Re: What causes lung, stomach, and side cramps during strenuous exercise?

REFERENCE(S) IF NOT READILY AVAILABLE:
1)Human Biology by George B. Johnson, Washington University, St. Louis;
Wm. C. Brown Publ., 1994 (pages 74-79 inclusive)
2)College texts on cell biology, biochemistry

Dear Mike,

Cell metabolism (muscle, liver, other cells) is often illustrated as a
vertical chain of chemical reactions called GLYCOLYSIS, which does not
require oxygen.  This chain is connected at its base to circle of chemical
reactions called OXIDATION, which by definition requires oxygen through
pulmonary respiration. This second reaction "ring" is also called the Krebs
Cycle from its discoverer.

During sustained excercise, glycolysis occurs immediately and, when oxygen
is present, rapidly initiates the oxidative pathway by converting the
glucose metabolite, PYRUVATE into two molecules of the "oxidation spark
plug" called acetyl-CoA.  The result?  The oxygen-requiring Krebs Cycle is
"off and running!"  Both pathways produce the high energy molecule ATP,
which is required for muscle contractility; but the oxidative reaction is
overwhelmingly more efficient in terms of ATP yield (i.e., 6 ATP molecules
from glycolysis versus 30 from oxidation)!

Without oxygen the body tires rapidly and eventually fails, as seen in high
altitude mountain climbing.  During oxygen deprivation, as in your example
of the "very strenuous part of a workout," the pyruvate produced in the
muscle during glycolysis can't be converted into acetyl-CoA!  Instead of
the pyruvate accumulating in your blood and reaching toxic levels, it is
converted (again without oxygen) into LACTIC ACID.  This molecule can be
converted BACK into pyruvate when oxygen becomes available!  Unfortunately,
no ATP is generated during lactic acid production.  Your skeletal muscles
will fatigue without oxygen generated ATP and you, the athlete, feel a
"tired, burning" muscle sensation, the so-called "oxygen debt."  Without an
oxygen supply a contracting muscle eventually can not relax or "recover,"
but will simpley remain in its contracted (or cramped, spastic) condition.

Speak with your coaches and the excercise physiologists at Vanderbilt; they
can help you with the necessary interval training that can often prevent
this muscle hypoxia.  Diet is important also!  Good luck in your sports and
lifelong learning.

Sincerely,
Scott