Evaluation of performance potential

CHAPTER 28


Evaluation of performance potential




Athletic performance


Successful athletic performance requires a complex interaction of physiologic mechanisms involving the musculoskeletal, nervous, respiratory, and cardiovascular systems. From a simplistic point of view, exercise demands the imposition of increased loads on the respiratory and cardiovascular systems to support the dramatic increases in metabolic rate occurring in contracting muscles during exercise. It is essential that the responses of each of these systems are appropriately integrated to ensure optimal physiologic performance. Dramatic increases occur in ventilation and cardiac output with increasing metabolic rate, and not surprisingly, the capacity and health of these systems will play a substantial role in determining the performance potential of a horse. Since superior athletic performance depends on tight integration of a number of body functions, it is to be expected that many elite equine athletes have metabolic characteristics indicating this potential for superior performance. For example, top-class Thoroughbred and Standardbred racehorses usually have values for maximal oxygen consumption (imageO2max) in the range of 150 to 200 milliliter per kilogram per minute (mL/kg/ min). Conversely, although a high imageO2max indicates substantial cardiorespiratory capacity, it does not ensure superior athletic performance. This is demonstrated in the horse that has a large cardiorespiratory capacity and yet is endowed with a conformation that does now allow the musculoskeletal system to withstand the rigors of training and racing. However, we also have conducted exercise tests on horses in which the cardiorespiratory capacity or conformation may not be considered ideal, yet these horses performed at a level above that predicted. Parallels for this latter observation exist in human athletes, and the reasons are likely to be related to (1) our inability to measure the metabolic determinants critical to performance, (2) physiologic integration, whereby the sum of all the components contributing to exercise capacity is greater than those indicated by measurement of the parts, and (3) intangible factors such as desire, capacity to withstand discomfort, and so on.


Evaluation of performance potential requires an understanding of the physiologic mechanisms involved in the energetics of exercise. Muscular work requires that the physiologic systems of the horse are integrated to minimize the stress imposed on the component mechanisms supporting the energetics. Muscular respiration depends on complex interactive systems that allow gas exchange between muscle cells and the atmosphere. Optimal gas exchange between muscle cells and the atmosphere requires (1) efficient lung function, (2) effective pulmonary circulation, which is able to match the requirements of ventilation, (3) blood with an adequate hemoglobin concentration, (4) a cardiovascular system that can deliver an appropriate quantity of oxygenated blood to the periphery to match tissue respiratory requirements, and (5) control mechanisms capable of regulating arterial blood gas tensions and pH.


Energy for muscular contraction is obtained predominantly by the oxidation of fuels in the mitochondria, with an additional portion delivered via biochemical mechanisms in the cell cytoplasm. This energy is used to form high-energy compounds, predominantly phosphocreatine (CP) and adenosine triphosphate (ATP). The energy from the terminal phosphate bond can be made available for cellular reactions involved in synthesis, active transport, and muscular contraction.



Oxygen-transport chain



imageO2max


During exercise, dramatically increased loads are placed on muscle bioenergetics, creating the need for the respiratory and cardiovascular systems to respond to support the increased gas-exchange requirements. Transfer of oxygen (O2) and carbon dioxide (CO2) between the mitochondria and air requires a finely coordinated interaction between the cardiovascular and respiratory mechanisms that is integrated with the cellular metabolic activity. The large increase in muscle O2 requirements during exercise demands that O2 flow to muscle increases. A simultaneous increase in CO2 production occurs, and CO2 must be removed from tissues to ensure that acidosis is avoided, since this can have profound adverse effects on muscular contractile activity. Components of the oxygen-transport chain that are integral to superior athletic performance include the airways and lungs, the cardiovascular system, blood volume and hemoglobin concentration, and the musculoskeletal system.



