In this week's FYI Fitness Tip, Phil Clark breaks down the science behind why running a marathon is so taxing on your legs.In this week's FYI Fitness Tip, expert trainer Phil Clark breaks down the science behind why running a marathon is so taxing on your legs. More tip videos:Why chains help in bench press exercisesDon't be intimidated by cable-pulley machinesA solution for your pull-up problemGetting the most from bench press machinesAb workouts that protect your backLunges minus the knee pain Report a correction or typo