Thanks for writing your query to us.
Quadricep muscle weakness
can occur due to recent injury
due to overexertion,ageing, period of inactivity or sickness, arthritis
of knee. The net result is that quadricep muscle doesnot get enough stimulation to remain strong & giving way of the knee occurs. This may occur while walking or while getting in & out of chair or while getting down on stairs.
I would advise you to fix an appointment with a experienced sports medicine
physiotherapist . Intensive quadricep exercise will help you to overcome weakness of the muscle.