The human body is a wonderful piece of engineering that can be straight forward in understanding in some cases and at the same time difficult to figure out when trying to add all the pieces together. I m alluding to the interregional dependence of joints and soft tissues.
One such exemple is the role and effect hip internal rotation has on your lower back, your pelvis and your knee with very mundane activities, such as in walking, running, squatting... Internal Rotation (IR) mobility of your hips is for the most part overlooked by many in the medical and fitness field.
IR is, to keep it simple, the rolling ( rotation) inward of your femur ( thigh bone) on your pelvis, inside your hip capsule.
Most activities require a limited amount of hip internal rotation and noticing/assessing a loss is not often easily remarkable
If mobility is lacking, your body will compensate...
If you re lacking the 35 deg of normal IR mobility, it is likely you wont be able to squat or perform rotating trunk motion correctly...Your foot will roll in or, you knee will collapse inward or your lower back will rotate too far.
You're of course at more risk of injuring your lower back if you ´re lacking IR and your occupation or hobby includes repetitive twisting/ rotation ( I am looking at you golfers).
IR occurs with hip extension( thigh/ femur going behind / backward) , particularly when you walk. If you miss IR mobility, your body could compensate by turning your foot further outward, by your pelvis rotating too far (directly affecting your back), by shortening your step, by shortening the time you stand on that one leg, by overextending (straightening) your knee or arching your lower back too far. All these compensations will lead to muscular imbalance(s) and potential injuries and will perpetuate weakness in specific muscles.
So I would definitely encourage anyone to work on maintaining their hips IR mobility and strength so you can improve your golf swing, your squat during your crossfit class ( or your daily chores) and/or simply walk normally without limping.
I am including a tutorial on how to mobilize and stretch your hip into Internal Rotation.