1)I can use tilt sensor to see how much I can stretch my knees while stretching my knees ( so if it is tilted then you are not flexible enough)
2) I can put the pressure sensor on the bottom of knees and see how hard I can press ( harder will show more flexibility on knees or legs)
3) Use fabric button on the bottom of knees and if it touches the point, could mean the flexibility.
4) Put one conductive fabric on the bottom of the knees and one on floor so when the knees touches the ground, it light the LED
5) Use capacitive sensor as the pressure sensor.