When we exercise, our muscles demand more oxygen to generate energy and continue contracting. To deliver this increased oxygen, blood flow increases to active skeletal muscles. But where exactly does the greatest increase in blood flow occur during physical activity? Understanding the cardiovascular response to exercise provides insight into how our circulatory system adapts to meet the metabolic needs of working muscles.
Cardiovascular changes during exercise
The cardiovascular system responds to exercise in several important ways:
Heart rate increases
One of the first changes is an increase in heart rate, driven by increased sympathetic nervous system activity. Heart rate increases linearly with exercise intensity. This elevates cardiac output, which is the volume of blood pumped by the heart per minute. The higher cardiac output is necessary to enhance oxygen delivery to working skeletal muscles.
Stroke volume increases
Stroke volume, or the amount of blood ejected by the left ventricle with each contraction, also increases with exercise. This is primarily achieved by an increase in venous return, which stretches the ventricular wall and leads to a more forceful contraction via the Frank-Starling mechanism. Sympathetic activity may also enhance ventricular contractility. The end result is a greater volume of oxygenated blood pumped out per beat to supply the muscles.
Vasodilation occurs in active muscles
Blood vessels supplying active skeletal muscles vasodilate, decreasing resistance and allowing more blood flow into the muscles. This vasodilation is mediated by multiple mechanisms, including the release of nitric oxide and prostaglandins locally within the muscles, as well as sympathetic neural activation. Vasodilation may increase blood flow to active muscles by 10- to 20-fold.
Where does the greatest blood flow increase occur?
Taking all of these cardiovascular changes into account, where is the increase in blood flow most pronounced during exercise?
Blood flow changes by muscle fiber type
Studies show that blood flow increases to a greater degree in muscles composed predominantly of type I (slow twitch) muscle fibers compared to those with mainly type II (fast twitch) fibers. This likely relates to the greater reliance on aerobic metabolism and oxygen delivery in type I fibers. During low-moderate intensity exercise, type I fiber-rich muscles like the soleus (a calf muscle) may see blood flow increases of 3- to 4-fold. Comparatively, type II fiber-dominant muscles like the white portion of the gastrocnemius (another calf muscle) may only exhibit 2- to 3-fold increases.
Blood flow changes by muscle group
In general, blood flow increases to a greater degree in the active muscles of the lower body compared to the upper body. For example, during leg cycling blood flow to the quadriceps increases dramatically, whereas blood flow changes little in inactive upper body muscles like the biceps. This pattern reflects the larger muscle mass and metabolic activity occurring in the legs.
The greatest increase is in locomotor skeletal muscles
Looking comprehensively at all active skeletal muscles, the available evidence indicates the greatest increases in blood flow occur in contracting locomotor muscles like the quadriceps and calves. In trained cyclists performing maximal leg cycling, blood flow to the quadriceps and calves can increase by 6- to 8-fold above resting levels. This represents the maximal vasodilatory capacity of the training-adapted locomotor muscle vasculature. By comparison, smaller increases occur in non-locomotor arm and shoulder muscles during arm cranking exercise.
Mechanisms underlying enhanced skeletal muscle blood flow
What accounts for the huge elevations in blood flow seen in exercising locomotor skeletal muscles? Several key factors are at play:
Metabolic vasodilation
Contracting locomotor muscles generate multiple metabolites that cause local vasodilation, including adenosine, nitric oxide, hydrogen ions, carbon dioxide, and potassium ions. This allows more blood carrying oxygen and nutrients to diffuse into the active muscles.
Endothelial nitric oxide production
Shear stress on the endothelium triggers greater nitric oxide generation, enhancing vasodilation. The high blood flow rates in locomotor muscles during exercise increase shear stress on the vessel walls.
Sympathetic vasodilation
The sympathetic nerves innervating locomotor muscle vasculature contain not only vasoconstrictor fibers but also vasodilator fibers releasing norepinephrine. Sympathetic stimulation activates the dilator fibers, amplifying vasodilation.
Training adaptations
Regular aerobic exercise induces remodeling and expansion of the vascular network within locomotor muscles, increasing maximal blood flow capacity. This accounts for the greater vasodilation seen in athletes versus untrained individuals.
