Study Design: Repeated measures (3 separate day sessions) to determine test reliability; single-session repeated measures to compare stability between limbs. Objectives: To develop a functional test measuring dynamic stability that is capable of differentiating between the injured and uninjured lower limb in 2 populations: (1) people with anterior cruciate ligament deficiency (ACLd) and (2) people with anterior cruciate ligament reconstruction (ACLr), and to establish the reliability of this test. Background: Many functional tests of the lower limb used by clinicians, such as the 1-legged hop for distance, the 1-legged hop for time, the vertical jump, the triple hop for distance, shuttle run, and single-limb standing, do not allow the clinician to discern differences between function in the injured and uninjured limbs. Methods and Measures: Twenty-five nonimpaired subjects (14 men, 11 women, aged 31.2 ± 9.1 years), 11 subjects with ACLr (9 men, 2 women, aged 26.3 ± 10.4 years), and 13 subjects with ACLd (5 men, 8 women, aged 40.4 ± 12.6 years) were tested. Twelve nonimpaired subjects participated in 3 testing sessions to determine the reliability of the force plate measures. Ground reaction forces (vertical, medial-lateral, and anterior-posterior) were measured while the subjects performed 1-legged hop and stepdown tests onto a force plate. Stability was defined as the ability to transfer the vertical projection of the center of gravity to the supporting base and keep the knee still. A repeated-measures analysis of variance (2-factor; limbs by trials) was used to compare the stability between limbs. Results: The majority of the measures used to calculate dynamic stability were reliable. Moreover, the data provide normal standards of functional knee stability for step-down and hop tests. In the step-down test, changes in vertical force did identify dysfunction in the injured limb (stabilization time = 1527 ± 216 ms) compared to the uninjured limb (stabilization time = 892 ± 498 ms) for subjects with ACLr. Conclusions: The normal standards may serve as a reference for comparing functional differences in ACLr or ACLd populations. The vertical force parameter during a step-down may be useful as an outcome measure to monitor progress during rehabilitation.

J Orthop Sports Phys Ther. 1999;29(8):444-454.

Key Words: dynamic stabilization, force plate, knee, rehabilitation