Introduction: Single leg hop for distance is a test often used as a measure of knee performance and stability during rehabilitation after knee surgery or injury. Both distance hopped and qualitative assessment of stability in landing is widely used as parameters of knee joint control. While hop distance is reported as highly reliable, no investigations have studied the reliability of the biomechanical parameters expressing the quality of the landing after a single leg hop. The aim of the present study was to investigate the reliability of hop distance and biomechanical landing parameters during a single leg hop test.

Methods: The study was designed as an intra-tester, inter-day test-retest reliability study. Fourteen (7 males, 7 females) physically active, healthy subjects volunteered to participate. The subjects performed 5 maximal single leg hops for distance including 2 trial jumps, and the mean of the last 3 hops was used for analysis. The test session was repeated after 1 week. In both sessions the same tester placed markers on the lower body (Helen Hayes model) and the subjects landed after maximal hopping on a force plate. The hops were recorded using an 8 camera Vicon 612 system filming at 200 Hz. Kinematic and kinetic data were calculated using inherent Vicon software. Intraclass Correlation Coefficient (2,1) was used for analysis of reliability on selected kinematic and kinetic knee joint parameters.

Results: The ICC of the maximal hop distance was excellent (0.93, p< 0.001). The reliability of the maximal knee joint flexion during landing was poor and non-significant, and also peak knee extensor moment during landing showed poor reliability (ICC: 0.48, p=0.037). The maximal external knee joint varus moment and the relative eccentric power production of the knee joint in comparison to the hip and ankle joints were moderately reliable (ICC: 0.56, p=0.015, and ICC: 0.64, p=0.005, respectively).

Conclusions: This study shows, that in healthy subjects the reliability of the maximal hop distance is excellent, however the underlying biomechanical parameters controlling the knee joint during landing is only moderately or poorly reliable. This may imply, that the subjects use slightly different strategies during landing from the hop and evaluation of knee joint performance based on landing biomechanics may be done with caution. Future reliability and validation studies of the take-off biomechanics may further reveal if the single leg hop test is reliable and valid as a measure of knee joint performance.