3d Fascicle Orientations in Triceps Surae 1 Journal of Applied Physiology (accepted Apr. 2013)

7 The aim of this study was to determine the 3D muscle fascicle architecture in human triceps 8 surae muscle at different contraction levels and muscle lengths. Six male subjects were tested for 9 three contraction levels (0%, 30% and 60% of MVC) and four ankle angles (-15°, 0°, 15° and 10 30° of plantar flexion), and the muscles were imaged with B-mode ultrasound coupled to 3D 11 position sensors. 3D fascicle orientations were represented in terms of pennation angle relative to 12 the major axis of the muscle and azimuthal angle (a new architectural parameter introduced in 13 this study representing the radial angle around the major axis). 3D orientations of the fascicles, 14 and the sheets along which they lie were regionalized in all the three muscles (medial and lateral 15 gastrocnemius and the soleus) and changed significantly with contraction level and ankle angle. 16 Changes in the azimuthal angle were of similar magnitude to the changes in pennation angle. The 17 3D information was used for an error analysis to determine the errors in predictions of pennation 18 that would occur in purely 2D studies. A comparison was made for assessing pennation in the 19 same plane for different contraction levels, or for adjusting the scanning plane orientation for 20 different contractions: there was no significant difference between the two simulated scanning 21 conditions for the gastrocnemii; however, a significant difference of 4.5° was obtained for the 22 soleus. Correct probe orientation is thus more critical during estimations of pennation for the 23 soleus than the gastrocnemii due to its more complex fascicle arrangement.