Agreement between different methods to calculate lactate threshold in standardbred racehorses

In equine sports medicine, the anaerobic threshold is traditionally defined as the Onset of Blood Lactate Accumulation (OBLA), fixed at 4 mmol/L [1]. Neverthless, in human medicine it has been recognized that a fixed value does not consider the interindividual differences in lactate metabolism. Therefore, several methods have been developed to define the individual Lactate Threshold (LT) [2]. Aim of the present study is to calculate the individual LT in racehorses by four different techniques applied in human medicine and to evaluate the agreement between the methods. From a large cohort of horses investigated for performance profiling, fifteen healthy and well performing standardbred racehorses (aged 3.1±0.8 years old) were selected. Each horse underwent an incremental treadmill test as previously reported [3]. Blood samples were collected for each speed step of the test by means of a jugular catheter. Plasma lactate was measured with an enzymatic colorimetric method. Lactate values were analyzed with a dedicated sotware [4] and LT was determined by the following methods: Inflection Point (IP) as the point of intersection between the two linear splines, Lactate Threshold by logaritmic transformation (Log-log LT), OBLA at 4 mmol/L and Initial Rise of 1 mmol/L post baseline. LT values obtained by the four methods were stastistically compared by Friedman test and Dunn's Multiple Comparison test. Correlation between the values calculated by different methods was evaluated by Spearman test. Statistical significance was set at p <0.05. Statistical analysis showed a highly significant difference (p<0.0001) between the different threshold values. In particular, differences were found between IP and Log-log LT (p<0.0001) and between IP and Initial Rise of 1 mmol/L (p<0.0001). Correlation was significant only between OBLA and Initial Rise of 1mmol/L (p<0.05). Results showed lack of concordance between the different methods, therefore they cannot be used interchangeably, as reported in human sport medicine [5]. Further studies are needed to determine the gold standard technique to calculate LT in horses and the method that better associates with performance.