Exercise physiology is the physiology of physical exercise, that is, study of the
acute responses and chronic adaptations to a wide range of exercise conditions.
In addition, many exercise physiologists study the effect of exercise on
pathology, and the mechanisms by which exercise can reduce or reverse disease
progression. Accreditation programs exist with professional bodies in most
developed countries, ensuring the quality and consistency of education. In
Canada, one may obtain the professional certification title – Certified Exercise
Physiologist for those working with clients in the health and fitness
industry. An exercise physiologist's area of study
may include but is not limited to biochemistry, bioenergetics,
cardiopulmonary function, hematology, biomechanics, skeletal muscle
physiology, neuroendocrine function, and central and peripheral nervous system
function. Furthermore, exercise physiologists range from basic
scientists, to clinical researchers, to clinicians, to sports trainers.
Energy expenditure Humans have a high capacity to expend
energy for many hours during sustained exertion. For example, one individual
cycling at a speed of 26.4 km/h through 8,204 km over 50 consecutive days
expended a total of 1,145 MJ with an average power output of 182.5 W.
Skeletal muscle burns 90 mg of glucose each minute during continuous activity,
generating ≈24 W of mechanical energy, and since muscle energy conversion is
only 22–26% efficient, ≈76 W of heat energy. Resting skeletal muscle has a
basal metabolic rate of 0.63 W/kg making a 160 fold difference between the energy
consumption of inactive and active muscles. For short duration muscular
exertion, energy expenditure can be far greater: an adult human male when
jumping up from a squat can mechanically generate 314 W/kg. Such rapid movement
can generate twice this amount in nonhuman animals such as bonobos, and in
some small lizards. This energy expenditure is very large
compared to the basal resting metabolic rate of the adult human body. This rate
varies somewhat with size, gender and age but is typically between 45 W and 85
W. Total energy expenditure due to muscular expended energy is much higher
and depends upon the average level of physical work and exercise done during a
day. Thus exercise, particularly if sustained for very long periods,
dominates the energy metabolism of the body. Physical activity energy
expenditure correlates strongly with the gender, age, weight, heart rate, and VO2
max of an individual, during physical activity.
Metabolic changes = Rapid energy sources =
Energy needed to perform short lasting, high intensity bursts of activity is
derived from anaerobic metabolism within the cytosol of muscle cells, as opposed
to aerobic respiration which utilizes oxygen, is sustainable, and occurs in
the mitochondria. The quick energy sources consist of the phosphocreatine
system, fast glycolysis, and adenylate kinase. All of these systems
re-synthesize adenosine triphosphate, which is the universal energy source in
all cells. The most rapid source, but the most readily depleted of the above
sources is the PCr system which utilizes the enzyme creatine kinase. This enzyme
catalyzes a reaction that combines phosphocreatine and adenosine
diphosphate into ATP and creatine. This resource is short lasting because oxygen
is required for the resynthesis of phosphocreatine via mitochondrial
creatine kinase. Therefore, under anaerobic conditions, this substrate is
finite and only lasts between approximately 10 to 30 seconds of high
intensity work. Fast glycolysis, however, can function for approximately
2 minutes prior to fatigue, and predominately uses intracellular
glycogen as a substrate. Glycogen is broken down rapidly via glycogen
phosphorylase into individual glucose units during intense exercise. Glucose
is then oxidized to pyruvate and under anaerobic condition is reduced to lactic
acid. This reaction oxidizes NADH to NAD, thereby releasing a hydrogen ion,
promoting acidosis. For this reason, fast glycolysis can not be sustained for
long periods of time. Lastly, adenylate kinase catalyzes a reaction by which 2
ADP are combined to form ATP and adenosine monophosphate. This reaction
takes place during low energy situations such as extreme exercise or conditions
of hypoxia, but is not a significant source of energy. The creation of AMP
resulting from this reaction stimulates AMP-activated protein kinase which is
the energy sensor of the cell. After sensing low energy conditions, AMP
kinase stimulates various other intracellular enzymes geared towards
increasing energy supply and decreasing all anabolic, or energy requiring, cell
functions. = Plasma glucose =
Plasma glucose is said to be maintained when there is an equal rate of glucose
appearance and glucose disposal. In the healthy individual, the rates of
appearance and disposal are essentially equal during exercise of moderate
intensity and duration; however, prolonged exercise or sufficiently
intense exercise can result in an imbalance leaning towards a higher rate
of disposal than appearance, at which point glucose levels fall producing the
onset of fatigue. Rate of glucose appearance is dictated by the amount of
glucose being absorbed at the gut as well as liver glucose output. Although
glucose absorption from the gut is not typically a source of glucose appearance
during exercise, the liver is capable of catabolizing stored glycogen as well as
synthesizing new glucose from specific reduced carbon molecules in a process