Airways and lungs


Following the onset of exercise, the increase in respiratory drive is thought to be mainly caused by increased neural stimuli. In elite athletic horses, this may involve an increase in minute ventilation from about 100 liters per minute (L/min) at rest to greater than 2000 L/min during strenuous exercise. The increase in ventilation occurs as a result of a small increase in tidal volume and a large increase in respiratory frequency of up to 150 breaths per minute. Entrainment of stride and respiratory frequencies restrict any greater increase in respiratory rate. As a result, peak airflows will be more than 6500 L/min. Achievement of flows of this magnitude will require production of transpulmonary pressures of more than 60 centimeters of water (cmH2O). For such enormous flows to occur, it is important that the upper respiratory tract be optimally dilated during exercise. This active dilatation allows a reduction in upper airway resistance during exercise. Not surprisingly, restrictions to the upper airway (e.g., idiopathic laryngeal hemiplegia) may substantially alter airflow dynamics and, therefore, gas exchange, resulting in reduced exercise capacity. Similarly, disorders that may alter elasticity of the lung or gas exchange in the alveolus (e.g., chronic obstructive pulmonary disease) also will reduce gas exchange and, therefore, restrict performance.



Cardiovascular system


Integration of cardiovascular and respiratory function during exercise is essential if superior athletic performance is to be achieved. In the transition from rest to exercise, dramatic alterations occur in the vascular system to accommodate the large increases in cardiac output. Initially, metabolic vasodilation occurs in the vascular beds of working muscle, resulting in an increase in imageO2 and stimulation of heart rate and cardiac output. Almost simultaneous dilatation of capillary beds occurs in the pulmonary vasculature to support the increased gas-exchange requirements imposed by the exercise. During intense exercise, heart rate increases to greater than 230 beats per minute (beats/min), which, in elite racehorses, is associated with increases in cardiac output to more than 350 L/min. Blood flow to working muscle has been shown to increase by more than 75-fold in ponies in response to intense exercise. These blood flows exceeded 160 mL/kg/min, almost twice the values reported to occur in humans.




Musculoskeletal system


During exercise, the major end point of the oxygen-transport chain is the contracting skeletal muscle. In its simplest form, skeletal muscle can be regarded as the apparatus that is fueled by the chemical energy sources derived from ingestion of food. As described above, most energy for muscular contraction is derived from the oxidation of fuel in the mitochondria. In horses, skeletal muscle has an intrinsically high oxidative capacity compared with humans and most other domestic species, and this may be enhanced by training.


For the oxidative metabolic pathways to be able to meet the energy demands imposed by exercise, delivery of oxygen to the working muscle via the pulmonary and cardiovascular systems must be adequate. Working muscle consumes the O2, and in response to the increased extraction of O2 and addition of CO2 to capillary blood by muscle, an almost immediate increase occurs in muscle blood flow. The initial vasodilation is thought to be centrally induced, with subsequent dilation occurring under the influence of local humoral control. This process is selective, allowing vasodilation in the muscle units with the highest metabolic rates.



Anaerobic energy delivery


Although the majority of energy during most intensities of exercise is provided by aerobic means, maximal exercise requires a substantial contribution from the anaerobic bioenergetic pathways. For this purpose, horses are endowed with intrinsically high activities of the enzymes involved in anaerobic energy production, with horses with the highest proportion of fast-twitch (type II) muscle fibers also having the greatest glycolytic potential.


From an energy point of view, it is important to consider that induction of energy production by the anaerobic pathways does not signal the downregulation of energy supply by the aerobic pathways. Lactate is a byproduct of anaerobic energy production, and at low exercise intensities, little or no change occurs in blood lactate concentration. As exercise intensity increases, a consequent formation of lactate takes place, with increases in the concentration of this metabolic byproduct in muscle and blood. The higher the intensity of exercise, the greater is the concentration of lactate accumulating in these tissues.

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  4. Energetic considerations of exercise

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Jul 8, 2016 | Posted by in EQUINE MEDICINE | Comments Off on Evaluation of performance potential

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