STUDY AUTHORS: Angelo Sabaga, Abdolrahman Najafib, Scott Michael, Tuguy Esgina, Mark Halakia, Daniel Hackett
STUDY SPOTLIGHT BY: Cody Haun, PhD, MA, CSCS
The purpose of this study was to conduct a systematic review of the scientific literature examining the effects of high intensity interval training (HIIT) and resistance training (RT) on strength and hypertrophy outcomes.
Electronic databases were searched for words pertaining to HIIT and RT to identify studies that met the following criteria:
1) included healthy adult subjects (≥ 18 years of age)
2) study was ≥ 4 weeks in duration
3) study had to compare a group performing HIIT combined with RT to a group performing the same RT program with no HIIT.
The authors described HIIT in identified studies as ≤ 5 minutes of exercise where intensities were described using at least one of the following criteria:
a) >80% maximal heart rate (maximal heart rate: an estimate of the maximum number of times the heart can beat in a minute, typically expressed as beats per minute [bpm]. This is typically estimated to be 220-age(years)=max heart rate[bpm].).
b) >100% lactate threshold (lactate threshold: lactate threshold is typically defined as the work rate or intensity during exercise during which blood lactate rises 1mmol above resting levels. This point hypothetically symbolizes when an individual is utilizing a notable amount of muscle glycogen for fuel compared to mostly aerobic processes not requiring a significant amount of glycogen use and therefore blood lactate increases.)
c) >90% maximal oxygen uptake (VO2max) (VO2max is measured and identified as the maximum amount of oxygen and individual consumes during a maximal exercise test.)
d) expressed as“sprinting or HIIT”
Studies also had to include assessments of:
a) changes in muscular strength via dynamic repetition maximum measurements
b) changes in lean muscle mass via biopsy, ultrasonography, computed tomography, dual x-ray absorptiometry, magnetic resonance imaging and/or densitometry.
14 studies were deemed eligible for the analysis resulting in the analysis of 263 study participants (182 men and 81 women) between the ages of 18–34 years. 4 of the identified studies involved“untrained” or“inactive” subjects, 6 studies included recreationally active subjects, and the remaining 4 studies included semi-professional or college-level athletes.
The authors interpreted these data to indicate the following:
1) HIIT can be completed during an RT program without negatively affecting increases in lean body mass.
2) The addition of HIIT to an RT program can reduce the magnitude of strength improvement compared to RT alone (HIIT+RT: ~+19% increase in strength on average; RT alone: ~+24% on average).
3) The addition of running-based HIIT to RT programs resulted in less of a negative effect on strength outcomes compared to cycling-based HIIT.
4) Some evidence suggested that this negative effect could be rescued by allowing at least 24 hours between HIIT and RT.
Interference between training characterized as “cardio” and resistance training has long been a concern in the fitness and sport communities. If you’d like a more thorough survey of the literature and introduction to the topic as it pertains to powerlifting specifically, I’ve written a good bit about this previously here and here. Some people think that “cardiovascular” or “aerobic” exercise (e.g., cycling, running, rowing, etc.) should be limited to very little volumes and intensities, or not completed at all, to maximize strength and hypertrophy. In short, as noted in the linked articles above, the state of the literature (and my practical and professional experience) suggest that this isn’t entirely true. In fact, to maximize strength and hypertrophy in the long-term, an appropriate amount of cardiovascular exercise might actually be necessary, or at least helpful. Of course, this depends on how one defines cardiovascular exercise as, technically speaking, resistance training is a form of cardiovascular exercise.
The intended division between “cardiovascular” and “resistance” exercise is a matter of emphasis. Generally speaking, cardiovascular exercise seeks to emphasize adaptations to the cardiovascular and respiratory systems while resistance exercise intends to emphasize adaptations to the skeletal, muscular, and nervous systems. Stated differently, one form of exercise tends to emphasize moving one’s own body mass and involves low levels of force production for longer periods of time while the other tends to involve moving external loads rather than one’s own body alone as a primary form of resistance along with high levels of force production for shorter periods of time. Therefore, high intensity interval training or “HIIT” can be considered a form of both; with its specific place on the exercise spectrum dependent upon the specific mode of exercise, duration, and work rate. This has made studying the compatibility of various types of exercise challenging, and is a confounding factor when deciphering a potential “interference effect” between two types of exercise. Some evidence has pointed to an interference effect while other evidence has failed to identify significant interference. Importantly, it’s critical to analyze methodology of the study to better understand why and to what extent interference occurred as often, many types of exercise are more similar than they are different and result in similar physiological responses.
If you read this or this, you now know the basics of muscle recruitment and contraction and that upon signal to recruit, a fiber twitches. Upon twitch, the fiber produces and therefore experiences tension (the primary mechanism underpinning hypertrophy). The volume and frequency of these twitches are keys to maximizing hypertrophy. Another key point from the articles linked above was that the number of fibers recruited depends upon the loading demands of the task, with lighter loads requiring less fiber actions and heavier loads more.
