The workload and plasma ion concentration in a training match session of high-goal (elite) polo ponies


  • G. C. FERRAZ,

    Corresponding author
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  • O. A. B. SOARES,

    1. Faculdades de Ciências Agrárias e Veterinárias, UNESP Univ Estadual Paulista, Campus de Jaboticabal, Departamento de Morfologia e Fisiologia Animal, Laboratório de Farmacologia e Fisiologia do Exercício Equino (LAFEQ) Jaboticabal, São Paulo, Brazil.
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  • N. S. B. FOZ,

    1. Faculdades de Ciências Agrárias e Veterinárias, UNESP Univ Estadual Paulista, Campus de Jaboticabal, Departamento de Morfologia e Fisiologia Animal, Laboratório de Farmacologia e Fisiologia do Exercício Equino (LAFEQ) Jaboticabal, São Paulo, Brazil.
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  • M. C. PEREIRA,

    1. Faculdades de Ciências Agrárias e Veterinárias, UNESP Univ Estadual Paulista, Campus de Jaboticabal, Departamento de Morfologia e Fisiologia Animal, Laboratório de Farmacologia e Fisiologia do Exercício Equino (LAFEQ) Jaboticabal, São Paulo, Brazil.
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    1. Faculdades de Ciências Agrárias e Veterinárias, UNESP Univ Estadual Paulista, Campus de Jaboticabal, Departamento de Morfologia e Fisiologia Animal, Laboratório de Farmacologia e Fisiologia do Exercício Equino (LAFEQ) Jaboticabal, São Paulo, Brazil.
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Reasons for performing study: This study was designed to consider the complexity of the physical effort inherent to horses in polo competitions and the absence of reports in the literature on the effort, intensity and electrolyte changes resulting from a collective team training session aimed at preparing for a polo championship.

Objectives: To determine the effort and ion changes caused by an outdoor polo training match for a 25 goal handicap (elite) based on physiological variables including acid-base status (venous pH, PCO2 and HCO3-), packed cell volume (PCV), haemoglobin (Hb), lactate, glucose, sodium, chloride and potassium and strong ion difference (SID) as well as creatine kinase (CK) activity.

Methods: Twenty-three clinically healthy ‘high-goal’ polo ponies were used, which included 10 geldings and 13 females. The horses performed a training match, as a preparation for a 25 goal tournament, consisting of 6 chukkas of 7 min duration each. Blood samples were collected during resting, and at 5 min, 6 and 12 h after each chukka. Data were analysed using ANOVA for repeated measures followed by Tukey's test.

Results: Differences (P<0.001) were evident mainly in post exercise for all variables studied. There was a reduction in pH, PCO2 and HCO3- and SID, together with an increase in PCV and Hb, lactate, glucose, Na+ and Cl-. K+ levels remained constant at all times of collection. The average resting value for CK was 255 ± 9 iu/l, and 6 h after effort there was a 35% increase in enzyme activity.

Conclusions: This study indicates that the horses participating in a training match underwent a high-intensity effort with alterations in electrolytes and acid-base equilibrium.

Potential relevance: Training matches should be carefully conducted, with a suitable recovery period before the main match.


The training of athletic horses involves the utilisation of periodic exercises that cause structural, functional and behavioural changes, preparing them for competitions. An effective response to training depends on the use of appropriate stimuli (Evans 2000) which should be guided by the principle of overwork which is made up of 3 classic variables represented by the triad intensity, duration and frequency.

Many studies carried out in the field have investigated physiological demands in some equestrian modalities such as horse racing (Davie and Evans 2000; Mukai et al. 2007), jumping (Sloet van Oldruitenborgh-Oosterbaan et al. 2006) and endurance (Teixeira-Neto et al. 2008). This information is important because it serves as a basis for the development of rational training programmes. In high-goal polo, there is a scarcity of reports in the literature with respect to effort intensity, principally during the training performed during a tournament season.

In equestrian polo, the handicap is determined by the individual ability of each player who is classified from –2 (low ability) to +10 (high ability). The handicaps of all the members of a team are summed determining the total handicap of the team (goals). Generally, above 10 goals is considered elite polo. According to Posner (2004), polo is an equestrian modality played in approximately 30 countries. The match takes place outdoors and is played by 2 teams of 4 players, where the game is made up of 6 periods (chukkas) of 7 min on a field of grass of approximately 10 acres.

