Due to the large amount of money available for two year old racing, horses are often placed into training well before they are two years of age (Hiney et al., 2000). Traditionally, training regimes have concentrated on improving muscle strength and cardiovascular function (Birch and Goodship, 1999). As the trainer starts to increase the work rate, the young horse’s ailments often increase; jarred joints, knocks, cracks, windgalls and curbs are all regularly seen in young racehorses. As well as the concussive forces which fracture bones, speed increases all the other forces applied to the horses’ body, stretching tendons and ligaments (Ivers, 1994, Marshall, 1994).
The physical demands imposed on the skeleton of a young racehorse by intense athletic activity are considerable, and as a result, injury to musculoskeletal tissues is the greatest cause of loss of horses from training (Birch and Goodship, 1999). McKee (1995) reported that the risk of bone fracture is highest during the first year of training and lowest in the period of 3-5 years in training, the risk of fracture being higher for horses who do no gallop work during training. Forelimb fractures account for the majority of catastrophic injuries, ruptured and severed tendons are also prevalent, with the majority being forelimb tendon injuries (Parkin, 2003).
Historically there has been a lack of knowledge and understanding of how each of the component tissues responds and adapts to different intensities and duration of exercise in the equine athlete. Due to the high incidence of injury, training regimes are now beginning to consider adaptation of skeletal tissues such that the system is able to withstand the rigours of racing (Birch and Goodship, 1999).
Ultimately, the integrity of the horse’s musculoskeletal system depends upon age, nutrition, mechanical forces and disease (Lawrence, Hoffman and Kronfeld, 1998). Optimum growth and maturation of the musculoskeletal system are therefore of vital importance to the conformation and ultimate athletic potential of the horse (Jeffcott, 1997).
The effects of early training on the physiological development of racehorses.
A study by Bathe (1994) on Thoroughbred racehorses in the UK reports an annual incidence of fractures of 9%, with the majority occurring during flat race training (cited in McKee, 1995). However, current studies provide sufficient evidence that it is humane to train and compete juvenile horses, disproving the theory that two year olds are more vulnerable than older horses (Sellnow, 2002; Wenholz, 2003).
Done correctly, exercising and competing a two year old horse can be beneficial as body tissues strengthen, remodel or reshape themselves to accommodate the forces they encounter, strengthening the musculoskeletal system of the horse and decreasing the long term risk of injury (Ivers, 1994; Jeffcott, 1997; Birch and Goodship, 1999; Wenholz, 2003). Studies conducted on racecourse fatalities have provided evidence that horses which started racing later in life are more likely to suffer subsequent fatal injury compared to those that started racing at the age of two (Wood, 2003).
The equine skeleton does not reach maturity until between four and six years of age, therefore a horse beginning training early in its second year still has an immature skeleton and both bone modelling and remodelling will occur at the same time (Nielsen et al., 1997).