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Cardiovascular, Respiratory, Skeletal, and Metabolic Response to Progressive Exercise

• Progressive exercise tests (graded exercise tests) are protocols that systematically and linearly increase exercise intensity over a defined period until the individual cannot maintain the workload.
• Common protocols use treadmills or stationary bikes.
• The progressive increase in exercise intensity is measured by predefined increments, or ramps, allowing for a continuous challenge.
• VO2 max is defined physiologically as the maximum ability of the respiratory and cardiovascular systems to take up oxygen (O2), deliver it to muscles, and for mitochondria to utilize the O2 to produce ATP.
• Graphically, VO2 max is indicated by a plateau or leveling off in VO2 despite increasing workload.
• The VO2 response to graded exercise is typically linear.
• A plateau is not a strictly horizontal line; a plateau is an increase in VO2 of less than or equal to 150 mL/min with an increase in workload (WR).
• During progressive exercise, the rate of appearance and disappearance of lactate may not always balance, which can cause a buildup of lactate and a shift to a non-linear response pattern.
• The rate at which individuals reach VO2 max can vary.

Secondary Criteria for Successful Maximum Exercise Test

• Individuals may exhibit "volitional fatigue" (feeling they absolutely cannot continue) during a cycling or treadmill exercise test.
• Respiratory Exchange Ratio (RER) of 1.15 or greater can also indicate a successful max test. The RER considers the metabolic exchange of carbon dioxide produced by the body (VCO2) compared to the volume of oxygen consumed (VO2).
• Reaching a maximum heart rate (HR) of 220 minus the individual's age or 208 minus (0.7 times age) provides another indication of a maximum test.
• Rating of Perceived Exertion (RPE) of 17 or greater using the Borg 6-20 scale can indicate a successful test.

VO2 Max (Peak) Values and Exercise Type

• VO2 max (peak) values on a treadmill tend to be higher than those observed during exercise on a cycle ergometer (bicycle).
• This difference may be attributed to factors like greater skeletal muscle mass use during running (vs. cycling) and varying levels of venous return.

Force Production During Progressive Exercise

• Skeletal muscle activation increase linearly with progressive exercise.
• Muscle fiber recruitment, particularly type I and IIx fibers, increases in a pattern in response to this progressive intensity.
• Linear increase in VO2 is caused by a linear increase in oxygen demand.
• The recruitment of type IIx fibers is associated with the greatest threshold to generate force.

Substrate Utilization During Progressive Exercise

• At light intensities, the primary energy source is fat. At higher intensities, muscle relies more on carbohydrates (glucose) to produce ATP.
• This "crossover" point occurs when the reliance for fuel shifts from fat to carbohydrates.
• Increased reliance on carbohydrates at higher intensities is partly due to the recruitment of type II fibers that are more adept at performing glycolysis.
• The higher reliance on glucose in skeletal muscles reduces fatty acid delivery from adipose tissue to skeletal muscles which will reduce the rate of lipolysis (fat breakdown.)

Blood Lactate Response to Exercise

• Lactate threshold is the work rate at which blood lactate begins to accumulate exponentially.
• The onset of blood lactate accumulation (OBLA) is the work rate where blood lactate reaches 4 mmol/L.
• Factors like rate of appearance and rate of removal of lactate can influence its level in the blood.

Relationship Between VO2, VCO2, RER, Blood Lactate, and VE

• A higher number of active muscle fibers correlates with a higher rate of CO2 production.
• The relationship between VO2, VCO2, RER, blood lactate, and ventilation is relatively linear across a wide range of workload intensities.
• At high intensities, CO2 production may exceed O2 consumption which results in an RER greater than 1.
• Lactate and ventilation rates increase exponentially as the rate of workload increases.

Metabolic Response to Progressive Exercise

• ATP production occurs through several biochemical pathways, such as the breakdown of phosphocreatine, and through the degradation of ATP resulting in ADP and inorganic phosphate (Pi) with the release of energy.
• The relative increase and stability of ATP in the presence of continual increased workload in progressive exercise are important features to note.

