Adaptation strategies of the Icelandic horse with induced forelimb lameness at walk, trot and tölt

Background and objective: Lameness assessment in the gaited Icelandic horse is complex. We aimed to describe their kinematic and temporal adaptation strategies in response to forelimb lameness at walk, trot and tölt. Study design: In vivo experiment. Methods: Ten clinically non-lame Icelandic horses were measured before and after reversible forelimb lameness induction. Upper body and limb kinematics were measured using 11 inertial measurement units mounted on the poll, withers, pelvis (tubera sacrale) and all four limbs and hoofs (Equimoves®, 500 Hz). Horses were measured on a straight line at walk and trot in-hand and at walk, trot and tölt while ridden. Linear mixed models were used to compare baseline and lame conditions (random factor = ‘horse’), and results are presented as the difference in estimated marginal means or percentage of change. Results: Lameness induction significantly (p < 0.05) increased head vertical movement asymmetry at walk (HDmin/HDmax_HAND: 18.8/5.7 mm, HDmin/HDmax_RIDDEN: 9.8/0.3 mm) and trot (HDmin/HDmax_HAND: 18.1/7.8 mm, HDmin/HDmax_RIDDEN: 24.0/9.3 mm). At the tölt, however, HDmin did not change significantly (1.1 mm), but HDmax increased by 11.2 mm (p < 0.05). Furthermore, pelvis vertical movement asymmetry (PDmax) increased by 4.9 mm, sound side dissociation decreased (−8.3%), and sound diagonal dissociation increased (6.5%). Other temporal stride variables were also affected, such as increased stance duration of both forelimbs at walk, tölt and in-hand trot. Main limitations: Only one degree of lameness (mild) was induced with an acute lameness model. Conclusions: Classical forelimb lameness metrics, such as vertical head and withers movement asymmetry, were less valuable at tölt compared to walk and trot, except for HDmax. Therefore, it is advised to primarily use the walk and trot to detect and quantify forelimb lameness in the Icelandic horse.