• Endurance_training

Physiological effects of endurance training

Introduction

Endurance training can be described as repeated isotonic contractions, aerobic exercises at submaximal intensity, with the objective of having better cardiovascular and metabolic effects, and to increase the anaerobic threshold. This can be achieved by, for example: running, swimming, and for humans, cycling. As with any type of physical exercise, the short term and long term effects are different. In thi student essay we will be focused on the long term effects, with three main aspects in mind: the molecular and metabolic changes, the cardiovascular changes, and the skeletal muscles changes. We will also be mentioning the eventual negative side effects that don’t necessarily come from the physical changes induced by a prolonged endurance training.

1. Molecular and Metabolic changes:
   1. General metabolic change
   The carbohydrate metabolism is affected by long term endurance training, more precisely the blood glucose level, the insulin sensitivity and the glycated haemoglobin. The insulin sensitivity is increased and the blood glucose level is decreased: this is important for individuals suffering from type 2 diabetes. The glycated haemoglobin level thus also decreases. These effects have been shown by a study conducted by Myoung Heo et al. in 2013, in which diabetic rats were the test subjects. Concerning the carbohydrate metabolism, in the same study, it has been shown that the total cholesterol level and triglyceride level of the blood plasma significantly decreases over time with endurance training, but high density lipoprotein (HDL) levels increase. A reminder that HDL carries cholesterol from the circulation to the liver to be reused or excreted, thus reducing the plaque formations of cholesterol in the circulation, and ultimately reducing the risk for heart diseases.- Immune system: effect on bone marrow not much info tho.. - RBC: increased RBC mass and increased plasma volume to have a more efficient oxygen transport (j’ai perdu le lien) -
   • b. Changes in enzymes and proteins
   When looking into studies concerning the enzymatic changes caused by endurance training, creatine kinase is a recurring subject. Indeed, it is a cause of concern, because it is secreted during exercise, and degrades the myocardium, so naturally there is a concern that high levels of exercise could cause, on the long term, myocardial damage. A study on dogs by T.D. Miller et al. (1989) proved that there is no significant difference in the quantity of myocardial creatine kinase secreted by sedentary dogs and that by regularly exercised ones. The higher level of myocardial creatine kinase can also only be observed in the left ventricle (Stuewe et al. 2001), and no damage to the heart is observable on the long term. to be completed ?
   • c. Changes in hormonal activity
   Endurance training increases fibroblast growth factor 21, which partakes in embryo development, cell growth, tissue damage repair, and reduces inflammation. During exercise, muscles are constantly getting damaged when used, so FGF21 is necessary to promote repairing. (released from bone marrow)il faut que je retrouve le lien It has been shown that prolonged endurance training can reduce the risk of ventricular fibrillation by decreasing the cardiac sympathetic activity, which in turn decreases the beta-2-adrenergic receptor (ADRB2) responsiveness. ADRB2 can bind epinephrine and results in smooth muscle relaxation and bronchodilation. an increased ADRB2 responsiveness can cause sudden death in susceptible animals According to a study by G. E. Billman et al. from 2006 , endurance training on the long term can reduce the risk of sudden death and cardiac fibrillations.
2. Effects of physical training on skeletal muscles metabolism Intracellular pathways To be delated: Regular / long term exercise positively influence skeletal muscle energy metabolism and function. Endurance training leads to an increase in mitochondrial volume density as well as the muscular capillarization. Regarding the intracellular pathway, PGC-1alpha plays a role in regulating mitochondrial biogenesis in skeletal muscle endurance training. In addition, proteins that are involved in mitochondrial dynamics correlate with PGC-1alpha. There is also a link between the activation of the calcineurin, and the expression of PGC-1alpha. As a result, the changes in intracellular calcium levels potentially regulate this metabolic transcriptional co-activator. Besides, there are other pathways are involved in the control of muscle oxidative pathways such as the MAPK and CaMKs pathways. (Ventura-Clapier, Mettauer, Bigard, 2007).
   1. Physiology of endurance training in skeletal muscles:
   ATP synthesis during an endurance training: During an endurance training, skeletal muscles need a huge amount of energy. The source of energy is the ATP which gives ADP and Pi. ATP + H2O + ADP + Pi + H+ + energy There are 3 metabolic ways of ATP production during endurance training. - Anaerobic alactic way - Anaerobic lactic way - Aerobic way The first one aims to produce energy very quickly for short and intense activities such as races for quarter horses or jumping. The second one produces ATP for longer efforts but less intense such as races of 1km for example. ATP is created by degrading glucose that comes from intramuscular glycogen or glucose in the blood. The Aerobic way is mostly used by trotters and horses of endurance (thoroughbreds) for long efforts. In this case, ATP is produced by degrading fatty acids present in the muscular cells or by degrading the intramuscular glucose. Besides, the metabolic reactions in aerobic pathways occur in presence of oxygen in the mitochondria.
   • b. Aerobic pathway:
   In the beginning of every training, the three pathways are used in the same time in order to produce energy. Then, depending on the activity, intensity and length of the exercise, the proportions of each way differ.
   • c. Type of muscle fibers: (according to the University of Veterinary Medicine, Budapest)

