Relationship between QRS-duration (heart score) and racing performance in trotters

Heart scores increased with age and training. Yearlings had average heart scores of approximately 90 milliseconds (msecs). This increased to approximately 110 msecs in 5-year-old horses, after which there was only slight further increase. In horses 4 years or older, regression analysis revealed a highly significant correlation between heart score and kilometre time. In younger horses the level of significance was less. There was a positive correlation between heart scores and earnings in races. Thus, horses with heart scores above 115 msecs had won considerably more than horses with lower heart scores. This relationship was closer in stallions than in mares.