Basically, polo training consists of 3 steps: vareo where a rider exercises up to 5 horses at the same time, utilising walk, trot or light canter for approximately 30 min and taqueo which is performed in a field designed for the practice of specific activities in polo. These 2 modalities are done at low intensities being part of a basic aerobic preparation. In the third step, a collective training is conducted, represented by a training match. This last one is performed as a preparation for the official matches. In accordance with the common sense of trainers and players, it is a preparatory match of relatively light effort with the objective of teaching the horses the specific activities inherent to an official high handicap championship.

The variables most commonly utilised for evaluation of workload effort include plasma lactate and glucose, PCV and haemoglobin (Rogers et al. 2007). According to Lindinger and Heigenhauser (2008), other variables can be used to determine the effect of exercise, such as acid-base equilibrium (pH, PCO2 and HCO3-) electrolytes (Na+, K+ and Cl-) and strong ion difference (SID). Therefore, the aim of this study was to determine the workload effort and electrolyte changes of a collective training session of horses belonging to an elite polo team.

Materials and methods

Horses and riders

In this study, 23 clinically healthy ‘high-goal’ polo ponies belonging to the same team were used, which included 10 geldings and 13 females, components of a Brazilian elite polo team. Horses had a mean ± s.e. bodyweight of 442 ± 28 kg and age 7.4 ± 2.2 years. All horses began the 2009 season in the month of March, where they were submitted weekly to the same training programme which essentially consisted of aerobic exercises (taqueo and vareo), 6 times a week. Once a week, there was a collective training match which was the target of evaluation of this study. The animals were kept in individual stalls, with access to water and to Medicago sactiva hay supplied twice daily (2.0 kg at 07.00 h and 2.0 kg at 17.00 h), plus mineralised salt. The diet with concentrate consisted of 30% feed1 (pelleted concentrate) and 70% oats. Each horse was supplied with 6–8 kg/day. All riders had international experience and their weight was mean 83 ± 3 kg.

The study followed the Ethical Principles in Animal Experimentation adopted by the Brazilian College of Animal Experimentation and was approved by the institutional animal care and use committee of the university.

Training match and sampling

The horses participated in a training match in preparation for a 25 goal tournament, composed of 6 chukkas of 7 min duration. Each horse participated in only one chukka. Therefore, as 4 horses were used in each chukka and as a match is composed of 6 chukkas, 24 horses should have been monitored. At the request of the team trainer, one mare was removed from the study because it appeared to be a very restive and frightened animal, where blood collections on the polo field could have hampered its management. Therefore, only 3 horses were monitored in one chukka, giving a total of 23 animals. The match took place outdoors, on a grass field with an area of 275 m by 180 m, which is located at south latitude -23°05′ 25″, west longitude 47°13′ 05″ and altitude 624 m. The weather conditions remained relatively constant throughout the match with the temperature and relative humidity between 22–25°C and 40–50%, respectively. A standard operating procedure for blood sample collection was created to establish proper procedures for collection, processing, and storage. After a period of 48 h of inactivity and 18 h before the collective training session, blood was collected from the jugular vein before and 5 min, 6 and 12 h after each chukka.

Blood analysis

Immediately after blood sampling, a portable chemical analyser2 (i-STAT, EC8+ cartridges) was utilised to determine PCO2, pH, HCO3-, PCV and haemoglobin, sodium, chloride and potassium. Strong ion difference [SID] was determined as the sum of the strong cations minus the sum of strong anions (Stewart 1983). Plasma glucose and lactate were measured in duplicate, using a lactate and glucose autoanalyser (YSI 2300 STAT Plus3). Ten ml of blood were obtained in tubes without anticoagulant for later determination of creatine kinase (CK) activity, where the blood was immediately centrifuged under refrigeration4 (ALC, Multispeed refrigerated centrifuge) and analysed by spectrophotometry5.

Data analysis

The values are given as mean ± s.d. The effect of exercise on physiological variables was evaluated statistically by the use of analysis of variance (ANOVA) for repeated measures using the general linear models procedure of SAS6, with the aim of determining significant differences for each step of collection, followed by Tukey's test when necessary. Significance was taken at P<0.001.


