Strength training volume is a key determinant of the amount of muscle growth that happens after a sequence of workouts. However, exactly how we should best measure volume is unclear.
Common methods of measuring training volume include counting the number of sets to failure or the volume load (sets x reps x weight), although many approaches have been used by researchers.
Even so, none of these current methods of measuring volume may be ideal, because they do not accurately record the volume of the mechanical stimulus that triggers muscle growth.
The mechanical stimulus that triggers muscle growth is the duration of time for which the muscle fibers controlled by high threshold motor units are activated and shorten slowly. Contractions that involve faster movement speeds, or which do not activate these particular fibers, do not stimulate much hypertrophy. Sets of light or moderate loads must therefore include some reps that cannot provide any stimulus at all. Despite this, most of our current methods for measuring volume will record them as part of the total.
When measuring volume for bodybuilding, it would be best if we only counted those reps that activate high-threshold motor units while bar speed is slow, and which therefore stimulate muscle fiber growth.
Let me explain.
What is the mechanical loading stimulus?
Hypertrophy is the result of single muscle fibers increasing in volume, usually by an increase in their diameter, but also by increases in length.
Single muscle fibers are stimulated to grow once they have detected mechanical loading or deformation with receptors located on their cell membranes, called mechanoreceptors.
The mechanical loading experienced by the whole muscle is therefore completely irrelevant for stimulating hypertrophy, because the whole muscle has no means of detecting this tension. The only thing that matters is the mechanical tension produced and detected by each muscle fiber.
The mechanical tension detected by a muscle fiber is determined by (1) whether the fiber is activated, and (2) the speed at which the muscle fiber shortens, because of the force-velocity relationship. According to the force-velocity relationship, slower shortening velocities allow greater tension to be exerted. These two factors are affected by the size of the external load used, and the degree of fatigue experienced.
Let’s look at each of those two factors more closely.
#1. Muscle fiber activation
Muscle fibers are activated when the motor units that control them are recruited.
Motor units are recruited in size order, with low-threshold motor units being recruited earlier than high-threshold motor units. Higher degrees of motor unit recruitment therefore increase the number of fibers that are activated. However, there is more to this story, for two reasons.
Firstly, the number of activated fibers increases exponentially as motor unit recruitment increases. This means that a relatively small number of high-threshold motor units typically control about half the fibers in a muscle. Failing to recruit the high-threshold motor units therefore leaves a huge proportion of the muscle fibers in a muscle unstimulated.
Secondly, the muscle fibers that are controlled by low-threshold motor units are less able to increase in size, partly because of their more oxidative nature, and partly because they are stimulated in everyday life. Stimulating them for a few dozen reps in a strength training workout pales into comparison with their usage in everyday life or during aerobic exercise (and as far as these particular muscle fibers can detect, these activities are identical).
For both of these reasons, it is mainly the muscle fibers controlled by the high-threshold motor units that grow after strength training. However, this does not mean that only type II fibers increase in size. The muscle fibers controlled by high-threshold motor units govern a range of type I, IIA, and IIX fibers, because most of the motor units in a muscle control type I fibers. Only the very highest-threshold motor units govern solely type II fibers.
So how does this work in practice?
Heavy loads, and either moderate or light loads under fatiguing conditions, can be used to reach high levels of motor unit recruitment during strength training, because of the size principle, which states that motor units are recruited in size order in response to increasing levels of effort. In the case of heavy loads, high threshold motor units are recruited from the beginning of a set because the size of the weight means that all the fibers of the muscle need to exert force in order to lift it. In the case of moderate or light loads under fatiguing conditions, high threshold motor units are recruited to compensate for the reduced force produced by each of the fibers controlled by the lower threshold motor units, as fatigue increases.
#2. Muscle fiber shortening velocity
The muscle fibers inside a muscle will not grow if they do not experience sufficient mechanical tension, even if they have been activated. Moreover, this tension can only be produced by the muscle fibers themselves, it cannot be imposed upon them from outside the muscle. To produce a sufficiently high level of tension, muscle fibers must shorten slowly (strictly speaking, they can also remain a constant length, or lengthen).
