Concurrent Training and the Interference Effect: What Meta-Analyses Say About Combining Strength and Cardio

Where the fear of combining cardio and strength came from

In 1980, Robert Hickson published a study in which the group combining cardio and strength training achieved smaller strength gains than the strength-only group. This gave birth to the term "interference effect" — the idea that aerobic training impairs hypertrophy and strength development. The following decades filled with cautious recommendations to keep "running" and "iron" separate.

Between 2011 and 2022, several meta-analyses examined the accumulated literature with rigorous methodology. The conclusion turned out to be surprisingly reassuring: the concerns were only partially warranted.

Hypertrophy and maximal strength: evidence from 43 studies

The meta-analysis by Schumann, Feuerbacher, Sünkeler, Freitag, Rønnestad, Doma and Lundberg (Sports Medicine, 2022) covered 43 studies and compared concurrent training with strength-only training. Key results:

• Hypertrophy (muscle volume by MRI/ultrasound): SMD = −0.01 (95% CI −0.16 to 0.18; p = 0.919) — no significant interference.
• Maximal strength (1RM, isometric tests): SMD = −0.06 (95% CI −0.20 to 0.09; p = 0.446) — similarly non-significant.
• Explosive strength (countermovement jumps, sprint): SMD = −0.28 (95% CI −0.48 to −0.08; p = 0.007) — a small but statistically significant deficit.

The conclusion is unequivocal: if the goal is muscle growth or maximal strength, cardio does not statistically interfere. Explosive strength is a separate matter.

Explosive strength: the only vulnerable target

Explosive strength (power, speed-strength qualities) depends on neuromuscular coordination and fast-twitch type II fibers, which are sensitive to accumulated fatigue and competing adaptation signals. This is where concurrent training creates a real limitation.

The subgroup analysis by Schumann et al. 2022 detailed the conditions:

• Cardio and strength in the same session: explosive strength SMD = −0.31 (significant).
• Session gap of ≥3 hours: significance disappears.

For most people training for hypertrophy or general strength, this nuance is of little practical importance. For jumpers, sprinters and weightlifters — it is critical.

Why running is worse than cycling for concurrent training

A separate meta-analysis by Lundberg, Feuerbacher, Sünkeler and Schumann (Sports Medicine, 2022) focused on muscle fiber hypertrophy from biopsy data across 15 studies. The overall effect on fiber size: SMD = −0.23 (95% CI −0.46 to 0.00) — small and on the border of statistical significance.

The key finding was in the subgroup analysis by cardio modality:

• With running: type I fiber hypertrophy — SMD = −0.81 (significant reduction).
• With cycling: no significant reduction detected.

The authors attribute this to the eccentric loading and stretch-shortening cycle in running: they create additional inflammatory stress in muscle fibers that competes with hypertrophy signals. Cycling is predominantly concentric and does not produce this irritation.

An important caveat: reduction at the individual fiber level does not necessarily translate into a loss of total muscle volume measured by MRI — which is consistent with the null SMD in Schumann 2022 for whole muscle.