Concurrent Training: When Cardio Holds Back Your Strength Gains, and When It Doesn't

Endurance doesn't kill your muscle. That's the short answer. The long one: hypertrophy (growth of muscle cross-section) and maximal strength survive parallel cardio almost untouched, while jumping power and sprint speed take a measurable hit. How big the loss is depends on the modality, the frequency, the session order, and how trained you already are.

What "concurrent training" actually means

The term comes from sports science and refers to something specific: resistance training AND endurance training within the same microcycle, often within the same day or even the same session. Three tiers are common.

• Same session, two stimuli back to back (e.g. 45 min legs, then 30 min bike). Highest interference risk.
• Same day, several hours apart (legs in the morning, bike in the evening). Moderate risk.
• Different days within the same week (Monday legs, Tuesday bike). Almost no interference.

Most recreational lifters are factually doing tier 2 or 3 while worrying about tier 1. That's the first reassurance.

Hickson 1980: how the whole topic started

Hickson 1980 had three groups train for 10 weeks: strength only, endurance only, or both with about two hours between stimuli. Through week seven, strength and VO₂max (maximal oxygen uptake in ml/kg/min) climbed together in the concurrent group. Then something odd happened: strength gains stalled and slipped slightly while aerobic capacity kept rising. The observation was named the interference effect and has been argued over ever since.

Worth flagging: Hickson's protocol was extreme. 6 days of endurance per week, parts of it 40-minute intervals, plus 5 days of heavy strength work. People who run an hour five times a week while pushing heavy squats are a rare species. Most hobby programs are much tamer.

What 43 newer studies turned the picture into

The most current full review is the meta-analysis by Schumann et al. 2022 in Sports Medicine. 43 studies, three outcome categories (maximal strength, explosive strength, hypertrophy), comparing concurrent training to pure strength training.

That's the central dividing line. If you want to build muscle or get stronger, you can integrate cardio well. If you're a volleyball player, sprinter, or combat athlete depending on explosive output, you face a measurable conflict.

What really slows you down: modality, frequency, training status

Wilson et al. 2012 in the Journal of Strength and Conditioning Research analyzed 21 studies with 422 effect sizes and could pinpoint which variables interfere most. Effect size (ES) for hypertrophy: 0.85 for concurrent versus 1.23 for strength only. The loss is real but moderate.

Three findings stood out.

Running interferes, cycling barely does

Running significantly reduced strength and hypertrophy gains. Cycling didn't. The most likely explanation: running is eccentrically demanding for an untrained leg, producing more local muscle damage that competes with the strength stimulus.

Frequency and duration scale the effect

Correlational analysis showed negative relationships between frequency of endurance training and strength gain (r = -0.26 to -0.35), and between session duration and strength gain (r = -0.29 to -0.75). Rule of thumb: two cardio sessions per week of 30 to 45 minutes are nearly cost-free, five sessions of 60 minutes start to bite.

Beginners don't have the problem

Petré et al. 2021 showed in a meta-analysis of 27 studies on 1RM (one-repetition maximum, the heaviest weight you can lift once) for leg press and squat: trained athletes lost mildly (ES = -0.35, p < 0.01), moderately trained tendentially (ES = -0.20, p = 0.08), untrained not at all (ES = 0.03, p = 0.87). If you start at zero, you can build both at the same time.

Session order and the spacing between sessions

If you're doing both on the same day: lift first, run second. Eddens, van Someren, and Howatson 2018 examined intra-session order in a meta-analysis. For hypertrophy and maximal strength they found no significant sequence difference, but Petré et al. 2021 reported that strength-before-endurance produced about 7 percent better dynamic leg strength than the reverse.

Things get cleaner with spacing. When sessions are at least 6 hours apart, the literature barely registers interference effects on 1RM values or jump performance. The mechanism runs through molecular signaling: mTOR (mechanistic target of rapamycin, the central growth sensor in the muscle cell) stays elevated for about 18 hours after lifting. AMPK, the energy sensor activated by endurance work, suppresses mTOR. Six hours of spacing gives both signals enough room to not directly compete in the same window.

What you can actually do

Three common programming mistakes

1. Stacking both hard stimuli into one session because training days are tight. Even moving them three or four hours apart helps if a 6-hour gap isn't possible. 2. Defaulting to running when a bike or rower delivers the same aerobic effect with less muscle damage. If running isn't the goal in itself, cycling or rowing is the better mixer. 3. Letting cardio volume creep up because you tacked a half-marathon goal on top of a strength block. That's where the dose (more than three hours of running per week) collides head-on with the Petré findings for trained lifters.

What the studies don't answer

Most data comes from 8 to 16 week programs with male subjects aged 20 to 30. For older adults, perimenopausal women, and multi-year applications, evidence at the same quality level is missing. The boundary between recreational and competitive athlete is also fuzzy: Petré classifies "trained" versus "untrained" through years of lifting and 1RM values, not VO₂max. Someone with three years at the rack but zero cardio history doesn't fit cleanly into any of the three buckets.

In short: this doesn't apply the same way to every goal. What stays solid: for hypertrophy you can integrate cardio almost freely, for explosive power you need careful dosing, and the cheapest win is almost always switching from running to a bike or rower when the cardio is just there to serve the heart.