The subjects in this group will perform isometric exercises, including isometric quadriceps, straight-leg raising, and isometric hip adduction exercises for 5 seconds.

All exercises will be performed in sets of 12 repetitions: one set twice a day for the 1st week, which will be progressed weekly until three sets twice a day by 3th week. Allow 5 seconds of rest between each repetition and 3 minutes of rest between sets, with a total contraction time of 60 to 180 seconds per session at 30 to 50% of maximum voluntary contraction (MVC).

The subjects in this group will perform isometric exercises, including isometric quadriceps, straight leg raising, and isometric hip adduction exercise for 10 sec.

All exercises will be performed in sets of 6 repetitions: one set twice a day for the 1st week, which will be progressed weekly until three sets twice a day by 3th week. Allow 10 seconds of rest between each repetition and 3 minutes of rest between sets, with a total contraction time of 60 to 180 seconds per session at 30 to 50% of MVC.

The subjects in this group will perform isometric exercises, including isometric quadriceps, straight leg raising, and isometric hip adduction exercise for 20 sec.

All exercises will be performed in sets of 3 repetitions: one set twice a day for the 1st week, which will be progressed weekly until three sets twice a day by 3th week. Allow 20 seconds of rest between each repetition and 3 minutes of rest between sets, with a total contraction time of 60 to 180 seconds per session at 30 to 50% of MVC.

Isometric quadriceps exercise: Patients lie in a supine position. A rolled-up towel was put beneath the knee. They were instructed to maximally activate their thigh muscles to straighten their knee and hold the contraction for 5 seconds (Group A), 10 seconds (Group B), or 20 seconds (Group C).

The Numeric Pain Rating Scale (NPRS) is the simplest and most commonly used numeric scale to rate pain, ranging from 0 (no pain) to 10 (worst pain). The NPRS will be used for subjective pain measurement, which has good test-retest reliability

We will focus only on stiffness and physical function. The Western Ontario and McMaster Universities Osteoarthritis (WOMAC) has been widely used by clinicians in assessing patients with knee OA since it was developed in 1986 (Bellamy, 1995) to evaluate the symptoms of pain, stiffness, and physical function in patients with hip and/or knee OA. It consists of 24 questions and is divided into 3 subgroups: the pain subgroup consists of 5 questions, the stiffness subgroup consists of 2 questions, and the physical function subgroup consists of 17 questions. Each question receives a value between 0 and 4, and the total score is calculated as follows: no difficulty is scored as 0, mild as 1, moderate as 2, severe as 3, and extreme as 4. Higher scores indicate greater physical dysfunction and disability, and thereby worse health-related quality of life

The muscle strength of the lower limbs was measured by the Five-Times-Sit-to-Stand Test (FTSST), in which participants rise from a chair and return to a seated position as quickly as possible with their arms folded across their chests. The time to complete five repetitions was recorded for two separate trials, with a 1-minute rest between each trial. The mean of the two trials was computed and used in the analysis

1RM test has good-to-excellent test-retest reliability. the participants will be asked to perform active knee extension against different pregressive increasing weigths to determine the 1 RM. 2 minutes rest will be allowed before each contraction effort to allow recovery of the tested muscle.