Johnson, J.L.; Moore-Colyer, M. Equine Veterinary Journal March 2009 41 3 301-303(3) Reasons for performing study: Research into kinematics of the healthy equine back, has been performed in the walk and trot. This study focuses on back kinematics during canter, over a range of velocities. Flexion extension (FE) movements in canter are greatest in the lumbosacral (LS) region. Previous research has focused on canter velocity of 7 m/s; therefore quantification of LS kinematics at varying velocities is required to understand LS functions in equine locomotion. Hypothesis: Range of flexion-extension movement through the lumbosacral joint increases with increasing velocity. Methods: Six Thoroughbred horses (mean age 9.6 years) cantered on treadmill at 4 velocities (6.0, 6.5, 7.0 and 8.0 m/s, respectively). Reflective markers were placed over the 5th lumbar vertebra (L5), the lumbosacral junction (LS) and the 3rd sacral vertebra (S3). Lumbosacral angle (LS) was defined as the angle formed between L5, LS and S3. Flexion-extension (FE) range of motion (ROM) was analysed using a 2 camera, 3D motion capture system ProReflex. Linear regression was used to determine strengths of relationships between speed of canter and lumbosacral FE movements. Results: Range of FE ROM seen at the lumbosacral joint increased linearly with speed. FE ROM ranged 6.1º ± 1.9 at 6 m/s, 6.3º ± 1.9 at 6.5 m/s, 6.6º ± 1.9 at 7 m/s and 7.2º ± 1.9 at 8 m/s. Linear regression showed positive associations between speed and LS FE range of motion (r² = 0.993; P = 0.003). Conclusions and potential relevance: Results show linear relationships between LS FE movements and submaximal canter velocities. These results provide information on the LS joint at canter. Understanding the effects of velocity on the back of healthy horses may aid our understanding of the demands placed on this joint in sport horses at this gait.