called gluconeogenesis. The ability of the liver to release glucose into the
blood from glycogenolysis is unique, since skeletal muscle, the other major
glycogen reservoir, is incapable of doing so. Unlike skeletal muscle, liver
cells contain the enzyme glycogen phosphatase, which removes a phosphate
group from glucose-6-P to release free glucose. In order for glucose to exit a
cell membrane, the removal of this phosphate group is essential. Although
gluconeogenesis is an important component of hepatic glucose output, it
alone can not sustain exercise. For this reason, when glycogen stores are
depleted during exercise, glucose levels fall and fatigue sets in. Glucose
disposal, the other side of the equation, is controlled by uptake of
glucose at the working skeletal muscles. During exercise, despite decreased
insulin concentrations, muscle increases GLUT4 translocation of and glucose
uptake. The mechanism for increased GLUT4 translocation is an area of
ongoing research. glucose control: As mentioned above,
insulin secretion is reduced during exercise, and does not play a major role
in maintaining normal blood glucose concentration during exercise, but its
counter-regulatory hormones appear in increasing concentrations. Principle
among these are glucagon, epinephrine, and growth hormone. All of these
hormones stimulate liver glucose output, among other functions. For instance,
both epinephrine and growth hormone also stimulate adipocyte lipase, which
increases non-esterified fatty acid release. By oxidizing fatty acids, this
spares glucose utilization and helps to maintain blood sugar level during
exercise. Exercise for diabetes: Exercise is a
particularly potent tool for glucose control in those who have diabetes
mellitus. In a situation of elevated blood glucose, moderate exercise can
induce greater glucose disposal than appearance, thereby decreasing total
plasma glucose concentrations. As stated above, the mechanism for this glucose
disposal is independent of insulin, which makes it particularly well-suited
for people with diabetes. In addition, there appears to be an increase in
sensitivity to insulin for approximately 12–24 hours post-exercise. This is
particularly useful for those who have type II diabetes and are producing
sufficient insulin but demonstrate peripheral resistance to insulin
signaling. However, during extreme hyperglycemic episodes, people with
diabetes should avoid exercise due to potential complications associated with
ketoacidosis. Exercise could exacerbate ketoacidosis by increasing ketone
synthesis in response to increased circulating NEFA's.
Type II diabetes is also intricately linked to obesity, and there may be a
connection between type II diabetes and how fat is stored within pancreatic,
muscle, and liver cells. Likely due to this connection, weight loss from both
exercise and diet tends to increase insulin sensitivity in the majority of
people. In some people, this effect can be particularly potent and can result in
normal glucose control. Although nobody is technically cured of diabetes,
individuals can live normal lives without the fear of diabetic
complications; however, regain of weight would assuredly result in diabetes signs
and symptoms. = Oxygen =
Vigorous physical activity increases the body's demand for oxygen. The first-line
physiologic response to this demand is an increase in heart rate, breathing
rate, and depth of breathing. Oxygen consumption during exercise is
best described by the Fick Equation: VO2=Q x, which states that the amount of
oxygen consumed is equal to cardiac output multiplied by the difference
between arterial and venous oxygen concentrations. More simply put, oxygen
consumption is dictated by the quantity of blood distributed by the heart as
well as the working muscle's ability to take up the oxygen within that blood;
however, this is a bit of an oversimplification. Although cardiac
output is thought to be the limiting factor of this relationship in healthy
individuals, it is not the only determinant of VO2 max. That is, factors
such as the ability of the lung to oxygenate the blood must also be
considered. Various pathologies and anomalies cause conditions such as
diffusion limitation, ventilation/perfusion mismatch, and
pulmonary shunts that can limit oxygenation of the blood and therefore
oxygen distribution. In addition, the oxygen carrying capacity of the blood is
also an important determinant of the equation. Oxygen carrying capacity is
often the target of exercise aids used in endurance sports to increase the
volume percentage of red blood cells, such as through blood doping or the use
of erythropoietin. Furthermore, peripheral oxygen uptake is reliant on a
rerouting of blood flow from relatively inactive viscera to the working skeletal
muscles, and within the skeletal muscle, capillary to muscle fiber ratio
influences oxygen extraction. = Dehydration =
Dehydration refers both to hypohydration and to exercise-induced dehydration. The
latter reduces aerobic endurance performance and results in increased
body temperature, heart rate, perceived exertion, and possibly increased
reliance on carbohydrate as a fuel source. Although the negative effects of
exercise-induced dehydration on exercise performance were clearly demonstrated in
the 1940s, athletes continued to believe for years thereafter that fluid intake
was not beneficial. More recently, negative effects on performance have
been demonstrated with modest (