Comparison of Maximum Isometric Strength of the Hip Joint Abductor and Knee Joint Extensor Muscles between Knee Osteoarthritis Patients with and without Self-reported Instability

Patients with knee osteoarthritis (KOA) often have impaired muscle function of the weight-bearing muscles, particularly in the knee and hip joints. This can lead to a significant loss of strength and power and may play a role in the perceived instability of the knee joint. The purpose of this study was to compare the maximum isometric strength of the hip abductor and knee extensor muscles between patients with KOA with and without perceived instability.Nineteen patients with KOA participated in this cross-sectional study and were divided into two groups. The first group (n = 10; women = 4, men = 6, mean age = 67.4 ± standard deviation [SD] 6.4 years) consisted of patients with self-reported instability in the knee joint, and the second group (n = 9; women = 5, men = 4, mean age = 69.6 ± SD 6.7 years) consisted of patients without self-reported instability. Functional and activity limitations were quantified using the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC). Maximum isometric strength of the hip abductors and knee extensors was measured using a belt-mounted handheld dynamometer and expressed as torque (Newton meters [Nm]) by multiplication with the determined lever arm. Torque was normalized to body weight and height.Patients with instability (median WOMAC score = 68) achieved a significantly lower mean torque in hip abduction than the patients without instability (median WOMAC score = 39) (p = 0.01; Cohen's d = 1.31). There was no significant difference in knee extension torque between the groups (p = 0.202; Cohen's d = 0.58).KOA patients with instability were able to develop significantly lower hip abductor strength than those without instability, suggesting that targeted strength training of this muscle group may be important for this group of patients.