There’s an ongoing debate on how important heavy weights are in order to get jacked. As with most trends, public opinion likes to cluster around the extremes. One day heavy weights are critical and the next they’re entirely unnecessary. Realistically, we need to have a more nuanced conversation about the merits and drawbacks to both high and low load approaches to hypertrophy. From there we can come up with some straightforward and practical recommendations that can be implemented into our training.
Mechanisms Of Muscle Hypertrophy
Hypertrophy is a term used to describe muscle growth. Essentially there are three primary drivers of muscle hypertrophy. Mechanical tension, volume, and metabolic stress.1 It was previously thought that muscle damage was a significant contributor to muscle hypertrophy. Although in limited circumstances it may act as a proxy to muscle growth; recent research shows the relationship does not appear to be causal or even reliably correlated.2
Hypertrophy is observed in individuals in an overtrained state who have accrued copious amounts muscle damage and yet, they may even lose LBM (lean body mass). Conversely, there are several instances where an individual experiences minimal delayed onset muscle soreness while continuing to build muscle mass. I do not believe muscle damage should be written off as entirely unimportant but because it is not a direct mechanism the topic of muscle damage it will not be covered extensively in this article. My personal stance on this is that if you’re never sore and simultaneously not making any progress it might mean you need to train harder. But beyond that, I don’t believe it’s a metric to reliably base your training decisions on.
Mechanical tension is where a stretch is applied to a muscle under load.1 As a 2011 paper found, “It is believed that mechanical tension disturbs the integrity of skeletal muscle, causing mechanochemically transduced molecular and cellular responses in myofibers and satellite cells.”3 The degree of mechanical tension is dependant on the load and the time under tension or the amount of stretch being applied to the muscle. Utilizing a combination of these factors that preferences a range in which all are optimized is likely to produce superior hypertrophic adaptations.3
This brings up the important topic of exercise selection. From a practical standpoint, the exercise selected largely dictates load prescriptions. For example, dumbbell chest flys versus barbell bench press will require dramatically different load selection based on the mechanical differences inherent in each movement.4 Since volume is one of the primary contributors of muscle hypertrophy, there is a clear benefit to preferencing compound exercises which allow for greater volume load and mechanical tension.1, 5
In addition to increasing the mechanical tension applied to the musculature, lifting heavy loads recruits high threshold motor units that would not be accessible at lower intensities.6 These findings have in some instances lead to an over-application of this approach—lifting too heavy too often. However, since hypertrophy is a complex adaptive response, it is not mediated by one single mechanism. Rather, mechanical tension is simply one aspect of a concomitant matrix that produces muscle growth.1
The fatigue cost associated with repeated bouts of high-intensity resistance training is robust and if left unchecked can lead to overtraining.7, 8, 9 Research demonstrates a significant benefit to the mindful inclusion of heavy loads as part of a resistance training protocol to maximize the hypertrophic response. In an attempt to prevent overtraining, effective program design must manage the frequency of high-intensity bouts and the associated fatigue.
Volume refers to the number of reps multiplied by number of sets completed (volume = reps x sets). As a stand-alone metric, volume does not provide much insight into the intricacies of a program. The simple reason being equal volumes may have a variety of different adaptive responses.
For example, the higher intensities prescribed to person A more closely resemble that of a strength program. The more voluminous prescription for person B more closely resembles a hypertrophy program. I understand this is a bit of an oversimplification, but it’s sufficient to demonstrate my point. Both volumes are identical, and in both cases, 24 total reps were completed. However, as I mentioned previously the adaptive response in each case is quite different.
For this reason, it’s common to see coaches use volume load which is calculated by multiplying the total number of reps by the load.10 In the table below you can see although volume and relative intensity is identical; volume load is 20% greater for Person A than for Person B.
Research has consistently shown that higher volumes produce greater hypertrophic gains compared to lower volume interventions.11 This is likely due to a combination of increased muscle tension, metabolic damage, and hormonal responses to resistance training.1 A 2019 paper found “muscle hypertrophy follows a dose-response relationship, with increasingly greater gains achieved with higher training volumes.”12 Essentially, more volume equates to greater gains so long as the athlete can sufficiently recover.
This leads into the next item up for discussion which is MRV also known as maximum recoverable volume. This is a term coined by Dr. Mike Israetel to define the maximum amount of volume an individual can sustain before overtraining. This is an important concept because as with most things that work well, more is often thought to be better. However, this dose-response relationship to hypertrophy is mediated by your ability to recover and continue subsequent training sessions of a productive nature.8
A 2018 paper titled “Effects Of Different Intensities Of Resistance Training With Equated Volume Load On Muscle Strength And Hypertrophy” found that “leg extension exercise performed at 30% 1RM until failure similarly increased quadriceps muscle volume compared to high-intensity exercise (80% 1RM) and was superior to a 30% 1RM non-failure condition.”13 Essentially what this means is that the intensity range at which we can build muscle is much larger than was previously assumed, approximately 40-80% 1RM.13 These findings also “indicate that the lowest [resistance training] intensity (20% 1RM) was suboptimal for maximizing muscular adaptations.”13
Although there is a wide spectrum of volumes and intensities that can induce productive adaptations, it’s important to be cognizant of where those rough boundaries exist and not venture unnecessarily too far in either direction.
