The purpose of this study was to establish the reliability of a frame-by-frame cross correlation method of assessing longitudinal sciatic nerve excursion motion using real time ultrasound imaging during a modified passive straight leg raise (SLR) test. Eighteen asymptomatic participants (age range 19-68 years) lay on their sides on a purpose made jig and the sciatic nerve in the posterior thigh was imaged during knee extension at 30° and then 60° of hip flexion (HF). Participants were re-tested ≥48 h later. The ultrasound images were analysed off-line using cross correlation software. Results demonstrated excellent repeatability of in vivo sciatic nerve excursion during a modified SLR (HF30° ICC 0.92, CI 0.79-0.97, SEM 0.69; HF60° ICC 0.96, CI 0.89-0.99, SEM 0.87). The authors also identify points of good practise to ensure an accurate as possible measurement of nerve excursion using this method. These include breaking down larger movements into sub-components, visually tracking the moving nerve during the tracking procedure, and ensuring the optimal image is captured prior to analysis. The use of ultrasound imaging in lower limb nerve dysfunction will enhance the understanding of how nerves move in vivo during neurodynamic testing, as well as being able to identify possible alteration to nerve movements in patients with neuropathic pain states.