Your knees are the largest, heaviest, hinge joints in your body. Because so many muscles of the upper leg are inserted into this joint, a multitude of factors can cause pain and tightness around the knee.
The major muscles of the thigh, inner thigh, and outer thigh all insert either directly into the knee or the region immediately surrounding the knee (See Reference 1). Any activity involving strenuous use of the leg muscles will generate a tightness and soreness in the thigh as well as around the knee.
There is a vast network of tendons that connect all of the thigh muscles into the areas into and around the knee. Strain inflicted on any of the tendons around the knee may also pull on the corresponding leg muscles, creating tightness and pain on multiple levels.
Fluid build-up around the knee can occur due to injury or overuse. (See Reference 2) The excess of fluid causes swelling, which adds tightness and pain around the knee that can also be felt in the muscles surrounding the knee.
According to the Mayo Clinic, common causes of knee pain include impact to the knee, repetitive motion or overuse, making sudden turns or pivots, poor landing from a fall or jump, or degeneration with age. (See Reference 3)
Regular exercise and stretching is a key preventive for knee pain. Keeping the muscles around the knee strong with weight-bearing exercise and keeping the tendons supple with stretching can reduce pain and tightness; it will also help to avoid further injury.