Volume also has an inverse relationship with intensity.14 What this means is that as intensity increases, volume necessarily decreases. This is also why you can squat 65% for 10 reps but 100% for only 1 rep and is depicted in the graph below.
A question I sometimes get is: “If increased volume decreases intensity, how can you maximize mechanical tension and volume simultaneously?” This is an excellent question, and although you may not truly be able to maximize both simultaneously you can certainly come close to optimizing them. Mechanical tension is not just the load being lifted, it’s also accumulative tension. This means even though you’re not lifting your 1RM, as the reps and sets progress, the voluminous training session induces significant mechanotransduction.1
Metabolic stress seems to have a large impact on muscular hypertrophy either directly or indirectly. A paper by Dr. Brad Schoenfeld found “Metabolic stress manifests as a result of exercise that relies on anaerobic glycolysis for ATP production, which results in the subsequent buildup of metabolites such as lactate, hydrogen ion, inorganic phosphate, creatine, and others.”1 Although lower loads lifted for high repetitions (15+) may not be sufficient to maximally recruit high threshold motor units, it can induce significant metabolic stress.1 Thus, there appears to be a clear benefit to incorporating higher repetition ranges at lower loads to take advantage of the metabolic stress pathway to hypertrophy.
Practical implementation of both low and high-intensity protocols vary dramatically. In a 2018 paper by De Souza et al, in order to produce similar hypertrophic responses with low loads the subjects were forced to take each set to muscular failure.13 This presents some very real limitations to this type of training due to the associated fatigue cost.
For instance, taking an isolation exercise like the leg extension to failure will create a significant hypertrophic response, however, the fatigue generated will likely be manageable. Compare that with a barbell squat taken to failure and the axial loading will result in more systemic fatigue which may also increase risk of injury.15 The fatigue generated from such a stressful training session may also bleed into subsequent training sessions, potentially having a negative impact on downstream performance. Beyond that, the psychological cost of training at this level of effort is extraordinarily taxing, and likely not sustainable for long periods. Thus exercise selection, sequence, undulation, and frequency of implementation should be considered when designing a program.
De Souza and colleagues also found that higher intensities not taken to failure are at least equally effective at eliciting a hypertrophic response during training.13 This is reflected by the recommendations made by Helms et al for natural bodybuilders where training intensities between 70-80% 1RM make up the majority of the intensity spectrum utilized.16
This again boils down to context. When looking at a single set, any intensity taken to failure will elicit a greater hypertrophic response than not taking the set to failure. This occurs because failure maximizes the combination of mechanical tension, volume, and metabolic stress accrued during the set.1 However, there is a strong correlation between the incidence of overtraining when an athlete exceeds their load/volume thresholds.17 Thus, training to failure as a primary strategy of program design is ill-advised and likely to result in injury and overtraining.
Endocrine Response To Resistance Training
Resistance training results in a cascade of endocrine responses that help facilitate the synthesis of muscle mass. Several questions still exist regarding the long term significance of acute alterations in hormones post-exercise. One paper found “Higher volumes of total work produce significantly greater increases in circulating anabolic hormones during the recovery phase following exercise.” 18 Ahtiainen et al attempted to determine the hormonal response to heavy resistance training with equated volume. The only difference in protocol between control groups was group A was instructed to do 4 sets at 12RM, where group B followed the same protocol but with a weight they could only complete 8 reps, and the remaining reps would be forced reps.
After measuring serum testosterone, free testosterone, cortisol, growth hormone, and blood lactate; both groups showed significant increases in concentration post-training.19 However, the forced rep group had a higher concentration upon measurement than the 12RM group. There is also evidence suggesting that training age of the athlete influences hormonal response to training. One paper found that trained subjects demonstrated lower responsiveness in hormone values (total testosterone, free testosterone, dehydroepiandrosterone, cortisol, and sex hormone-binding globulin) post resistance exercise.20 Therefore, we can speculate that the endocrine response to resistance training is likely attenuated over time.20 This may at least in part explain the requirement of higher volumes in trained athletes to stimulate myogenesis.
Insulin-like growth factor-1 (IGF-1) is a hormone that, along with growth hormone (GH), helps promote normal bone and tissue growth and development. Although the mechanism by which mechanical load modulates IGF-1 expression is unclear, there is emerging evidence in support of this observation.21
The image below is a visual represents of a dose-response relationship between volume, load, and endocrine response to resistance training (ie. greater loads and volumes resulting in a larger acute elevation). As mentioned previously, it’s still unclear how acute elevations in anabolic hormone concentrations impact long term outcomes. However, if the acute elevations in anabolic environment resulting from resistance training are frequent enough and at a large enough magnitude, it would be reasonable to assume they would be reflected in downstream gains.