Acid-base status

Compared to levels before the exercise, the samples obtained immediately after the chukkas had lower values of PCO2, pH and HCO3- (Table 1). By 6 h after the effort, all variables returned to baseline levels.

Table 1. PCO2, pH and HCO3- (mean ± s.d.) before and at 5 min (post exercise), 6 and 12 h after a practice game of polo in preparation for a championship of 25 goals
 Steps of assessment
Variable (n = 23)BeforePost exercise6 h12 h
  • *

    indicates decrease in relation to rest.

pCO2 (mmHg)47.9 ± 0.5532.0 ± 0.68*48.6 ± 0.4949.8 ± 0.91
pH7.42 ± 0.017.15 ± 0.04*7.40 ± 0.017.42 ± 0.01
HCO3- (mmol/l)31 ± 0.2513 ± 0.49*30 ± 0.2532 ± 0.54

PCV and haemoglobin

The analysis shown in Table 2 indicates that there was an increase in PCV and Hb when compared to resting levels, immediately after effort. After the end of exercise, PCV and Hb values tended toward initial values.

Table 2. Packed cell volume, haemoglobin (Hb), Na+, K+, Cl-, Strong ions difference, lactate and glucose (mean ± s.d.) before and at 5 min (post exercise), 6 and 12 h after a practice game of polo in preparation for a championship of 25 goals
 Steps of assessment
Variable (n = 23)BeforePost exercise6 h12 h
  • a–c 

    Different letters in the same row indicate significantly (P<0.001) different values.

PCV (%)37 ± 5.03a56 ± 5.7b40 ± 6.17a38 ± 6.40a
Hb (g/dl)12.7 ± 1.71a19.1 ± 2.05b13.7 ± 2.06a12.9 ± 2.06a
Na+ (mmol/l)137 ± 2a141 ± 1b137 ± 2a137 ± 1a
Cl- (mmol/l)108 ± 0.24a112 ± 0.56b109 ± 0.42a104 ± 0.43c
K+ (mmol/l)3.68 ± 0.23a3.66 ± 0.33a3.63 ± 0.47a3.71 ± 0.26a
SID (mmol/l)32 ± 2a14 ± 1b30 ± 2a35 ± 2c
Lactate (mmol/l)0.64 ± 0.18a18.70 ± 5,36b0.76 ± 0.2a0.79 ± 0.2a
Glucose (mmol/l)4.73 ± 0.32a7.42 ± 1.30b5.00 ± 0.39a4.77 ± 0.37a


There was an elevation in concentrations of sodium (Na+, mmol/l) as well as chloride (Cl-, mmol/l) just after the end of the polo training match. However, potassium concentrations (K+, mmol/l) remained the same at all times studied (Table 2).

Strong ion difference (SID)

Table 2 shows that SID declined with exercise, where this behaviour was verified by pH (Table 2). Six h after exercise, values returned to those observed during rest.

Plasma lactate and glucose concentration

Just after effort, mean levels of both lactate and glucose were increased, returning after 6 and 12 h to values equivalent to those at rest (Table 2).

Creatine kinase

As revealed in Figure 1, the average value of CK was 255 ± 9 iu/l before effort and 6 h after effort there was a 35.3% increase in enzyme activity, giving a mean of 345 ± 17 iu/l.

Figure 1.

Serum CK activities before, at 5 min (post exercise) 6 and 12 h after a practice game of polo in preparation for a championship of 25 goals.*indicates increased after the match.


With regard to the complexity of the physical challenge to which polo horses are submitted and to the high monetary value, there are few publications that report the physiological demands of the animals in high-goal polo. With respect to monitoring of training of this modality, scientific information is practically nonexistent. In contrast to the majority of equestrian sports, the physiological demand of a game or a collective polo match is variable (Marlin and Allen 1999), because the games are intercalated with moments of pause with much change in speed and direction of movements, requiring great muscular and articular involvement.

For the quantification of workload effort expended by horses during sport activity, it is necessary to determine, essentially, the intensity and duration of the exercise which could help to better understand changes in physiological variables. Art and van Erck (2008) point out that during exercise, coordinated responses of body systems determine the participation of aerobic and anaerobic pathways, for the synthesis of ATP, maintenance of acid-base control and control of body temperature. In the present study, the effort of a training match produced alterations in variables responsible for the maintenance of plasma acid-base stability due to a series of biochemical and physical-chemical reactions associated with a marked contribution of the glycolytic anaerobic pathway for the production of energy.

