The interference effect: what the research actually shows

The interference effect is the observation that training strength and endurance at the same time produces smaller strength gains than training strength alone. It is not folklore: it was measured directly, in a controlled trial, in 1980, and it has been replicated and refined since. What has changed since 1980 is the size of the effect researchers now report, and how one-directional it turns out to be.

Where the finding comes from

The original study, by Robert Hickson in 1980, put separate groups through 10 weeks of strength-only, endurance-only, or combined training. The strength-only group added roughly 44% to their squat 1RM. The combined group added roughly 22%, about half as much, despite doing the identical strength protocol. Endurance measures (VO2max) improved similarly in the endurance-only and combined groups. That asymmetry, endurance capacity barely dented, strength capacity cut roughly in half, is the finding that launched four decades of follow-up research.

What later meta-analyses revised

Hickson's single study used a small sample and a specific protocol (six days per week of combined training, a demanding volume by any standard). Wilson and colleagues' 2012 meta-analysis, pooling many subsequent trials, reported a more moderate picture:

• Endurance adaptation: negligibly affected by added strength work in the pooled data.
• Maximal strength: reduced by an effect size the authors describe as small-to-moderate when meaningful endurance volume is added.
• Hypertrophy: the biggest hit, a larger effect size than for pure strength, meaning muscle growth is more sensitive to interference than 1RM strength is.
• Power output (jump and sprint-type measures) showed the most interference of the strength-adjacent qualities, particularly when the endurance work was high-volume running.

A 2022 systematic review and meta-analysis by Schumann and colleagues, focused specifically on hypertrophy and muscle function, reached a similar conclusion: the interference on muscle size is real but modest, and highly dependent on how the two training modes are arranged in the week, not just how much of each you do.

What moderates the size of the effect

Three variables show up consistently across the literature as changing how much interference an athlete actually experiences:

• Endurance volume: the dose-response relationship is real. Two low-intensity endurance sessions per week interferes little; five to six sessions, especially at higher intensity, interferes measurably more.
• Session proximity: same-day sessions separated by several hours show less interference than sessions performed back-to-back; different-day sessions show the least.
• Training status: untrained and recreationally trained individuals show smaller relative interference than highly trained specialists in either strength or endurance, per a 2024 review of concurrent-training moderators (Wang & Bo), likely because there's more "free" adaptation available to a less-trained system regardless of what else it's doing.

What this means in practice

A hybrid athlete training both qualities seriously should expect measurably slower strength and hypertrophy progress than a dedicated strength athlete doing the identical lifting program, and should expect that cost to scale with how much endurance volume gets added on top. That is a real, replicated finding, not a training myth. What the research does not support is the stronger claim sometimes made in fitness culture, that combined training produces no meaningful progress in either quality. The pooled effect sizes in the modern literature are described as small-to-moderate, not as a wash.

What remains genuinely unsettled

Mechanistic explanations for the interference effect (competing AMPK/mTOR signaling pathways, acute fatigue masking chronic adaptation, or simply a finite recovery budget) are still debated in the exercise-physiology literature, and different studies emphasize different mechanisms. This piece describes the measured outcomes the field agrees on; it does not adjudicate between the competing mechanistic models, which is a live research question rather than a settled one.

FAQ

Does lifting weights hurt your running, or does running hurt your lifting?

The effect runs mostly one direction. Added strength work has little measurable effect on endurance adaptation, while adding substantial endurance volume measurably reduces strength and hypertrophy gains. Pooled analyses describe the reduction as small-to-moderate, with hypertrophy and power measures generally showing more interference than maximal strength.

Can you eliminate the interference effect entirely?

No study shows a way to fully eliminate it while training both qualities seriously. Time-separating sessions by several hours, or training them on different days, reduces the measured interference compared to back-to-back sessions, but does not remove it.

Is the interference effect the same for everyone?

No. Untrained and moderately trained individuals show smaller relative interference than highly trained strength or endurance specialists, and endurance volume is the biggest moderating factor: two easy sessions per week interferes far less than six.