Training regimens: Physiologic adaptations to training

CHAPTER 19


Training regimens: Physiologic adaptations to training



The major objectives of training are to prepare a horse for the rigors of athletic competition, to decrease the risk of injury, and to increase the work capacity. By nature, horses are gifted athletes that are also capable of undergoing substantial adaptations in response to training. Investigations examining the adaptability of the musculoskeletal, cardiorespiratory, hemolymphatic, and thermoregulatory systems have been undertaken over several centuries. Some of the findings will be summarized here. Most of the measurements discussed have been defined and used only in experimental laboratories. However, more recently many more “in-field” measurements have been procured.



Muscular responses to exercise



Enzymes in muscle



Aerobic enzymes


Muscle is a remarkably plastic tissue that is remodeled when exposed to the stresses of training. In general, although by no means universally, training results in an increase in mitochondrial density, with resulting increases in the activities of enzymes in the tricarboxylic and lipid metabolic pathways, both of which contribute to increased oxidative capacity of skeletal muscle. Endurance training produces the greatest increase in the activities of aerobic enzymes, and in the first few months of training, overall increases of more than 100% over pretraining values occur. These changes also may be associated with an increase in the number of oxidative muscle fibers within the working muscle. Additionally, an increase occurs in the density of capillaries surrounding muscle fibers.


Although the metabolic advantages of increases in oxidative capacity in skeletal muscle will be greatest in animals required to undertake more prolonged exercise, for example, steeple-chasing, eventing, or endurance racing, positive effects occur in animals participating in more intense activities, with a prolongation of work capacity.


The mechanism by which the increase in oxidative capacity exerts its effects is by more efficient utilization of substrates by the metabolic pathways within the skeletal muscle. This occurs as a result of a more rapid translocation into the mitochondria of the adenosine diphosphate produced during muscular contraction. Since an increased ratio of adenosine diphosphate–adenosine triphosphate (ADP:ATP) within the cytosol of working muscle is one of the stimuli for an elevation in glycolytic (anaerobic) energy production, the increase in oxidative capacity serves to keep this ratio low via the rapid mitochondrial uptake of ADP. A decrease in the ADP:ATP ratio reduces the stimulus for glycolysis and increases the contribution of fat to total energy production. The capacity for greater utilization of fatty acids by muscle during submaximal exercise results in a sparing of glycogen within the working muscle. This glycogen-sparing effect assists in delaying the onset of fatigue during endurance events because of a direct relationship between exhaustion of the intramuscular glycogen store and the onset of fatigue.


The lower intracellular ADP:ATP ratio is also likely to have important effects during exercise at higher intensities. A beneficial effect would be provided by the augmented aerobic capacity, since this allows a greater proportion of energy to be produced by the aerobic pathways early in the exercise. Thus, the production of lactate and hydrogen ions will be delayed, reducing the potential of these byproducts to adversely affect the contractile apparatus, a factor that contributes to fatigue. This increase in oxidative capacity is reflected by increases in values for the metabolic variables such as in response to standardized exercise tests, or more practically improved racing times.



Glycolytic enzymes


Equine skeletal muscle possesses an intrinsically high glycolytic capacity, which is reflected by the high activities of the glycolytic enzymes. Such is the magnitude of the activities of the glycolytic enzymes that when compared on the basis of protein concentration, the glycolytic activity is more than tenfold greater than for the aerobic enzymes. However, some changes occur in the activities of glycolytic enzymes in response to most routine training programs. In some cases, modest decreases in the activity of lactate dehydrogenase occur in response to training. These findings of few, if any, changes in the activity of the glycolytic enzymes are similar to those reported for several other mammalian species.


In contrast, when training involving short-term, intense bursts of exercise is undertaken, it results in an increase in the activities of several glycolytic enzymes. This also parallels the findings in human and murine studies, in which intense training has been shown to result in an increase in the glycolytic potential.




Capillarity


The number of capillaries surrounding muscle fibers has been shown to increase in response to training. The purpose of this increased capillarity appears not to be related to an increase in the supply of blood to the working muscle per se but to prolongation of the transit time for blood through the capillary bed of the muscle. This increased transit time improves the potential for exchange of substrates to and metabolic byproducts from muscle fibers. During prolonged exercise, these effects allow greater uptake by muscles of glucose and free fatty acids, which are ideal fuels for the metabolic pathways, whereas during intense exercise, the capability for the offloading of oxygen and glucose and the removal of carbon dioxide, lactate, and hydrogen ions from the contracting muscles is increased.

< div class='tao-gold-member'>

Related posts:

  1. Training the racing quarterhorse
  2. Training working horses
  3. The cardiovascular system: Anatomy, physiology, and adaptations to exercise and training
  4. Energetic considerations of exercise

Stay updated, free articles. Join our Telegram channel
Tags:
Jul 8, 2016 | Posted by in EQUINE MEDICINE | Comments Off on Training regimens: Physiologic adaptations to training

Full access? Get Clinical Tree
Get Clinical Tree app for offline access