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My questions semi-relates to these two items: Where does extra blood come from to fill your muscles during exercise? , and Blood pressure during exercise .

While reading Exercise Physiology by Dr. Brooks, I've come to the following quote, which is a bit puzzling to me:

Muscle blood flow increases exponentially with metabolism. At maximum exercise, some vasoconstriction occurs in active muscle in order to maintain blood pressure. Maximum muscle blood flow is largely determined by the amount of active muscle. The larger the portion of active muscle mass, the less total muscle blood flow.

Questions:

  1. Why is it that the larger the active muscle mass, the lower the total muscle blood flow? I'd expect it to be greater.

  2. Say that I am doing the bench press, does that qualify as an exercise with large active muscle mass?

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  • $\begingroup$ I think that vasoconstriction from the quoted part refers only to maximum exercise. Because, in general, blood flow in the active muscles during exercise increases. $\endgroup$
    – Jan
    May 6, 2019 at 17:42
  • $\begingroup$ Re #2, think about the difference between doing it with heavy weight & few reps, vs light weight & many reps, or in the extreme, a highly aerobic exercise like rowing where endurance rather than strength is needed. $\endgroup$
    – jamesqf
    May 7, 2019 at 17:30
  • $\begingroup$ @jamesqf not sure I am following the comment james. Are you saying that Q#2, only applies to endurance / aerobic exercise? i.e anaerobic exercises like bench press would not necessarily qualify? $\endgroup$
    – angryip
    May 7, 2019 at 18:12
  • $\begingroup$ @angryip: No, I'm saying that apparently (I have never seen detailed studies) blood flow increases during aerobic/endurance exercise, but doing things like a bench press with heavy weights gets the muscles "pumped", which seems to decrease the blood flow. (This is just my casual observation, of course, and could easily be wrong.) $\endgroup$
    – jamesqf
    May 8, 2019 at 4:53
  • $\begingroup$ @jamesqf interesting. I'll try to dig it up in this book and observe $\endgroup$
    – angryip
    May 8, 2019 at 5:07

1 Answer 1

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I think there might be some missing context here leading to confusion of which muscles are being referred to in that passage.

During exercise, blood flow increases in active muscle due to local vasodilation. Cardiac output also increases, which ensures that total blood flow in the body is sufficient to meet the increased demand.

At higher levels of exercise, there is also systemic vasoconstriction. Systemic vasoconstriction acts to maintain blood pressure. You simply have too much total muscle for maximum cardiac output to supply all the muscle with full vasodilation plus the rest of the body without a loss of pressure. For an athlete with greater maximum cardiac output, it would be possible to fully perfuse more muscles at high effort than for someone untrained.

At maximum exercise, some vasoconstriction occurs in active muscle in order to maintain blood pressure

Because vasoconstriction is systemic, it has an influence on all the muscles, including the active ones. You should think of this vasoconstriction as relative to already dilated vessels, not necessarily compared to rest.

Maximum muscle blood flow is largely determined by the amount of active muscle.

This sentence could be rewritten as:

Maximum blood flow through a given muscle is largely determined by how much muscle is active in the rest of the body.

That is, the maximum blood flow through an arm muscle would be greatest if your legs are resting.

The larger the portion of active muscle mass, the less total muscle blood flow.

This is I think the most confusing sentence...I believe what they are referring to is the total muscle blood flow for a given muscle (total meaning over time rather than over space), which will be maximum when fewer other muscles are active at the same time.

Joyner and Casey 2015 explain all this in more detail; their Figure 8 is most directly related to the question but the rest of the background is also useful.

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.

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  • $\begingroup$ that makes a lot more sense! thank you! the wording in ch 16&17 of the book is a bit challenging to understand. $\endgroup$
    – angryip
    May 7, 2019 at 11:54

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