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2011-06-07T09:22:00.005-04:00

Miles make you faster....

2011-05-05T22:01:00.000-04:00

The glycogen depletion leads to glycolysis not vice versa as you wrote above.

Explanation in Layman's Terms:

Exercise causes the release of a hormone called epinephrine from a part of the adrenal gland call the medulla. Epinephrine is the same thing as adrenaline. So, that jolt you get when startled--that's epinephrine.

Cells throughout the body have receptors that bind epinephrine. When these receptors bind epinephrine cellular processes are altered. One type of EPI receptor found on cells including skeletal muscle cells is called the beta-2 adrenergic receptor. (The word adrenergic comes from the word adrenal. The word adrenal means near the kidney: ad-renal.)

When EPI docks on the beta-2 receptors on skeletal muscle it triggers the break down of glycogen and the formation of lactate through a signaling pathway called the cAMP/PKA (cyclic AMP/Protein Kinase A) pathway. The epinephrine is causing the glycolysis.

The lactate released from the non-working muscles can be used as fuel by tissues throughout the body including sketetal muscle and heart. The heart is very good at using lactate for fuel becuase of it large number of mitochondria. The heart can be up to 8% mitochondria by weight. Well trained skeletal muscle can be ~2% mitochondria by weight, higher in slow twitch fibers and less in fast twitch fibers. Thus slow twitch fibers are much better at using lactate for fuel than fast twitch fibers. Lactate can be reconverted into glycogen in skeletal muscle and liver. Additionally the liver can convert it back into glucose for immediate release into the blood stream.

This system is beneficial because it allow the fast twitch fibers which are not suitable for use in endurance running to serve as a fuel tank for the slow twitch fibers. Hence even after the slow twitch fibers are glycogen depleted their energy supply can be maintained somewhat by lactate from the fast twitch fibers and glucose from the liver and fat.

Fasting empties the liver of glycogen but will NOT empty the muscles of glycogen. Another hormone called glucagon acts through the same cAMP/PKA pathway to cause glycolyis in the liver but the release of glucose into the blood. Glucagon becomes elevated while fasting. Skeletal muscle lacks glucagon receptors. So, the only way to access the skeletal muscle fuel take is by exercising.

But, do remember that the glycogen depletion occuring in fast twitch fibers that are not recruited on long runs is almost certainly an adaptation stimulus of some kind. Glycogen is a known inhibitor of one pathway for mitochondrial biogenesis. Purposely restricting food intake is to keep muscle glycogen low by not restoring muscle glycogen after workouts in hope of better stimulating mitochondrial growth is probably a bad idea. To stay healthy you should try and recover in every legal way possible. A study just came out showing that interleukin-6, a potent stress chemical with sometimes harmful effects, is released from skeletal muscle if a training session is started with extremely glycogen depleted muscles. However, starting with adequate glycogen and then depleting during the run didn't seem to have the same effect. So, the necessary pattern seems to be empty duing the run, but fill back up a some before the next run. This is really too new of a finding to say much though.

J Appl Physiol. 2003 Aug 22 [Epub ahead of print].

Interleukin-6 release from human skeletal muscle during exercise: relation to
AMPK activity.

MacDonald C, Wojtaszewski JF, Pedersen BK, Kiens B, Richter EA.

Human Physiology, Copenhagen Muscle Research Centre, Institute of Exercise and
Sport Sciences, University of Copenhagen, Copenhagen, Denmark.

Read more: http://www.letsrun.com/forum/flat_read.php?thread=4030549&page=2#ixzz1LX9F2pTh
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2011 Race Hat Ideas

2011-02-20T03:53:00.004-05:00

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Miles make you faster....

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