BACKGROUND: Ankle dorsiflexion ROM is commonly measured during the weight-bearing lunge test (WBLT) as horizontal knee distance travelled or tibial inclination. These measures are assumed to represent talocrural dorsiflexion, yet have not been validated against radiographic images. OBJECTIVES: To determine the: a) contribution of the talocrural joint to tibial inclination during the WBLT; b) validity of inclinometer and photographic measures of tibial inclination compared to radiographic images; and c) association between tibial inclination and horizontal distance measures. METHODS: Tibial inclination using an inclinometer, horizontal distance via a ruler, radiographic and photographic images were recorded on 20 participants in standing and end-range WBLT. Two assessors used computer software to measure talar rotation and tibial inclination from digital radiographs and photographs. Limits of agreements (LoA) between photographic and inclinometer measures against radiograph measures, and correlation between measures were calculated. RESULTS: At end-range WBLT, 91.8% of motion occurred at the talocrural joint with 8.2% occurring distally. There were very strong correlations (all r=0.88, p<0.001) between end range radiographic and photographic measures, radiographic and inclinometer measures, and radiographic, inclinometer, photographic measures of tibial inclination and horizontal lunge distance. Calculation of LoA indicated unacceptable agreement between inclinometer and radiograph measures (-7.84, 5.92) and acceptable agreement between photographic and radiographic measures (-2.17, 2.49). CONCLUSION: Tibial inclination during the WBLT primarily occurs at the talocrural joint. While inclinometer and photographic measures of tibial inclination can reliably be used clinically to measures dorsiflexion ROM during the WBLT, inclinometer results will differ slightly than those obtained from radiographs. J Orthop Sports Phys Ther, Epub 23 Jul 2019. doi:10.2519/jospt.2019.8697.