Combined strength-endurance training: easy bedfellows?

New research provides some answers about the wisdom of concurrent strength and endurance training sessions

Many athletes may need to combine strength and endurance training into the same workout, especially where time or facilities are at a premium. Whilst being a time-efficient mode of training, there’s a potential downside in the form of ‘interference’. This ‘interference effect’ describes a reduced level of strength development following a combined strength and endurance training session compared to the same strength session conducted in isolation^(1,2). For athletes who sports require both good levels of strength and endurance, this conflict between opposing sides of the strength-endurance training adaptation continuum can be troublesome, both at the elite and recreational levels (although some research suggests elite athletes may be more susceptible than recreational athletes to interference⁽³⁾).

Understanding the interference effect

The current consensus is that the interference effect is thought to occur due to a molecular interference between two separate biochemical and signalling pathways in the body – one to do with protein synthesis in and remodelling of muscle fibers, and the other to do the cellular pathway that regulates energy production for endurance activity⁽⁴⁾. In particular, it is believed that the production of a key anabolic signalling molecule known as ‘mTORC1’ (mTORC1 can be considered a growth or strength molecule – see this article for more info) is impaired by the production of a protein known as ‘AMPK ‘, which is of key importance to the endurance adaptation training process, due to its function of monitoring the energy status of the muscle and initiating aerobic adaptive responses⁽⁵⁾. In short, performing endurance training increases the activity of AMPK, which according to the theory, in turn suppresses the production of mTORC1 production, which reduces the amount of muscle growth and strength that occurs following strength training in the same session.

Given the huge range of possible variations in endurance training options (ranging from very high-intensity, short intervals to long-duration continuous low-intensity exercise) there is limited research addressing the role of endurance exercise intensity and how this might be implicated in the interference effect. Also, if endurance training stimuli can impede the anabolic signaling processes, another unknown is how the endurance training status of an athlete affects this interference. Surprisingly perhaps, there has been no research into the role that endurance exercise intensity plays in producing a molecular interference amongst trained endurance athletes. This knowledge would be of great importance to endurance athletes who want to introduce strength training into a program because of the great performance benefits it can bring – see this article for a more in-depth discussion.

New research

A newly published study by British scientists has tried to answer these questions⁽⁶⁾. Published in the journal ‘Scientific Reports’, this study set out to investigate whether combining strength and endurance training (independent of intensity) really does result in the inhibition of anabolic signaling proteins - ie blunts strength adaptations - relative to the same amount and type of strength training performed on its own. In addition, the scientists looked to see whether the intensity of the endurance training undertaken in conjunction with a strength session affected AMPK, and therefore mTOR levels, and what the implications are for combined strength-endurance training. To do this, eight male cyclists completed the following trial:

• Resistance exercise consisting of six sets of 8 reps of squats at 80% 1RM, and no endurance training.
• Resistance exercise as above combined with moderate intensity cycling consisting of 40 minutes at 65% VO2peak (moderate intensity). This was the strength plus low-intensity endurance combination.
• Resistance exercise as above combined with high-intensity interval cycling consisting of 40 minutes incorporating six sets of 3-minute intervals at 85% VO2max, interspersed with recovery intervals at 45% VO2max. This was the strength plus high-intensity endurance combination.

The design was a ‘crossover trial’, which meant that each cyclist had to undergo each task on three completely separate occasions. Before and after each trial, muscle biopsies were taken from each cyclist to measure the levels of mTORC1 and AMPK, and to see what effect if any adding endurance work had on the production of mTORC1 and thus potential strength gains.

The findings

The key finding was that (contrary to what was expected), adding in endurance work did NOT diminish the production of mTORC1. In fact, compared to resistance only training (which should according to the theory produce the highest increase in mTORC1), there was a significantly lower increase in mTORC1 levels compared to the resistance and moderate intensity training (see figure 1). Adding in high-intensity intervals meanwhile also resulted in a slightly higher increase in mTORC1, although this was not a big enough difference to be considered significant. Furthermore, the rise in AMPK levels did not seem to be affected by the intensity of the endurance work added.

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Figure 1: Changes in mTORC1 levels following different combinations of strength/endurance training

Individual and average responses in phosphorylation of the mTOR signaling pathway in the resistance-only (RES), resistance plus high-intensity endurance (RES + HIC), and resistance plus moderate intensity endurance (RES + MIC) trials. Grey dots represent individual responses and black lines represent the mean.

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What do these findings mean for athletes?

The authors concluded that their data did not support a molecular interference effect when adding endurance to strength training in cyclists under controlled conditions. These findings are significant because they call into question the current consensus that (in trained athletes at least) mTORC1 production is reduced when endurance training is added, which then leads to poorer strength adaptations. Indeed, adding in moderate-intensity endurance work in this study increased mTORC1.

There are two possibilities here. The first is that these findings suggest that because mTORC1 wasn’t reduced when strength training was added, there is actually little interference between strength and endurance. However, this is unlikely as a studies show that there is a real interference effect⁽²⁾. A more likely explanation is that the production of mTORC1 alone is a poor indicator of how much actual growth and strength gains will occur. In this study, muscle growth and strength gains were not assessed; were they, it might have been discovered that while mTORC1 increases were lower in the resistance-only group, the actual growth/strength gains were higher. Another difficulty is that this study did not look at the longer-term levels of mTORC1, how they may have been affected by nutrition, or look at differing types of strength training routines. This all makes it hard to draw definitive conclusions.

However, taken in the round, we can perhaps say that despite its limitations, this study does NOT provide solid evidence for separating strength and endurance training, especially when that endurance training is of moderate intensity and duration. This in turn should be reassuring to athletes who are tight for time and wish to add some strength training to a workout. While we can’t be sure the strength benefits are optimized, we can say that athletes still stand to make significant gains. One tip however would be to perform moderate-intensity endurance work before the strength work, to minimize the possibility of the strength stimulus being ‘watered down’ by an endurance session performed afterwards. Meanwhile, high-intensity endurance work is best avoided before strength work - in order ensure the target muscles are relatively fresh and that mental motivation levels are high going into the strength session!

References

1. J Strength Cond Res. 2012 Aug; 26(8):2293-307
2. J Strength Cond Res. 2016 Mar; 30(3):693-702
3. Aviat Space Environ Med. 2007 May; 78(5):457-62
4. J Physiol. 2017 May 1; 595(9):2883-2896
5. Med. Sci. Sport. Exerc. 2004;36:28–34
6. Sci Rep. 2021 May 24;11(1):10785