Why do my legs tire so quickly when I run on the treadmill?

National Academy of Sports Medicine Answered:
Leg fatigue while running on a treadmill or outside is due to either overtraining or training at an intensity that is too high for your fitness level. A sign of overtraining is when you can no longer achieve the same workloads (speed and incline) that you usually achieve. Your body cannot generate the power it normally does. In this case, your legs will feel very heavy and slow. This is your body telling you to take a break and do some recovery exercises or enjoy a recovery day. Sometimes the fatigue is a good thing. If you push yourself to a higher intensity than you normally do and your legs fatigue at the end, it is a sign of overloading the muscles which ,in return, will help you take your fitness to the next level. If done correctly, you will recover before the next workout.
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