The Interference Effect: Does Endurance Training Blunt Strength Gains?
Quick Intro
The interference effect is the idea that training for endurance and training for strength at the same time can reduce gains in one or both. It was first described by the researcher Robert Hickson in 1980 and has been discussed ever since. This article covers the theory and the biology, and the concurrent training article covers how to schedule both in practice.
The Idea in Plain Terms
Hickson’s study had participants train for strength alone, endurance alone or both together. After ten weeks, the combined group gained strength more slowly than the strength-only group, and the difference grew toward the end of the study. That result raised the possibility that the two types of adaptation compete.
Explanations include:
- Molecular signaling. Endurance exercise activates a cellular energy sensor called AMPK, which prepares cells to make more energy and tends to slow energy-expensive processes. Muscle building depends on a pathway called mTORC1. In laboratory experiments, activating AMPK can dampen mTORC1 signaling. Keith Baar summarized this idea in a 2006 review.
- Fatigue. After a long run or ride, the muscles are tired, which can reduce the quality of the next strength session.
- Energy and recovery. A large amount of endurance work uses energy and time that would otherwise support muscle repair.
- Overtraining. Very high total volume may exceed recovery.
The signaling story is attractive and has an obvious weakness: it comes from cell experiments and short studies of molecular markers, and it is not clear that these shifts translate into a measurable loss of muscle over months of training.
A Worked Example
Two athletes combine training in different ways:
| Athlete | Weekly training | Likely interference |
|---|---|---|
| A | 3 strength sessions, 2 easy 30-minute cycles | Small, likely none for size or strength |
| B | 3 strength sessions, 6 hard runs of 60 minutes | Greater, because endurance volume is high and recovery limited |
The amount, intensity and type of endurance work all influence the outcome.
What the Research Shows
Hickson’s 1980 study in the European Journal of Applied Physiology started the discussion. A 2012 meta-analysis by Wilson and colleagues found that endurance frequency and duration were associated with smaller gains in size, strength and power, with running appearing worse than cycling. A larger 2022 meta-analysis by Schumann and colleagues of 43 trials found no interference with gains in maximal strength or muscle size, whatever the type or frequency of aerobic exercise, with the open question being explosive power.
The reconciliation is probably that interference is real in some conditions, such as very high endurance volume, high-intensity endurance work and insufficient recovery or food, and small or absent in typical recreational programs. The molecular findings help explain what could happen and do not predict how large the effect is.
Who It Suits
This concept helps:
- Lifters who worry that cardio will cost muscle.
- Athletes combining strength and endurance.
- Coaches balancing training types.
How to Use It
- Keep most endurance work easy to moderate if strength is the priority.
- Separate hard strength and endurance sessions by hours or days.
- Eat enough to support both.
- Prioritize the quality that matters most.
- Watch recovery and adjust.
- Do not avoid cardio out of fear.
Common Misunderstandings
It is not that cardio destroys muscle.
It is not proven to matter for most recreational lifters.
It is not a reason to skip health-supporting aerobic exercise.
FAQ
Is AMPK bad for muscle? It is a normal signal and part of adaptation.
Does running interfere more than cycling? Some studies suggest so, and others do not.
Does it affect power? Possibly, and the evidence is less settled.
How much cardio is safe for gains? Moderate amounts are compatible with progress.
Where This Comes From
Hickson RC, 1980, interference of strength development by simultaneously training for strength and endurance (European Journal of Applied Physiology, 45, 255 to 263). Baar K, 2006, training for endurance and strength: lessons from cell signaling (Medicine and Science in Sports and Exercise, 38, 1939 to 1944). Wilson JM and colleagues, 2012 (Journal of Strength and Conditioning Research). Schumann M and colleagues, 2022 (Sports Medicine, 52, 601 to 612).
Related to: concurrent training, body recomposition, HIIT and MICT.