• • d. Effects:
  Fiber type transition: According to American Journal of Veterinary Research, there is a modification of the type of muscular fibers in the case of Thoroughbred horses. Indeed, thoroughbred horses possess more oxidative fiber of type I and type IIa than the other breeds. These characteristics allow the thoroughbreds to adapt themselves better in endurance races for example. Regarding untrained horses, an increase of the muscular fibers type II is observed (Yamano et al., 2002). Trained thoroughbreds do not have an obvious change in fiber type due to endurance training. How to measure a physiological muscular adaptation due to training There are different ways to measure the endurance adaptation of horses. First, some parameters can be measured such as heart rate, speed of the animal and the level of lactic acid. Indeed, blood acid lactic is used to evaluate the aerobic capacity of exercise for training according to Mitsutoshi Kobayashi.Second, it is possible to measure the glycogen content as well as the enzyme activity in the skeletal muscles.
• Cardiovascular adaptation The heart can improve structurally and functionally engaging a larger muscle mass with regular exercise. Endurance training leads to cardiovascular adaptations that markedly increase aerobic power and improve performance. Adaptation is due to long term physiological changes in response to training that allows the body to meet new demands. Studies demonstrate that endurance training results in :
  • ● Increased cardiac output ● Increased vascular conductance ● A greater perfusion capacity in muscle ● A greater oxygen extraction
  • Central adaptations :
  Cardiac output (beats per min) and Stroke volume (mL per min) increase in elite female and male athletes. According to several studies, we observe an increase in end-diastolic (end of filling) dimensions of the left(LV) and right(RV) ventricles, LV hypertrophy, and increased LV mass, and an increased volume of the left atrium (LA).

  LV end-diastolic diameter, LV wall thickness, and LV mass have been reported in endurance athletes. The dynamics of the blood changes that occur during exercise is the origin of cardiac remodeling. 2 principals form of training : ● endurance exercise with an elevation in cardiac output with a reduced peripheral vascular resistance e.g long-distance running, cycling ==> LV enlargement ● strength training characterized by an increase of the peripheral vascular resistance and low cardiac output. ==> LV hypertrophy ( increased wall thickness/size) The increased wall thickness improves the heart's ability to circulate blood è it produces a more powerful contraction which helps to push the blood. Either endurance and strength training lead to adaptations in cardiac structure. However, endurance training induces enhanced diastolic LV filling and a faster filling of the heart during exercises.

  • b. Blood volume responses :
  Plasma volume is reduced proportionally to the metabolic and thermal demand during exercise. It results in a loss accompanied by increased electrolyte concentration and leads to the activation of the renin-angiotensin-aldosterone è water retention Endurance training leads to an expansion of blood volume (hypervolemia) via an aldosterone sodium retention with an increase in plasma albumin concentration. Data shows that trained and elite athletes have a higher blood volume than untrained individuals. The hypervolemia is also associated with a larger body water volume, which helps in the conductance of perfusion at the level of the skin, but also followed by a lower hematocrit value and blood viscosity. This kind of training is also associated with an increased hemoglobin mass, which increases the buffer capacity of the blood. It facilitates the lactate release from active skeletal muscle.
  • c. Blood pressure
  Controlled by a complex mixture between neural, hormonal and intrinsic factors involving brain, heart, and kidney (controls fluid balance). It has been shown that endurance training reduces blood pressure at rest, due to vascular remodeling and changes in the peripheral vascular function.
  • d. Arterial remodeling
  Arterial diameter: Endurance trained individuals display a reduced arterial wall thickness and an increased lumen diameter for the transport of blood from the heart to the different organs. Optimally, the diameter and the elastic properties of the arteries should be adequate for the blood flow. (from 6 to 12 weeks of training to increase the femoral artery). Some studies showed that the coronary arteries enlargement might be induced by training, but a genetic contribution must be taken into consideration.
  • e. Peripheral adaptation
  Blood flow: During exercise, the blood flow increases to accommodate the increased demand for oxygen by the skeletal muscle. This elevation is achieved by the increased cardiac output with the improvement of vascular conductance in the active blood. The increased cardiac output is originated by the sympathetic activity, which induces a vasoconstriction tone in organs and active muscles. Training induces the other adaptation that may influence the level of blood flow, it is the better distribution of the flow by capillarization and therefore increases the oxygen diffusion. This vascular distribution may originate from the nervous system.

Conclusions

Finally, we can demonstrate that through endurance training, which improved cardiac and arterial function and increases the number of microcirculation vessels in cardiac and skeletal muscles, the sedentary individuals susceptible to cardiovascular disfunctionment can improve their health by exercising regularly.