To suggest that classical musicians are physical workers, like athletes, is to invite a wave of snickers and snide jokes about the fat guy in the back of the second violins. But the repetitive motion of playing a string instrument, for example, is tremendously stressful to the muscles involved, and increasingly, orchestra musicians have been sustaining career-threatening injuries from the simple act of pulling a bow back and forth. Enter Janet Horvath, who is on a one-woman crusade to teach orchestral musicians how to avoid such injuries. Horvath’s credentials: she’s a cellist who, back in college, was hurting so badly that she couldn’t turn a doorknob, but who has now served as associate principal cello of a major American orchestra for 20 years.