Many people combine strength and endurance training. You want to be strong, capable and resilient – often within the same sessions, for reasons of time as well. Yet some experience their muscle building stagnating, even though they train regularly. This is exactly where a concept comes in that is referred to in training science as interference.
Interference describes how certain training stimuli can affect one another. This becomes especially relevant when the same muscle is exposed within a short period to both intense strength training and intense endurance training. This does not mean that combined training is fundamentally bad – rather, that the training goals should be clear.
What interference in training means
The term interference comes from English and means something like impairment. In the training context it refers to the impairment of muscle protein synthesis – that is, the process by which the body forms new muscle proteins and builds muscle tissue.
In concrete terms this means: if a target muscle experiences both a strong strength stimulus and a strong endurance stimulus within minutes or a few hours, the muscle may indeed become functionally more resilient, but builds less new muscle mass than with pure strength training.
Why strength and endurance training can affect each other
Strength training pursues a clear goal: the muscle should become stronger and more resistant. To achieve this, the body responds with increased production of muscle protein. This muscle protein synthesis is a central prerequisite for muscle growth.
Intense endurance training, however, sets a different focus. It creates high energy stress. The body is forced to provide energy efficiently and to conserve it. For this, different molecular signalling pathways are activated than in classic muscle building.
When these two stimuli occur very close together in time, the energy stress from endurance training can slow down the very processes that would actually be responsible for muscle building. It is precisely this overlap that is called the interference effect.
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What this means for strength development
A clear distinction is important: interference primarily concerns muscle building, not necessarily strength capacity.
Strength arises not only from muscle mass, but also from neural adaptations. This refers to the nervous system's ability to control muscles efficiently, to activate more muscle fibres simultaneously and to build tension better. This neural component of strength is barely impaired by combined strength and endurance training.
In practical terms this means: you can become stronger, more stable and more capable, without necessarily building visibly more muscle mass. This is precisely where the benefit of interference-oriented training lies – when the goal is not primarily hypertrophy.
What interference-oriented strength training can look like
In strength training, the target muscle is first deliberately placed under tension. Often 6 to 10 repetitions are performed at a deliberately slow, controlled tempo. This is less about maximum weights and more about a long duration of tension.
When lifting the weight, the muscle shortens under tension – this is the so-called concentric phase. In the end position the weight is held briefly, the muscle stays contracted without changing its length. This holding work is called isometric. During the subsequent lowering, the muscle works to brake and lengthens in the process – an eccentric phase. In the stretched position too, the tension can be held briefly before the next repetition begins.
Through this interplay of shortening, holding, lengthening and holding again, a tension duration of about 60 to 100 seconds arises. The muscle is strongly challenged mechanically, without extreme loads being necessary.
The second stimulus: targeted energy stress
Following the strength training – or very close to it in time – the same muscle is exposed to an intense endurance stimulus. Particularly suitable are high-intensity interval formats, in which the energy stress is deliberately high.
This energy stress activates adaptations that improve metabolic capacity, but at the same time can slow the building of new muscle protein. This is exactly where the interference effect arises: functional performance increases, but muscle building remains limited.
Who this form of training can make sense for
An interference-oriented training approach is not a mistake, but a targeted tool. It can be especially useful for people who:
- want to develop strength without significantly gaining muscle mass
- want to stay functionally capable
- have to combine strength and endurance demands (e.g. team sports)
- have little training time available
- value resilience over maximum muscle gain
For classic muscle building with a focus on maximum hypertrophy, however, this form of training is less suitable.
Conclusion – using interference consciously rather than avoiding it
Interference does not mean that strength and endurance training cannot be combined. It means that different training stimuli trigger different adaptations – and partly affect each other.
Those who prioritise muscle building should separate intense strength and endurance training in time. Those who, on the other hand, want to combine strength, stability and metabolic capacity can benefit specifically from interference effects.
What is decisive is not the form of training itself, but clarity about the training goal.
