Why does muscle blood flow decrease during exercise?
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., &amp; 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.