Concurrent training is when an athlete is performing both resistance training and endurance training in the same program with the goal of improving both. The interference effect is when the endurance training impacts the ability to get stronger, gain size or increase power. This is due to the AMPK response that turns down the mTOR response. mTOR is responsible for signaling protein synthesis and the more protein synthesis the more muscle we can grow. If we can find a way to attenuate the blunting of mTOR we can see and improvement in all gains. When looking through the research it seems that concurrent training can be effective for a period of 6-8 weeks before seeing some negative effects.

Factors That Play a Role

There are a plethora of factors that can play a role in attenuating the interference effect:

  • the duration of the training
  • the modality
  • how the sessions are organized
  • the training status of the athlete.
Duration

Research shows that the longer the sessions are the more impactful it can be on blunting the strength training adaptations. It seems that anything over 20-30 minutes seems to be too much for the body to overcome. Again it is not so much the intensity of the endurance session but how long the last. This makes sense in terms of total volume. If duration is extended that means that total volume will become greater thus if a reduction in strength training volume does not happen to make up for the added volume and fatigue state begins to appear.

Modality

What form of endurance training seems to matter as well. The cycle ergometer seems to be less impactful then running. The reason for that is running creates more muscle damage and great perception of fatigue and effort than a bike does. Those two factors can lead to a greater amount of fatigue being built up.

Programming

Where the cardio is in accordance with the training session really matters. Research shows that if cardio is done first followed by strength training that the blunting effect of mTOR is not as strong. The downside is you go into the training session fatigued and won't get the most out of it. This strategy could be effective if the goal was strength or muscle maintenance while pushing for an endurance race. When the sessions are spread out for 3-6 hours wit the most important session going first is the best strategy for implementing concurrent training successfully. The further they are apart the better, along with the proper nutrition in place and thing are looking good.

Training Status

Whether or not an athlete is trained or untrained and what they are trained in matters. In untrained athletes it is a very favorable outcome and that is because untrained athletes pretty much adapt to anything and are not strong enough to cause really damage. if an athlete is endurance trained and very efficient at the economy of movement there will be less muscle damage and thus less fatigue. That along with having a huge ceiling to improve strength gains doesn't hurt them either.

Does the Interference Effect Matter?

In short, no, as long as these guidelines are met and the true understanding of fatigue management is put into place you will see progress. It has been shown time and time again in the research and that usually does not account for fatigue management and proper nutrition strategies. Understanding that volume for endurance and volume you from training all come from the same pool and if one goes up the other must go down will play the biggest role in success. Consider the strategies above, learn to autoregulate your training and know you don't have to be ultra intense to see progress in endurance. These are the biggest takeaways and why I have been able to have mild success in my adventures into concurrent training.

References:

Petré H, Hemmingsson E, Rosdahl H, Psilander N. Development of Maximal Dynamic Strength During Concurrent Resistance and Endurance Training in Untrained, Moderately Trained, and Trained Individuals: A Systematic Review and Meta-analysis. Sports Med. 2021 May;51(5):991-1010. doi: 10.1007/s40279-021-01426-9. Epub 2021 Mar 22. PMID: 33751469; PMCID: PMC8053170.