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Relationship between muscle strength and knee pain in knee osteoarthritis patients

      Purpose: Knee osteoarthritis is a motor disorder that leads to decreased QOL and physical function in old age, and is one cause of the need for nursing care. The prevention of knee osteoarthritis and amelioration of its symptoms are an urgent issue today with rapidly increasing elderly populations. The importance of knee muscle strength training in conservative treatment is well known, and it effects have been verified. To elucidate the relationship between muscle strength and pain symptoms, we investigated the relationship between muscle strength, measured with a knee extension and flexion strength measurement instrument we are currently developing, and knee pain in activities of daily living.
      Methods: The subjects were 92 women (mean age 73.8 ± 8.6 years) with knee osteoarthritis who were being treated in the orthopedics department of our center, comprising 160 knees (right 75, left 85) that had not been treated surgically. Standing frontal X-ray images of the knee were evaluated and knee pain during activities of daily living (level ground walking, stair climbing, lying down, standing up, and sitting on floor with legs folded underneath the body) was surveyed with a questionnaire. Knee extension and flexion strength was measured using a prototype measuring instrument developed jointly with the Department of Gerontechnology at our center. The knee extension and flexion measuring instrument can be transported on a cart, and uses an Imada Co., Ltd. force gauge for measuring the precision of industrial products. With subjects in a sitting position with legs flexed 90° and a strap on the ankle joint, knee extension and flexion strength were measured isometrically for 3 seconds. Knee flexion strength and extension strength were measured two times each in the left and right legs and the better value for each side was utilized. Knee extension and flexion strength were expressed as the proportion to body weight, and the difference of the two, (extension strength/weight) minus (flexion strength/weight), was also calculated. The correlation between those indices and the knee pain score during activities of daily living was investigated.
      Results: In the right knee, significant correlations were seen between knee extension strength/weight and the scores for knee pain during level ground walking, stair climbing, and sitting on the floor with legs underneath the body (walking, stair climbing p < 0.01, sitting on the floor with legs underneath p < 0.05). In the left knee, significant correlations were seen between knee extension strength/weight and knee pain scores during level ground walking, stair climbing, standing up, and sitting on the floor with legs underneath the body (walking, standing up, sitting on the floor with legs underneath p < 0.01, stair climbing p < 0.05). Knee flexion strength/weight was found to be significantly correlated with knee pain during level ground walking in the right knee (p < 0.05). The correlation with the knee pain score and the difference between extension strength/weight and flexion strength/weight were significant in all activities in the right knee (walking, stair climbing p < 0.01, others p < 0.05), while in the left knee these values were significant during walking, standing up, and sitting on the floor with legs underneath (standing up p < 0.01, walking, sitting on the floor with legs underneath p<0.05).
      Conclusions: In knee osteoarthritis patients, knee pain during activities of daily living increases with decreases in knee extension strength in both left and right legs. Differences were seen between right and left in the activities in which a relation between muscle strength and knee pain appeared. Investigations that consider leg strength in proportion to weight or include flexion strength are considered to be useful.