Participants in this arm will receive low intensity pulsed ultrasound therapy on the affected knees for 20 minutes per day, 5 days a week for 4 weeks. Conventional physiotherapy of quadriceps muscle exercise and health education about life style will also be given.

Participants in this arm will receive high intensity continuous ultrasound therapy on the affected knees for 10 minutes per day, 5 days a week for 4 weeks. Conventional physiotherapy of quadriceps muscle exercise and health education about life style will also be given.

Low intensity pulsed ultrasound therapy will use ultrasound with frequency of 1.5 MHz, power intensity of 30 mW/cm^2 (0.03 W/cm^2 ) and 20% duty cycle.

It is one of the most commonly used measures of pain intensity and it is divided into 10 ordinal ratings with 0 being "no pain" and 10 being "pain as bad as it could be".

This index measures clinically important symptoms of pain, stiffness and function. It consists of 23 questions (5 pain, 2 stiffness and 16 physical function) and the maximum score is 100, indicating the worst outcome while the minimum score is 0, indicating the best outcome.

Skin irritation, a possible complication of ultrasound therapy, will be recorded as a dichotomous variable (i.e. present or absent).

Tingling sensation, a possible complication of ultrasound therapy, will be recorded as a dichotomous variable (i.e. present or absent).

Oedema, a possible complication of ultrasound therapy, will be recorded as a dichotomous variable (i.e. present or absent).

Burns, a possible complication of ultrasound therapy, will be recorded as a dichotomous variable (i.e. present or absent).