The interference effect, accurately
The classic 1980 Hickson study showed that adding intense cardio to a strength program reduced strength gains by ~35%. This single result spawned the "cardio kills gains" mantra. But the methodology was extreme: 6 days/week of 40-minute high-intensity intervals plus 5 days/week of strength work, far more than any normal trainee does.
Subsequent research has been more nuanced. The 2022 meta-analysis by Schumann et al. on concurrent training showed:
- Concurrent training does not reduce hypertrophy in most populations
- Strength gains are only meaningfully blunted when cardio exceeds 5 sessions/week or includes long high-intensity sessions
- Lower-body strength is more affected than upper-body
- Trained athletes are more affected than novices
For the average lifter doing 3-4 strength sessions and 2-3 cardio sessions per week, the interference effect is small to negligible.
Why combining them is usually better than either alone
Lifters who never do cardio:
- Have lower aerobic capacity (limits work capacity within sets and between sessions)
- Recover slower from training
- Have higher cardiovascular disease risk
- Often have higher resting heart rate and worse HRV
Cardio enthusiasts who never lift:
- Lose muscle as they age
- Have lower bone density
- Have worse metabolic health (less glucose disposal capacity)
- Get injured more often (weak connective tissue)
Combining both produces the best healthspan outcomes, by far. Research consistently shows the lowest all-cause mortality in people who do BOTH 150+ min of cardio and 2+ resistance sessions per week.
The three rules for combining them
1. Separate sessions by 6+ hours when possible
If you can lift in the morning and do cardio in the evening (or vice versa), do that. Six hours is enough recovery for the muscle protein synthesis window from lifting to be largely complete before cardio's molecular signals (AMPK activation) compete with it.
2. If same-day and same-session, lift first
Cardio first depletes glycogen and fatigues the nervous system, blunting strength performance for 4+ hours. Lifting first lets you train at full strength, then do cardio when fatigue won't compromise the strength stimulus.
3. Choose modality wisely
Different cardio creates different amounts of interference, ranked from least to most:
- Walking, hiking: almost zero interference, can be done daily
- Cycling, rowing: low-moderate interference (low impact, supports recovery)
- Swimming: low-moderate interference
- Running: highest interference (eccentric loading on legs competes directly with leg training)
If your goals are 70% physique and 30% endurance, lean cycling and rowing. If you love running, do it on lower-body off-days.
The principle: Modality interference scales with how much eccentric muscle damage and how much glycogen depletion the activity causes. Pick low-impact, lower-intensity cardio if you want to maximize muscle gain.
Zone 2 vs. HIIT
Zone 2 (conversational pace, ~60-70% of max heart rate) interferes less than HIIT and provides most of cardiovascular benefit:
- Builds mitochondrial density
- Improves fat oxidation
- Easier to recover from
- Compatible with more lifting volume
HIIT (intervals at 90%+ effort) is more time-efficient but creates more central fatigue. The current recommendation for most lifters: 2-3 zone 2 sessions of 30-60 minutes per week, plus optionally 1 HIIT session per week.
A sample week that works
- Mon: Lower body lift
- Tue: Upper body lift + 30 min zone 2 cycling (after lift or evening)
- Wed: 45 min zone 2 walk or hike (rest from lifting)
- Thu: Lower body lift
- Fri: Upper body lift + 30 min zone 2 (optional)
- Sat: 45-60 min zone 2 hike, swim, or bike
- Sun: rest or active recovery walk
This is 4 lifting sessions and 3-4 cardio sessions weekly. Total weekly cardio: 2-3 hours. Total weekly lifting: ~4 hours. Sustainable, balanced, and produces both physique and cardiovascular results.
Hormones and recovery
Concurrent training increases total recovery demand. If you're under-sleeping, under-eating protein, or have suboptimal hormones, the interference effect compounds. Common signals you've crossed your recovery line:
- Lifts stalling for 3+ weeks despite consistent effort
- Resting heart rate creeping up
- Sleep quality degrading
- Persistent low mood or low libido
- Frequent illness
If you've optimized sleep and nutrition and you're still struggling, a comprehensive lab panel often catches low T, low ferritin, or thyroid issues that are silently capping your recovery capacity.
The bottom line
The "cardio kills gains" myth is mostly outdated. For 90% of trainees, combining cardio and lifting produces better physique, health, and longevity outcomes than either in isolation. The keys are spacing, modality, and intensity discipline. Done right, your bench press, your VO2 max, and your 80-year-old self all benefit.