Results The posterior compartments of the soleus comprised 80 5% of the total muscle volume (356 58 cm3) At the short muscle length, the average fascicle length, pennation angle and curvature was 37 8 mm, 31 3deg and 17 4 /m, respectively. We did not find differences in fascicle lengths between compartments. However, pennation angles were on average 12deg larger (p 0.01) in the posterior compartments than in the anterior compartments. For every centimetre that the muscle tendon unit lengthened, fascicle lengths increased by 3.7 0.8 mm, pennation angles decreased by 3.2 0.9deg and curvatures decreased by 2.7 0.8 /m. Fascicles in the posterior compartments rotated almost twice as much as in the anterior compartments during passive lengthening. Discussion The homogeneity in fascicle lengths and inhomogeneity in pennation angles of the soleus may indicate a functionally different role for the anterior and posterior compartments. The data and techniques presented here demonstrate how DTI can be used to obtain detailed, quantitative measurements of the anatomy of complex skeletal muscles in living humans.
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