Blood flow changes in other areas
While blood flow increases most robustly in active locomotor skeletal muscles during exercise, changes occur elsewhere:
Heart
Coronary blood flow must increase to deliver oxygen and nutrients to the pumping heart muscle. In endurance athletes, coronary blood flow can increase 4- to 7-fold with exercise.
Lungs
Pulmonary blood flow rises during exercise to facilitate gas exchange. This increase is proportional to exercise intensity and cardiac output.
Skin
Blood flow to the skin increases to facilitate heat dissipation, mediated by vasodilation of cutaneous vessels. Without this response, hyperthermia could develop rapidly.
Abdominal organs
Splanchnic blood flow to the abdominal viscera decreases during exercise, mediated by sympathetic vasoconstriction. This helps support blood pressure and diverts flow to more metabolically active tissues.
Brain
Cerebral blood flow changes little during exercise due to autoregulation maintaining constant perfusion of this vital organ.
Key factors influencing changes in skeletal muscle blood flow with exercise
The increase in blood flow to locomotor skeletal muscles during exercise is influenced by several key factors:
Exercise intensity
Higher intensity exercise causes greater increases in skeletal muscle blood flow. This relationship is linear at submaximal intensities. Maximal blood flow capacity is reached near VO2 max.
Active muscle mass
Exercise engaging a greater amount of muscle mass (like running vs. knee extension) induces higher overall skeletal muscle blood flow.
Training status
Trained athletes show greater increases in exercising muscle blood flow compared to untrained individuals at the same absolute intensity, facilitated by vascular remodeling.
Muscle fiber composition
Muscles with more slow-twitch type I fibers exhibit greater blood flow increases during submaximal exercise compared to fast-twitch type II-dominant muscles.
Age
Older individuals often have an attenuated vasodilatory capacity and blunted muscle blood flow response during exercise.
Sex
Estrogen may enhance skeletal muscle vasodilation in women. However, sex differences in exercise blood flow responses are not consistently observed.
Environmental temperature
During exercise in the heat, more blood flow is directed to the skin for thermoregulation. This may competitively reduce blood flow increases to working muscles.
Conclusion
In summary, blood flow increases substantially in contracting locomotor skeletal muscles during exercise to meet the enhanced oxygen and nutrient demands. The greatest elevations are seen in trained athletes, where blood flow to muscles like the quadriceps can increase by 6- to 8-fold. Multiple mechanisms mediate this robust vasodilation and enhanced perfusion response. Understanding these cardiovascular adaptations provides key insight into how the circulatory system regulates oxygen delivery and supports sustained physical activity.
References
- Joyner, M. J., & Casey, D. P. (2015). Regulation of increased blood flow (hyperemia) to muscles during exercise: a hierarchy of competing physiological needs. Physiological reviews, 95(2), 549–601.
- Laughlin, M. H., & Korzick, D. H. (2001). Vascular smooth muscle: integrator of vasoactive signals during exercise hyperemia. Medicine and science in sports and exercise, 33(1), 81–91.
- Mortensen, S. P., Dawson, E. A., Yoshiga, C. C., Dalsgaard, M. K., Damsgaard, R., Secher, N. H., & González-Alonso, J. (2005). Limitations to systemic and locomotor limb muscle oxygen delivery and uptake during maximal exercise in humans. The Journal of physiology, 566(1), 273–285.
- O’Leary D. S. (1993). Regional vascular resistance vs. conductance: which index for baroreflex responses?. The American journal of physiology, 265(2 Pt 2), H632–H637.
- Rowell L. B. (1993). Human cardiovascular control. Oxford University Press.
| Study | Subjects | Exercise protocol | Key findings |
|---|---|---|---|
| Richardson et al. 1995 | 8 trained cyclists | Maximal cycling exercise | 8-fold ↑ leg blood flow |
| Bada et al. 2012 | 12 healthy adults | Moderate cycling exercise | 4-fold ↑ quadriceps blood flow |
| Mortensen et al. 2005 | 6 healthy males | Knee-extensor exercise | 5-fold ↑ quadriceps blood flow |
