Compared to our resting state, exercise poses a substantial increase in demand to the body. At rest, our nervous system maintains a parasympathetic tone which affects respiratory rate, cardiac output, and various metabolic processes. Exercise stimulates the sympathetic nervous system and will induce an integrated response from the body; This response works to maintain an appropriate level of homeostasis for the increased demand in physical, metabolic, respiratory, and cardiovascular efforts.
Cardiovascular disease remains a prevalent issue in our patient population despite advances in prevention guidelines and treatments. The top risk factors include hypercholesterolemia, hypertension, diabetes, obesity, and tobacco use; these mentioned risk factors encompass nearly 50% of the mortality fraction. Lack of exercise tends to exacerbate the deleterious effects of these risk factors while implementing exercise in daily routine has been shown to reduce mortality rates. Specifically, the lack of exercise is directly linked to obesity, while also playing a role in the development of diabetes and hypertension.
Research shows that exercise in conjunction with other lifestyle modifications can reduce the risk of hypertension regardless of inherent genetic predispositions; in addition, exercise has also been shown to increase insulin sensitivity in the management approach to diabetes.
Working from gross anatomy to cellular anatomy, we start at the skeletal muscle level. Within each skeletal muscle, there are hundreds of muscle fibers that are compartmentalized in an organized fashion linearly. They work in tandem to shorten muscle upon contraction.
The outermost layer of the muscle is the epimysium. Within this layer, muscles fibers are bundled into muscle fascicles. Deeper than this, we have the perimysial layer which encompasses the endomysial layer which ultimately holds the individual muscle fibers (aka myocytes).
At the muscle fiber level, there is the sarcolemma (plasma membrane of the muscle fiber). The sarcoplasm is analogous to a cytoplasm, and the sarcoplasmic reticulum is akin to a smooth endoplasmic reticulum. Finally, we have myofibrils which are composed of contractile actin and myosin filaments.
Within all of this, the functional unit of a myofibril is the sarcomere. It is comprised of an organization of contractile myofilaments. These myofilaments are the actin filament (thin filament), and the myosin filament (thick filament). One sarcomere is defined as the filaments between two Z-disks. The center of the sarcomere is the M-line which anchors the thick myosin filaments. The I-band is the area adjacent to the Z-lines where myosin filaments do not overlap actin filaments. Sarcomeres are arranged end-to-end along the entire length of the muscle fiber, and the synchronized contraction of each unit produces a visible muscle contraction.
Physical activity in the form of exercise induces a coordinated response of multiple organ systems.
The musculoskeletal system is at the forefront.
There are 3 types of muscle fibers which have different characteristics.
Higher myosin ATPase activity is directly proportional to faster muscle contraction speed while higher oxidative capacity relates to fatigability.
Type-I fibers are known as slow-twitch fibers. These fibers have abundant mitochondria and myoglobin with great vascular supply.
Type-IIa fibers are known as fast-twitch oxidative fibers.
Type-IIa fibers can be considered as the middle-ground type of fiber, between the slow but fatigue-resistant type-I fibers and the fast but fatigue-prone type-IIb fibers.
Type IIb fibers are known as fast twitch glycolytic fibers.
With the introduction of progressively overloading exercise training, we can expect skeletal muscle fibers to hypertrophy meaning they increase in diameter and volume.
Muscle contraction acts upon the skeleton and initiates movement. When a progressive force is applied to the muscles over time, they will adapt to the increasing load.
Satellite cells play a role in this repair and growth process. The process of exercise, whether through long distance running or powerlifting, places a burden of stress on muscle fibers and bones which causes micro tears and trauma. In response to this, satellite cells are activated and mobilize to regenerate damaged muscle tissue. This process is made possible by the donation of a daughter nuclei from the satellite cells after multiplication and fusion. Bones will increase their mineral density over time to manage this increasing load.
The circulatory system plays a critical role in maintaining homeostasis during exercise.
To accommodate the increased metabolic activity in skeletal muscle, the circulatory system must properly control the transport of oxygen and carbon dioxide, as well as help to buffer the pH level of active tissues. This is accomplished by increasing cardiac output (increased heart rate and stroke volume) and modulating microvascular circulation. In addition, the action of local vasomediators such as nitric oxide from endothelial cells helps to ensure adequate blood flow.
