The fatigue of high-intensity exercise is now believed to reside primarily within the excitation–contraction coupling processes associated with the plasma membrane of skeletal muscle (sarcolemm) and calcium-mediated events leading to myofilament sliding. This paper summarizes recent developments and advances in the identification of factors that contribute to changes in sarcolemmal excitability of mammalian skeletal muscle as a consequence of high-intensity exercise. There is an increasing recognition of the probable role that is played by the transverse tubular system (T-system), a system that comprises c. 80% of the total sarcolemmal surface capable of ion exchange. Furthermore, the fluid within the T-system has limited access to interstitial fluid bathing myofibres; hence, T-system fluid is probably markedly different from interstitial fluid during high-intensity exercise. Mechanically skinned fibre preparation is providing many new insights into functions of the surface membrane and T-system in fatigue. A scenario is developed whereby accumulation of potassium within the T-system ([K+]o) contributes to reduced membrane excitability, as well as lowering of T-system sodium and chloride, concomitant with loss of intracellular potassium ([K+]i) and accumulation of intracellular sodium ([Na+]) and chloride ([Cl−]). Lowering the [Na+]o/[Na+]i ratio and raising myoplasmic [Na+]i have been shown to decrease membrane excitability and impair action potential propagation. Maintained high [Cl−]o may also have a protective effect in maintaining membrane excitability, and this effect appears to be very pronounced in the presence of raised [K+]o. In contrast to dogma associating high [H+] to fatigue, recent studies have also shown that induced acidosis that results in increased [H+]o and [H+]i restores force production in muscles and skinned fibres fatigued by intermittent tetanic stimulation. This effect may be due to a decrease in surface membrane Cl− permeability that serves to restore membrane excitability. During high-intensity exercise, simultaneous changes in trans-membrane ion concentrations and membrane ion conductances may serve to reduce impairment of membrane excitability that provides for a maintained, though reduced, contractile function.

The exercise performed by Thoroughbred (TB) and Standardbred (SB) racehorses is similar. Nevertheless, warm-up regimes seem to differ between trainers of the two breeds. The aim of this study was to collect some preliminary results on warm-up strategies used by trainers of SB and TB in Sweden and the physiological response to two warm-up regimes of practical relevance. A questionnaire was answered by eight and nine SB and TB trainers, respectively, and showed that the SB trainers tended to perform longer warm-ups than TB trainers. In the exercise study, a long and a short warm-up (LWU and SWU) were designed by two trainers (SB and TB, respectively) and performed prior to an intensive exercise session. The study showed that the recovery period in the LWU trial in SBs was prolonged compared with the SWU trial and associated with increased body temperature (39.6±0.3 versus 39.1±0.1°C 15 min post-exercise), heart rate, and breathing frequency (66±10 versus 55±6 beats min−1 and 86±11 versus 56±6 breaths min−1 15 min post-exercise). In addition, the body weight loss was increased by 129% compared with the SWU. In the TB trial, the post-exercise breathing frequency was higher following SWU compared with LWU (97±7 versus 71±8 breaths min−1 15 min post-exercise), indicating increased anaerobic metabolism. Although this study involves few animals, it indicated that the LWU for TBs and the SWU for SBs was most beneficial.

Pregnant mares are often removed from work during gestation. However, little is known about the effects of exercise on the pregnant mare and her foetus. In the present study, maternal and foetal heart rates were monitored by electrocardiography before and after exercise in four equine pregnancies (days 250–285). Exercise consisted of lungeing mares in 20 m circles for 5, 10 and 20 min. Following exercise, significant increases in maternal heart rate were detected (P<0.0001) but not in foetal heart rate (P = 0.4331). All mares gave birth to normal foals uneventfully. While these findings suggest that moderate exercise of the pregnant mare does not appear to be harmful to the equine foetus, data are insufficient to draw conclusions. A larger, more controlled study is necessary.

Because athletic horses become hypoxaemic and hypercapnoeic during high-intensity exercise but ponies do not, six Thoroughbred horses and five ponies performed an incremental exercise test at speeds with calculated energy requirements that were 40, 60, 80 and 115% of V˙O2max, with the objective of comparing their blood gas and ventilatory responses to exercise. Expired gas and blood samples were taken and breathing mechanics were assessed before exercise and during the last 15 s at each intensity. Maximal V˙O2 and V˙CO2 in horses were 153±5 (SEM) and 187±4 ml kg−1 min−1, respectively, while corresponding values in ponies were 92±4 and 112±7 ml kg−1 min−1. During heavy and supramaximal exercise, horses, but not ponies, became hypoxaemic and hypercapnic. There was no significant difference for V˙E kg−1 between groups during maximal exercise, but PAO2, PaO2 and PvO2 were lower and PaCO2 and [(A−a)O2D] were greater in horses than in ponies. Additionally, the horses' maximal transpulmonary pressure difference was higher and their total pulmonary resistance and ventilatory equivalent lower than in ponies. Flow-volume loops suggested that horses experienced expiratory flow limitation but that ponies did not. These results indicated that horses like Thoroughbreds appear to be expiratory flow-limited and become hypoxaemic and hypercapnic when the demand for gas exchange associated with their high V˙O2max and V˙CO2max is greater than can be met by their ventilatory system. Ponies, which are less capable athletes, could better match their ventilatory response with their metabolic capabilities and so were able to maintain PaO2 in the pre-exercise range and decrease PaCO2 to a tension that was more compatible with acid–base homeostasis.

