From the Guidelines
During muscular exercise, the cardiorespiratory system undergoes significant changes to meet increased metabolic demands, with cardiac output increasing up to 6 times that at rest, and minute ventilation increasing in proportion to the increase in work rate, as noted in the 2010 study by the American Heart Association 1. The increase in oxygen uptake by the working muscles is facilitated by an increase in cardiac output, which is achieved through an increase in heart rate and stroke volume.
- Cardiac output is redistributed away from nonactive tissues to the skeletal muscles, facilitating greater O2 delivery.
- A concomitant increase in blood flow to the lungs occurs, both by the increase in cardiac output and by vasodilation of the pulmonary vessels.
- A greater extraction of O2 from the blood also occurs as the blood perfuses the muscles, resulting in a widening of the arteriovenous oxygen difference. Respiratory adaptations include:
- Increased breathing rate from 12-15 breaths per minute at rest to 40-50 during exercise, along with greater tidal volume.
- Minute ventilation increases from 6 liters per minute to potentially 100-150 liters during maximal exertion. These adaptations are driven by metabolic demands, neural stimulation via the sympathetic nervous system, and local chemical changes including decreased pH and oxygen levels in working muscles, as discussed in the 2001 study on exercise testing in chronic heart failure patients 1. Regular exercise training enhances these responses through physiological adaptations like cardiac hypertrophy and increased capillary density, leading to improved cardiorespiratory fitness and reduced global cardiovascular risk, as outlined in the 2007 update by the American Heart Association 1. The most critical aspect of these changes is the increase in cardiac output and minute ventilation, which are essential for meeting the increased metabolic demands of the working muscles during exercise, and are supported by the highest quality evidence from the 2010 study 1.
From the Research
Cardiorespiratory Changes During Muscular Exercise
The cardiorespiratory changes during muscular exercise are complex and involve various physiological responses. Some of the key changes include:
- Increased heart rate and cardiac output to supply oxygen and nutrients to the muscles 2
- Increased ventilation to take in more oxygen and expel carbon dioxide 2
- Increased blood flow to the muscles to deliver oxygen and nutrients 2
- Increased aerobic and anaerobic metabolism to generate energy for muscle contraction 2
Metabolic Response to Exercise
The metabolic response to exercise involves the breakdown of energy stores to fuel muscle contraction. The key metabolic pathways involved include:
- Anaerobic glycolysis for high-intensity, short-duration exercise 2
- Aerobic metabolism for low-to-moderate intensity, long-duration exercise 2
- Fat oxidation for low-intensity, long-duration exercise 2
- Protein metabolism for prolonged exercise or when energy stores are depleted 2
Exercise Recommendations
The American College of Sports Medicine recommends the following exercise guidelines for adults:
- At least 150 minutes of moderate-intensity aerobic exercise per week 3, 4
- At least 75 minutes of vigorous-intensity aerobic exercise per week 3, 4
- Resistance training exercises for each major muscle group 2-3 times per week 3, 5, 6
- Flexibility exercises for each major muscle group 2-3 times per week 3, 5, 6
Individualized Exercise Programs
It is essential to design an individualized exercise program that takes into account a person's needs, goals, and initial abilities 3, 5, 6. The program should include a combination of cardiorespiratory, resistance, and flexibility exercises to improve overall fitness and health.