The Volume Equation

Correlation doesn't always equal causation.

Volume is one of the most thoroughly researched training tools as it relates to influencing muscle growth. As a rule of thumb, an increase in volume performed appears to stimulate greater expected results – at least in certain populations, for a certain amount of time, under the right circumstances. However, volume is a raw numerical value. What it fails to describe is the underlying mechanisms which stimulate growth. This, along with a hazy understanding of terms combines for a general misunderstanding of the concept – which continues to influence programming decisions to this day.

Volume Defined

Volume is the sum total of sets and repetitions. You can take it one step further and integrate the load used during the exercise, for example:

[sets] x [reps] x [load] = volume

[3] x [10] x [100 lb] = 3000 lbs of volume performed (or volume-load).

This is usually where most people land when tracking volume. Daily or weekly tonnage seems like a logical form of quality assurance when you consider the amount of research suggesting increased volume over time drives hypertrophy.

However, it isn’t quite as simple as that. If all you did was track volume through tonnage and creep it ever higher – you would certainly see results, but we can do better with a touch of physiology.

Hypertrophy How-To

To better understand how to interpret volume, a few principles of hypertrophy go a long way. The overarching goal of hypertrophy training is to grow new tissue. In order to do so, you’ll need a few key components.

First, actual recruitment of the full breadth of motor units that controls your target tissue. Second, adequate stimulation without overkilling the muscle in order to see the best stimulus-recovery cycle for ongoing progress.

Motor Unit Recruitment

A motor unit is the interface between muscle and the nervous system that controls it. It’s composed of an alpha motor neuron and all of the muscle fibers it commands – thus, motor unit recruitment is highly important for your growth. You’ll want to recruit as many motor units per set as possible to maximize the efficiency of your workout, and this works through Henneman’s size principle of motor recruitment.

Henneman’s size principle of motor recruitment states that motor units are recruited in orderly fashion based upon necessity. As you begin any movement, the smallest, weakest, and lowest threshold (the degree of challenge needed to recruit them) motor units are called into action. With increased force demands or accumulated fatigue, higher threshold motor units (typically your type-2) are then brought into the equation in escalating fashion. 

Complete recruitment of the complement of available motor units can be inferred by the degree of challenge experienced during the set. If there is an orderly recruitment of motor units, reaching high degrees of effort (close to failure) would by extension indicate that most (if not all) available motor units had been recruited. This is particularly important for hypertrophy as the higher threshold motor units tend to be the ones with the greatest hypertrophy potential.

Stimulation of Motor Units

Stimulation of the recruited motor units is extremely important for growth. As introduced through motor unit recruitment, high degrees of effort are essential to capture adequate stimulus. Approaching muscle failure (and thereby recruiting and stimulating motor units) is not just an arbitrary set and repetition scheme – or even when the set begins to feel difficult. It is an involuntary slowing of concentric contraction speed. The involuntary part is critical, as it indicates that despite greater and greater input, you can no longer overcome the resistance. Thus, recruitment and the greatest compliment of tissue reaching a proximity to failure all at once. Mechanical tension is the driving force behind stimulation – a topic for another day.

Training is about striking a balance between stimulus and recovery. Motor unit recruitment and stimulation guide how your sets should look – but every set takes something away from your pool of resources. Fatigue accumulates from the onset of the workout and mounts as you perform greater and greater volume throughout the day. This means that peak efficiency is to recruit as many motor units as possible and stimulate them all together within the confines of recoverable volume before cutting yourself off.

Stimulating Repetitions

Stimulating repetitions is the proposed stratification of value received from repetitions performed across a workout. Stimulating repetitions (or more accurately, more stimulating repetitions) are typically the ones described as having a high proximity to failure. This means that within a single set, the most motor units have been recruited and stimulated simultaneously. It doesn’t mean that anything performed short of this has no value, but rather, that over time the goal should be to train towards greater levels of efficiency.

Each set performed is inherently less effective due to growing fatigue. Motor unit recruitment becomes more difficult and recovery costs increase from additional sets or repetitions beyond an individualized threshold. Aiming to tailor your workouts around the stimulating repetition count ideal for you is a maturation process over time.

Basically, cut the fat from your workouts for another boost to your progress.

Putting It All Together

Volume represents the numerical sum of proxy metrics used to stimulate growth – but is easy to misinterpret and subsequently inappropriately program. Motor unit recruitment and stimulation is the key to hypertrophy; however, simply applying greater degrees of volume is not the same thing.

Progressive increases in volume helps stimulate early growth because you are still learning to train. Movement pattern efficiency is improving, true proximity to failure per set is being realized, and your volume threshold for recovery is being landmarked. 

In reality, increased volume in this stage is actually just nudging you closer to achieving previously untapped stimulating repetitions via accumulated fatigue. You gain more net stimulation by pressing closer to failure due to previously non-stimulating sets. This method will inevitably experience diminishing returns.

Past a certain stage, refinement must occur. This is where progressive increases in volume ceases to provide more growth and actually may become detrimental. As you get better, volume [sets x reps] tends to decrease – or at least slow to a crawl. You will continue to get stronger, which by virtue of increased load will steadily “increase volume-load” (sets x reps x load) but you’re actually chasing the stimulating repetitions per set through whichever means helps you get there as efficiently as possible.

There are consequences to excessive volume without stimulation. This doesn’t mean that landmarking your working weight or using warm-up sets will be fatal to your progress, but it does mean that arbitrary increases in sets or repetitions without adequate challenge will inevitably stop working. The mechanisms of growth are becoming more fleshed out, which means surrogates such as volume require contextualization moving forward.

Volume ≠ Stimulation

In absence of quality, there is volume.

You can see tremendous progress by utilizing ever increasing (borderline overwhelming) volume. A lack of precision does not mean a complete sacrifice of growth, but the possibility of growth despite inaccuracy leaves a huge runway of progress potential on the table. As you gain experience in the weight room, volume as defined by sets and repetitions loses its value and must be further contextualized. Volume will always linger as a metric within programs – but the real question is, volume of what?