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 why?
Here are the main factors contributing to stimulating growth:
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 recruit.
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 Output: 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 process).
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. 1980).
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 by:
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 exercises.
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
Exercise 2: Moderate rep movement (8-10) using another multi-joint exercise
Exercise 3: High rep movement (12-15) using a secondary exercise
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 war.