Muscle Mania Part VII: Training Talk 03
Looking at how rest intervals and exercise selection interact with hypertrophy
Welcome back to the Muscle Mania Series (I, II, III, IV, V, VI), where we are continuing the thread of how different training variables alter hypertrophy outcomes. Here, we’ll dive into rest intervals – how much time you take in between sets – and exercise selection – especially compound vs. isolation movements – and uncover how to manipulate each one in favor of inducing hypertrophy. Prior to getting started, check out the recap below for a refresher on muscle anatomy, hypertrophy-related cell biology, mechanisms of hypertrophy, and other topics we’ve looked at so far.
Check out my reference list for Part VII here.
Recap
Muscles are ultimately composed of contractile proteins, called myofilaments, which are organized into structural units, called sarcomeres.
Muscular hypertrophy, or muscular growth, involves building and incorporating myofilaments into sarcomeres to grow the overlying muscle.
Muscle hypertrophy and atrophy are the results of an interplay between muscle protein synthesis (MPS) and muscle protein breakdown (MPB); interestingly, both processes are essential for building muscle.
Protein synthesis involves a macro-level signal (like lifting weights or eating protein) inducing a cellular signal at the micro-level, which provokes transcription of specific parts of the cell’s DNA (i.e. specific genes) into mRNA, the translation of that mRNA by ribosomes, and the construction of the correlating proteins. During muscle protein synthesis, those genes code for specific muscle proteins that will be incorporated into sarcomeres.
Our muscle cells utilize 3 pivotal processes to initiate and propagate hypertrophy: ramping up muscle protein gene expression by activating transcription factors, increasing nuclear capacity via satellite cell addition, and expanding translational capacity through ribosome biogenesis.
Key molecules involved in hypertrophy-related cell-signaling include mTORC1, Akt/PI3K, Myostatin, MAPKs, and intracellular calcium; additionally, key hypertrophy transcription factors include MEF2, SMAD2/3, SRF, UBF, and Pax7.
Mechanical tension, the force placed on a muscle fiber during contraction, is likely the key mechanism for initiating hypertrophy-related cellular pathways; however, some suggest that metabolic stress and/or muscular damage play supportive roles in the process of instigating muscle growth.
It makes the most sense to take hypertrophy research with a grain of salt, due to small sample sizes, small effect sizes, and biological variability; furthermore, rather than blindly following the research, it is most reasonable to combine expert opinions and study results with your own experience when considering your own exercise strategies.
Progressive overload, the process of gradually switching up the stimuli in your training in order to challenge your body and produce further adaptations, underlies all exercise programming.
Volume load, the total amount of weight moved during a training session – easily calculated as (weight) x (reps) x (sets) – is likely the cardinal variable in a resistance training program. In terms of sets per week, volume and hypertrophy seem to exist in a graded-dose response relationship, meaning that greater volumes produce greater hypertrophy up until a maximum threshold of beneficial volume. In terms of volume per session, it appears that more sets produce more hypertrophy up until about 6-10 sets per muscle group per session where the benefits begin to plateau.
In general, it appears that training to absolute failure does not provide superior hypertrophy as compared to training near failure, provided that volume is equated; nonetheless, it is reasonable to imagine that there is a minimum threshold of proximity to failure for inducing hypertrophy, and some research suggests that training to failure becomes more important for more advanced lifters and when training with lighter loads.
Research shows that you can achieve significant and comparable amounts of hypertrophy with weights ranging from 30%-90% of 1-rep-max(1RM), provided the sets are taken to failure and volume load is equated.
Aside from meeting weekly and per-session volume targets, it appears that whatever weekly frequency and training split(s) allows you to train consistently, comprehensively, and with high quality and effort is ideal.
For the most part, it appears, provided that weekly volume, per-session volume, a minimum of 30% 1RM intensity, and a close proximity to failure (~5-0 reps away) are all accounted for, choice of tempo within 0-10 seconds will not have a large impact on hypertrophy outcomes.
Tempo’s greatest contribution could be maintaining proper form and maximizing tension on the target muscle by limiting momentum and swinging during the given movement.
