Perhaps you sit like the woman pictured above: any seated, cross-legged position causes your knees to rise. You aren’t able to sit like this for long without shifting your weight around, let alone use the posture for meditation. Rising knees are usually caused by tight hips, a condition which also may cause straining in the lower back.
To remedy the discomfort and allow yourself to sit longer and more comfortably, simply sit on a yoga block or a stack of blankets. This will elevate your hips just enough to let gravity pull your knees and quadriceps downward, allowing you to sustain the posture. Want more yoga tips? Please head over to the Innerspace mothership!