Within exercise science, the “principle of specificity” goes mostly unquestioned, and for good reason. In general, if you want to improve a particular aspect of performance, your training should closely resemble the type of performance you’d like to improve (and the evidence bears this out). If you want to get stronger, lift heavy. If you want to improve velocity and power output, do training that requires high velocity and power outputs. There are at least a dozen other examples of the principle of specificity in action.
The principle of specificity isn’t a completely unerring north star, but it’s an incredibly useful heuristic. However, when a heuristic works too well, too often, I think there’s a tendency to become increasingly reliant on the heuristic, in place of critical thinking and careful data analysis. Notably, I think over-reliance on the principle of specificity has over-simplified the way that many people think about training to improve strength endurance. If you ask 100 fitness professionals how to improve strength endurance, you’ll probably get at least 80-90 answers along the lines of, “do lighter, higher rep training.” That’s a perfectly logical answer, given the principle of specificity – strength endurance has been defined as, “the ability to maintain submaximal muscle actions,” so it stands to reason that improving muscular endurance would involve training that requires you to perform more submaximal muscle actions. However, when you actually dig into the data, you’ll find that the actual answer depends pretty heavily on how you define and measure strength endurance, since “strength endurance” is a surprisingly nuanced concept. It’s not clear-cut that high-rep training is always best.
A recent meta-analysis by Hackett and colleagues (1) beautifully illustrates this nuance. The researchers started by identifying all studies meeting these inclusion criteria:
- The studies needed to directly compare the effects of two different rep ranges on measures of strength endurance, following a dynamic resistance training intervention lasting at least four weeks
- The studies needed to be peer-reviewed, and use adult subjects with no known medical conditions or musculoskeletal injuries
- The studies needed to assess muscular endurance via a reps-to-failure test, using either the same percentage of pre-training 1RM at pre- and post-training testing sessions, or using the same percentage of moment-in-time 1RM at both testing sessions.
Furthermore, studies would be excluded if the intervention involved any supplement besides protein, if the training intervention involved training modalities other than resistance training, or if the details of the training intervention didn’t allow groups to be neatly classified as the “higher-rep group” and “lower-rep group” within each study.
The researchers performed two primary meta-analyses. The two meta-analyses assessed the impact of lower- versus higher-rep training on strength endurance a) when pre- and post-training assessments of strength endurance were both performed with the same percentage of the subjects’ pre-training 1RMs, and b) when pre- and post-training assessments of strength endurance were performance with the same percentage of the subjects’ moment-in-time 1RMs. In other words, if a subject trained for 8 weeks, increased their squat from 125kg to 150kg, and performed reps-to-failure tests with 70% of 125kg pre- and post-training, their data would be included in the first meta-analysis. For the rest of this article, I’ll refer to this as an increase in “absolute strength endurance.” If they performed a reps-to-failure test with 70% of 125kg at pre-training, and with 70% of 150kg post-training, their data would be included in the second meta-analysis. For the rest of this article, I’ll refer to this as an increase in “relative strength endurance.” Six studies were included in the first meta-analysis on absolute strength endurance, and nine studies were included in the second meta-analysis on relative strength endurance.
The researchers found that higher- and lower-rep training were similarly effective for improving absolute strength endurance (Figure 1).
Conversely, higher-rep training was considerably more effective than lower-rep training for improving relative strength endurance (Figure 2).
Furthermore, the researchers performed a meta-regression (with segmental linear regression) to assess the point at which performing more reps in a set no longer led to further increases in relative strength endurance. They found that the breakpoint was 24 reps per set, with a 95% confidence interval spanning from 11 to 39 reps per set (Figure 3).
Finally, the researchers performed moderator analyses to see if any subject or training characteristics were predictive of larger or smaller increases in absolute and relative strength endurance. The only noteworthy result was that gains in maximal strength were predictive of smaller gains in relative strength endurance (or even losses in relative strength endurance; β = -0.89; p = 0.002), but that changes in maximal strength weren’t predictive of changes in absolute strength endurance (β = 0.06; p = 0.76).
The basic finding of this study is that if you want to be able to perform more reps with a particular percentage of your 1RM, even as your 1RM changes over time, it certainly makes sense to train with higher reps. However, if you want to improve your strength endurance performance with a particular absolute load, you don’t necessarily need to do higher-rep “strength endurance” training.
