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September 11, 2022
The bodybuilding community is forever seeking to develop advanced training methods that enhance muscle growth. Loaded stretch has long-been proposed as a potential strategy to increase gains. My close friend and colleague, the late, great John Meadows, was a huge proponent of including loaded stretch as a component in bodybuilding program design. Commonly, but not always, this practice is incorporated into the inter-set rest period between sets of an exercise.
Research in animals has consistently shown a benefit to loaded stretch protocols. However, there have only been a few human studies to date that attempted to investigate the topic in a controlled fashion. Thus, our lab decided to conduct a study to determine if loaded stretch performed between sets in a resistance training routine influences muscular adaptations in the calf muscles. The study, Loaded inter-set stretch may selectively enhance muscular adaptations of the plantar flexors, was recently published in PLoS One (and is open-access so that all are able to read for free).
For those who want the consumer-friendly version, here’s the scoop….
What We Did
The study was led by my master’s degree student, Derrick Van Every, with support from our terrific team of research assistants. To reduce potential confounding from genetic and lifestyle factors, we employed a within-subject design where all participants performed both conditions during the same session in counterbalanced fashion. As such, we randomized the legs of young, untrained men to perform 4 sets of straight- and bent-knee calf raises with 2 minutes rest between sets. One of the participants’ legs rested passively during the rest period (consistent with traditional training protocols) while the other leg descended into a loaded stretch (i.e., dorsiflexion) immediately after the end of the set. The stretch condition was held for 20 seconds and then participants rested passively for the remaining duration of the rest interval. Each set targeted 8-12 sets carried out to volitional failure. The training component of the study lasted 8 weeks.
To determine if the strategy influenced hypertrophy, we assessed pre- to post-study changes in muscle thickness of the calf muscles (lateral and medial gastrocnemius, as well as the soleus). We also measured changes in isometric strength with the knees straight and bent.
What We Found
Soleus hypertrophy was modestly greater for the loaded stretch condition. The magnitude of effects ranged from negligible to relatively substantial compared to passive rest (~9% greater). The lateral gastroc showed a slightly advantageous effect for loaded stretch, but the range of values were of questionable practical meaningfulness. The medial gastroc showed neither a benefit nor detriment to loaded stretch.
Somewhat surprisingly (at least to me), strength was modestly enhanced by loaded stretch. The effects ranged from negligible to potentially meaningful (?10% of the baseline strength).
What are the Practical Implications of Findings
Our study suggests that adding brief (20 seconds) loaded inter-set stretch bouts to a resistance training program may in fact promote modest improvements in muscle growth, perhaps specific to certain muscles more than others. In addition, there may be modest improvements in muscle strength as well. Interestingly, the beneficial effects occurred despite a decrease in volume load (sets x reps x load) of ~5 to 12%.
Previous research shows conflicting results on the value of intense inter-set stretch, with some studies indicating a hypertrophic benefit and others not. Although it’s difficult to speculate on the reason(s) for these discrepancies, it’s interesting to note that our study found that beneficial effects were greatest in the soleus muscle. The soleus is a slow-twitch dominant muscle, comprised of ~80% type I fibers. It’s possible that type I fibers may be more anabolically responsive to the higher time-under-tension compared to type II fibers. It also is possible that these fibers may have additional inherent properties predisposed to loaded stretch following performance of eccentric actions (e.g., differences in muscle architecture). Of note, research in animal models also tends to show greater gains in slow-twitch dominant muscles. Alternatively, the results may simply be a chance finding that occurred irrespective of fiber type composition. Replication is needed to provide more insight on the topic.
A limitation of the study is that participants were untrained men. This population was chosen so that we could isolate training only the calves without confounding from other multi-joint leg training (e.g., compound lower body exercises involve plantarflexion); it would have been near impossible to recruit trained subjects willing to give up training their thighs for a couple of months. Thus, it remains to be determined if similar results are achieved in trained individuals as well as in women. In addition, the findings are specific to the calf muscles; we cannot necessarily extrapolate similar results in other muscles of the body. Each study is but a piece in the puzzle of theory development, so further research is needed to better understand the nuances of the topic.
Take-Home Conclusions
From a practical standpoint, loaded stretch performed between sets may be a viable strategy to increase muscular development. Based on our findings, the improvements are relatively modest and their practical meaningfulness would depend on your ultimate goals (e.g., more relevant to those interested in bodybuilding vs general fitness). Although speculative, benefits may be best achieved when the stretch is performed immediately after the final eccentric action to take advantage of the associated passive force enhancement effects on the muscle .
I’d note that the strategy employed in our study did not increase the duration of the workout, making it a time efficient option. Given that there does not seem to be a downside to inclusion of loaded inter-set stretch, it’s a strategy worth experimenting with if your goal is to maximize gains.
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Interesting. Could this be followed up by a study on elite level natural powerlifters. One group continues there training as usual. The other adds, for some muscle groups / exercises, 20 second loaded stretches between sets (note they may have to cut down on load or reps due to fatigue caused by stretching). Thus, one can perhaps gauge if 1) the effect only applies to untrained lifters, 2) there is a significant difference in progress between the stretching and non-stretching group. Groups should of course be comparable in level of training achieved (elite level, they are squeezing out the last bit if there is any last bit of progress left). Taking powerlifters from a country, following often quite the same progression model / competition program, could perhaps be an idea.
Comment by Dr. Egbert van de Schootbrugge — October 2, 2022 @ 3:35 pm