You want to build muscle through strength training. You also want cardiovascular fitness and maybe some fat loss. But you’re confused: will doing cardio ruin your strength gains? Should you do them on the same day or separate days? Cardio before weights or after?
Here’s what usually happens: you Google “cardio and strength training” and find terrifying warnings about the “interference effect”—how cardio “kills your gains.” You’re told to never mix them, or you’ll lose all your muscle.
The frustrating truth: research shows moderate cardio does NOT significantly interfere with strength gains for most people—especially beginners and intermediates. The “interference effect” has been massively overstated.
I’m Will Duru, a personal trainer with over 10 years’ experience in London. I’ve programmed combined cardio and strength training for hundreds of women, and I can tell you from real-world experience: when done properly, both improve simultaneously—you don’t have to choose.
My observation from 10+ years: Women who do 2-3 cardio sessions weekly (20-30 minutes moderate intensity) alongside their strength training build muscle and strength just as effectively as those doing strength training only. The ones who struggle? Those doing 60-90 minute intense cardio sessions 5-6 days weekly whilst trying to lift heavy—that’s genuine overtraining, not the interference effect.
This guide explains what the interference effect actually is, what current research says (spoiler: it’s less scary than claimed), how to combine cardio and strength training effectively based on my decade of client experience, when to do cardio vs when to skip it, and how 12REPS balances both.
What Is the "Interference Effect"?
The interference effect is the theory that combining endurance training (cardio) with strength training reduces strength, power, and muscle gains compared to strength training alone.
The fear: Cardio interferes with muscle recovery and growth, making your strength training less effective.
Where this comes from: A famous 1980 study by Hickson found that participants who did both strength training (5 days weekly) AND endurance training (6 days weekly)—11 total sessions weekly—saw reduced strength gains after week 7 compared to strength training only.
Why this scared people: It suggested cardio fundamentally conflicted with building muscle and strength.
The problem: That study used extreme volumes (11 sessions weekly) that most people would never attempt. Modern research paints a very different picture.
Will’s insight: In 10+ years, I’ve never had a client training 11 sessions weekly. Most women train 3-4 days total (strength + cardio combined). The classic interference effect study doesn’t reflect real-world training scenarios.
What Current Research Actually Shows
2022 Meta-Analysis: No Significant Interference for Most People
A 2022 systematic review and meta-analysis examined 43 studies with 1,090 participants comparing strength training alone vs strength training + cardio.
Findings:
- Maximal strength: No significant difference
- Muscle hypertrophy (growth): No significant difference
- Explosive strength/power: Small reduction (may matter for competitive athletes, insignificant for general population)
Conclusion: “Aerobic training does not appear to compromise maximal strength development or muscle hypertrophy when added to a resistance training programme.”
Will’s real-world confirmation: This matches exactly what I see with clients. Women doing 2-3 strength sessions weekly + 2-3 moderate cardio sessions build muscle, increase strength, and improve body composition effectively. I track every client’s weights lifted—the progression is consistent whether they do cardio or not, provided cardio is moderate.
When Does Interference Actually Occur?
According to research reviewed by Stronger by Science, interference effects are minimal or non-existent when:
1. You’re untrained/beginner Research shows NO significant interference effect for untrained individuals. Your body responds so readily to both stimuli that they don’t meaningfully conflict.
Will’s observation: Beginners (first 6-12 months) respond to everything. I’ve had complete beginners add cardio from day one—they still make excellent strength gains. Their squat goes from 30kg to 60kg, deadlift from 40kg to 80kg, all whilst doing 2-3 cardio sessions weekly. The interference effect simply doesn’t apply.
2. You separate cardio and strength into different sessions
Research shows NO interference when cardio and strength are done on separate days (or separated by 3+ hours same day).
Will’s practical application: I programme strength Monday/Wednesday/Friday and cardio Tuesday/Thursday for clients prioritizing muscle gain. They get strength benefits without fatigue carry-over. This is ideal if you have 5 days available.
