Can exceeding your target heart rate during aerobics have a detrimental effect on muscle gain and fat loss?

If you are in a fairly good physical condition going a little higher every now and then can be safe but over exertion should be avoided. It is possible to exceed the upper limit of your target heart rate without any ill effects, as long as you do not have coronary artery disease or are at risk for a heart attack.

What it may do, though, is leave you with a musculoskeletal injury or sore joints and muscles. Exceeding you target heart rates (THR) would not allow you exercise for longer duration which is most important while exercising.

Also remember to reevaluate your THR about every month or so based on your RHR (resting heart rate). The best way is to stay on equal carbohydrates, proteins, and fats combined with exercise.