Five minutes after the end of effort, there was an evident reduction in pH, HCO3- and venous PCO2, indicating that this collective training match session was of high intensity, characterised by the development of metabolic acidosis. According to Robergs (2001), when the intensity of exercise is high, there is an increase in the production of protons (H+) and lactate due to the partial contribution of anaerobic metabolism for the production of energy demanded by muscle contraction. In fact, lactacidaemia after exercise was significantly elevated. The increase observed was more than twice that reported by Craig et al. (1985), which could be related to the type of polo monitored by these authors who studied indoor polo, probably played with less intense effort.

Although intense exercise leads to an increase in the production of protons as well as lactate, some authors still attributed observed acidosis to lactic acid (Böning and Maassen 2008; Ba et al. 2009).

As a counterpoint to this theory, Robergs and Parker (2006) argued that lactate production actually consumes and does not produce protons by means of reoxidation of NAD+ at the end of anaerobic glycolysis. Moreover, these authors report that one of the main sources of H+ during exercise is hydrolysis of ATP, which is intensified as effort increases. They state, however, that at physiological plasma pH and even close to 7.0, as obtained in this study, the principal form of this substance in physiological systems is sodium lactate, considering that the pK of lactic acid is 3.86 and that the term ‘lactic acidosis’ would therefore be unsuitable.

Although this question is still polemic, it appears that the majority of investigators agree on the unsuitability of the use of the term ‘lactic acidosis’ to define the acidosis observed after intense exercise (Prakash 2008; Robergs 2008), since there is concordance on the idea that the events that determine acid-base status are complex, involving the contribution of variables such as strong ion difference (SID), active and inactive tissues, plasma, red blood cells and blood compartments.

In horses, the main lactate transporters are the proteins monocarboxylate transporters (MCT). During intense effort, each molecule of lactate transported from the muscle to the plasma, by a MCT, carries a proton (Pösö 2002). In man, Juel et al. (2004) demonstrated that interval training of high intensity increases the activity of isoforms MCT1 and MCT4 in the sarcolemma, allowing a greater influx of lactate in the blood. Certainly, the game of polo can be characterised as a high intensity intermittent exercise. Therefore, training match sessions of high-goal polo horses can contribute to the elevation in the buffering capacity of muscles and blood, enhancing tolerance to acidosis. Therefore, this hypothesis can be considered, notwithstanding the need for more studies involving interval training to see if this response also occurs in horses.

In the present study, splenic contraction was responsible for the elevation in PCV and Hb of 51 and 50%, respectively, in line with Bayly et al. (2006). In this latter study of 6 Thoroughbred horses during and after supramaximal exercise at 115% VO2max, there was an increase in PVC of 46%, reinforcing that monitored match training represented an exercise of high intensity. These same authors propose for horses, as with human individuals, a directly proportional relation between degree of splenic contraction and the removal of lactate from the blood, guaranteeing in this manner the maintenance of the concentration gradient between compartments, plasma and muscle. In the last analysis, this biological mechanism would favour the removal of lactate from the muscle during exercise, thereby postponing fatigue.

However, on acid-base control during exercise, Stewart (1983) reported that concentration of the ions H+ and HCO3- are determined by the independent variables SID and PCO2 and by the total concentration of weak acids (Atot). The reduction in SID observed in the present study was induced, principally, by the increase in lactate production as a result of intense effort. This elevation in lactacidaemia was followed by increased plasma levels of the ions Na+ and Cl-, without modification of calcium levels. During exercise, a possible mechanism that explains the elevation in plasma sodium levels and maintenance of calcium levels, is the movement of ions and water, which occurs between active and inactive muscles (Schott et al. 2002).

