High intensity, short duration pulling in heavy horses: physiological effects of competition and rapid weight change.

Background: The Heavy Horse Pull is a competition where teams of two horses pull an increasingly heavy sled for a short distance. Similar to human wrestlers, some horses may undergo rapid weight change in order to enter a lower weight category. The objectives were to study the physiological effects of this practice as well as of the pulling competition in draft horses.

Results: Fifty horses were divided into light-, middle- and heavyweight categories based on their arrival weights and competed 1-3 days after. Body weight was measured upon arrival and pre-competition. Blood was sampled for chemistry and high sensitivity cardiac troponin T (hscTnT) at arrival, pre- and post-competition in 34, 26 and 20 horses, respectively. Body weight increased significantly between arrival and pre-competition for light (7.2% (Median: 62.8Kg (41.7-77.0)) and middle (8.6% (Median: 80.5Kg (62.7-90.9)) weight categories. Change in body weight was correlated (r = 0.69, p = 0.002) with competition ranking for middleweights. The ratios of weight pulled to team body weight were 2.7 (1.9-2.8), 2.6 (2.5-2.6) and 2.4 (2.2-2.5) for the lightweights, middleweights and heavyweights, respectively. Blood chemistry indicated hemoconcentration on arrival in the middleweight and lightweight horses. Hemoconcentration was not seen on arrival in some horses with marked rapid weight change. Overall, no chemistry parameter changed between pre- and post-competition. The hscTnT stayed within normal range post-competition.

Conclusions: While horses arrived to the event with indications of hemoconcentration, they appeared to have sufficient time to rehydrate prior to competition, and the effects of the competition were reversible within 3 h.