Hello, and welcome @rhea Yes, Running, whether outdoors or on a treadmill, is a high-impact exercise that can be tough on your knees. The knees are a complex system of joints and are susceptible to injury because of the weight they are required to bear. High-impact exercise like running creates a great deal of stress on your knees. The repetitive motion required to maintain speed on a treadmill places extra strain on your knees. Over time and with heavy treadmill use, this can increase the risk of stress injuries to the knees. As you said you are sometimes experiencing pain in the knees kindly request you to visit a doctor.