Hi Jim, the route to Muir is mostly class 2 (after you leave the main Whitney trail) with a very short section of class 3 to gain the summit block. The exposure is not bad. If the climbing is more difficult than that, or you encounter a lot of exposure, you are probably off route. For detailed info on this ascent, check out this <a href="http://www.summitpost.com/mountains/route_link.pl/route_id/168/object_id/359">site</a>. Mt. Muir is well worth the extra few minutes it takes to get up there.