Why do my legs fatigue on my stationary bike before my heart rate gets up?

Leg fatigue is a common problem when cycling and is often simply the result of weak legs. Often when individuals start cycling they may accustomed to other exercise modes such as running. Running incorporates different muscles in the legs than cycling, so you need to strengthen your legs for cycling by actually cycling. You will need to strengthen your legs through hill climbs on the bike before you can start to really work on heart rate training. You must get your legs strong enough to be able to push your cardiovascular system. Once you are strong enough to train both your cardio and legs on the bike, you need to watch for signs of overtraining. Signs of overtraining include when your legs give out earlier than usual (at a lower watt or gear) and before you get your heart rate to your normal zones. This is a sign that you will need some recovery periods at a low intensity or complete rest before trying the high levels again.