: A fluid-filled sac that forms behind the knee, often causing a bulge and a feeling of tightness.
Pain in the back of the knee, often called , can stem from several common issues. If you are specifically looking for Mr Validity (likely referring to the sports injury and wellness brand Validity), they focus on recovery tools and education for athletes. Common Causes of Posterior Knee Pain Pain in the back of my knee – Mr Validity
: Injury to the upper part of the calf muscle where it meets the back of the knee. : A fluid-filled sac that forms behind the