On the surface, stimulating muscle growth is easy: just lift the
damn weight! This will work as long as you're progressively
increasing the demand placed on the muscle over time (either by
adding more weight, doing more reps at a given weight, doing more
sets, increasing density, etc.). But it's still interesting to
understand the factors that can contribute to making your muscles
grow. We know that lifting weights builds muscle, but

Here are the main factors contributing to stimulating

1) Intramuscular Tension: This refers to how hard a muscle must
be contracted during the performance of an exercise. As such, it's
directly correlated with the amount of force you have to produce.

More force equals great intramuscular tension. A high level of
intramuscular tension can influence muscle growth because it leads
to a high rate of protein degradation (more tension = faster muscle
damage). In that regard, see tension as a punch: the harder you
punch someone, the more damage it'll do.

However, the more you put into a punch, the fewer swings you can
take at your opponent. It's impossible to effectively throw 60
uppercuts in one round of boxing, but it is possible to throw that
many jabs.

It's the same thing with weights: the more tension you produce,
the less time you can sustain that tension. So while a high level
of tension will cause rapid muscle damage, if it's too high (e.g.
1-3 maximal reps) the time spent causing damage to the muscle might
not be long enough to elicit a maximal growth response.

But still, high tension will stimulate muscle growth. It also
has another interesting impact: the recruitment of the
high-threshold motor units (fast-twitch fibers). The more force
(tension) you produce, the more HTMUs you'll

2) Time Under Tension: If a high intramuscular tension
represents an uppercut, a long time under tension is kinda'
like hitting your opponent 60-80 times in a round. The more often
you hit your adversary, the more potential damage you can cause.

If a set lasts longer, the time spent causing muscle damage is
more important and thus can lead to more growth stimulation. The
problem is that there's an inverse relationship between the
magnitude of the tension and the time that it can be sustained: if
you shoot for a very long set, you won't be able to do the set
under a lot of tension.

So while you might spend a lot of seconds working at causing
muscle damage, you're actually not causing a lot of damage per
second. You can bitch-slap an opponent a thousand times, but it
won't knock him out! Just like with tension – increasing the time
under tension of a set will lead to more growth stimulation, but
only if the tension level stays relatively high.

3) Blood Vessel Occlusion, Metabolite Accumulation, Hormonal
You might have heard of "kaatsu" or tourniquet training.
It's a training method that relies on lifting light weights (20-30%
of your maximum for 15-30 reps) while wearing a special cuff that's
tightened up around the proximal end of a limb to restrict blood
flow to the muscle.

Studies have shown that despite the light weights being used,
the muscle growth response is as big as lifting heavy weights (80%
and more). The reason is the blood occlusion, which has been shown
to create a deprived oxygen state (since blood flow to the muscle
is limited, so is oxygen transport). This leads to an accumulation
of lactate which increases the production of both hGH (growth
hormone) and IGF-1 (Takareda et al. 2000).

The lack of oxygen (hypoxic state) and increase in acidity has
also been shown to increase the recruitment of fast-twitch muscle
fibers (Shinohara and Moritani, 1992). In fact, oxygen restriction
to the muscle increases fast-twitch recruitment, firing rate, and
spike amplitude (Yasuda, 2005).

Finally, exercise with restricted oxygen/blood entry in the
trained muscle also leads to the production of Reactive Oxygen
Species (ROS), which increase muscle satellite cell activation and
proliferation (two key phenomenon involved in the muscle growth

The good news is that you don't have to use kaatsu training
(which can be risky) to create this oxygen/blood flow restriction
to the muscle. Sustained muscle tension (as in never allowing a
muscle to relax during a set) can make muscle hypoxic even without
external occlusion (Bonde-Peteron et al. 1975, Mitchell et al.

A recent study compared several training protocols' effects on
oxygen levels during the execution of an exercise. With kaatsu
training, oxygen levels were at around 22% of the rested/normal
state, compared to 32-35% for normal, heavy training – a
difference that can explain the efficacy of kaatsu training.

However, they also found that performing sets without blocking
blood flow, but using a 303 tempo and never allowing the muscles to
relax during the set (always flexing as hard as possible during
every inch of every rep) with 50-60% of the maximum performed to
failure, led to oxygen levels of 23-24%. Lactate, hGH, and IGF-1
levels were also the same as with kaatsu training.

The moral of the story is that constant tension exercises can
build size and strength despite using relatively light weights and
even if muscle damage is fairly low. However, if the muscle is
allowed to relax during the set, oxygen and blood will flow into
the muscle and you won't reach optimal benefits.

So, we could say that muscle growth can be stimulated

1. Heavy lifting (4-6 reps), which promotes a high rate of
mechanical damage/protein degradation.

2. Relatively high reps (up to 12-15 for the upper body and
15-20 for the lower body), which promote a high mechanical
degradation due to the combination of moderate time under tension
and intramuscular tension magnitude.

3. Constant tension sets. To do these properly, you must flex
the target muscle hard during every inch of every rep. You can
never allow the muscle to relax. This means no rest between reps
either. This method is best kept for isolation

You could take advantage of all three methods by designing your
program according to this template:

Exercise 1: Heavy lift (4-6 reps) using a basic, multi-joint

Exercise 2: Moderate rep movement (8-10) using another
multi-joint exercise

Exercise 3: High rep movement (12-15) using a secondary

Exercise 4: Constant tension movement (303 tempo, 50-60% of
maximum, 8-12 reps) using an isolation exercise

A chest workout might look something like this:

A. Decline bench press

4-5 x 4-6 reps
90-120 seconds of rest

B. Incline dumbbell press

3 x 8-10 reps
75 seconds of rest

C. Cable crossover or lying crossover

3 x 12-15 reps
60 seconds of rest

D. Squeeze press (pressing the dumbbells together as you
simultaneously lift them)

3 x 8-12 using a 303 tempo
45 seconds of rest


It's clear that muscle is stubborn; that it often resists
our best efforts to prod it into growth, but perseverance, a little
smarts, and varying strategies will win the battle and the

Christian Thibaudeau specializes in building bodies that perform as well as they look. He is one of the most sought-after coaches by the world's top athletes and bodybuilders. Check out the Christian Thibaudeau Coaching Forum.