How “high knees” can be misleading.
Sometimes high-knee drills are used for conditioning. This might transfer over to be an emphasis in running mechanics and can lead to some inefficiencies. Problems may follow from connecting an upward high knee movement with a downward movement. Although the thigh must always move down at some point in the stride, it can be done with a downward linear force, or with a backward rotational force. The downward force is the problem that can come from thinking of high knees and the vertical component of the leg movement. The downward force will drive the body upward rather than forward.
To prevent this problem, the emphasis should be on creating backward force to pull on the ground. The knee should still be high, but rather than thinking of a high knee, you should emphasize the rotation of the thigh. By thinking of rotation, the backward force can be generated on ground contact as the thigh continues to rotate. This rotation will also make the foot contact the ground closer to the point below the hips. When thinking of a vertical motion, the movement may emphasize the downward force at the time of ground contact ending any rotation of the thigh and decreasing the backward force on the ground.
Emphasizing high knees and a vertical motion can be beneficial if transferred to the drive phase of the first few strides in a sprint. The same movements done while the body is upright can be done while the whole body system is angled forward near a 45-degree angle. At this angle, the high knee movement (formerly vertical movement) is now driving at a 45 degree angle. This accelerates the body forward rather than upward.