We know this because high-velocity movements like jumping involve very high levels of motor unit recruitment, but fail to trigger muscle growth.
The amount of tension that muscle fibers exert is dependent upon their contraction velocity because contraction velocity determines the number of simultaneously attached actin-myosin crossbridges, and this is what determines the force exerted by the fiber.
We know this because researchers have found that if they experimentally increase the force produced by a single muscle fiber, the number of attached crossbridges increases. Conversely, when they experimentally increase the contraction velocity of the fiber, the number of attached crossbridges decreases. This happens because the detachment rate of the crossbridges at the end of their working stroke increases at faster contraction velocities. When the crossbridges detach at a faster rate, this reduces the number of simultaneously attached actin-myosin crossbridges.
So how does this work in practice?
Heavy loads, and either moderate or light loads under fatiguing conditions can be used to produce slow fiber shortening velocities during strength training. In the case of heavy loads, slow speeds are used from the beginning of a set because the size of the weight means that the fibers of the muscle must shorten slowly in order to lift it. In the case of moderate or light loads under fatiguing conditions, the contraction speed of the muscle fibers gradually reduces, as fatigue increases.
How do we know that there is a dose-response effect of the mechanical loading stimulus?
Many studies have shown that there is a dose-response effect of strength training with increasing volumes, where volume is traditionally defined as the number of sets per muscle group in a week.
This dose-response relationship is not linear, but is likely to be a curve, with the greatest increases in the hypertrophic effect occurring when volumes are increased from low-to-moderate levels, and much smaller increases occurring when volumes are increased from moderate-to-high levels.
However, muscles grow when single muscle fibers are stimulated to increase in size by mechanical tension, and the loading experienced by whole muscles may not reflect the true stimulus. If the level of motor unit recruitment is not sufficiently high, or the contraction velocity of the fiber is not slow enough, then a rep will not produce a hypertrophic stimulus.
So how do we know that there is a dose-response relationship between volume and hypertrophy when considering the mechanical stimulus on each individual muscle fiber?
We know this because research has shown that activating high-threshold motor units while their muscle fibers shorten slowly does not trigger them to grow, if this stimulus happens only a couple of times in a workout. Doing one repetition-maximum (1RM) tests on a daily basis for 21 days fails to trigger muscle growth, but doing 1RM tests in addition to training with multiple sets of a moderate load does cause hypertrophy. A 1RM effort certainly involves full motor unit recruitment, and also involves a slow contraction velocity. Clearly, for hypertrophy to occur requires more than a few reps of this mechanical loading stimulus in a single workout.
How can we measure the volume of this mechanical loading stimulus?
We can measure the volume of the mechanical stimulus to the single muscle fibers by recording only the number of reps performed at a high level of motor unit recruitment, and at a slow contraction velocity. We can call the reps that provide this mechanical stimulus to single muscle fibers “stimulating reps.”
Heavy loads (1–5RM), which correspond to >85% of 1RM, typically involve full motor unit recruitment on all of the reps in a set, and also involve moving with a slow speed.
Therefore, we might expect all of the reps of these sets to count as “stimulating reps.”
In contrast, moderate (6–15RM) and light (15RM+) do not involve full motor unit recruitment on all of the reps in a set. Also, so long as we move with maximal intent, the muscle fibers do not shorten at a slow speed. Rather, levels of motor unit recruitment increase gradually as the set progresses, while bar speed reduces progressively because of fatigue. In fact, the bar speed reached on the final rep of a set to muscular failure with any load is the same as the bar speed achieved in a 1RM effort.
Therefore, we might expect only the final reps of sets with moderate and light loads to count as “stimulating reps,” while the earlier reps will not.
One other thing that is important to bear in mind is that because of the curved dose-response relationship between training volume and hypertrophy, we should expect the effect of increasing numbers of stimulating reps to increase rapidly from low-to-moderate numbers, but to increase far less from moderate-to-high numbers.
