I have a partially torn right acl (had the mensicus trimmed after getting a flap tear). The cruciates act to limit front to back shear in the knee. If you take your hand and cross your middle (signalling) finger over your index finger and put your hand on you knee (same side) the topmost finger is the anterior cruciate, the backmost the posterior cruciate. The anterior cruciate is tightest with your knee straight. In fact the last few degrees of extension, your lower leg actually turns outward a bit because of the tightnes (screw home mechanism). Strengthing the muscles that cross your knee also help stabilize the joint. The quads stress the ACL, the hamstrings don't.

Riding fixed may cause some stress on the ACL, because your quads would be active during resistance but I'm not sure how much would actually be a problem. Someone who is more math inclined than I may be able to figure it out; a healthy ACL fails at 1725 N.