Exercise, Hypertrophy

October 20, 2014

New Insight into Rest Intervals for Muscle Growth

I recently collaborated with my colleague Menno Henselmans on a review paper that sought to provide clarity on the effects of rest intervals on muscle hypertrophy. Based on the current literature, we concluded that evidence was lacking to support the contention that rest interval length has an impact on growth. Problem is, there have been very few studies carried out to investigate the topic. Thus, it’s difficult to say with any degree of confidence as to whether there are or aren’t any benefits to varying how long you should rest between sets. For more on specifics of the review paper check out my blog post where it is discussed in detail.

Fast forward to today: A new study has just been published titled, Short rest interval lengths between sets optimally enhance body composition and performance with 8 weeks of strength resistance training in older men that sheds further light on how the duration of rest intervals may affect muscular adaptations. If you just read the abstract, you might think the answer is clear.

Not so fast…

Here’s my take:

Study Design
22 older men (mean 68 yrs) were recruited for participation. Subjects were healthy but were not involved in resistance training. All subjects underwent a 4-week “break in” phase where they performed a “hypertrophy-type” total body routine consisting of 2-4 sets of 8-15 reps per set. The subjects were then tested for various measures including strength and body composition, and then pair-matched based on 1RM bench press to perform an 8-week strength-type program with short rest (1 minute) or long rest (4 minutes) between sets. The strength-type routine consisted of 2-3 sets of 4-6 reps carried out 3 days a week. The exercises included leg press, flat bench machine chest press, lat pulldown, seated row, dumbbell step-ups, dumbbell Romanian deadlifts, knee extension, and knee flexion. Reps were performed with the intent to move the loads as fast possible while maintaining proper form. All sessions were supervised by trained personnel.

Results
Results were determined over the final 8-week strength phase of the program. Significantly greater increases in fat free mass (FFM), 1RM bench press, and 1RM leg press were noted for the short-rest group compared to those who took long rest periods. The researchers investigated a wide array of additional outcomes including power measures, which also generally favored the short rest group as well.

Commentary
Based on these results, it would appear that limiting rest between sets is beneficial to enhancing strength and hypertrophy. The increase in FFM for the short-rest group was 1 kg vs just a 0.3 kg increase for the long-rest group. The effect size — a measure of the meaningfulness of the results — was 0.37 indicating a fairly small effect. That said, a difference of 0.7 kg (equating to ~1.5 pounds) could certainly be meaningful for those seeking to maximize hypertrophy — particularly over a fairly short period (8 weeks). The effect sizes for strength were fairly large (0.65 and 0.76 for the 1RM bench and squat, respectively). Combined, these findings indicate that muscular adaptations are enhanced by taking short rest periods between sets.

But…a closer scrutiny of the study’s methodology gives reason for caution when drawing conclusions.

First and foremost, the researchers used DXA to measure body composition. The authors reported results for FFM, which as stated were higher for the short rest group. However, FFM encompasses all tissues in the body other than fat mass. This includes bone, connective tissue, and importantly water. You can probably rule out any differences associated with bone and connective tissue, which almost certainly would be minimal over an 8 week resistance training in terms of contribution to body mass. However, variances in water weight could easily have accounted for a large portion of the the reported 0.7 kg difference in FFM. It’s curious why the researchers did not choose to quantify the subject’s segmental muscle mass. There are equations that can be employed with DXA to obtain these values, which would have given a better sense as to true increases in muscle. Unfortunately, the reported data do not allow for a true understanding of changes in the lean component of body composition between groups.

Second, the subjects did not train to failure in either condition. The researchers stated that this was done to reduce neuromuscular fatigue and thus ensure that the subjects could tolerate the program over its duration. While I have no problem with that reasoning, it does raise a major issue: Since those in the short rest interval group had to lift again after only 60 seconds, they would have been taxed to a greater extent on each successive set. The long-rest group on the other hand would have sufficient time to recover prior to the next set, and thus would not have been substantially taxed at point during the workout. Now it is a bit difficult to determine how much the subjects were actually challenged on each set based on the study write up. Ideally the researchers should have quantified the level of effort exerted (perhaps by RPE or similar scale) to provide context. Without this info, I’m left wondering if the design was biased to produce a greater effect with shorter rest periods.

Finally and importantly, the study was carried out on elderly, untrained subjects. These individuals would no doubt have been sarcopenic, and their response to an exercise stimulus therefore would not necessarily mimic that of young, fit individuals. Thus, generalizability of results is limited to the population studied.

In conclusion, this is an interesting study that adds to the body of literature. However, caution must be exercised when attempting to draw conclusions as to the effects of rest interval length on muscular adaptations. The limitations of the study preclude extrapolation of results to those seeking maximal muscle mass.

The good news is that I am currently collaborating on a study on the topic that directly measures hypertrophy in well-trained subjects. Target completion for data collection is early next year. I will update when results are available.