Running as swinging the legs vs running as a more circular motion.
Some people may think of the leg movement while running mainly as the legs swinging forward and backward. This is the only movement the the thighs can make, but the lower leg and foot have the freedom to move in a more circular motion. Not creating a more circular motion may result in a few different problems. One problem may be that the strides are reduced to reaching ahead with the foot and falling onto it. In this movement, the passive lead leg is not pulling on the ground to propel the body forward. It is probably just absorbing the impact and pushing the body more upward than forward. This movement usually involves a heel strike. It may also involve an awkward forefoot strike since the foot is not under the body.
Another problem with the swinging motion might be a long and passive trail leg. The hamstring may be inactive decreasing the amount that the leg bends. If the leg does not bend to an optimal angle, the leg will be longer and slower. With these mechanics, the hip flexor may be emphasized. This may result in overusing the hip flexor if the leg is brought forward quickly, or if the effort from the hip flexor is being conserved, the leg will just move forward slowly. The leg will probably move forward when the lead foot contacts the ground preventing the knees from being close to aligning at ground contact.