Research Spotlight: An important update on low-load training

Research Spotlight articles share concise breakdowns of interesting studies. The study reviewed is "Changes in Body Composition and Strength after 12 Weeks of High-Intensity Functional training with Two Different Loads in Physically Active Men and Women: A Randomized Controlled Study."

Low-load training is a topic we’ve discussed pretty frequently in MASS Research Review. At this point, we have very strong, consistent evidence that low-load training (with <50% 1RM loads) leads to just as much muscle growth as moderate-load training (with 60-85% 1RM loads), when sets are performed to failure. However, there are still a few lingering concerns about low-load training. First, there’s very little research investigating the impact of low-load, non-failure training. Of the studies that do exist, most involve training that’s basically performed to failure (2), or training that’s performed very far from failure (3). We know that stopping a couple reps shy of failure with moderate load training is perfectly fine for hypertrophy, but similar studies haven’t been performed for low-load training. So, it’s unclear if low-load training needs to be performed to failure in order to produce a robust hypertrophy stimulus. Second, within the online fitness community, I’ve seen it posited that low-load training can only work as a short-term intervention. In other words, it may cause hypertrophy that’s comparable to moderate-load training for a while, but over time, rates of hypertrophy with low-load training will drop off sooner than hypertrophy with moderate-load training. To be clear, I’m not aware of any strong evidence to substantiate this claim, but it’s still a claim I frequently encounter in the wild.

A recent study by Kapsis and colleagues doesn’t fully address both of these concerns, but it does provide us with a bit of evidence to suggest that low-load training doesn’t need to be performed to failure in order to generate hypertrophy comparable to moderate-load training, and a bit of evidence to suggest that rates of hypertrophy don’t slow down faster with low-load training than moderate load training (1).

In the presently reviewed study, two mixed-sex groups of active but untrained subjects completed a 12-week circuit training program. There was also a non-training control group, which will not be referenced in the rest of this research spotlight (unsurprisingly, the control group did not experience any notable changes throughout the duration of the study). The training program for both experimental groups consisted of five exercises (bench press, back squat, bent-over row, deadlift, and dumbbell shoulder press) performed in a circuit. Subjects performed four circuits, with 2.5 minutes of rest between circuits. Each circuit consisted of one 30-second set of each exercise, with 30 seconds of rest between exercises. Subjects completed as many reps as possible during each 30-second set. One group (the moderate-load group) trained with 70% 1RM loads, and one group (the low-load group) trained with 30% 1RM loads.

Strength and body composition were assessed before the start of the training program, after six weeks of training, and after the twelfth week of training. Strength was assessed via 1RM testing for all five exercises, and the strength test after six weeks of training was used to adjust training loads for the final six weeks of the intervention. Body composition was assessed using 4-point bioelectrical impedance (BIA). It’s worth noting that the BIA device used in the present study has been validated against DEXA (4). Even medical-grade four-point BIA devices aren’t as good as DEXA for assessing body composition, but they’re considerably better than BIA “smart scales” or the handheld devices used in some gyms.

Total volume load (sets × reps × weight) was similar between groups, averaging ~19,700kg in the low-load group and ~19,000kg in the moderate-load group (p > 0.8). Both groups significantly reduced fat mass, but the change tended to be larger in the low-load group (-3.19 ± 1.59kg versus -1.64 ± 1.44kg; p = 0.052). Lean mass gains were similar between groups (1.11 ± 0.65kg for the low-load group, and 1.25 ± 1.59kg for the moderate-load group). However, the pattern of lean mass accretion differed between groups. The moderate-load group gained an average of 1.05kg of lean mass over the first six weeks, and only 0.2kg of lean mass over the last six weeks. Conversely, the low-load group gained an average of 0.45kg of lean mass over the first six weeks, and an additional 0.66kg over the last six weeks. Strength gains were similar between groups for all five exercises, though they were nominally (but non-significantly) larger in the moderate load group.

Circling back to the introduction to this research spotlight, I’d like to discuss how these findings address the two ongoing questions about low-load training: a) does low-load training need to be performed to failure in order to be effective for hypertrophy? and b) do the hypertrophic effects of low-load training wane faster than the hypertrophic effects of moderate-load training?

The present study clearly addresses the second question. Body composition was assessed after 6 weeks and 12 weeks of training. While the total lean mass gains were similar between groups, the low-load group actually tended to accrue more lean mass than the moderate load group throughout the second half of the study. Since body composition was assessed via BIA, I wouldn’t recommend interpreting the reported changes as outrageously precise, and I would have loved to see direct assessments of hypertrophy (i.e., measurements muscle thickness or cross-sectional area). That said, this study does provide at least a bit of evidence to suggest that the hypertrophic effects of low-load training aren’t attenuated faster than the hypertrophic effects of moderate-load training.

The way the present study addresses the first question requires a bit more explanation. Remember, volume load was similar between the two groups. Baseline strength levels were also similar between groups. With that in mind, we can be quite confident that the moderate-load group was training quite a bit closer to failure than the low-load group. In general, you can complete greater volume loads during a set to failure with lower loads. There are bioenergetic reasons for that (with lower loads, a larger relative proportion of the ATP used for muscle contractions comes from aerobic respiration, which is virtually limitless, rather than the ATP/PCr system or anaerobic glycolysis, which are quite limited), but it’s easy to see this relationship in practice using a standard rep max equation. I constructed the table below based on the classic Epley equation (1RM = 0.033 × reps completed × weight lifted + weight lifted).

So, volume load should be considerably greater when training with lower loads, assuming you’re performing the same number of sets and training at a similar proximity to failure. The fact the volume loads were comparable in both groups suggests that the low-load group was training quite a bit further from failure. And, logically, that makes sense. If you’re up for a bit of masochistic fun, pick any lift, load 30% of your 1RM on the bar, perform a set to failure, and note how long it takes you to complete the set. A few days later, repeat the process with 70% of your 1RM. You’ll find that it takes much longer to complete a set to failure with 30% of your 1RM. More importantly, you’ll also find that it takes longer than 30 seconds. Since the present study used circuit-based training, with each set lasting for 30 seconds, it’s likely that the moderate-load group was training quite a bit closer to failure than the low-load group. However, gains in lean mass were comparable in both groups.

Of course, caveats apply. I’d love to see this finding replicated in a study using “normal” resistance training (rather than time-based circuits) that actually quantifies proximity to failure, and takes direct measures of hypertrophy. Those caveats are the reason I’m writing about this study in a research spotlight, rather than as a primary article. However, this study does provide us at least a bit of evidence that low-load training doesn’t need to be performed to failure in order to effectively promote hypertrophy.

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