I do my "sprinting" during spin classes. I spin about four or five days a week (four hours total (max) per week), and usually do not exceed 70% to 75% of my max heart rate. One day a week I sprint all out on the bike, usually above 170 RPM for about 15-20 seconds then recover to below 70% max heart rate. I do this for, usually, five or six "sprints". During the sprint I get to, sometimes, 90% or even 100% of my max heart rate of 168 beats per minute. (I am 56 years old with a 39-43 resting heart rate.)

Is a 95% to 100% max heart rate effort too extreme for sprinting, even though it's only for a few seconds at a time, usually for no more than five or six times, and no more than one day a week?

For what it's worth, I spin since I have had knee surgery, and am a rabid snow skier. I save my knees for Phat Rocky Mountain Pow!