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Hello Homoeopathic constitutional treatment will be really effective in these problems as it treat patient as a whole and medicine given will stimulate your vitality and initiate natural healing process without any side effects We have very good medicines to treat your condition from the root itself without having any side effects I need your detail case history to guide u properly if u wish to take proper treatment consult online through practo app for proper management

Need to discuss to find exact reason

Though you are not under weight,  body composition also need to consider. Strength training exercise/ functional training along with right nutrition will help you to improve your strength and muscle mass.

Too weak could mean that your bone and muscle strength is on the lower side and may be your reflex to quickly adapt to positions as you exercise is low it could be due toeither  lack or over physical activity of + defeciency in your diet. to support your skeletal system interms of bone nutrients such as calcium,vitamin D,,vitamin K ,protein etc.

Gud evening u need to have a proper healthy balanced diet with some light forms of yoga 20mins minimum to be done for strengthening your body.

You are not underweight but your body doesn't have enough nutrition to support your weight and the knees are the total support of your body.. So it has affected it more.