PHYS-19 · Principle
Will my running or riding hurt my strength gains?
A little, mostly for explosive power: in meta-analyses maximal strength and muscle size were largely spared, and the power loss sat mostly in same-session designs.
How the body works, agreed across the reviews below; a study entry shows what one experiment measured.
Practical Application
Will my running or riding hurt my strength gains? A little, mostly for explosive power: in meta-analyses maximal strength and muscle size were largely spared, and the power loss sat mostly in same-session designs.
What it does not mean: that lifting costs endurance (it improved economy and time trials in trained endurance athletes, REF-071; aerobic gains matched endurance-only training across 17 meta-analyses, Held et al. 2026), or that cycling is safe and running not (modality mattered in REF-512, not in REF-330).
Mechanism
In 21 studies, adding endurance work lowered pooled gains in size, strength and power, power alone significantly below strength-only; running and more frequent or longer endurance sessions went with more interference (REF-512).
In 43 trials of healthy untrained-to-active adults, maximal strength (SMD −0.06) and hypertrophy (−0.01) were not compromised and explosive strength was (−0.28), mostly in same-session designs (REF-330).
Narrower costs elsewhere: lower-body 1RM in trained, not untrained, people (ES −0.35, 27 studies, Petré et al. 2021); lower-body strength in men (−0.43), not women (59 studies, Huiberts et al. 2024); fibre size (SMD −0.23; type I fibres with running, not cycling; Lundberg et al. 2022).
Endurance signalling can inhibit protein-synthesis pathways in theory (REF-511), yet acute concurrent exercise has not shown such molecular interference in humans (Fyfe, Bishop & Stepto 2014); data in highly trained athletes are scarce (Held et al. 2026).
Common misreading
"Cardio kills gains"; "lifting makes an endurance athlete slow"; "cycling causes no interference".
Backs
SYNTH-21 "Interference exists, and it is smaller and narrower than the folklore." · SYNTH-21 "Do not talk an endurance athlete out of strength work on interference grounds." · SYNTH-31 "endurance-exercise AMPK activation can inhibit mTOR signalling to the protein-synthesis machinery" (proposed, not shown in humans after acute exercise)