The concept of the “interference effect” has persisted in strength and conditioning discourse for over four decades, yet its interpretation remains overly reductionist in many applied settings. Originally demonstrated by Robert C. Hickson (1980), the effect is often summarised as endurance training impairing strength development. While directionally correct, this interpretation fails to capture the complexity of concurrent training adaptations.
A more accurate framing is that interference is not an inevitable consequence of combining modalities, but rather a context-dependent outcome shaped by programming variables. The interaction between endurance training intensity and volume has emerged as particularly important. Contemporary literature suggests that training volume is the primary driver of interference, while intensity may be manipulated to mitigate its effects though this position is not without limitation.
Mechanistic considerations: competing signals or compatible systems?
At the molecular level, the interference effect is commonly explained through competing intracellular signalling pathways. Resistance training stimulates the mechanistic target of rapamycin complex 1 (mTORC1), promoting muscle protein synthesis and hypertrophy. In contrast, endurance training activates AMP-activated protein kinase (AMPK) and peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α), facilitating mitochondrial biogenesis and metabolic adaptations.
The prevailing hypothesis suggests that AMPK activation may inhibit mTORC1 signalling, thereby attenuating anabolic responses when endurance exercise is performed in close proximity to resistance training. However, this model is incomplete. Acute molecular responses do not consistently predict long-term adaptations, and importantly, the magnitude and duration of AMPK activation are influenced by the nature of the endurance stimulus, particularly its duration and metabolic cost.
This provides a mechanistic basis for distinguishing between endurance training modalities, rather than treating all aerobic work as equally disruptive.
At a molecular level, the interference effect is often explained through the interaction of key signalling pathways that regulate adaptation to training. Resistance training primarily activates the mTOR (mechanistic target of rapamycin) pathway, which plays a central role in muscle protein synthesis and the development of strength and hypertrophy.
In contrast, endurance training activates pathways such as AMPK (AMP-activated protein kinase) and PGC-1α, which are associated with energy regulation, mitochondrial biogenesis, and improvements in aerobic capacity.
The traditional explanation is that AMPK activation, particularly during prolonged or energetically demanding exercise can inhibit mTOR signalling. In theory, this creates a conflict: one pathway promotes muscle growth, while the other prioritises energy efficiency. When endurance training is performed in close proximity to resistance training, this has been proposed as a mechanism for reduced anabolic adaptation.
However, this explanation is incomplete. Acute changes in signalling do not reliably predict long-term training outcomes and the degree of AMPK activation is highly dependent on the type of endurance work performed. Longer-duration, high-volume training tends to produce a more sustained metabolic stress, whereas shorter, high-intensity efforts may result in a different signalling profile.
This distinction is critical. It suggests that interference is not simply a consequence of combining strength and endurance training, but rather a function of how endurance training is structured, particularly in terms of its volume and metabolic demand.
Re-examining the evidence: what actually interferes?
Modern systematic reviews and meta-analyses have clarified that interference is outcome-specific. While early work suggested substantial impairments in strength development, more recent analyses indicate that maximal strength and hypertrophy are often only minimally affected, particularly in trained populations.
In contrast, explosive strength and rate of force development (RFD) appear more susceptible to impairment under concurrent training conditions. This is likely due to the sensitivity of neuromuscular performance to fatigue and residual training stress, rather than purely molecular interference.
Crucially, when interference is observed, it is strongly associated with specific training characteristics, including:
- High total endurance volume
- High session frequency
- Short recovery intervals between modalities
- Mechanically demanding modalities such as running
These findings shift the discussion away from modality conflict per se, towards the structure and organisation of training.
The role of volume: a consistent driver of interference
Across the literature, endurance training volume emerges as the most consistent predictor of interference. High-volume, moderate-intensity endurance training imposes sustained metabolic stress, prolongs AMPK activation, and generates significant peripheral fatigue.
High training volumes also increase cumulative recovery demands, particularly when combined with resistance training targeting similar musculature. This is especially relevant in running-based endurance training, where eccentric loading contributes to muscle damage and prolonged recovery.
From a programming perspective, this suggests that weekly endurance volume is a primary variable governing the extent of interference, particularly when strength and hypertrophy are key outcomes.
The case for intensity: a more compatible stimulus?
In contrast, low-volume, high-intensity interval training presents a distinct physiological profile. Despite inducing large acute perturbations, these sessions are typically brief and do not sustain metabolic stress to the same degree as prolonged aerobic work.
This distinction has several important implications.
First, higher intensity intervals can elicit central cardiovascular adaptations comparable to traditional endurance training, including improvements in maximal oxygen uptake (VO₂max), with substantially lower training volume. Second, the reduced duration of metabolic stress may limit prolonged AMPK activation, allowing anabolic signalling processes to proceed with less interference. Third, lower overall training volume reduces cumulative fatigue, preserving neuromuscular performance.
Empirical evidence supports this distinction. Studies examining concurrent training models suggest that interval based endurance training results in similar or reduced interference with strength and hypertrophy compared to moderate-intensity continuous training, particularly when total training volume is controlled.
This has led to the increasingly adopted view that intensity is a more manageable variable than volume when attempting to minimise interference.