It has been suggested that the presence of a bit reflexly increases salivation but, at the same time, interferes with the horse's ability to swallow. The objective of this study was to compare swallowing frequency in 12 horses exercising at canter while wearing a head collar, a bitless bridle, a jointed snaffle bit and a Myler correctional-ported barrel bit. Laryngeal movements were recorded videoendoscopically as the horses cantered (8 m s−1) on a high speed treadmill, with the use of side reins to flex the poll. Swallowing was stimulated artificially by infusion of sterile water at a constant rate of 5 ml min−1 through a cannula in the endoscope's biopsy port. The results showed large differences in swallowing frequency between horses. Swallowing frequency was lower for the Myler snaffle than for the other conditions (P<0.05). It is concluded that the presence of a bit does not preclude swallowing during exercise at canter with the poll in a flexed position, but certain types of bits may be associated with a reduction in swallowing frequency.

Parathyroid hormone (PTH) has been shown to have anabolic and catabolic effects on the skeleton, and in young racing horses the anabolic effects of PTH on bone might be of great importance in reducing the risk of skeletal injury during race training. The aim of this pilot study was to elucidate the effects of intermittent exogenous application of human parathyroid fragment (hPTH 1-37) on calcium homeostasis and bone turnover in resting horses. Five horses were used in this study. Horses were treated subcutaneously with hPTH (hPTH 1-37, 0.5 μg kg−1 BW) daily at 600 h over a period of 28 days. A foregoing control trial was conducted under the identical test protocol without hPTH application. Blood samples were taken at defined times for the control and hPTH treatment to analyze ionized Ca (Ca++, AVL), total Ca (AAS), inorganic Pi (flame photometry), intact PTH (RIA), osteocalcin (ELISA) and ICTP (carboxyterminal telopeptide of type-I collagen, RIA). Eight hours after hPTH application, blood ionized Ca++ (days 1 and 28) and plasma Pi (days 1–28) increased significantly in comparison to the control. At day 1, 8 h after hPTH application, the decrease in intact plasma PTH was more pronounced after hPTH treatment than in the control, and plasma PTH levels were higher after treatment at day 14 (16 h post-injection, P<0.05), day 18 (16 h post-injection, ns) and day 24 (0 h post-injection). There were no treatment-related differences in total plasma Ca and bone markers. Intermittent PTH administration in healthy horses affected Ca and P homeostasis as well as endogenous PTH secretion, but bone turnover was not affected during the treatment period of 28 days.

The aim of this study was to determine which oxidant–antioxidant blood markers are of interest for a field exercise test (ET) performed on a racetrack. Healthy Standardbred horses (S: n = 12) and healthy eventing horses (E: n=12) were investigated. Exercise was monitored by measuring velocity (V), heart rate (HR), and plasma lactate (LA). Whilst maximal LA did not differ (11.8±0.88 mmol l−1), maximal V (S: 12.3±0.17 m s−1versus E: 11.1±0.24 m s−1, P<0.05) and final HR (S: 222±1 versus E: 203±8 beats min−1, P<0.05) were significantly different between groups. Venous blood was collected at rest (R) prior to ET and the following oxidant–antioxidant markers were determined: uric acid (UA), ascorbic acid (AA), α-tocopherol (Vit E), vitamin A (Vit A), superoxide dismutase (SOD), glutathione peroxidase (GPx), glutathione (reduced: GSH and oxidized: GSSG), glutathione redox ratio (GRR), copper (Cu), zinc (Zn) and selenium (Se), oxidized proteins (Protox), lipid peroxides (Pool), antioxidant capacity of water-soluble components (ACW) and antioxidant capacity of lipid-soluble components (ACL). The following markers were further determined 15 min (E15) after the ET: UA, ACW, AA, GSH, Protox, Pool, ACL. Standardbreds had significantly higher concentrations of ACW, GSH, ACL and Protox, whilst Se, Zn and SOD were significantly lower than in eventing horses. Exercise induced a significant increase in ACW and UA. GSH decreased in eventing horses and Pool significantly decreased in both horse groups. This study describes a field ET of high intensity for Standardbred and eventing horses, which could be performed by all animals tested. By sampling blood at rest and at E15, changes of the hydrophilic antioxidant defence were partially assessed, whereas no interpretable changes of the lipophilic antioxidants and of oxidation markers (Protox, Pool) could be detected.

The aim of this study was to examine the influence of an experienced rider and a novice rider on the stride kinematics of experienced riding horses. SVHS video recordings (50 Hz) were made of ten experienced riding horses jumping a 1.05 m-high vertical fence. The horses were randomly assigned to jump the fence under two experimental conditions: ridden by an experienced rider and ridden by a novice rider. Three trials for each ridden condition were analysed, and the effects of the rider type on four kinematic variables were examined using a repeated measures ANOVA. No significant differences were found between the riders for velocity and stride length during the approach, or for the take-off and landing distances from the fence. The results suggest that the rider's body position and body movement had no effect on the horse's jumping kinematics as measured in this study, and that each horse jumped the fence in its own manner, regardless of what the rider was doing. This is contrary to the current belief that a horse's jumping technique is strongly influenced by the rider. These findings have relevance for both horses and riders, in that if an experienced horse does not respond to a rider's instructions as expected, then the implications for training of the horse and the rider are considerable.