When excess phosphates are stored in the bones, muscles and tendons, it interferes with the body's ability to produce ATP, which it uses for energy. Eventually, "lumps and bumps" form in the muscles. Guaifenesin is supposed to help the kidneys excrete phosphates, preventing the buildup in the body.
(Source: Natural Treatments: An A to Z Guide, by Kenna Simmons. An official publication of the Arthritis Foundation. To purchase, visit www.arthritis.org.)