Since there is a lot of conjecture with regard to the relationship between long term outcomes and acute elevations in anabolic hormones, I would not spend much time attempting to alter your biochemistry. Simply focus on the variables that have been well established to cause muscle growth and let your body sort the rest out on its own.
Training Frequency and Fatigue Management
All progress in training is predicated on adequate recovery, allowing for subsequent bouts of training that over time yield a positive adaptive response. The repeated bout effect is a sports science concept that describes the bodies adaptive response to stressors resulting in increased resiliency.22 There is a limit to the rate of our adaptive ability and exceeding this limit can predispose you to injury and reduced performance.9 Fatigue management, therefore, is a fundamental tenant of every effective training protocol. The SRA (stimulus recovery adaptation) curve charts the adaptive process to resistance training and is depicted in the image below.
There are three main points to highlight here. The first is that exercise generates fatigue, the magnitude of which is determined by several factors but primarily volume and load. The second point is that if you wait too long before introducing another training stimulus adaptive dissolution occurs. This means you regress because subsequent training exposures were either insufficiently overloading, insufficiently frequent, non-specific or a combination of these. The third point is as you accumulate fatigue through overloading training sessions your ability to express athletic performance declines temporarily.
Knowing this, frequency of training plays a significant role in the proper application of various loading strategies. For instance, if you were to do 10×10 squats to failure, you may not be able to train legs for a whole week. So, when looking at the magnitude of the stimulus produced in a vacuum it’s huge which is positive. But the fact that you can’t train legs for an entire week likely makes the opportunity cost of this strategy a poor trade-off. In most cases frequencies higher than 1x per week are required to really optimize muscle growth. Thus, a phasic structure and effective program design can help prevent the exacerbation of a single pathway, manage fatigue, and also potentiate future gains.
Practical Takeaways And Recommendations
With regard to the compound lifts, the majority of your hypertrophy gains will likely come from the following recommendations:
- Reps: 6-15
- Sets: 4-8
- Intensity: 60-80%
- Rest: 2-3 minutes
However, this does not exclude the implementation of low load training taken near or to absolute muscle failure. It simply means that it needs to be applied intelligently. Since the physiological and psychological fatigue generated from taking sets to absolute muscular failure is significant as well and an all-around terrible experience I would use it in moderation. Its implementation would likely be most effective for smaller muscle groups or exercises that are limited in the amount of load that can be lifted (ie. bicep curls, tricep press downs, calf press, DB shoulder press, etc).
Implementing a phasic structure that emphasizes specific adaptive pathways can be very effective. The ideal structure would be based on each phase potentiating subsequent phases. Thus one potential approach could be a linear periodization model where volume starts high and declines over time as intensity rises. An example of which is below:
- Phase 1: Metabolic (high volume, low load)
- Phase 2: Volume (moderate volume, moderate load)
- Phase 3: Mechanical Tension (moderate volume, moderate to high loads)
Below is an example of a similar workout adapted to each phase to give you an idea of what your training might look like:
As you can see from the sample workouts, each phase may look relatively similar. This brings me to an important point—complex training isn’t synonymous with effective training. The basics are what produce the bulk of your results anyway, and no matter how amazing it would be to find “hacks” that yield better progress it generally doesn’t work that way in practice. Your best bet is to use the complete spectrum of reps, sets, and intensity ranges while still maintaining the bulk of your work within the guidelines mentioned above.
The use of tactics such as giant sets, rest-pause sets, supersets, negative sets, etc can be useful in eliciting metabolic stress. These can be implemented at your desecration, but I would recommend either using them on multi-joint machine-based exercises or isolation exercises with free weight or machines. This will help limit the amount of fatigue you can generate from this type of training while still producing a significant stimulus.
Hopefully, this clears up some of the confusion and offers some practical application for implementing various loading strategies into your hypertrophy program. Lift big.
1. “The Mechanisms of Muscle Hypertrophy and Their Application”, The Journal of Strength & Conditioning Research”, LWW.
2. Flann, Kyle L, et al. “Muscle Damage and Muscle Remodeling: No Pain, No Gain?” The Journal of Experimental Biology, U.S. National Library of Medicine, 15 Feb. 2011.
3. “The Use of Specialized Training Techniques to Maximize”, Strength & Conditioning Journal.” LWW.
4. “A Biomechanical Comparison of the Traditional Squat”, The Journal of Strength & Conditioning Research. LWW.
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19. Ahtiainen, Juha P, et al. “Acute Hormonal Responses to Heavy Resistance Exercise in Strength Athletes versus Nonathletes”, Canadian Journal of Applied Physiology = Revue Canadienne De Physiologie Appliquee, U.S. National Library of Medicine, Oct. 2004.
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22. McHugh, Malachy P. “Recent Advances in the Understanding of the Repeated Bout Effect: the Protective Effect against Muscle Damage from a Single Bout of Eccentric Exercise”, Scandinavian Journal of Medicine & Science in Sports, U.S. National Library of Medicine, Apr. 2003.