Rest Intervals
Similar to intraset tempo, which we discussed last week, timing comes into question in between sets as well, as rest interval length plays a role in dictating the degrees of both mechanical tension and metabolic stress induced when resistance training. Firstly, rest intervals impact mechanical tension via their effects on intensity, as restricting rest ultimately limits the amount of weight you can use in consequent sets; furthermore, shorter rest intervals elevate metabolic stress by cutting down on the time your body has to alleviate the build-up of metabolites and local hypoxia that come with each set. (I, V, VIII, IX, X, XI) Consistent with the supposition that mechanical tension is the most important mechanism of hypertrophy, some research points to rest intervals of 2 minutes or greater between each set as superior for muscle growth when volume load is not equated, likely because such rest enables you to use the greatest amount of load across your sets.(I, V, VII) When volume load is equated, on the other hand, it appears that rest intervals do not heavily impact hypertrophy outcomes; though, equating volume load with shorter rest intervals likely requires conducting a greater amount of sets to reach the same results. (I, II, IV, VI, VII) With that said, some experts in the field suggest that incorporating some training with shorter rest intervals (<60 seconds between sets) can facilitate mechanical tension in the long run, in that doing so improves your body’s capacity for buffering metabolic stress over time, ultimately allowing you to utilize heavier weights in more metabolically demanding rep ranges (i.e. 8+ reps per set). (I, XI) In this way, you could use your rest intervals to induce progressive overload in your training by cycling through the generally recommended range of ~1-2+ minutes of rest between sets to induce different levels of metabolic stress and mechanical tension.
For a summary of the literature on how rest intervals influence hypertrophy, check out Grgic et al. and Henselmans et al.’s reviews here and there, respectively.
From a more practical standpoint, similar to that for frequency, it may make the most sense to – instead of adhering to a strict rest interval – simply take as much time as is needed to recover sufficiently to complete the next set with high quality, intensity, and effort. (I, X, XI) This will vary from person to person, exercise to exercise, and likely day to day, in that some people recover faster than others, some exercises tend to require more time for recovery than others, and your personal recovery rate likely fluctuates depending on environmental factors (stress, sleep, etc.). So, where you may need to rest a full 3 minutes before hitting another set of squats, you may be sufficiently recovered 1 minute or less after a set of bicep curls or calf raises. In this way, using a self-regulated approach could optimize time efficiency while maintaining volume load. Check out Dr. Mike Israetel’s simple system for determining your rest intervals below.
Exercise Selection
Though we’ve covered a lot of specifics, nothing we’ve discussed thus far answers what is probably the first question for somebody walking up to the weight rack: what exercises should I use? Exercises can be placed into two general buckets, compound movements (a.k.a. multi-joint or MJ) that involve many muscles across many joints and isolation movements (a.k.a. single-joint or SJ) that mainly utilize the motion of 1 joint and target the activation of 1 or 2 muscles. For example, the back squat, bench press, and deadlift are all examples of compound movements that involve motion across many of the major joints (shoulder, elbow, hip, knee, and ankle) and draw from a spectrum of muscle groups. On the contrary, the bicep curl, tricep extension, and lateral raise are all classic isolation movements that elicit major motion from 1 joint and focus tension on 1-2 muscles. In theory, each category offers its own benefit from a mechanical tension point of view, in that MJ movements generally allow you to use greater total load, and SJ movements generally allow you to focus a greater percentage of the load to the target muscle. If you consider the biceps brachii (biceps), for example, most people can use considerably more weight with a lat pull-down (MJ) than they can with a barbell curl (SJ); however, a majority of the load is carried by the latissimus dorsi (lat) muscles in a lat pull-down, whereas a higher percentage of the strain is placed directly on the biceps during the curl. In this way, depending on the exact loads used and the degree of lat vs. biceps involvement in the lat pull-down, the MJ and SJ movements could offer different amounts of mechanical tension on the biceps. Check out Figures 1 and 2 below to visualize this discrepancy.
