Lactate measurements in equine sport science

Lactate measurements in equine sport science

Accumulation of lactate in muscle cells and in the blood  is a normal consequence of fast exercise in  the horse. At low speeds, the horse is able to generate sufficient energy by catabolism of glycogen, glucose and fat. This metabolic process uses oxygen to generate ATP, and is referred to as aerobic. At higher speeds, aerobic metabolism does not regenerate ATP quickly enough. Pyruvate accumulates in muscle cells, and it is converted to lactate ions at increased rates.

Exercise at speeds greater than approximately 700–800 m/min recruits fast twitch skeletal muscle fibers. These fibers can be classified as being highly oxidative or highly glycolytic in nature. At fast speeds, recruitment of fast twitch, highly glycolytic muscle fibers results in accumulation of lactate anions and hydrogen ions in the muscle cells, due to the contribution of anaerobic glycolysis to ATP resynthesis. Both these ions diffuse into the extracellular fluid. It is generally thought that the stimulus for anaerobic glycolysis in skeletal muscle fibers during fast exercise is a limitation to the supply of oxygen at the cellular level.

The production of many molecules of lactate and hydrogen ions results in acidosis of both the skeletal muscle cells and the blood. The increasing acidity of muscle cells is implicated in fatigue during intense exercise, but the direct cause and effect is still debated. In any horse at top speed for about 800 m, the accumulation of lactate and the concomitant cellular acidosis has a negative effect on energy production by anaerobic glycolysis. The rate of ATP production is decreased, and the animal reduces speed.

Resting blood lactate concentration in the horse is approximately 1–1.5 mmol/L. At low speeds this value does not change greatly from the resting value. At moderate speeds lactate begins to accumulate in the blood. Accumulation of lactate in blood occurs most quickly when the work speed is faster than that at which blood lactate is about 4 mmol/L. This work speed at which blood lactate is 4 mmol/L is often referred to as the anaerobic threshold, or the speed at onset of accumulation of blood lactate (OBLA). It is also frequently referred to as VLa₄. VLa₄ is therefore the work velocity which results in a blood lactate of approximately 4 mmol/L. This value is derived from inspection of graphs of exercise speed (on the X axis) plotted against blood lactate concentration (on the Y axis). The rate of lactate accumulation generally parallels the accumulation of adrenaline in the blood.

At speeds greater than VLa₄, lactate accumulates rapidly in the blood. The general relationship between velocity and blood lactate is therefore usually described as exponential. However, if sufficient steps are used in the exercise test, the relationship is described by two straight lines, with an obvious velocity at which the blood lactate begins to accumulate in blood.

After a race, blood lactate concentrations are usually greater than 20 mmol/L. It is normal for the blood and muscle lactate concentration to then gradually decrease over a 1–2 h period after a race or fast work. Acidosis of muscle and blood is a normal result of fast work, and this acidosis is rapidly reversed by the horse’s own metabolism. Racehorses do not develop chronic acidosis due to lactate accumulation in training, and therefore alkaline supplements are not required.

The rate of decrease in blood lactate after exercise is affected by the activity during this period. For example, blood lactate decreases more quickly after strenuous exercise if the horse is trotted for 3 0minutes, rather than walked. However, such a practice might delay cooling of the horse.

Many studies of horses trained on both treadmills and on racetracks consistently demonstrate that training results in lower blood lactate concentrations at the same work speed. The speed at which blood lactate begins to accumulate rapidly, VLa₄, also increases. The horse is able to work at a higher speed without accumulating lactate.

Repeated tests of the blood lactate relationship with velocity are suitable as a means of measuring increasing stamina with training. If more than one horse is tested, measurements of VLa₄ enable comparisons of the relative stamina in each horse. VLa₄ measurements every 2–3 weeks  also enable measurement of changes in fitness through the training program. However, VLa₄ measurements have the disadvantage of requiring several blood collections and analyses. A simple, one step exercise test can be designed to give the same information. For example, the blood lactate concentration 3 minutes after a two minute gallop on a treadmill was correlated with racing performance in thoroughbreds (Evans et al., 1993). The exercise test for race-fit horses was as follows. The treadmill angle was set at 6° (or a 1 in 10 slope). The horse is trotted for 2 min (at 4 m/s), followed immediately by 2 min slow cantering (6 m/s). The horse is then walked on the treadmill for 4 min, and then given 2 min exercise at 10 m/s. Racehorses with superior stamina, or endurance fitness, have blood lactate concentrations of <4 mmol/L after the test.

The blood lactate response is therefore a guide to racing ability, and the other factors which contribute to racing success must not be ignored. However, a treadmill study of 12 English Thoroughbred racehorses indicated that 47% of the variability in Timeform rating (a handicap rating system) was due to variability in their blood lactate response to treadmill exercise, using a test similar to that described above. The better horses had a lower blood lactate after treadmill exercise at 10 m/s on a treadmill inclined at 10%. Similar results have been reported in trotting horses. The blood lactate response to exercise is a very important determinant of likely success or failure on the racetrack.

  

When blood is collected for lactate assays, it should be added to tubes containing a suitable anticoagulant and inhibitor of glycolysis. Fluoride and oxalate combinations are suitable, as the blood stores well at room temperature for at least two days in those chemicals. However, if possible, blood should be stored in a refrigerator or on ice until analysis. Individual laboratories should be consulted concerning ideal storage conditions, as different centres use different techniques to conduct the analysis.

Lactate assays can also be conducted on plasma. However, plasma and whole blood lactate concentrations in blood collected after exercise are not equivalent. Plasma lactate concentrations are 140–150% of concentrations found in whole blood, due to unequal distribution of lactate between plasma and erythrocytes in horse blood after exercise rainger etal., 1995). Plasma and whole blood lactate concentrations should therefore not be directly compared.

  

In the past, lactate assays have been complicated and not readily available. The analysis of plasma or serum for lactate concentration has been greatly simplified by the development of rapid analysers. However, these technologies should be used with great caution. They express a value for blood lactate, but blood lactate is not directly assayed. The assay is of plasma lactate, and if haematocrit exceeds 50% large errors occur. (Evans and Golland 1996). After exercise haematocrit will usually exceed 50% in horses.

Incremental treadmill exercise tests, with measurements of heart rate and blood lactate at various speeds, can also be conducted. Such tests enable calculation of indices of fitness such as V₂₀₀ and VLa₄. Collection of blood at each speed necessitates stopping the treadmill after each step, or placement of an IV catheter. Sterile heparinized saline solutions can be used to maintain catheter patency during the tests. Such tests are now common at research centres with treadmills for assessment of poor performance. Measurements of heart rate, blood lactate and oxygen consumption are usually conducted during submaximal exercise and during exercise at maximal intensities. However, blood lactate concentrations after maximal exercise have not been correlated with racing performance.

Blood lactate measurements after field exercise are not advisable for assessments of fitness in thoroughbreds. Small variations in speed or distance of the gallop can have large effects on the post-exercise blood lactate concentration.

Blood lactates after exercise have been recommended for guiding the training intensity. A recent study found no difference to the aerobic fitness produced after training at lactate concentations of 2 or 4 mmol/L. However, post exercise blood lactates may be useful for guiding training at higher intensities. It is known that muscle adaptations to training occur at 80-100% of HRmax. At these intensities PLASMA lactate concentrations immediately after exercise could be expected to be in the range 10-15 mmol/L.