Maybe I'm just not understanding what is meant by "cardiac output" here, but
most of what I've read about exercise physiology says it's more complicated
than that.

Mark Drela wrote
aerobic power output is limited mainly by cardiac output,
not by how much muscle mass can be recruited.

If I'm understanding Mark right, this is a key issue in choosing among
techniques. Because if (A) most of us are limited by central VO2max, then
there's little point in looking for more muscles to use -- instead we should
just figure out which subset can most effectively convert VO2 into
forward-motion power, and focus only on those few. But if (B) the
performance limit for most of us is something like Lactate Threshold with a
strong "peripheral" component, then looking for more muscle groups to engage
is good, even if somewhat less effective (provided they don't interfere with
the more effective subset).

My current analysis:
It may be that elite XC ski racers are limited by their central
cardio-vascular capacity (heart chamber volume, max rate, central
artery-vein capacity) -- but most of the rest of us are not operating
anywhere near our personal genetically possible VO2max, and certainly not in
a 50 km marathon race.

Lots of authorities say that the aerobic limit for most of the rest of us is
strongly influenced by how well-trained our body is at transporting that
oxygen out to the specific muscles that need it. And that "peripheral"
capacity depends on lots and lots of tiny capillaries out there right near
those specific muscles -- also on how good the specific muscles are at
quickly and effectively using the oxygen that arrives, and quickly and
effectively transporting bad byproducts out ("lactate" seems to be the prime
offender that the authorities mention). My reading says that (B) is the
condition for most of us.

So spending time building those peripheral capacities makes sense, and all
the programs I've seen in books devote lots of training hours to exercises
for that. And to me, finding clever ways to use more muscles make sense --
and when I look carefully at videos of Alsgaard and Swenson skating, I see
them making very precise use of a large number of muscles.

I don't see how the elite XC ski racers could have _trained_ themselves to
such a high VO2max central cardio-vascular capacity without having figured
out how to recruit a larger muscle mass. Same for the rest of us: the more
muscles we learn to engage in our skiing, the more stimulus we can provide
to raise our central CV capacity to higher levels.

That's my current theory anyway -- if somebody's got corrections or
improvement, I'd be glad to hear them, so I can do better on my technique
and training choices.

Ken