Comparative performance of isometric and isotonic quadriceps strength testing in total knee arthroplasty

Background: Early quadriceps muscle strength assessment after a total knee arthroplasty (TKA) provides timely information on progress, but little is known about the pain profile and predictive validity associated with common clinical muscle strength tests. This study aimed to, in patients with a recent TKA, examine the associations of isometric and isotonic quadriceps strength with gait speed, accounting for knee pain experienced during testing. Methods: A sample of 76 patients (mean age 68 years; 46 women) with a recent TKA (median, 1.5 months) participated. Quadriceps strength was measured on both limbs using a knee extension machine. Isotonic strength was assessed with a one-repetition maximum test. Isometric strength was measured at 40 degrees and 70 degrees of knee flexion using a custom-built load cell. To allow for valid comparisons between the tests, quadriceps strength symmetry ratios were calculated. Knee pain during testing was measured using an 11-point pain scale. Fast gait speed was measured using the 10-m walk test. Results: Compared with isotonic test, quadriceps strength ratio was higher for the 40 degrees flexion isometric test (P = 0.01), and this difference may be explained by the lower knee pain intensity elicited during the isometric tests (Ps < 0.001). All strength measures were closely associated with fast gait speed after adjustment for knee pain and covariates (Ps < 0.001). Conclusions: Early in the post-TKA period, isometric and isotonic strength tests may be used to assess quadriceps strength but these tests are not interchangeable. Isometric quadriceps testing may be preferable to isotonic testing as it was associated with lower knee pain intensity.