When beach volleyball player Kerri Walsh won the gold medal in the 2008 Olympics, the black web on her right shoulder got nearly as much attention as her game. The mysterious material turned out to be kinesiology tape, which was stabilizing a rotator-cuff injury. One curious viewer, John MacKay, sensed a business opportunity after he discovered that the tape was being marketed only to physicians. His KT Tape, which comes in 10 colors, is sold over the counter to consumers who, say, pull a muscle or suffer from tendinitis. When placed strategically, the elastic adhesive strips, which sell for $13 to $20 a roll, can either activate or inhibit a targeted muscle. MacKay's company sold more than two million rolls of the stuff last year through some 22,000 drugstores and other retailers.