I think an illustration will clarify the difference. Let’s assume you currently squat 300 pounds, and you want to be able to squat as many reps as possible in a single set with 225 pounds. Right now, you might be able to complete 10 reps.
If you do higher-rep, lower-load training over a number of months, your squat 1RM might only increase to 320 pounds, but the local aerobic and anaerobic capacity of your prime movers should increase considerably. You should be expected to complete more than 10 reps with 225, because 225 now represents a lower percentage of your moment-in-time 1RM (70% now, versus 75% of your pre-training 1RM), but without increases in local aerobic and anaerobic capacity of your prime movers, you might only be expected to complete about 12 reps with 225. However, since your prime movers are more highly conditioned now, you might be able to complete 18 reps with 225 – a large improvement in strength endurance performance, relative to your absolute strength levels.
Conversely, you might do lower-rep, higher-load training over a number of months, and increase your 1RM to 400 pounds. With this large increase in strength, 225 only represents 56% of your current 1RM, so you might be able to crank out 18 reps, even if the relative aerobic and aerobic conditioning of your prime movers hasn’t meaningfully changed. So, you might say that both approaches were similarly effective for improving your strength endurance.
However, if your goal was to maximize the number of reps you can perform with 75% of your 1RM, this little experiment would have very different results. In both cases, you could complete 10 reps with 75% of your 1RM at baseline. Following lighter, high-rep training, you may be able to complete 15 reps with 75% of your current 1RM (240 pounds; 75% of 320). Following heavier, low-rep training, you may only be able to complete 8 reps with 75% of your current 1RM (300 pounds; 75% of 400). With this metric of strength endurance, you’d conclude that higher-rep training was considerably more effective for improving strength endurance.
If you disambiguate absolute and relative strength endurance, the results are considerably clearer. With higher-rep training, you experience a small increase in strength (+20 pounds), a larger increase in relative strength endurance (+6 reps at the same percentage of 1RM), and an even larger increase in absolute strength endurance (+8 reps with a fixed load). With lower-rep training, you experience a large increase in strength (+100 pounds), a small reduction in relative strength endurance (-2 reps at the same percentage of 1RM), but the same increase in absolute strength endurance that was observed with higher-rep training (+8 reps with a fixed load).
Ultimately, when evaluating the effects of a particular approach to training on strength endurance, it’s worth asking which metric(s) of strength endurance matter the most for you. I’d argue that in most contexts, assessing absolute strength endurance – rep performance with absolute load (or with a fixed percentage of your pre-training 1RM) – is more representative of the carryover you can expect between the gym and competitive or “real world” scenarios. For example, if a strongman contest has a max reps event for log press or deadlift, all competitors in each weight class use the same load – you don’t get penalized with a heavier load if you’re stronger than your competitors. So, you can gain an advantage by either having better relative strength endurance than your peers, or by simply being a lot stronger than your peers. Similarly, the muscular fatigue you experience in most athletic or “real world” scenarios comes as a result of simply moving your body around for a prolonged period of time. Assuming your body mass doesn’t fluctuate wildly, your body will be less challenging to move around for a prolonged period of time if local aerobic and anaerobic capacity improves (increases in relative strength endurance), but it will also be less challenging to move around for a prolonged period of time if your body mass represents a much smaller percentage of your maximal capacity to generate force. So, unless you simply want to be able to perform a ton of reps at 60% of your 1RM (as an example) for its own sake, I’d argue that lighter, high-rep training isn’t actually better than heavier, lower-rep training in most contexts. In most contexts, I’d contend that absolute strength endurance is a more useful metric than relative strength endurance.
Before moving on to applications, I’d just like to make two more quick notes about the present meta-analysis. First, if you want to do high-rep “strength endurance training” to increase your reps-to-failure performance at a given percentage of your moment-in-time 1RM, you don’t necessarily need to do ultra high-rep training. The meta-regression found that sets of ~20-30 reps are likely sufficient to maximize gains in relative strength endurance. So, you could do sets of 50 reps if you feel particularly masochistic, but sets of 20-30 reps are likely sufficient.
