Does maximum weekly training volume differ between muscles?
When we take the time to look carefully at the anatomy and biomechanics of different muscles, we can learn a great deal about how we might best train them. Usually, our investigations reveal a lot about exercise selection, but we can also draw conclusions about optimal training volumes and frequencies as well, and this has an impact on how we might implement some popular high volume training routines.
What’s the background to training volume?
Talking about training volume in the fitness industry is difficult, partly because there are several standard definitions of “volume” floating around in the research literature, and partly because even the standard definitions do not account for situations when sets are not taken to muscular failure (as you may know, I think we can get around this latter problem by counting the number of stimulating reps in a set).
Even so, it is slowly becoming accepted that we can use the total number of sets performed to muscular failure in a workout as a measurement of workout volume, because that measure is a consistent assessment of the hypertrophic dose achieved by that training session, regardless of the weight on the bar. In contrast, volume load (sets x reps x weight) is not.
An increasing body of literature also indicates that we might be able to use the total number of sets performed to muscular failure in a week as a measure of weekly volume. However, this picture is much less clear, because some studies have failed to show an increasing effect of higher training frequencies (when using higher training frequencies to add more volume, rather than to spread the same volume over more workouts). This is most likely because of CNS fatigue resulting from one workout interfering with the hypertrophic stimulus of the next workout, if it is done too soon afterwards. As a result, we need to be much more cautious when making statements about weekly volumes than when making statements about workout volumes.
What does CNS fatigue have to do with this?
Introduction
Fatigue involves a highly complex set of mechanisms inside the central nervous system (CNS) and inside the muscle itself that reduce strength during and after a workout. To a large extent, peripheral fatigue during dynamic strength training does not seem to inhibit the hypertrophic stimulus. Yet, CNS fatigue does, because it prevents the highest threshold motor units (which are the ones that control large numbers of the most responsive muscle fibers) from being recruited during exercise. During normal strength training, if you cannot activate a muscle fiber, then you cannot make it grow.
During a workout
CNS fatigue always occurs during a strength training set, and peaks at task failure. Normally, it dissipates quite shortly after finishing the set, although some CNS fatigue can still be present for a few minutes (this is why short rest periods are not a great idea). CNS fatigue also seems to increase gradually over a strength training workout, being highest at the end (this is why we get the best results in muscles that we train first in a workout, and the worst results in muscles that we train last, albeit this effect is more marked for strength gains than for hypertrophy. It might also be an argument against doing really long workouts, although that is a topic for another day).
After a workout
CNS fatigue often occurs for days after a strength training workout (despite what some experts have argued over the years). Even so it dissipates faster than the local muscle fatigue that we tend to call “muscle damage” but which is more likely normally just sustained excitation-contraction coupling failure, in addition to some myofibrillar disruptions.
There are a few hypotheses about the cause of this CNS fatigue, but I tend to think that it is triggered by the inflammatory response that can result from the myofibrillar disruptions being repaired, when they are sufficiently severe. This has the benefit of explaining why CNS fatigue usually only lasts for a couple of days after heavy strength training in strength-trained individuals, but can last for a week after unaccustomed eccentric contractions.
Importantly, we know that the CNS fatigue must occur due to a feedback mechanism from the muscle, and cannot arise from a workout that simply involves a high level of motor unit recruitment, as power training workouts do not cause CNS fatigue, but in fact cause a short period of potentiation. Also, it actually makes a lot of sense that a badly-damaged muscle would send an afferent feedback signal to the CNS to tell it to reduce high-threshold motor unit recruitment, since it is the high-threshold motor units that control the largest number of the most easily-damaged fast twitch muscle fibers. Thus, switching of these muscle fibers stops them from being loaded while they are being repaired or regenerated.
Is post-workout CNS fatigue muscle-specific?
When people first hear about CNS fatigue, they tend to assume that it is a whole-body phenomenon that affects all muscles equally. This is not true. During a workout, trained muscles are affected more than untrained muscles. After a workout, it is also unlikely to be true, since the amount of CNS fatigue that occurs is most likely determined by the amount of local muscle damage that is caused, through an afferent feedback pathway.
There is also a tacit assumption that similar types of training will cause similar amounts of CNS fatigue in each muscle. Again, this is almost certainly untrue. During a workout, smaller muscles with higher normal levels of voluntary activation seem to be affected less than larger muscles with lower normal levels of voluntary activation. After a workout, it is also unlikely to be true, since the amount of CNS fatigue that occurs is most likely determined by the degree of muscle damage that is caused. Differences in muscle fiber type and voluntary activation capacity can cause some muscles (such as the soleus) to remain largely undamaged after an unaccustomed eccentric workout. In contrast, other muscles (such as the biceps brachii) experience a lot of damage, and can even undergo necrosis, which is very uncommon after voluntary (rather than electrically-stimulated) muscular contractions.
