The athletic demands in the Standardbred trotters industry require young, still growing horses, to be trained well above light exercise level. The energy needs for such young horses consist not only of maintenance and growth but also of requirements for work. The risk of occurrence of energy imbalance during that period is high. In man, the lack of energy homeostasis is considered as the basic problem in the development of chronic fatigue potentially leading to overtraining (OT) syndrome. Moreover, chronic fatigue leads to downregulation of hypothalamic hormonal and sympathoadrenergic responses and catabolism (Steinacker et al. 2005).
In horses, overtraining is also considered as a cause of poor performance. As in human athletes, frequent signs of OT horses include behavioural disturbances, weight loss with correct feed intake, muscle soreness, increased occurrence of musculoskeletal accidents and higher sensitivity to infections. Overtraining-induced studies have been conducted over the past 15 years (Bruin et al. 1994. Tyler et al. 1996, 1998; Golland et al. 1999, 2003; Tyler-McGowan et al. 1999; Hamlin et al. 2002; McGowan et al. 2002; Wijnberg et al. 2008; de Graaf-Roelfsema et al. 2009). In these studies, the experimental difficulties reported are the identification of a clear loss of performance sustained for at least 2 weeks and the inclusion of a control group (Rivero et al. 2008). However, little is known about the occurrence of maladaptation to training under real training conditions. That field longitudinal study compared morphological, haematological and endocrine changes in a group of 14 young Standardbreds presenting signs of maladaptation to training to those observed in a group of 40 individuals serving as a control and the aim was to identify objective biomarkers of early maladaptation to training in young racehorses under field conditions.