Aging Baby Boomers obsessed by fitness will sympathize with the plight of President Clinton, who must suffer the consequences of taking a pratfall and ripping up his knee. A little advice for the First Jogger: don't do it. As a so-called impact-loading exercise, jogging puts maximum pressure on the knee, especially once it has been weakened by a major tendon tear. TIME Daily's prescription for fitness: ride a bike. It wi ll provide the same aerobic value while putting no damaging stress on that weakened knee -- and the spandex clothes are cool.