What does this mean for programming?
Introduction
Currently, there is a lot of support for high volume training programs for bodybuilding. From a practical point of view, this presents problems, because few lifters want to maintain a high training volume for all muscles, all of the time. Consequently, many strength training experts have produced solutions to allow high volumes to be used on an ongoing basis, rather than just as a two-month experiment before taking a month-long vacation. One broad category of solutions involves gradually ramping up volumes, while another category implements specialization blocks.
#1. Ramping volumes
Ramping volumes involves adding a set (or two) per week to each of the exercises in a workout for the duration of a training block, before backing off for a week or two and then starting again. This involves a gradual increase in workout volume, and also a gradual increase in weekly volume.
While popular at the moment, this strategy has the severe drawback of making it impossible to track progress from one workout to the next. Indeed, many training blocks can pass without it being obvious whether any gains are being made (especially given that muscle damage causes swelling that masks the underlying size of the muscle). While a deload week usually implemented between training blocks, which may allow an occasion for progress testing, the constant fluctuations in the levels of local muscle fatigue, muscle-specific CNS fatigue, and systemic CNS fatigue make it hard to gauge progress in this week by testing strength under comparable circumstances.
#2. Specialization blocks (volume allocation)
Specialization blocks involve allocating a high, fixed level of (both workout and weekly) volume to one or two muscle groups for a single training block. During this time, the other muscle groups are usually trained with lower volumes. Sometimes these lower volumes are still sufficient to cause hypertrophy, but at a slower rate (this is the approach usually taken by bodybuilders bringing up a lagging body part), while other times they are not. In such cases, the volume is set only at a level which can maintain any hypertrophy that was achieved in previous specialization blocks.
While they still suffer from the same problem as ramping volumes (insofar as excess fatigue can mask whether any progress is being made), specialization blocks do not suffer to the same extent, for two reasons. Firstly, while local muscle fatigue and muscle-specific CNS fatigue can be very high during a specialization block, systemic CNS fatigue need not be (since overall volume across all muscle groups can be sensible). Secondly, when muscles are put back onto maintenance after a specialization block, they should rise to a higher level of strength than before the specialization block, at least once muscle-specific fatigue has dissipated. If this does not occur, then we can see that the specialization block has clearly not worked.
Susceptibility of muscles to CNS fatigue
So how does the susceptibility of each muscle to CNS fatigue affect how we implement these two training methods in practice?
Well, since some muscles are much easier to damage than others, this affects how much weekly volume can be done for each muscle (see the penultimate section of this article for a list of muscles and their characteristics). While some muscles (like the calves and quadriceps) can handle a very high volume in a workout, and still recover from their muscle damage and associated CNS fatigue in time for the next workout a couple of days later, others (like the biceps and triceps brachii) cannot. This has major implications for the fine-tuning of both ramped volume and specialization block approaches to high volume training.
When using ramped volume programs, it makes sense to cap weekly volumes at lower levels for easily-damaged muscles than for less easily-damaged muscles. In practice, this might mean that a training block involves adding one set per workout per week for four weeks for the quadriceps, but only one set per workout per week for the first two of those weeks for the biceps, before maintaining that level for the remainder of the training block.
When using specialization blocks, it actually makes most sense to exclude easily-damaged muscles from the rotation, while rotating through the less easily-damaged muscles in sequence. Thus, direct arm work (biceps brachii and triceps brachii) might remain at the same volumes at all times, while other muscle groups rotate through high and low volume cycles. This is no real logic to implementing a specialization block for the arms, since the upper limit on the amount of weekly volume that these muscles can handle is too low to justify putting everything else on maintenance. You might as well do a normal, intermediate-level balanced routine.
What is the takeaway?
Some muscles are much easier to damage than others. Easily-damaged muscles have a high capacity for voluntary activation and a large fast twitch fiber proportion (like the biceps brachii), both of which make it easier a large number of fast twitch muscle fibers to be fatigued (and therefore damaged) by strength training. Easily-damaged muscles likely experience greater and more long-lasting muscle-specific CNS fatigue than other muscles, due to the much more pronounced inflammatory response that accompanies the greater muscle damage.
This has key implications for high volume programming methods. Specifically, when using ramped volume programs with deloads between training blocks, it makes sense to cap weekly volumes at lower levels for easily-damaged muscles than for less easily-damaged muscles. When using specialization blocks, it makes sense to exclude easily-damaged muscles from the rotation, while rotating through the less easily-damaged muscles in sequence. In practice, this means keeping direct arm work at the same volume at all times, while rotating other muscle groups through high and low volume cycles.
