Concurrent training may compromise strength, power, and/or hypertrophic adaptations, which is commonly referred to as the interference effect.

Some results indicated that there might be a decrease in adaptation to resistance training when it is combined with an aerobic training program. It seems that resistance training enhances aerobic training, but not vice versa (Hickson 1980; Hakkinen et al., 1993; Kraemer et al., 2005). The first meta-analysis on concurrent training in 2012 found that concurrent training limited (by about ~20%) the amount of strength, power, and hypertrophy that could occur compared to strength training alone.

A few years later some different scientists looked at a very similar data set, and what they found was that when you compared concurrent to strength training, the farther you separated the endurance component from the strength component, the better. There’s actually a nice dose-response up to about 24 hours of separation between training types.

Endurance exercise primarily increases the number of mitochondria (aka mitochondrial biogenesis). Resistance training mainly increases the size of the muscle. These are somewhat competing pathways.

Endurance training stimulates molecular pathways such as PGC-1, CaMK, calcineurin, AMPK, and MAPK which underly the cellular processes that promote mitochondrial biogenesis and angiogenesis, which lead to an increase in endurance capacity.

AMPK is a key energy sensor of the cells and is an inhibitor of the AKT-mTOR pathway.

Even though the animal studies have been impressive at showing that AMPK can directly inhibit mTORC1 activity and muscle growth, acute studies in humans are not definitive. Plus, most studies have examined acute molecular responses up to 4 h post-exercise, whereas mTORC1 signaling persists for up to 24 h post-exercise while AMPK is active for a much shorter period (~3h).

There was a more recent meta-analysis in 2018 that found HIIT and RT does not negatively impact hypertrophy or upper body strength, but does have a possible negative effect on lower body strength. However, the negative effect can be ameliorated by incorporating running based HIIT and increasing time between exercise modes.

Finally, the newest meta-analysis found that concurrent aerobic and strength training did not interfere with the development of maximal strength and muscle hypertrophy compared with strength training alone. Yet, the development of explosive strength was reduced by concurrent training. The subgroup analysis was only significant within the explosive strength measurements, indicating that there was a detrimental effect of doing concurrent training within the same session, but if separated by 3 hours the effect disappeared.

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