In article
,
" wrote:

On Apr 16, 1:41*am, datakoll wrote:
what is "normal" for an upper exercise limit?
muscles giving out first,
or heart rate topping out as a limit?

A little of both, and as age increases, HR can more often become the
limiting factor. And training changes the balance too.

Riding along at a steady easy level, HR is low and is pumping some
amount of blood and thus delivering some amount of oxygen to the
working muscles. The muscles are able to use this oxygen to do work.
But some of the muscles do not get all the oxygen they need to do the
work asked of them. So they work anaerobically and generate lactate.
This is only a small amount, and the body is able to easily deal with
this lactate before it builds up.

At a higher intensisty (more power) level, more oxygen is needed, so
the HR picks up to deliver more oxygen. But still not all the muscles
get all the oxygen they need, so they produce more lactate. If the
amount of lactate produced can be used by the body so that the amount
of lactate in the blood does not keep rising, the effort level is said
to be below the LT. If more lactate is produced than can be disposed
of, the level of lactate in the blood keeps rising, even though the
effort level remains constant. This is above LT. This is not
sustainable because eventually with high enough concentrations, the
hydrogen ions released as part of the lactic acid creation interfere
with the mucels ability to do work, and it cannot continue to produc
ethe same power, so you slow down. You may have slowed down already,
because unpleasantness ensues usually well befor ethis point.

But LT isn't at max HR, and more work can be done (power generated)
above LT, but just for a limited time. The body can still supply more
ans use more oxygen to do work above LT, but it is inhibited from
maintaining this level of output for very long. This is also different
from peak power, which is more akin to brute strength and usually is
at levels well above LT.

One addition. As you state, the problem with exercise at
high levels is acidosis. Production of lactic acid buffers
the H+, and the lactic acid is then transported out of the
cell. Check out
Biochemistry of exercise-induced metabolic acidosis.
Robert A. Robergs,Farzenah Ghiasvand,and Daryl Parker

http://ajpregu.physiology.org/cgi/content/full/287/3/R502

--
Michael Press