Sprinting affects muscles in almost the exact same way intense weight lifting does. For proof, look at the thighs of an Olympic marathoner and an Olympic sprinter, and you’ll get the idea. The confusion the average person seems to be laboring under is that if they do a couple of sprint or intense weight lifting sessions a week they might “suddenly” wake up one morning looking like those kinds of athletes, instead of like the super model they ache to emulate. If only either of those outcomes ere possible! They are delusions. Doing some high intensity training, appropriate for your age and current level of fitness will only speed up the process of you getting a little fitter than you are now. Don’t worry. Just do it!