The high intensity of exercise observed in this study resulted in an increase in the intramuscular production of CO2. Most of the CO2 that diffuses into the venous circulation is transported in the form of HCO3- to the plasma and into the erythrocytes by means of the catalytic mechanism of carbonic anhydrase, facilitated by chloride exchange (Carlson 1995). Therefore, it was observed that the plasma Cl- concentration after effort, was elevated probably due to the exchange between plasma and RBC compartments, represented by the influx of bicarbonate into erythrocytes and efflux of chloride to the plasma (Bayly et al. 2006) and also due to pulmonary hyperventilation for the elimination of excess CO2 (Jones 2008). This last affirmation is corroborated by the data in Table 1 revealing that horses showed hypocapnia after effort.

Glycaemia can also be utilised for the evaluation of intensity of effort. In both man and horses, during and after intense effort of short duration, there is an increase in glucose availability (Simões et al. 1999; Ferraz et al. 2008), mainly immediately after exercise. The results obtained in Table 2 confirm this finding. Such fact is related to the increase in the activity of hormones that regulate energy metabolism, such as catecholamines and glucagon, which upon release promote hepatic glycogenolysis and neoglucogenesis, increasing plasma glucose concentration. This is important for the maintenance of glycaemia during exercise (McKeever 2002).

Currently, there is great interest in the scientific community that studies the physiology of exercise in man (Váczi et al. 2009) as well as horses (Piccione et al. 2009) with regard to the relation between serum CK activity, clinical condition during training, occurrence of musculoskeletal injuries and intensity of effort. According to McGowan et al. (2002), the incidence of exertional rhabdomyolysis (ER) per season in polo horses is 7%, which causes considerable athletic and financial damages. Rivero and Piercy (2008) in studying the prevalence of clinical cases of ER in sport horses established 3 semi-quantitative categories based on CK. At rest, the group of polo horses utilised in this work showed a CK activity of 255 ± 9 iu/l, being classified as grade 1, equivalent to a moderate increase. This result for CK for elite polo horses can be classified as grade 0, because none of the horses studied showed any sign of rigidity, weakness or shortening of step, principally of pelvic limbs.

Figure 1 reveals that 6 h after the training match there was an increase in CK activity of 35% with return to baseline values at 12 h after the end of effort. It is established in the literature that exercises of high intensity cause elevations in plasma CK activity (McGowan 2008) which can be related to damage of or transient increase in the permeability of the muscle fibre membrane (Teixeira-Neto et al. 2008). Some authors utilised the study of CK for evaluation of training, where evaluation is recommended for up to 24 h after exercise to differentiate between horses with physiological responses and those with morbid responses (Lindner et al. 2006). However, the standardisation of the CK kinetic study after exercise, training or clinical evaluation, considering its half-life of 6 h (Snow and Valberg 1994), is the more technical procedure, because it improves the accuracy of significance of the response.

In comparing the effort exerted by horses in this training session of elite polo with other equestrian modalities and pointing out the changes in some variable such as pH, PCO2, lactate and glycaemia, similarities are noted with 3-day event horses (Foreman et al. 1999) and racehorses (Davie and Evans 2000; Mukai et al. 2007). On the contrary, Arabians performed aerobic activities of long duration where compensatory responses were related to the need for thermoregulation and where the marked changes included intense perspiration which alters water-electrolyte balance (Teixeira-Neto et al. 2008).

An important aspect in the rational recommendation of training protocols in horses is the establishment of recovery periods (Rogers et al. 2007) prior to official matches, where this should be taken into account by trainers and players. Planning the season applying the concepts of macrocycle, mesocycle and microcycle should be employed by those responsible for recommending training protocols.

In conclusion, from a practical point of view, this collective training session caused alterations in electrolytes and acid-base equilibrium, representing a type of high-intensity effort.


Supported by Fundation de Amparo à Pesquisa do Estado de São Paulo (2007/08671-0). The authors wish to thank the staff of the Maragata polo team for their support in conducting this study.

Conflicts of interest

The authors have not declared any potential conflicts.

Manufacturers' addresses

1 Supra - Tonnus, São Leopoldo, Rio Grande do Sul, Brazil.

2 Heska Corporation, Fort Collins, Colorado, USA.

3 YSI Inc., Yellow Springs, Ohio, USA.

4 Multispeed refrigerated centrifuge PK121R, ALC, Princeton, New Jersey, USA.

5 Quick Lab Chemistry Analyser, Hameln, Germany.

6 SAS Institute Inc., 1988, Cary, North Carolina, USA.