With this in mind, along with the concepts we discussed in Part IV, you can understand how, in theory, one or the other – MJ vs. SJ – could be better for inducing hypertrophy. In this regard, the data are mixed, in that some research shows no significant difference with MJ vs. SJ nor with MJ vs. MJ + SJ – even without equating volume load (VIII, XIV, XV, XVI); whereas, other data suggest the opposite, showing a hypertrophic benefit to SJ or MJ + SJ as compared to MJ. (XVIII, XX, XXI) So, does it matter which type of exercise you use or not? Before drawing any conclusions, it’s important to touch on some nuances.
Firstly, the data supporting MJ + SJ over MJ alone come from studies where volume was not equated between groups – both groups performed the same MJ protocol, while one group added in the SJ exercises; therefore, the benefits shown in the MJ + SJ group could simply be a reflection of greater weekly volume load. Interestingly, the 0.25% increase in hypertrophy per additional weekly set that Schoenfeld and colleague’s found in their review on volume closely predicts the differences between MJ and MJ + SJ in this study. (XXI, XXIII)
Secondly, as we discussed above, some research shows that improving the mind-muscle connection can significantly improve hypertrophy outcomes; consequently, if subjects were guided towards activating specific musculature during MJ movements – for example, focusing on using the biceps during the lat pull-down – it’s possible that any potential gap in stimulation between SJ and MJ movements could be attenuated. (III)
Check out Ribeiro and colleagues’ take on the utility of SJ exercises in hypertrophy training here.
Thirdly, essentially all of the direct research on this topic looks at hypertrophy of the upper arm, especially the biceps; consequently, we cannot generalize the findings to other muscle groups. This limitation is particularly impactful when considering that not all muscle groups are sufficiently stimulated during MJ exercises the way that the biceps might be. For example, some research shows that, though they induce hypertrophy in the other quadriceps muscles, back squats do not lead to significant rectus femoris growth; contrarily, implementing other leg exercises, such as the leg press and lunges, alongside back squats can grow all aspects of the quadriceps. (I, XVII, XVIII, XIX) Considering that other muscle groups – like the hamstrings (during the squat), rear and side deltoids, and calves – are minimally activated during many MJ movements, it makes sense to at least implement SJ movements for these muscles.
Fourthly, when considering exercise variation, it is important to understand the anatomical design of different muscle groups; particularly, it is important to consider that many muscle groups, such as the biceps, triceps, hamstrings, and quadriceps consist of multiple heads, some of which cross joints that the others do not. For example, the biceps consists of a long head that only contributes to motion at the elbow joint and a short head that contributes to motion at both the elbow and shoulder joints. (XXIV) Together, the two heads facilitate the brachialis – the main elbow flexor – with elbow flexion and create forearm supination; additionally, the short head aids in shoulder flexion, adduction, and stabilization. For this reason, increasing or decreasing shoulder flexion and/or adduction during biceps exercises, such as the curl, can impact the degree of load placed on the different portions of the muscle. Considering that this anatomical pattern – in which different heads of the same muscle cross different joints and can produce different motions – is consistent across most major muscle groups, it is sensible to attack each muscle from a plethora of positions and angles – as Dr. Brad Schoenfeld describes in his textbook, Science and Development of Muscle Hypertrophy – supporting the incorporation of SJ exercises. (I) This means varying your body orientation – for example, performing a bicep curl sitting straight up vs. leaning at a backwards angle vs. leaning at a forwards angle with your chest supported – altering your grip placement, and shuffling the exercises themselves. Similar to the idea from Muscle Mania Part V about varying intensity to freshen up training psychologically, I find that rotating through different exercises can help keep training interesting. Check out the clip below with JM Blakley, bench press legend, for a visual explanation of how manipulating angle can impact load on a muscle.
So, where does this leave us? Together, these points suggest that MJ exercises alone can create significant hypertrophy and are potentially even more useful when manipulating the mind-muscle connection; however, if your goal is optimization, implementing SJ exercises and diversifying exercise selection as a whole, over time is likely valuable – especially for muscle groups that are not significantly activated during MJ movements. Lastly, though it appears that SJ exercises can lead to similar amounts of muscle growth when compared to MJ exercises, solely using isolation movements requires more exercises per training session and sacrifices time efficiency.