You only need the muscles to position the body. Structure and grounding toward the earth is the source of strength, not actual muscle exertion. If you can properly use your body you will not need the muscles for more than position, not resistance.
Real speed and strength comes from complete relaxation and not tension, which weight lifting only promotes.
The problem, in my eyes, of current weightlifters, is that they believe that they are healthy because they can lift certain weights under certain conditions. Move to a different context and that "strength" does no good and generally is a detriment both to coordination and to speed.
Let me add that this is from a martial artist perspective. Of course muscles are needed to do work like climb a mountain or such, but your are better at growth trying to climb that mountain than some form of gym weightlifting activity.