How many stimulating reps does a set involve?
For the sake of creating a model, let us say that only reps that correspond to (1) the level of motor unit recruitment, and (2) the bar speed used when training with heavy loads (1 — 5RM), can trigger muscle growth.
In practice, the number could be lower (1 — 4RM) or higher (1 — 6RM), and it might differ slightly between muscles and between individuals.
In this model, when using moderate or light loads (which are lighter than 5RM), the earlier reps provide no stimulus. Only the final 5 reps of a set are stimulating, as fatigue builds up.
We can show this graphically.
As you can see from the chart, this model produces two predictions for how the load used in a set should affect muscle growth.
Firstly, we should expect that when using heavy loads (1 — 5RM), the stimulating volume will increase with increasing repetition maximum, even when the number of sets is the equated.
Specifically, training using 3 sets of 1RM should stimulate little muscle growth, because it involves just 3 stimulating reps. In contrast, training with 3 sets of 3RM should produce a moderate amount of muscle growth, because it involves 9 stimulating reps. Moreover, the amount of hypertrophy caused by training with 3 sets of 3RM should be less than with 3 sets of 5 — 15RM, because these programs involve 15 stimulating reps. Finally, doing at least 5 sets of 3RM should compensate for using 3RM loads, because this approach involves >15 stimulating reps.
This is pretty much what the research shows us.
Secondly, we should find that when using moderate (6 — 15RM) or light (15RM+) loads, the number of stimulating reps does not change with increasing repetition maximum, even though volume (sets x reps) and volume load (sets x reps x weight) change. Training using 3 sets of 10RM and training using 3 sets of 25RM should produce the same amount of muscle growth, because they all involve 15 stimulating reps.
Again, this is exactly what the research shows us.
N.B. It is noteworthy that the difference in hypertrophy between numbers of stimulating reps is large when the numbers of reps are small (from 3 to 9 reps), moderate when the numbers of reps are moderate (from 9 to 15 reps), and non-existent when the numbers of reps are large (from 15 to 21 reps). This agrees with the decreasing dose-response effect with increasing volume.
What does this mean in practice?
This model sheds some light on why certain groups of trainees tend to gravitate towards particular training programs.
Firstly, it explains why few bodybuilders use heavy (1 — 5RM) loads. It becomes progressively harder to achieve a sufficiently high number of stimulating reps in a workout, the further from 5RM that you go. When equating the number of stimulating reps, training programs involving 15 x 1RM, 8 x 2RM, 5 x 3RM, 4 x 4RM, and 3 x 5 — 15RM should all lead to similar muscle growth. Needless to say, 3 x 5 — 15RM is much easier to fit into in a workout than 15 x 1RM.
Secondly, it explains why the 3 x 5RM model is one of the most popular training methods for increasing maximum strength at the same time as increasing muscle mass, because 5RM is the heaviest load you can use while also maximizing the hypertrophic impact of each set (assuming you train to failure).
Thirdly, it explains why training with light loads has never really caught on as a popular training method, unless training to enhance muscular endurance as a strategy to increase volume load. The first 25 reps of a 30RM set are just bringing about the fatigue necessary for the last 5 stimulating reps to occur, and probably do not contribute in a meaningful way to hypertrophy.
What is the takeaway?
When training for muscle growth, the best way to measure volume is to count only those reps that involve the recruitment of high-threshold motor units while bar speed is slow. We can refer to these as “stimulating reps.”
We do not yet know exactly how many reps in each set to failure involve the activation of high-threshold motor units while bar speed is slow, but it is likely to be approximately 5 reps, because 5RM is the boundary between heavy and moderate loads.
When lifting loads heavier than 5RM, extra sets will need to be done to compensate for the smaller number of stimulating reps per set. When lifting loads lighter than 5RM, the same number of sets to failure will cause similar muscle growth, regardless of the weight or volume load.