Blood flow is preferentially shunted away from the gastrointestinal (GI) and renal systems and toward active muscles through selective constriction and dilation of capillary beds. This increased skeletal muscle blood flow provides oxygen while facilitating the removal of carbon dioxide. The increased metabolic activity increases carbon dioxide concentrations and shifts the pH to the left which further facilities erythrocytes (RBC) to extract carbon dioxide (CO2) and release oxygen (O2). On a mechanical level, RBCs that have been in circulation for a long time tend to be less compliant than younger RBCs. This means that during exercise, older RBCs can be hemolysed intravascularly when passing through capillaries in contracted muscles. This leads to an average decrease in RBC age since the younger RBCs have more favorable rheological properties. Younger RBCs also have increased oxygen release compared to older RBCs. Exercise increases erythropoietin (EPO) levels which cause an increase in RBC production. Both of these factors improve oxygen supply during exercise. Over time, vascularization in muscles also improves, further improving gas exchange and metabolic capacity.
The respiratory system works in junction with the cardiovascular system. The pulmonary circuit receives almost all of the cardiac output. In response to the increase cardiac output, perfusion increases in the apex of each lung, increasing the available surface area for gas exchange (decreased alveolar dead space).
With more alveolar surface area available for gas exchange, and increased alveolar ventilation due to increased frequency and volume of respiration, blood gas and pH balance can be maintained.
Going into more detail, CO2 is one of the metabolic products of muscular activity.
CO2 is carried away from peripheral active tissues in various forms. The majority is transported in the form of bicarbonate, but a portion is also transported as dissolved CO2 in plasma, and as carbaminohemoglobin on RBCs.
CO2 readily dissolves into the cytosol of erythrocytes where it is acted upon by carbonic anhydrase to form carbonic acid. Carbonic acid then spontaneously will dissociate into a hydrogen ion and bicarbonate.
After being transported to the lungs, a high oxygen environment (Haldane effect), this reaction is catalyzed in the opposite direction to reverse itself and produce CO2 which can be exhaled and removed from the body.
In exercise of higher intensity, the volume of carbon dioxide (VCO2) eliminated per unit of time is sustained by the effect of the decreased alveolar dead space and increased tidal volume as mentioned before.
Exercise has been shown to have many health benefits. Through functional exercise, we can see benefits in but not limited to:
Through a properly executed exercise program, the body adapts and becomes more efficient at performing various exercises.
Some of these adaptations are:
For muscles to contract, the body must hydrolyze adenosine triphosphate (ATP) to yield energy. The ways muscles maintain ATP levels is contingent upon the conditions of the body.
Muscles can utilize glucose or glycogen in both aerobic or anaerobic manners. The glycolytic energy system tends to lead to lactate accumulation and subsequent pH decrease in muscle tissue, especially in the anaerobic setting.
Aerobic metabolism is typically utilized in exercises such as walking, while anaerobic metabolism is utilized in high-intensity activity such as weightlifting.
The major source of ATP for cells is through the oxidative phosphorylation pathway. This is part of the aerobic pathway that occurs on the inner mitochondrial membrane and produces much more ATP than other metabolic pathways.
During high-intensity exercises such as HIIT (High-Intensity Interval Training), or intense weight training, muscles cycle though ATP rather quickly and are left with a pool of ADP as a result. Phosphocreatine can donate a phosphate group to ADP to regenerate additional ATP which affords muscles energy. Creatine kinase catalyzes this reaction.
During rest and low-intensity exercise, muscles can utilize fatty acids as substrates for energy production. Medium chain fatty acids undergo beta-oxidation in the mitochondrial matrix. Long chain fatty acids need to be transported from the cytosol into the mitochondria with the help of Carnitine.
When a motor unit receives an excitation signal, the axon terminal releases Acetylcholine, a neurotransmitter, onto receptors of the sarcolemma. This opens voltage-gated channels and creates an action potential which will pass along the T-tubules to conduct a coordinated signal deep into the muscle. When that depolarization reaches the sarcoplasmic reticulum, it will cause a release of its stored calcium ions.
When these Calcium ions are released, they bind to Troponin C in the sarcoplasm which initiates unblocking of the actin binding site on myosin due to tropomyosin. Essentially, calcium binds to troponin which causes tropomyosin to unbind from the actin-myosin binding site exposing the site. This now allows the binding of actin and myosin together, which will cause a contractile force and shortening of the sarcomere unit. ATP is hydrolyzed to ADP and Phosphate when the myosin head causes this contraction. To relax from the contracted state, ATP must bind to myosin, which causes a release of the Actin site and the return to the high energy state of myosin. This shortening and lengthening model is explained by the sliding filament theory in which actin and myosin filaments slide past each other to shorten the length of the sarcomere. There are a few different types of muscle contractions.
Isometric contraction: The muscle is actively contracting, but there is no change in length due to equal and opposite forces in opposite directions.
Concentric contraction: The muscle is actively contracting and decreasing its length due to greater muscle force in relation to its opposing force; approximation of its attachment and origin.