The role of the resistance training stimulus.
An important consideration often overlooked in discussions of concurrent training is the quality of the resistance training stimulus itself. The interference effect is only meaningful when there is a meaningful strength or hypertrophy adaptation to interfere with.
If resistance training is performed at low intensities or with insufficient effort, the stimulus for adaptation is already limited. Under these conditions, any potential interference from endurance training becomes largely irrelevant, as strength development is constrained by the quality of the resistance training rather than the presence of concurrent work.
This reinforces a key principle: concurrent training does not eliminate the need for adequate loading and intent within resistance training. When strength is a priority, training must still be performed with sufficient intensity and proximity to failure to drive adaptation. Only then does the interaction with endurance training become a factor worth managing.
Limitations of the “intensity over volume” perspective
Despite its practical appeal, the argument that intensity is inherently superior to volume is not universally supported.
Firstly, when total work is equated, some studies report similar interference effects between high-intensity interval training and moderate-intensity continuous training. This suggests that total training load and accumulated fatigue, rather than intensity alone, may be the primary mediators.
Secondly, the ecological validity of reducing endurance volume must be considered. For endurance-dominant athletes, high training volumes are essential for developing peripheral adaptations such as mitochondrial density, capillarisation, and movement economy. Reducing volume in favour of intensity may therefore compromise sport-specific performance.
Thirdly, high-intensity training itself imposes significant autonomic and neuromuscular stress. When performed frequently, it may impair recovery and contribute to interference through alternative mechanisms, including central fatigue.
Taken together, these points suggest that while intensity is a valuable tool, it cannot be considered a universal solution.
A more useful framework: interference as a programming problem
The evidence supports a reframing of the interference effect as a programming issue rather than a fixed physiological constraint.
Key principles include:
- Interference is minimised when endurance volume is controlled
- High-intensity, low-volume endurance can be used strategically
- Resistance training should be prioritised when strength is the primary goal
- Temporal separation between modalities reduces acute interaction effects.
- Modality selection influences mechanical fatigue and recovery
Importantly, the significance of interference is entirely dependent on the athlete’s performance priorities. For strength and power-dominant athletes, even small decrements may be meaningful. In contrast, for tactical or hybrid populations, the ability to concurrently develop multiple qualities may outweigh minor compromises.
Key takeaways:
The interference effect is neither a myth nor an unavoidable limitation. It is a manageable consequence of concurrent training, governed by the interaction of volume, intensity, frequency, modality, and recovery.
While current evidence supports the strategic use of high-intensity, low-volume endurance training to minimise interference, volume remains the most consistent driver of impaired strength and hypertrophy adaptations. However, intensity alone does not eliminate interference, and its application must be aligned with the specific demands of the athlete.
Ultimately, effective concurrent training requires a shift in perspective from avoiding interference to deliberately managing trade-offs within a structured training system.
Reference List
Fyfe, J.J., Bishop, D.J. and Stepto, N.K. (2014) ‘Interference between concurrent resistance and endurance exercise: molecular bases and the role of individual training variables’, Sports Medicine, 44(6), pp. 743–762.
Fyfe, J.J., Bartlett, J.D., Hanson, E.D. and Stepto, N.K. (2016) ‘Endurance training intensity does not mediate interference to maximal lower-body strength gain during short-term concurrent training’, Frontiers in Physiology, 7, p. 487.
Hickson, R.C. (1980) ‘Interference of strength development by simultaneously training for strength and endurance’, European Journal of Applied Physiology, 45(2–3), pp. 255–263.
MacInnis, M.J. and Gibala, M.J. (2017) ‘Physiological adaptations to interval training and the role of exercise intensity’, The Journal of Physiology, 595(9), pp. 2915–2930.
Methenitis, S. (2018) ‘A brief review on concurrent training: from laboratory to the field’, Sports, 6(4), p. 127.
Petré, H., Hemmingsson, E., Rosdahl, H. and Psilander, N. (2018) ‘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 Medicine, 48(8), pp. 1799–1812.
Schumann, M., Feuerbacher, J.F., Sünkel, M., Freitag, N. and Rønnestad, B.R. (2022) ‘Compatibility of concurrent aerobic and strength training for skeletal muscle size and function: an updated systematic review and meta-analysis’, Sports Medicine, 52(3), pp. 601–612.
Seiler, S. (2010) ‘What is best practice for training intensity and duration distribution in endurance athletes?’, International Journal of Sports Physiology and Performance, 5(3), pp. 276–291.
Vikmoen, O., Raastad, T., Seynnes, O., Bergstrom, K. and Ellefsen, S. (2016) ‘Effects of endurance training intensity on strength development in concurrent training’, European Journal of Applied Physiology, 116(10), pp. 1933–1943.
Wilson, J.M., Marin, P.J., Rhea, M.R., Wilson, S.M.C., Loenneke, J.P. and Anderson, J.C. (2012) ‘Concurrent training: a meta-analysis examining interference of aerobic and resistance exercises’, Journal of Strength and Conditioning Research, 26(8), pp. 2293–2307.
Will Strathdee 
Leave a comment