You've been told that cardio kills gains. That is not exactly wrong — but it is not exactly right either.

The concurrent training interference effect is one of the most cited and most misapplied concepts in performance fitness. Athletes hear it and draw the wrong conclusion: that conditioning and strength cannot coexist. The research says something more specific, and more useful, than that. It says how you combine them is what determines whether you get both adaptations or sacrifice one for the other.

At No Tomorrow Athletics, we program strength and conditioning together every week. That is the No Tomorrow Method. Understanding the interference effect is exactly why we do it the way we do.

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What the Interference Effect Actually Is

In 1980, Robert Hickson published a foundational study showing that athletes who trained for both strength and endurance simultaneously made smaller strength gains than those who trained for strength alone¹. That paper gave this phenomenon its name. It also launched decades of research trying to understand why.

The leading explanation is molecular. Endurance training activates AMPK, an energy-sensing enzyme that signals adaptation to aerobic stress. Resistance training activates mTOR, the primary driver of muscle protein synthesis and hypertrophy. The problem: AMPK suppresses mTOR². When both pathways are triggered in close proximity, they compete. Strength adaptation is typically the one that loses.

That is the mechanism. It is real. But mechanism is not the whole story.

Understanding where each session lands in the supercompensation cycle is what determines whether concurrent training accumulates fitness or accumulates fatigue.

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What Most Athletes Get Wrong

The interference effect is not a flat rule. It is a conditional one. The research shows it is highly sensitive to three variables: modality, timing, and volume.

Modality

Not all cardio creates equal interference. A 2021 meta-analysis in the Journal of Strength and Conditioning Research found that running produces significantly greater interference with lower body strength and hypertrophy than cycling does — likely because running involves a higher eccentric load and greater muscle damage³. Rowing and ski erg work show similar low-interference profiles to cycling.

This is not a minor distinction. If your conditioning is all running, you are creating more interference than necessary. If you are using mixed modalities — bike, rower, ski erg, and structured running — you are managing the signal more intelligently.

Timing

Session proximity matters. A 2022 review in Sports Medicine found that performing endurance work within three hours of a resistance session produced measurable suppression of anabolic signaling⁴. Separating the sessions by six or more hours significantly reduced that suppression. Training them on separate days produced the least interference of all.

The practical implication: when you must train twice in a day, lift first. Do your conditioning later. Your mTOR pathway gets its window without competition.

Volume

This is where most athletes actually go wrong — not by doing cardio, but by doing too much of it. High-volume endurance training creates systemic fatigue that compounds across a training week. The interference is not just molecular at that point. It is structural: you are arriving at your strength sessions pre-fatigued, under-recovered, and unable to produce the mechanical tension needed to drive adaptation⁵.

The solution is not less conditioning. It is calibrated conditioning — enough to build the aerobic and metabolic capacity you need, not so much that it cannibalizes your strength work.

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Why This Matters for Hyrox and CrossFit Athletes

If you compete in Hyrox or CrossFit, you do not have the option of training only one quality. You need power output, local muscular endurance, aerobic capacity, and functional strength. These are not optional trade-offs. They are all requirements.

The interference effect is not an argument against conditioning. It is an argument for programming it correctly.

Hyrox athletes in particular face a programming challenge that pure powerlifters or pure runners never have to solve: the race demands that you be strong and fit at the same time, in the same body, on the same day. If your training treats strength and conditioning as adversaries, you will show up to the start line with a gap in one or the other.

Research on concurrent training in functional fitness populations confirms that well-structured programs — those that sequence sessions intentionally, control weekly conditioning volume, and prioritize recovery — produce meaningful gains in both maximal strength and aerobic capacity simultaneously⁶. The key word is structured.

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How the No Tomorrow Method Addresses It

The three pillars of the No Tomorrow Method — Strength, Conditioning, and Mobility — are not independent tracks. They are designed to interact without undermining each other.

In practice, that means a few things:

• Strength sessions are protected. They are not programmed immediately after high-volume conditioning. The sequencing is deliberate.
• ‍Conditioning modality is varied. We use bike, rower, ski erg, and running — not because variety is a value in itself, but because different modalities create different interference profiles, and rotating them reduces cumulative damage to the same muscle groups.
• Volume is tracked and periodized. Conditioning volume is not arbitrary. It scales with the training block — higher during conditioning-priority phases, lower when we are chasing strength adaptations.
• Mobility is not optional. Tissue quality and range of motion directly affect how much mechanical tension you can produce in a strength session. An athlete who cannot sit into a clean rack position is leaving strength on the platform before they ever touch the barbell.

This is not a program built around avoiding the interference effect. It is a program built around understanding it well enough to use it.

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The Practical Takeaway

If your conditioning is killing your strength gains, the answer is rarely to stop conditioning. It is to look at what you are doing, when you are doing it, and how much you are asking your body to absorb. Strength remains the foundation regardless of how much conditioning you layer on top — and that principle doesn't change as athletes age. HRV tracking gives you a daily readout of whether your concurrent training balance is producing adaptation or burying it.

The concurrent training interference effect is real. It is also manageable. Athletes who train at No Tomorrow Athletics do not have to choose between being strong and being fit. That is the whole point of how this program is built.

If you want to see what that looks like in practice, come train with us.

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Sources

1. Hickson RC. Interference of Strength Development by Simultaneously Training for Strength and Endurance. European Journal of Applied Physiology, 1980.
2. Fyfe JJ, Bishop DJ, Stepto NK. Interference Between Concurrent Resistance and Endurance Exercise: Molecular Bases and the Role of Individual Training Variables. Sports Medicine, 2014.
3. Wilson JM, Marin PJ, Rhea MR, et al. Concurrent Training: A Meta-Analysis Examining Interference of Aerobic and Resistance Exercises. Journal of Strength and Conditioning Research, 2012.
4. Methenitis S. A Brief Review on Concurrent Training: From Laboratory to the Field. Sports, 2018.
5. Murach KA, Bagley JR. Skeletal Muscle Hypertrophy with Concurrent Exercise Training: Contrary Evidence for an Interference Effect. Sports Medicine, 2016.
6. Eddens L, van Someren K, Howatson G. The Role of Intra-Session Exercise Sequence in the Interference Effect: A Systematic Review with Meta-Analysis. Sports Medicine, 2018.