Second, there probably is a point at which low-rep training becomes too low-rep to be particularly useful for improving absolute strength endurance. If you look back at Figure 1, you can see that for seven strength measures from five different studies, lower-rep and higher-rep training were similarly effective for improving absolute strength endurance – the individual effect sizes are all trivial-to-small, and the individual confidence intervals all crossed the “zero effect” line. However, one measure from one study sticks out like a sore thumb – changes in knee extension strength endurance, from the study by Mattocks and colleagues (2). In that study, higher-rep training was considerably more effective than lower-rep training for improving absolute strength endurance. This could just be a fluke, but it’s noteworthy that the Mattocks study was the only study included in this meta-analysis where the low-rep group was performing single-rep sets. Sets of 3 reps were just as effective as sets of 10 reps in the study by Schoenfeld and colleagues (3), but single-rep sets were trounced by 10-rep sets in the Mattocks study. So, while heavier, lower-rep training seems to be effective for improving absolute strength endurance, it’s certainly possible that absolute strength endurance could be negatively affected when training gets too heavy and the reps get too low. However, it seems like that may only be a concern if you’re living on a diet of exclusively single-rep sets.
Finally, let’s step away from the research and into the real world. This present meta-analysis purposefully set up a dichotomy to examine the effects of lower-rep versus higher-rep training on strength endurance. If you’re trying to isolate the effect of a particular variable, that’s precisely how you need to conduct your analysis. However, in the real world, you don’t have to choose one or the other – if you want to maximize gains in absolute strength endurance, I can’t think of a good reason why you wouldn’t do both: some heavier, lower-rep training to increase maximal strength, and some lighter, higher-rep training to increase relative strength endurance.
If you’re training for the goal of maximizing strength endurance, I’d recommend structuring your training in one of three ways:
- Use a reverse linear periodization approach, starting with heavy training to increase your 1RMs, followed by lighter, higher-rep training to maintain strength while maximizing relative strength endurance.
- Use an approach employing daily undulating periodization, with heavy, low-rep training in one training session per week (for each lift), and lighter, higher-rep training in one training session per week (for each lift).
- Use a hybrid approach, with shades of Westside or the Hatfield method. You could either do some heavy training and some high-rep training for most exercises within most training sessions, or you could train your compound lifts heavy with sets of 3-8 reps, but opt for 20-30 rep sets for most accessory exercises.
Ultimately, if we consider a strength-endurance curve (Figure 4), training designed to maximize absolute strength will pull up on the left side of the curve, while training designed to maximize relative strength endurance will pull up on the right side of the curve. You can’t maximize absolute strength endurance without doing both.
Note: This article was published in partnership with MASS Research Review. Full versions of Research Spotlight breakdowns are originally published in MASS Research Review. Subscribe to MASS to get a monthly publication with breakdowns of recent exercise and nutrition studies.
Update: November 2023
I recently came across a paper by Fliss and colleagues that was published around the same time as the meta-analysis reviewed in this article. I’m just sharing it because its data more-or-less perfectly reflects the conceptual illustration above. The study examined changes in strength endurance after training with high versus low loads. From this testing, the researchers were able to sketch out strength endurance curves. As you can see, heavier training (Post-RET HL Leg Model) led to larger increases in strength endurance with heavier loads, whereas lighter training (Post-RET LL Leg Model) led to larger increases in strength endurance with lighter loads.
So, this study certainly doesn’t change the takeaways from this article – it simply reinforces them. However, I felt compelled to share it, because I’m always a little nervous about sharing conceptual illustrations that are extrapolations from what the data directly support. Seeing a near-identical figure in a similar study helps put my mind at ease, and helps solidify the takeaways of this article.
References
- Hackett DA, Ghayomzadeh M, Farrell SN, Davies TB, Sabag A. Influence of total repetitions per set on local muscular endurance: A systematic review with meta-analysis and meta-regression. Science & Sports. 2020 Sep-Oct; 37(5-6):405-420.
- Mattocks KT, Buckner SL, Jessee MB, Dankel SJ, Mouser JG, Loenneke JP. Practicing the Test Produces Strength Equivalent to Higher Volume Training. Med Sci Sports Exerc. 2017 Sep;49(9):1945-1954. doi: 10.1249/MSS.0000000000001300. PMID: 28463902.
- Schoenfeld BJ, Contreras B, Vigotsky AD, Peterson M. Differential Effects of Heavy Versus Moderate Loads on Measures of Strength and Hypertrophy in Resistance-Trained Men. J Sports Sci Med. 2016 Dec 1;15(4):715-722. PMID: 27928218; PMCID: PMC5131226.
