Muscle forces or gravity--what predominates mechanical loading on bone? Introduction.

This article describes the background, rationale, significance, and objective of the recent American College of Sports Medicine symposium entitled "Muscle Forces or Gravity-What Predominates Mechanical Loading on Bone?" (55th Annual Meeting, Indianapolis, IN, May 28, 2008) and introduces a series of papers representing the positions taken by three of the speakers at that symposium. Our goal was to reinvigorate discussion on a topic that will inform many, provoke some, and, most importantly, stimulate ideas that will encourage progress in the field of exercise prescription for bone.