Strength and physique athletes have been advocating for training with a high intensity for a long time. However, “intensity” is often poorly defined. We walk you through four domains of intensity and which are the most important for your goals.

Chapters

00:00 Intro
01:27 Load
02:27 Proximity to Failure
05:21 Intended Velocity
07:40 Psychological Acuity
09:02 Summary

Transcript

Strength and physique athletes have been advocating for training with a high “intensity” for a long time. We can look to the Bulgarian era of weightlifting, where lifters would max out, sometimes even multiple times a day, on exercises like the snatch, clean and jerk, and squat. Or we can look to esteemed bodybuilders like Dorian Yates and Mike Metzer who took most of their working sets to or even past failure. Proponents of “high intensity” training like this have been around for decades. The issue however, is that “intensity” is often poorly defined in these contexts, which may end up diluting our attention away from the things that actually move the needle for strength and muscle growth, respectively. In today’s video, we’ll cover how to better define training “intensity” and how doing so can improve the gains you see in the gym.

Questioning the ambiguity of the term “intensity”  is not new. In 2012 Dr. James Steele wrote a paper in the British Journal of Sports Medicine making the case that the term “intensity” shouldn’t be used without further clarification of the training variables being referred to. Specifically in the context of resistance training, the paper advocates for the separation between intensity of “load” and intensity of “effort”. While I personally like to use slightly different terminology, the idea of separating different components of “intensity” lays the groundwork for a useful mental model. I like to break down “intensity” into 4 specific domains:

1. Load

2. Proximity to Failure

3. Intended Velocity

4. Psychological Acuity

Let’s discuss each of these domains, and their relevance to training for strength and muscle growth, starting with load.

LOAD:

We can define load as the external resistance relative to an individual's maximal capacity. In practice, load is probably best measured by the percentage of 1RM on the bar. As Josh discussed in our last video, a recent preprinted meta-analysis of over 100 studies from Swinton and colleagues suggests that the load used has a strong relationship with 1RM strength gains, such that training with heavier loads tends to improve strength more than lower loads. These results seem to abide by the principle of specificity. Since a 1RM is essentially a test of maximal force production and peak forces rise linearly alongside increases in load, heavier loads are more specific to a 1RM; which may explain why they result in greater strength gains. 

On the other hand, so long as sets are being terminated pretty close to failure (more on this in a bit) the load doesn’t seem to influence muscle growth much at all. We can see in a recent meta-analysis from Refalo and colleagues that there is essentially no difference between conditions training with loads <60% of 1RM and those training with loads 60% of 1RM or heavier. To tie this together mechanistically, we need another domain of intensity in the mix, proximity to failure. 

PROXIMITY TO FAILURE:

To keep it simple, proximity to failure can be defined as the number of repetitions in reserve, or RIR, following the termination of a set. We could spend some more time on this definition, so if you’d like to see a video discussing the nuances of the definition of “failure” leave a comment below. 

For strength, the RIR in which a set is terminated doesn’t seem to have an independent influence on strength gains. In other words, once the load has been determined, for example 75% of 1RM, the RIR in which a set is terminated doesn’t seem to alter strength gains a whole lot. This can be visualized with our recent preprint that examines the dose response relationship between RIR and strength gains at a given load; as you can see the curve is essentially flat suggesting no meaningful relationship. 

Once again, the principle of specificity in regards to force production may help us to explain the observed relationship. Consider an example where you complete a set of squats with 75% of your 1RM on the bar. As you get closer to failure, and the bar speed slows, you’re actually going to be producing less force than when you initially began the set. Because the reps closer to failure are actually less specific with regards to force production, this may help to explain why training closer to failure doesn’t seem to lead to greater strength gains.

In fact, a meta-analysis by Zhang and colleagues further supports this point by showing that reps early in the set provide the greatest relative strength gains. This analysis is partially confounded by the additional training volume performed in sets terminated closer to failure, but given the low weekly set volumes used in the studies included, I have a hard time believing that difference completely explains this relationship.

Looking at the similar dose-response curve for hypertrophy, you can see gains in muscle size seem to improve as sets are terminated closer to failure. However, this curve flattens out when using loads somewhere around 80% of 1RM or greater, suggesting heavy loads may not require sets to be terminated as close to failure to maximize outcomes. If we turn to a recent paper by Murphy and colleagues, we may be able to cautiously speculate the mechanistic origins of this relationship. 