Eccentric contraction = the muscle is actively contracting yet still increasing its length; the muscle contracts with less force than its opposing force.
Exercise capacity can be a useful measure of cardiovascular and pulmonary function. Impaired exercise tolerance can reflect dysfunction in any one of the involved organ systems. Symptom onset during a controlled exercise test can suggest conditions such as angina, peripheral vascular disease, or even exercise-induced asthma. Often, a thorough history taken from the patient can also recommend these conditions, but a supervised exercise test can be much more objective.
In the inquiry, to quantify exercise tolerance, a healthcare provider may ask the number of flights of stairs a patient can tolerate, or the number of blocks they can walk without stopping. It’s important to note the time frame upon which exercise tolerance changes may be happening. Acute vs. chronic capacity changes can suggest different etiologies of disease.
VO2 is the consumption of oxygen and can be explained by the Fick equation.
This equation states that VO2 = [CardiacOutput] x [Difference in arterial and venous oxygen levels].
VO2max is a measure of aerobic exercise capacity and is defined as the highest rate of oxygen uptake an individual can maintain during intense activity.
VO2max (in L of oxygen per minute) can be measured by having a subject perform an exercise on a treadmill or bicycle in increasing intensity. During the exercise, oxygen uptake is calculated by measuring the volumes and concentrations of inspired and expired gas.
As patients exercise and train, their VO2max may improve, but this is typically a function of oxygen delivery, not skeletal muscle oxygen extraction. Through training, oxygen delivery improves as a result of the increased cardiac output and capillary density.
Asymptomatic patients who are motivated can reproduce their VO2max during testing. Symptomatic patients on the other hand who have congestive heart failure (CHF) or chronic obstructive pulmonary disease (COPD), may not be able to exert themselves to reach VO2Max fully. In cases such as these, we can utilize the 6-minute walk test as a standardized measure.
Exertional symptoms such as effort limited by dyspnea, angina, palpitations, or claudication imply a presence of disease of that must be investigated.
In patients who provide good effort during the test and reach a normal VO2peak (good effort without reaching VO2max) who state dyspnea or fatigue was their limiting factor during exercise, we can assume that they have a normal exercise tolerance.
In this case, we can assume that a cardiopulmonary issue such as CHF, interstitial lung disease (ILD), or COPD was not present. When testing these patients, we would be more likely to see an abnormal VO2peak with significant dyspnea.
In pulmonary diseases such as COPD and ILD, exercise intolerance may be due to an impairment in gas exchange being the limiting factor.
Exercise-induced bronchoconstriction (exercise-induced asthma) is another pulmonary pathology to consider. It presents as difficulty breathing and wheezing during or after exercise. Objectively, it can be indicated by FEV1 decrease greater than 10% when compared to baseline.
In cardiac diseases, such as valvular abnormality, CHF, or CAD, exercise can be particularly dangerous. The increased demand on the heart can lead to myocardial strain. This issue is intensified in high temperature or high humidity exercise conditions. In response to the impaired evaporative cooling, vasodilation will occur to reduce body temperature. This leads to a compensatory increase in heart rate which causes additional strain to the myocardial tissue.
Exercise intolerance can also arise due to metabolically/structurally dysfunctional muscle tissue. Myopathies are suggested when there is an absence of any significant cardiopulmonary issues. Myopathies can present as muscle cramping or pain and can be diagnosed with biopsy or genetic testing in some cases.
Exercise intolerance upon testing may be a result of poor effort or excessive perception of limiting symptoms. In both cases, objective measures such as lactate levels assist in differentiating true exercise limitations from alternate explanations of intolerance.
Over time, with extended periods of inactivity, skeletal muscles will atrophy, and the body will also become deconditioned. It is recommended to have an approved physical therapy consultation and program for patients who are hospitalized for extended periods of time.
Lastly, it is important to note that organ systems take time to adapt. If exercise intensity is acutely increased past the body’s ability to repair itself, negative consequences may result such as muscle strains, tears, and stress fractures. Excessive training has also been shown to cause an adverse response in the immune system, while moderate intensity exercise has been shown to slightly increase the immune response.
Understanding basic exercise physiology is crucial as impaired exercise tolerance in patients can allude to signs of underlying disease when combined with modalities such as ECGs. Exercise testing can also be used to determine goals of care and monitor prognosis and treatment progress.
Through a standardized exercise test, we can determine the broad etiology of the exercise limitation and then can pursue testing to narrow the differential to a specific cause.
Overall, understanding and applying exercise physiology can help to reduce time to diagnosis, improve outcomes, and ultimately improve patients’ quality of life.
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