There is an increased risk of both injuries and cardiovascular events during speed sessions. Be sure to get your doctor's OK before beginning a speed program. Don't ever continue to run - even 10 yards - when you suspect that you may have an injury or a real cardiosvascular problem. Stop, take an extra 2-3 days off and get medical advice if necessary. When in doubt, take more rest, more days off, and run slower. In other words... be more conservative.