Quantification of Energy Expenditure in Field Conditions: Comparison of Direct VO 2 Versus VO 2 Estimation from Heart Rate Measurements

Abstract

Welfare issues are under continuous scrutiny especially in the media. There are many studies of injury and fatalities in racehorses. However, little data are available for horses undertaking endurance exercise [28]. A fatality rate of 67 out of 252,738 horse starts was recorded in the American Endurance Ride Conference (AERC) sanctioned during 2002-2013 [29]. Twenty-one of the 67 horses completed the ride and 81% of the fatalities related to acute abdominal pain with gastric rupture in 12%. Other studies of endurance horses evaluated water intake and dehydration both of which may have an impact on elimination during endurance exercise and ultimately on fatalities. The 'tucked up' appearance of endurance horses was correlated with decreases in abdominal circumference and bodyweight (bwt) [30]. Another study trained 24 endurance horses for 12 weeks on a treadmill and confirmed an individual variation in voluntary fluid intake. This was inversely related to bwt loss [31]. Finally, a novel gastrointestinal pill for continuous measurement of core body temperature in endurance horses [32] shows promise for further studies of thermoregulation in hot weather conditions. Drugs in equestrian sports A recent systematic review indicates that there is high quality evidence that furosemide prevents or limits the occurrence of exercise-induced pulmonary haemorrhage [33]. Two studies reported at ICEEP 9 examined the implications of furosemide administration in TB horses: furosemide induced a linear dose-dependent increase in plasma total CO 2 concentration to near the threshold value used for screening of prerace administration of alkalising agents [34]. A second study showed that a single dose of furosemide increased urinary and faecal mineral excretion (Ca, P, Na, Cl) and affected calcium balance for 72 h [35]. The ergogenic effects of sodium bicarbonate (NaHO 3) was investigated and preloading with NaHO 3 60 min before high intensity exercise to fatigue did not improve performance in 4 horses but both KCl and NaCl administration resulted in improved performance, with a longer run time to fatigue [36]. Several studies investigated effects of anti-inflammatory drugs. A reversible model of equine foot pain was used to investigate the effects of different drugs and demonstrated that dexamethasone (0.045 mg/kg bwt) did not have any analgesic effect [37] while firocoxib (0.09 mg/kg bwt) showed lower efficacy than phenylbutazone (4.4 mg/kg bwt) in a placebo-controlled trial [38]. Treatment with phenylbutazone (4.4 mg/kg bwt) for 7 days adversely affected the gastric mucosa by increasing gastric permeability and exacerbating gastric ulceration [39]. Muscle physiology Electromyography was used to evaluate fatigue in the superficial and deep digital flexor muscles (SDFM and DDFM) during maximal exercise in 6 TB horses on an inclined treadmill [40]. The DDFM may fatigue earlier than the SDFM during long distance running, potentially increasing load on the superficial digital flexor tendon. High resolution respirometry was to investigate potential differences in mitochondrial oxidative phosphorylation (OXPHOS) capacity in horses with exertional rhabdomyolysis (ER). Horses with ER had severely decreased OXPHOS capacity which may play an important role in the pathogenesis of polysaccharide storage myopathy (PSSM) [41]. Furthermore, it was suggested that high resolution respirometry might be useful for identifying horses with dysfunctional muscle energy metabolism that might subsequently develop ER [42]. Biopsies were taken from the semimembranosus muscle to measure glycogen synthase (GS) activity in Belgian draught horses which were PSSM1 homozygotes, heterozygotes or controls [43]. Glycogen synthase activity was significantly higher in PSSM1 homozygotes than in heterozygotes and control horses but was not associated with increased expression or degradation of the mutant enzyme. Subsequent in silico 3-dimensional (3D) modelling of mutant GS associated with PSSM1 revealed that although the PSSM1 mutation lay close to the G6P binding site, it did not confer any clear structural rearrangement on the 3D configuration of the protein [44]. Genetics of performance The sequencing of the equine genome [45] has enabled new and rapid developments in equine genetics and genomics, providing biomolecular tools for investigations of performance evaluation and markers of health and disease. The influence of variations in the myostatin gene (MSTN g.66493737C/T) on racing performance has been of particular interest [46] and a number of studies presented at ICEEP 9 further explored the relationships between this 'speed' gene and athletic performance. One investigated whether or not these genetic speed markers could be used to optimise management of racing careers of Turkish TB horses [47]. They found genotype frequencies of 0.23 (CC), 0.52 (CT) and 0.25 (TT) and determined that sprinters (CC) and middle distance horses (CT) were not being entered in races that fit with their genetic ability. In another study, no association was found between MSTN genotype and _ VO 2peak in untrained TB yearlings [3]. This may be a reflection of the small number of TT horses included in the study (n = 3) or related to the need for a period of training before MSTN-associated changes in _ VO 2peak become evident. Two other studies from the same research group looked at additional genetic factors that might potentially influence exercise performance. PGC-1a and irisin have a coordinated role in the skeletal muscle adaptive response to exercise independent of MSTN-determined muscle type [48]. Significant associations were identified between SNPs in the insulin-like growth factor 1, protein tyrosine phosphatase, nonreceptor type,1 and leptin receptor genes and fat-free mass (FFM) at different time points during training, suggesting a genetic contribution to variation in FFM [49]. Recurrent laryngeal neuropathy (RLN) has long been suspected to have a strong genetic component. An association between the BIEC2-808543 C-allele and RLN appeared to be independent of height; however, a strong trend of association was observed (P = 0.06) in a small group of TB horses (16 RLN, 32 controls) [50]. Subsequently, another study involving a larger number of TB horses (282 with RLN, 268 controls) found that height and LCORL/NCAPG gene were significantly associated with RLN [51]. It is clear that genetic studies will continue to play an important role in expanding our knowledge of factors associated with athletic performance and disease susceptibility in the near future.