3. Cardio volume is moderate (not excessive)
Small amounts of cardio (2-3 sessions weekly, 20-30 minutes moderate intensity) don’t interfere. Excessive volume (6+ sessions, 60+ minutes, high intensity) potentially does.
Will’s guideline: For women focused on building strength and muscle, I cap cardio at 90-120 minutes total weekly at moderate intensity. That’s typically 3× 30-40 minute sessions or 2× 45-60 minute sessions. This provides cardiovascular benefits without compromising recovery.
The Real Issues That Get Mistaken for Interference Effect
Issue 1: Total Training Volume Exceeds Recovery Capacity
The actual problem: You’re doing too much total training (strength + cardio combined), not that cardio inherently interferes.
Example of overtraining:
- Strength training: 5 days weekly, 90 minutes per session
- Cardio: 6 days weekly, 60 minutes per session
- Total: 11 sessions, 13.5 hours weekly
Why this fails: Insufficient recovery time, inadequate sleep, inadequate nutrition to support that volume.
Will’s experience: I see this with women who add intense spin classes or boot camps on top of already demanding strength programmes. It’s not the cardio itself—it’s that their total training load outstrips recovery capacity. Solution: reduce total volume, ensure adequate nutrition/sleep, or accept slower progress in both.
Issue 2: Inadequate Nutrition
The actual problem: Not eating enough to support both strength training adaptations AND cardio energy demands.
What happens: If you’re eating in a large calorie deficit whilst doing both strength training and cardio, muscle growth is compromised. This gets blamed on “interference” when it’s actually insufficient calories.
Will’s insight: Women come to me doing 4 strength sessions + 5 cardio sessions weekly whilst eating 1,200-1,400 calories. They’re not gaining strength or muscle, and they blame the cardio. Reality: they’re severely underfed for that activity level. When we increase calories to 1,800-2,200 (appropriate for their activity), strength gains resume immediately—despite continuing the same cardio.
Issue 3: Poor Programming Sequencing
The actual problem: Doing intense cardio immediately before strength training, causing fatigue that impairs lifting performance.
Bad sequence:
- 45-minute hard run
- Immediately attempt heavy squats
- Legs are fatigued, can’t lift usual weight
- Blame “interference effect”
Reality: That’s just acute fatigue, not physiological interference.
Will’s solution: Always do strength training first if combining same day, or separate by minimum 3 hours.
How to Combine Cardio and Strength Training Effectively
Strategy 1: Separate Days (Best for Strength Prioritisation)
Structure:
- Monday: Strength (lower body)
- Tuesday: Cardio (moderate, 30 minutes)
- Wednesday: Strength (upper body)
- Thursday: Cardio (moderate, 30 minutes)
- Friday: Strength (full body or lower)
- Saturday: Optional cardio (20-30 minutes light)
- Sunday: Rest
Why this works: Each strength session starts fresh, no interference from cardio fatigue.
Will’s recommendation: This is my default for women whose primary goal is building muscle and strength. Cardio days provide active recovery and cardiovascular benefits without compromising strength sessions.
Strategy 2: Same Day, Strength First (Good for Time Constraints)
Structure:
- Monday: Strength (45 min) + Cardio (20 min moderate)
- Wednesday: Strength (45 min) + Cardio (20 min moderate)
- Friday: Strength (45 min) + Cardio (20 min moderate)
Why strength first: You lift with full energy, ensuring progressive overload continues. Cardio performance might suffer slightly, but that’s acceptable if strength is priority.
Will’s application: I use this for busy clients who can only get to the gym 3 days weekly. Total session time: 65-70 minutes. They still make consistent strength gains. Example: Client squatted 40kg → 75kg over 12 months using this exact approach.
Strategy 3: Separate by 3+ Hours Same Day
Structure:
- Morning: Strength training (6:30am)
- Evening: Cardio (6:00pm)
Why this works: Research shows 3+ hour separation minimises interference. You’ve recovered sufficiently from morning session.
