Something in the way you move : studies of movement asymmetry in young Standardbred trotters

Lameness is a major welfare issue in athletic horses. Traditional lameness evaluation is based on subjectively assessing whether the horse shows asymmetrical movement, and this can be challenging. Sensor-based systems that accurately quantify equine movement asymmetry are now available. However, knowledge is lacking regarding the biological and clinical relevance of measured movement asymmetry. This is especially true for specific types of horses, such as the Standardbred trotter. We conducted a prospective, longitudinal study to look at movement asymmetry in young Standardbred trotters. The main areas of interest were the prevalence and magnitude of movement asymmetry at the initiation of training (paper I), any changes seen in asymmetry with increasing age and increasing training (paper III), as well as under different training circumstances (paper II). We were also interested in reporting the occurrence of orthopaedic veterinary interventions during the study period. Finally, we wanted to look at the number of horses that completed a qualification race, as well as how many went on to participate in regular races before the end of their three-year-old season. Standardbred trotters were recruited as yearlings, just as they were starting their training. The study period was two years, until the end of the three-year-old season. Movement asymmetry was measured utilising an inertial measurement unit (IMU) system during both in-hand trotting and during driven exercise. Data was collected approximately every third month at 13 different training yards; 114 yearlings were recruited to the study. Study drop-out was substantial during the study period, as many horses changed trainers or were taken out of training for different reasons, including lameness. Our main findings were: 1) During the first months of training, a high proportion (94%) of Standardbred yearlings showed movement asymmetries. Most horses showed mild asymmetry. There was considerable variability (i.e. uncertainty) in the data, mainly due to the behaviour of inexperienced, fresh young horses. When asymmetry data from in-hand trials were compared to trials when the yearlings were driven on the track, there was no overall difference in asymmetry. However, substantial individual variation was observed, and some horses showed one asymmetry pattern in-hand and another altogether when driven on the track. This illustrates the importance of assessing the Standardbred on the track as well as in-hand during clinical lameness evaluation. 2) In two-year-old Standardbreds, trotting through a curve during driven exercise induced systematic changes in measured movement asymmetry. These changes did not entirely correspond to previously described changes in asymmetry for horses lunged in circles. The changes seen in our Standardbreds were overall small. When collecting IMU asymmetry data under similar circumstances (large diameter trotting track, non-banked turns and jogging speed) for clinical lameness evaluations, there is little need to differentiate between data from the straight part of the track and data from the curved part of the track. 3) During the first two years of training, movement asymmetry in our cohort of Standardbred trotters was substantial, ranging from mild to severe in magnitude. Most horses showed mild or moderate movement asymmetry. There was some evidence of an increase in asymmetry magnitude over time for hindlimb parameters, however, these results need to be interpreted with care due to the large number of horses dropping out during the study period. The most frequent orthopaedic veterinary intervention was therapeutic joint injections, which were more common for three-year-old horses in our cohort. Despite this, completion rates for qualification races were high (> 70%) and the percentage of study horses that entered regular races (64%) is above the national average for this age group. Our results contribute to the knowledge base of objectively measured movement asymmetry in young Standardbred trotters. Suggested future research should focus on the possible causal relationship between the magnitude of measured movement asymmetry and the development of clinical lameness. Further knowledge of the relationship between these two entities could potentially lead to earlier detection of clinical lameness