6.2 Muscle moment arm variation with joint angle

Moment arms determine the moment-generating potential as well as the amount a muscle changes length with joint rotation. Moment arms vary with joint angle. They are determined by the geometry of the joint: center of rotation, and the relative locations of the attachment points of the muscle with respect to the joint. This question allows you to explore this relationship:

(a) Two important parameters about the moment arm vs. joint angle relationship are: the peak moment arm and the angle at which the moment arm peaks. In order to get a larger peak moment arm, would you want your muscle attachments to be closer to or further from the joint?

(b) Based on the triangle model below, determine the value of the peak moment arm () and the angle () at which the moment arm peaks, in terms of the distances from the attachment sites to the joint center,  and  . Assume that  < .

(c) What are the limitations to assuming a model such as this one?

(d) Why are peak moment arm and the angle at which the moment arm peaks important?