Thus, although fibers are recruited during running, cycling, rowing, etc and can be grown from such activities (to a point), traditional resistance training more effectively recruits larger numbers of fibers given the increased loading demands and on a per-unit basis, is a better catalyst for hypertrophy and strength. Therefore, in many studies investigating potential interference of “aerobic” exercises vs “resistance” exercises, the aerobic exercises can simply function as an increased number of contractions for fibers in various muscles (although to a lower extent than resistance exercise) and result in improvements in fiber size and potentially strength. For the sake of brevity, we’ll postpone a thorough discussion of many of the cursory benefits of aerobic exercise related more to metabolic factors and benefits to the cardiovascular and respiratory systems and I must note that the above is quite the oversimplification. But, future articles can clear some of that up.
Since ground reaction forces during running have been measured at ~2-3x’s bodyweight and electromyography data clearly indicates increased muscle activation of a variety of muscles during running, considering running exclusively as aerobic exercise is incorrect. Furthermore, as far back as the 80s we knew resistance training could result in positive adaptations to the cardiovascular and respiratory systems. Considering this, all exercise can be considered on a spectrum of emphases. Since all exercise involves some form of resistance, categorizing exercise as either one or the other is somewhat puzzling. Rather, exercise can emphasize adaptations to various body systems to different extents depending on the nature of the exercise and physiological status of the individual. Scientific authors have started using “resistance exercise training” (RET) to describe training with weights that are utilized to resist movement and “aerobic exercise training” (AET) to describe training utilizing one’s own body mass and the force of gravity as resistance with a focus on improving one’s ability to inspire and utilize oxygen. I like these terms as I feel it provides a bit more division and acknowledges that both are forms of exercise.
Now that I feel we have an appropriate foundation, my interpretation of the study can be more clear. Multiple studies have now shown that AET can induce hypertrophy and be included in the same program with RET and produce significant hypertrophy, with some evidence even suggesting greater hypertrophic outcomes. So, I’m not surprised by the findings of Sabaga et al in this regard, but am encouraged that evidence is mounting that cardio isn’t the gainz goblin the field once thought. Separating bouts by a few hours provides insurance that the two modes won’t interfere for muscle hypertrophy purposes. To be fair, as in this analysis by Sabaga et al, many investigations that have reported these observations have included untrained participants in the analysis and high-level trainees shouldn’t simply start doing loads of cardio to get yoked. Not what I’m saying here. High-level trainees require a bit more nuanced structures and careful balance between volumes and intensities of exercise for best results.
The fact that HIIT+RT resulted in slightly less strength accrual during short-term training studies (~5% less) is likely related to residual fatigue effects both centrally and peripherally. For example, given the high-intensity nature of the interval training, it’s likely that some larger primarily fast-twitch fibers possessed lower glycogen levels important for force production during resistance training sessions and this could have impeded the stimulus necessary for larger improvements in strength. Centrally, a decreased desire to train during various bouts could have been related to increased serotonin or decreased glycogen stores in the brain, reductions in serum testosterone and elevations in cortisol, or other potentially mood-altering hormonal changes. However, the authors noted that a subanalysis pointed to a lack of interference if bouts of HIIT were separated from RT by at least 24 hours. This concept is quickly becoming a theme in this area of research, although more work is needed to parse out very specific minimum time differences to avoid interference. Hence, the first piece of advice I’d provide is: If you perform HIIT in combination with your RT, do so ~6-24 hours after your RT. However, HIIT is likely not the best option if you’re trying to maximize strength. Although HIIT can be creatively peppered in to a program aimed at maximizing strength in the long-term, lower intensity, steady state conditioning for low volumes provides a host of cardiovascular and respiratory benefits and is unlikely to impede strength progress given its dissimilar nature and low fatigue generation. A few light runs or rows for ~20 minutes per week are examples. Further, the potential improvement in recoverability in the weight room and between strength training sessions, along with improved blood flow and other responses are additional reasons to consider this mode. Variation at some point in the annual cycle is likely a good idea, however.
Intriguingly, the authors noticed worse interference from cycling compared to running. This is in contrast to other data. This one is somewhat puzzling to me, but I’d chalk it up to greater local fatigue of the knee extensors from cycling from HIIT and direct negative effects on strength tests that required forceful use of the knee extensors (e.g., squatting, leg pressing). From this, if HIIT is employed, it seems running is a better option than cycling to avoid worse strength adaptation interference. Take-homes:
1) HIIT and RT can be performed in the same short-term program without significantly negative effects on strength or hypertrophy outcomes.
2) Separate bouts by ~24 hours.
3) Running-based HIIT seems to result in less interference on strength outcomes than cycling-based HIIT.
4) Similar to other recent findings, this evidence suggests greater hypertrophy can occur from short-term concurrent training compared to RT alone although differences in this analysis didn’t reach statistical significance.