In this study the authors examined motor unit behavior in response to isometric contractions performed to momentary failure with either a high or low % of an individual's maximal isometric force capacity. The authors observed that while high loads might elicit greater muscular excitation at the beginning of the set, the two conditions converged as they neared momentary failure. 

If we assume that high levels of muscular excitation are permissive to delivering mechanical tension, which is thought to be the primary driver of hypertrophy, to the most muscle fibers, this would help to explain that ideas that:

1. Load doesn’t influence muscle growth so long as sets are taken near failure

2. Training close to failure becomes less necessary with heavy loads

INTENDED VELOCITY:

At a given load and RIR, the repetitions of a set can be performed in a variety of tempos. Think of one lifter performing each rep as explosively as possible, while another deliberately slows them. This example illustrates the next domain of intensity, intended velocity. 

For strength, we have a decent amount of research to determine the best intended velocity. For the concentric portion of a rep, a recent meta-analysis by Hermes et al. suggests that conditions performing a “fast” concentric outperform those with a deliberately slowed concentric. Once again, the principle of specificity in regards to force production can help to explain outcomes here. At a given mass, aiming to accelerate the barbell as fast as possible increases the amount of force produced on each rep - thereby making each repetition more specific to a 1RM. 

Alternatively, there is considerably less data on the preferred intended velocity for the eccentric portion of the repetition. On one hand, faster eccentric speeds have been shown to improve concentric performance - potentially allowing for heavier loads to be lifted. However, performing an eccentric too quickly may result in the lifter losing control, and negatively impacting technique - which can make all the difference in a 1RM attempt. Tentatively, performing the eccentric as fast as possible while maintaining control seems to be the best bet for strength. 

For muscle growth, the evidence is less clear. A recent review paper by Wilk and colleagues suggest that while the tempo of repetitions seems to be minimally influential, particularly when sets are taken to failure an explosive concentric paired with a slower eccentric seems to have the most support. Conceptually, this seems to fit in line with the data from Murphy and colleagues. 

As more force is produced when a repetition is performed with an explosive concentric and controlled eccentric, one could reasonably assume that greater muscular excitation would follow. In theory, this would allow for mechanical tension to be delivered to more muscle fibers early in the set. However, when training to failure, you’re likely going to end up at a similar end point very much analogous to the different patterns of motor unit behavior with high versus low loads.

PSYCHOLOGICAL ACUITY:

The final domain of intensity is probably best introduced with an example. Consider a lifter performing a set with the same load, RIR target, and intended velocities. In example A, the lifter is focused, listening to their preferred music, and has a high degree of psychological arousal in preparation of the set. In example B, the lifter is distracted with emails, talking to some friends at the gym up until the start of the set, and goes into the set with minimal arousal. Despite all of the aforementioned training variables being the same, one lifter is clearly training with a higher “intensity”.

This illustrates the final domain well, psychological acuity. I like to think of this domain as the ability to access our maximal performance potential for a given set of task demands. In the research, there are a few examples where we can see a lifter's psychological acuity impacted. 

The first example is mental fatigue where there is a decrease in repetition performance following a cognitively demanding task, despite maximal force capacity seemingly being unaffected. Another example is when lifters exhibit pacing strategies, ultimately limiting performance, throughout a set despite being told to train with maximal effort.

The point here is that training with greater psychological acuity is likely going to improve our performance acutely and while this is a bit of a logical leap, that will likely optimize both strength and hypertrophy outcomes over time.

SUMMARY:

As you can see, the aspects of intensity that are the most important are dependent on your training goal. For strength, allocating your recovery resources to tolerating heavier loads and the appropriate intended velocities is going to be your best bet. For hypertrophy, proximity to failure is probably much more important and gives you access to a range of intended velocities that are acceptable. For both goals, training with a high degree of psychological acuity is probably going to maximize progress over time and is an important aspect of intensity that shouldn’t be overlooked.

Hopefully this video helped to break down how to best conceptualize “intensity” and what domains are the most pertinent to your training goals. Understanding these concepts can make sure that your time in the gym is both efficient and effective.

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