Lameness is the most common reason for elimination of horses from endurance competitions. Due to the nature of these competitions, the causes of lameness are not quantified or determined. Exercise associated increases in serum muscle enzyme concentrations are commonly reported in endurance horses during competition. It is unclear what role, if any, muscle damage may play in lameness in endurance horses. The aim of the present study was to investigate changes in muscle enzyme concentrations in horses eliminated for lameness from a 160 km endurance ride.
Horses in the study were competitors in Tom Quilty Gold Cup National Championship 160 km ride held in Western Australia in September, 2007. Blood samples were collected before the ride and at the completion of the ride (either after the final veterinary examination at 160 km or after the horse was eliminated at one of the during-ride veterinary check points). Veterinary examinations were performed on horses before, during, and at the finish of the ride as per standard endurance ride protocol.
Of the 48 horses participating in the study, 18 (38%) completed the ride, 16 (33%) were eliminated for lameness, 11 (23%) were eliminated for metabolic concerns (insufficient recovery to continue) and 3 (6%) were withdrawn at the riders' discretion. Mean speed of the finishers was 8.7 ± 0.6 km/hr. There was no clinical evidence of muscle damage or cramping in any of the pre-ride or post-ride veterinary examinations. All horses except for one had creatine kinase (CK) and aspartate aminotransferase (AST) concentrations within normal reference range in pre-ride blood samples. Seven (15%) of the 48 horses had post-ride CK activities greater than 10,000 iu/l, of these 4 were eliminated due to lameness, 1 was eliminated for metabolic concerns and 2 successfully finished the ride. The CK and AST concentrations for finishers, lameness eliminations and metabolic eliminations were 3722 ± 6886 and 1028 ± 1040 iu/l; 11926 ± 24788 and 1272 ± 1259 iu/l; and 2288 ± 2296 and 735 ± 796 iu/l, respectively. Although there was no significant difference in CK and AST activity at the end of the ride between finishers and non-finishers, 2 horses eliminated for lameness had the greatest increases in CK and AST activity.
Similar to the results of previous studies, the majority of horses eliminated from endurance competition had clinical signs of lameness. While a number of the lame horses also showed significant increases in muscle enzyme activities, a direct association between lameness and muscle enzyme activity could not be demonstrated. Despite being the major cause of elimination from endurance competition, there is limited information on causes of lameness during endurance events. Further studies exploring reasons for lameness in endurance competitions are needed.