Heres a decent article I found regarding the science behind the ECA stack
ECA Supplement Stack For Muscle Growth and Fat Loss
by Stimulating the ß-Agonist System - The Role of Ephedrine,
Caffeine, and Aspirin
By Michael C. Prevost Ph.D.

The role of the ß-agonist (beta agonist) pathway in controlling
the body's fat stores, muscle hypertrophy (growth), the body's
response to exercise and even muscle fiber type has been
brought to light in the last 10 years through the efforts of
hundreds of scientists with literally thousands of papers
being published on the subject. Clearly, the ß-agonist pathway
is one of the most important signalling pathways in the body.
To understand how the ß-agonist pathway works and what role
Caffeine, Ephedrine and Aspirin plays in the system one must
first understand what a ß-agonist is and what they do.

On the surfaces of many of the cells of the body (for our
purposes the most important tissues are muscle and fat cells)
are located b receptors. These b receptors bind b-agonists
(adrenaline and noradrenaline). When a ß-agonist is bound
to a b receptor, the receptor initiates a series of chemical
reactions that results in the production of a chemical
messenger called C-AMP. This C-AMP then activates
enzymes that phosphorylate proteins. Why is thisimportant?
Well, many of these proteins are enzymes and
phosphorylation activates some enzymes and de-acitvates
others. In fat cells enzymes are activated that induce
lipolysis (fat breakdown). In muscle cells enzymes are
activated that increase metabolism and cause a host of
other important reactions which control muscle growth, fiber
type and enzyme concentration. So, how do ephedrine,
caffeine and aspirin fit into this pathway? Ephedrine
enhances ß-agonist production and even acts as a
ß-agonist itself. Caffeine inhibits the breakdown of C-AMP.
Aspirin inhibits the negative feedback loop that would
reduce ß-agonist production. So taken together these
agents enhance three to four different steps in the ß-agonist
pathway.

Ephedrine's role as a lipolytic agent (one that breaks down
body fat to be used as energy) has been known for some
time. It has been shown that ephedrine when taken in
therapeutic doses is mildly effective in the management
of obesity. The problem was that the initial lipolytic effects
of ephedrine were soon diminished as other steps in the
pathway were reduced in a negative feedback cycle. In an
effort to enhance these steps that were being
downregulated due to negative feedback, scientists added
caffeine and aspirin to the regime. The result was a very
effective combination in the management of obesity
(dosages given were 20mg ephedrine, 300mg caffeine, and
80mg aspirin, roughly equivalent to one typical ephedrine
tablet, a cup of coffee and one aspirin). In fact, not only
did the results (lipolysis) not decrease over time as with
most drugs, but they actually increased with time. The
effects of the combination of ephedrine, caffeine and
aspirin were categorized into desirable and undesirable
effects. The desirable effects were lipolysis and protein
sparing (subjects on the drug combination retained
more muscle mass while dieting than the subjects on
placebo). Again, these desirable effects did not diminish
over time. The undesirable effects, increased heart rate
and muscle tremors, lasted only a few days and never
returned. In fact, after 1 year of supplementation subjects
were experiencing no side effects but were still
experiencing the desired effects of lipolysis and protein
sparing. To date it has not been determined if the ephedrine,
caffeine and aspirin combination can enhance muscle
growth and fat loss in healthy, exercising adults. However,
based on what we know about the ß-agonist system, it is
certainly possible. It is important to note, however, that a
small handful of subjects among the large subject pool
had to drop out of the study due to an intolerance to the
supplementation regime. So it appears that most
people can use the ephedrine, caffeine and aspirin
combination with no problems, while a small handful of
people may be intolerant. A consultation with a
physician is suggested before beginning this ECA
supplement regime.

The ß-agonist pathway may effect processes other than
muscle sparing and fat loss. In fact in chickens ß-agonists
were shown to be stronger growth promoting agents than
steroids (believe it or not but poultry science folks are
doing a lot of research trying to produce more muscular
chickens since the muscle is the meat that we eat). In fact
ß-agonists seem to produce muscle growth without the
stimulus of exercise, something steroids have failed to
do. However ß-agonists are not as effective in rats and
the effects on humans is unknown. The ß-agonist
clenbuterol can cause slow twitch muscle fibers to
be converted to fast twitch fibers. It can also prevent
muscle atrophy due to disuse. Other studies have shown
that b-antagonists (agents that block b-receptors and
prevent them from functioning) cause muscles to shift
from fast twitch to slow twitch and cause muscle atrophy
(muscle wasting or breakdown). These studies indicate
that ß-agonists might play an important role in the
maintenance of fast twitch muscle fibers and in maintaining
and perhaps increasing muscle mass. It may be no
coincidence then that large amounts of ß-agonists
(adrenaline and noradrenaline) are produced during
high intensity training sessions. Perhaps these
ß-agonists are necessary in initiating the muscle growth
stimulus.

It is important to note that some ß-agonists can be
dangerous (for example clenbuterol and cocaine) and
illegal and these substances are not recommended. Also,
as mentioned earlier, although the ß-agonist combination
of caffeine, ephedrine and aspirin appears to be safe (and of
course legal) a small number of people are intolerant and
consultation with a physician is suggested. Everyone
should visit there doctor prior to using any of these
substances for a clean bill of health.