When you mention joint mobility, posture is not the first thing to come to mind. But if you stop and think about it, for those with “bad” knees who are not able to crouch down, you can easily see how that could affect your posture. For example, when you bend forward to pick something up, weak knees can cause back sprain and increase spinal pressure if not done correctly.
Causes of poor joint mobility can include age, gender, existing condition or injury, opposing muscle tightness, genetics, connective tissue elasticity and activity. According to the American Chiropractic Association, long-term practices of standing, sitting and lying down incorrectly can be corrected with time and vigilance. For some, a bit of guidance mixed with a good exercise and stretching program can help alleviate the problem. But for others, seeking medical advice may be the route needed.
For the purpose of this article, lets look at how poor knee flexibility can affect posture during the squat. In the following picture, the image on the left is showing good posture during the squat. His torso is tall, elbows are under the bar, and his hips are back. The only note I would mention here is that I do not recommend everyone go below parallel like that in their squat until they understand the mechanics of the exercise and how to activate the correct musculature. In the case of someone who has poor knee mobility, if they were to attempt going deep like that, the image on the right is probably what they would look like. Notice how the body compensates for a weakness in the kinetic chain, allowing you to get deeper in the squat by bending at the waist.