Relationship between Slope of the Plasma Lactate Accumulation Curve and Working Capacity in Andalusian Horses

Muñoz A. , R. Sant is teban, M. D. Rubio, C. Riber , E. I . Agüera, F. M. Castejón: Relationship between Slope of the Plasma Lactate Accumulation Curve and Working Capacity in Andalusian Horses. Acta Vet. Brno 1999, 68: 41-50. The main objective of this study was to analyse the relationship between the slope of the plasma lactate accumulation in response to exercise and working capacity in two groups of Andalusian horses of different age. The horses were subjected to increasing intensity tests, comprising both submaximal and maximal velocities. Heart rate was monitored throughout the exercise and jugular venous blood samples were withdrawn at rest, after each workload and during the first 30 min. of recuperation. Working capacity was assessed by means of these functional indices: HRmax (maximum mean heart rate during exercise), Vmax (maximum velocity), STEPmax (maximum number of workload completed), VLA2 (aerobic threshold), VLA4 (anaerobic threshold), PLA (lactate peak), pHmin (minimum pH) and PCVmax (maximum packed cell volume). HRmax, pHmin, VLA2, VLA4, STEPmax and Vmax were positively correlated with working capacity, whereas PLA and PCVmax were negatively correlated with working capacity. The slope of the plasma lactate accumulation curve were inversely related to VLA2, VLA4, Vmax, STEPmax and pHmin and positively related to HRmax and PCVmax. Besides, the plasma lactate slope was higher in the youngest Andalusian horses. These results indicated that the slope of the plasma lactate concentrations after several exercise intensities might be considered as a marker of aerobic potential, providing useful information about fitness and training degrees in the Andalusian horse. Exercise, fitness, heart rate, packed cell volume Physical exercise requires a balance between production and consumption of energy within working muscles. It has been postulated that there is a critical intensity or threshold above which the cardiovascular, respiratory and/or muscular responses are of insufficient magnitude to supply energy through aerobic pathways. As a consequence, anaerobic pathways become predominant, and large amounts of lactate (LA) are produced. LA is considered to be a small and easily diffusable anion, and it may be translocated rapidly between the water compartments of the body. Muscle LA produced during exercise can be utilised by the active muscles, exported via the blood to other organs, principally liver and heart, or taken up by other muscles with adequate oxygen supply and utilised as a source of fuel (Brooks 1986; Marl in et al. 1987). Three different models of plasma LA accumulation have been proposed. The single breakaway point is the classical model of aerobic-anaerobic transition. Marga r i a et al. (1933) established that lactic acid was formed and plasma LA accumulated during times of local muscle hypoxia, such as the onset of exercise and during times of oxygen deficit. ACTA VET. BRNO 1999, 68: 41–50 Address for correspondence: Dr. Ana Muñoz Dept. of Animal Biology, Section Physiology Faculty of Veterinary Medicine, Univ. Córdoba Campus Univ. Rabanales C1, 140 71 Córdoba, Spain Phone: +34 957 21 86 85 Fax: +34 957 21 20 02 E-mail: [email protected] A three-phase, double breakaway model was defended by Kinderman et al. (1979) in response to threshold LA values of both 2 and 4 mmol/l. According to this second model, 2 mmol/l represents the upper limit of exclusive aerobic metabolism, and therefore, it should be referred to as the ‘aerobic threshold’. An aerobic-anaerobic transition period occurred between plasma LA concentrations of 2 and 4 mmol/l. Finally, a plasma LA level of 4 mmol/l reflexes the true ‘anaerobic threshold’, a value above which LA production exceeds its removal and endurance work performance is severely limited. The exponential model was described by Hughson et al. (1987), after finding that plasma LA concentrations increased as a continuous function during progressive exercise. This third model suggests that there is no breakaway point at which there is an increased reliance on anaerobic metabolism. By contrast, an exponential increase in the production of energy through glycolytic pathways from the beginning of incremental exercise occurs. This model challenges the existence of critical intensities, LA and ventilatory threshold, upholding the belief that glycolytic energy sources supply an ever-increasing proportion of the total energy expenditure from the onset of incremental exercise. Hughson et al. (1987) hypothesised that a LA slope index would be an indicator of fitness, replacing the previously applied threshold concept. In the present study, Andalusian horses performed an incremental exercise, during which plasma LA accumulation was studied. The slope and the y intercept of the LA vs. velocity curve were determined. Moreover, working capacity of these animals was assessed by calculating functional indices according to the procedure described in detail by Muñoz (1997) and Muñoz et al. (1997a). Two are the purposes of the present research: 1) To establish if there is a relationship between slope and y intercept of the plasma LA accumulation curve and working capacity in Andalusian horses, as suggested by Hughson et al. (1987) in humans and 2) To detect differences in these variables between horses with different training level and used for different purposes. Materials and Methods Animals Thirteen healthy male Andalusian horses were included in this study, seven between 3 and 4 years (group A) and six between 7 and 10 years (group B). Standardised exercise test (SET). The group A performed a SET (SETa) with two phases of different intensity, separated by a 2-min-rest period. The first phase, of submaximal intensity, was divided into four exercise bouts at velocities of 4.15, 5.50, 6.90 and 8.30 m/sec. A distance of 1 000 m was covered at each velocity and a 2-minuterest period was allowed between them. The second phase, of maximal intensity, consisted of 1 000 m at the maximum velocity the horses were able to reach. The group B carried out a SET (SETb) with six workloads (4, 6, 8, 10, 12 and 14 m/sec), covering distances of 400, 600, 800, 1 000, 1 200 and 1 200 m, respectively, at each exercise level. A total distance of 4 000-5 200 m was covered. Some horses were unable to achieve the last velocities and the number of steps completed was recorded. Thirty sec. of rest were given between velocities in order to obtain blood samples. Both SETs were made in a semielliptical sandy track. The procedure used to standardise the velocity has been given elsewhere (Agüera et al. 1995; Muñoz et al. 1997b). An ANOVA showed the lack of significant differences between theoretical or calculated velocities and real or chronometered velocities (p < 0.01) in both tests. Training protocols Group A. These horses were subjected to the training programme followed traditionally in their Centre of Training, whose characteristics has been previously reported (Muñoz et al. 1997a). In summary, this programme had a duration of 5 months and was composed of two stages: 1) First stage, lasting for 14 weeks, consisted of daily exercise sessions at trotting and galloping velocities. The duration of these daily sessions was progressively increased from 10 min to 30 min, at a frequency of 5 times/week. 2) Second stage, lasting for the remaining 6 weeks. At the same frequency as the first stage, the daily schedule included 20-30 min of walking, 25-30 min of trotting and 12-18 min of galloping. This training programme is not intended as a means to prepare horses for competitions. The main aim is to improve physical fitness to guarantee a proper reproductive function. Group B . The animals of the group B were trained for harness events for at least the last 24 months before the experiment. The training protocol consisted of track exercises 2 days/week for approximately 60 min. During these sessions, animals performed exercise workloads at trotting and galloping velocities (between 4-6 m/sec) in order to improve movement coordination. Moreover, they carried out at least 30 min of draft-loaded exercises. The 42