Tissue Metabolite Concentrations and Exercise Intensity

• Tissue metabolite concentrations increase proportionally with exercise intensity.
• These metabolites produced in skeletal muscle cells, diffuse or are transported from the cells, to generate an increase in interstitial fluid.
• Higher concentration of metabolites in the interstitial fluid is proportional to the metabolic rate or exercise intensity. This influences blood flow.
• Potassium concentration and lactate concentration increase in the interstitial fluid (ISF) as exercise intensity increases.

Heart Rate Response to Progressive Exercise

• Heart rate (HR) increases linearly from rest to low to moderate workloads.
• Higher levels of HR are associated with the withdrawal of parasympathetic (PNS) stimulation and the activation of sympathetic (SNS) nerves in the sino-atrial (SA) node.
• The release of norepinephrine (NE) from the SNS nerves significantly increases heart rate.

Control of HR - Resting Membrane Potential

• The resting membrane potential (Em) is the electrical voltage across the cell membrane during rest, and its value in cardiac cells is -90mV.
• The Em is determined by the concentration differences of positively and negatively charged ions across the cell membrane, the membrane's relative permeability and the ion pumps that transport ions across the membrane, along with the presence of intracellular non-diffusible negatively charged proteins.

Control of HR - Action Potentials

• Action potentials that are generated in response to a stimulus are a consequence of rapid changes in membrane potential across the cell, depolarizing from a resting negative value to a more positive one, and then reverting back to the resting potential.
• During action potentials in cardiac cells, changes in the permeability and movement of ions across the cell membrane are crucial to membrane depolarizations and repolarizations.

Stroke Volume Response to Exercise

• Stroke volume (SV) is the difference between the end-diastolic volume (EDV) or preload and end-systolic volume (ESV) or afterload, and inotropy.
• SV typically plateaus at approximately 40–50% of maximal VO2.

Regulation of Stroke volume

• End-diastolic volume (EDV): Volume of blood in the left ventricle before contraction.
• End-systolic volume (ESV): Volume of blood in the left ventricle after contraction.
• Stroke volume (SV): EDV - ESV

Factors Determining SV

• Preload: Ventricular stretch at the end of diastole. Increased EDV increases preload and SV (Frank-Starling law)
• Afterload: The pressure the ventricle must overcome to eject blood during systole. Increased afterload will increase ESV and reduce SV.
• Contractility: The force of ventricular contraction during systole. Increased contractility decreases ESV and increases SV.

Venous Return

• Venous return (VR) is the volume of blood that returns to the heart from the systemic circulation.
• Factors influencing VR include venoconstriction (constricting veins), skeletal muscle pump, and the respiratory pump.

Pulmonary System

• Ventilation and gas transport are determined by ventilation, respiration, hemoglobin concentration and saturation.
• Various factors influence the oxygen content dependent on metabolic rate; blood or tissue perfusion.

Respiratory Musculature

• The diaphragm and intercostal muscles, as well as accessory muscles, regulate breathing, specifically inspiration (inhaling) and expiration (exhaling).
• During exercise, these muscles increase their activity to enhance ventilation.

Uptake of Oxygen in the Lung

• Oxygen exchange takes place within the alveoli and pulmonary capillaries, due to the large pressure gradient between the alveolar air in the lungs and the venous blood.

Adjustments in Pulmonary Circulation

• Adjustments in blood flow to the lungs are crucial during exercise to maintain gas exchange.
• The relationship between pulmonary blood flow, ventilation or partial pressure of oxygen, or partial pressure of carbon dioxide, depends on the amount of activity and blood flow to the tissues which in turn correlates oxygen delivery to the tissues.

Hemoglobin and Oxygen Transport

• Hemoglobin is an oxygen-carrying protein in red blood cells that transports most of the oxygen in the blood. Hemoglobin binds oxygen in the lungs, where oxygen levels are high, and releases it to the tissues, where oxygen levels are lower.
• Hemoglobin, or Hb, carries oxygen which is critically important in physiological function. Oxygen is important because of the role it plays as a final substrate needed by mitochondria to produce ATP via oxidative phosphorylation or aerobic respiration.

The O2-Hemoglobin Dissociation Curve

• The O2-hemoglobin dissociation curve describes the relationship between the partial pressure of oxygen (PO2) and the percentage of hemoglobin saturated with oxygen (SO2).
• The shape of the dissociation curve is sigmoidal/s-shaped under resting conditions, but it can be altered linearly in active conditions.