on the front (anterior) of the thigh and the hamstrings
on the back (posterior) of the thigh represent two large groups of gross motor muscles that move both the hip and knee joints.
The rectus femoris, is one of the "quads." It crosses both the hip and the knee, and moves the hip joint into flexion; all the quads together, attach below the knee, and extend or straighten the knee. The hamstrings extend the hip, bringing the leg behind us (as in walking) and flex or bend the knee.
These two groups of muscles are supposed work in a finely orchestrated interaction of the movement of the hips and knees in an efficient, smooth, graceful, easefull way. Unfortunately, imbalance between these two muscle groups is very common. Some examples include, one or both of these muscle groups, or their individual muscles, may carry excess tension or contraction, or the quads may become over developed in relationship to the hamstrings. In either case, an inefficient coordination between the hips and knees can easily lead to range of motion imbalance where one hip may flex or extend well, but the other is "limited." Pain, stiffness, limping, favoring of one leg over the other - an experience often producing the sense of having a "strong" and "weak" leg - swelling, inflammation, and more, can result.
If you feel you carry tension, pain, and/or stiffness in the hip or knee joints I encourage you to watch all four videos and practice these movements. They demonstrate self-pandicular movement, that is voluntary movement, done with awareness, to reset the resting tonus of the muscles. That means the level of tension is reduced, the brain can coordinate their actions more efficiently, and you move with more ease and grace, less stiffness, less pain, and more fluidity.
Unfortunately, exercise, does not necessarily reduce levels of pain and discomfort. Have you ever done a great workout or attended a beloved yoga or exercise class, but still continue to have "that nagging pain or stiffness." Exercise is good for us and can contribute, for example, to cardiovascular health, an increase in energy and well being, but it may not relieve that low back pain, or sense of joint stiffness in the hips and knees that we had hoped it would.
Learning to do movement in a pandicular way, incorporating slow, comfortable, focused, voluntary movement, emphasizing the role of the motor cortex of the brain, may be the answer for you and an important contribution to your over-all health and exercise plan. Pandicular movement is done voluntarily and with awareness. I recommend you both move into and especially out of the contraction slowly, with as much control as possible, giving the motor cortex a chance to release excess contraction and more efficiently refresh and or re-configure motor programs.
Remember to move slowly and comfortably. Focus on the sensations of the movements. Always stay within your comfort zone.