It was important that our model stayed within the tested joint ranges and kept the muscle fiber lengths within the active portion of the force-length curve. To satisfy the first condition, we lowered the feet and tilted the pelvis. We tired a couple different pelvic tilt angles and ultimately settled on 45 degrees because that gave us realistic fiber lengths and was a reasonable pelvic tilt. In addition, we lengthened the hamstrings and rectus femoris to account for the leg being shifted in the middle of the pelvis instead of its normal position.
To evaluate the second condition, we plotted the normalized fiber length through the knee flexion range while the hip was both maximally extended and maximally flexed (within the tested range). The curves are different for different initial position because OpenSim varies the knee angle without taking into account the constraints we set. But in any case, the curves are within the active portion of the force length curve, and therefore the passive forces are not predominant. The hamstrings are especially tight, and the rectus femoris loose: they are not working in the optimal range because of the seated position, which is what we expected.
First Results - Challenges
We would like to thank Scott Delp and the entire ME 485 teaching team, especially to Calvin Kuo for all the help and guidance with our model.
1. Gibbons, RS., McCarty, ID., Gall, A., Stock, CG., Shipppen, J., Adrews, BJ. Can FES-rowing mediate bone mineral density in SCI: a pilot study. Spinal Cord. 52, S4-S5 (2014).