Will’s observation: Popular with clients who train before work (strength) then do evening cardio class or run. Works well provided they eat adequately between sessions—snack post-strength, proper lunch, pre-cardio snack.
Strategy 4: Cardio Only on Non-Strength Days (Moderate Approach)
Structure:
- Monday: Strength
- Tuesday: Cardio
- Wednesday: Rest or light walk
- Thursday: Strength
- Friday: Cardio
- Saturday: Strength
- Sunday: Rest
Why this works: Balances both without excessive weekly volume. Strength sessions always start fresh.
Will’s recommendation: Ideal for intermediate trainees (6-18 months training) who want balanced fitness. They improve cardiovascular fitness measurably whilst continuing strength progression.
What Type of Cardio Matters
Running: Highest Interference Potential
Research shows running creates more interference than cycling or rowing.
Why: High-impact, uses similar muscles as squats/deadlifts (quads, glutes, hamstrings), greater muscle damage from eccentric loading.
Will’s guideline: If strength is priority, limit running to 2× weekly maximum, 20-30 minutes moderate pace. Avoid running immediately before or after leg day. Example schedule: Strength Monday (legs), run Tuesday, Strength Wednesday (upper), run Thursday, Strength Friday (legs).
Cycling: Lower Interference
Why: Less muscle damage than running, non-impact, different muscle recruitment pattern.
Will’s preference: For women prioritising strength gains, I recommend cycling over running. Can do 3× weekly, 30-40 minutes without noticing interference.
Rowing: Moderate Interference
Why: Works upper body significantly—could fatigue back/arms before upper body strength session.
Will’s application: Programme rowing on lower body strength days or separate from upper body sessions by 24+ hours.
Incline Walking: Minimal to No Interference
Why: Low intensity, minimal muscle damage, easily recoverable.
Will’s observation: I’ve never seen incline walking interfere with strength gains. Clients do this 4-5× weekly, 30-45 minutes without issues. Perfect for active recovery days or post-strength session cardio.
HIIT: Context-Dependent
High-Intensity Interval Training (HIIT):
- 20-30 seconds maximum effort
- 60-90 seconds rest
- 10-20 minutes total
Potential benefits: Improves cardiovascular fitness without excessive volume, may enhance muscle retention during fat loss.
Potential drawback: Very demanding on nervous system—can impact recovery if done frequently.
Will’s guideline: Maximum 2× weekly for clients prioritising strength. More than that, I see reduced lifting performance.
When to Prioritise Cardio Over Strength (or Vice Versa)
Prioritise Strength Training If:
✅ Primary goal is building muscle, strength, or changing body composition
✅ You’re a beginner (first 12 months)
✅ You’re in a fat loss phase (strength training preserves muscle)
✅ You’re over 40 (strength training combats age-related muscle loss)
Cardio recommendation: 2-3 sessions weekly, 20-30 minutes moderate intensity for cardiovascular health. Enough for benefits, not enough to interfere.
Prioritise Cardio If:
✅ Training for endurance event (half marathon, marathon, triathlon)
✅ Primary goal is cardiovascular fitness for health reasons
✅ You genuinely enjoy cardio more than lifting
Strength recommendation: 2 sessions weekly minimum to maintain muscle mass, bone density, and functional strength.
Will’s note: Even marathon runners benefit from 2× weekly strength training—it improves running economy, prevents injury, maintains muscle mass that’s otherwise lost with high-mileage training.
Sample Weekly Schedules
chedule 1: Strength Priority (Muscle Gain/Body Recomposition)
Monday: Lower body strength (50 min)
Tuesday: Moderate cardio—incline walk or cycle (30 min)
Wednesday:Upper body strength (50 min)
Thursday: Rest or light walk Friday: Full body strength (50 min)
Saturday: Moderate cardio (30 min) Sunday: Rest
Weekly totals:
- Strength: 3 sessions, 150 minutes
- Cardio: 2 sessions, 60 minutes
- Result: Prioritises progressive overload whilst maintaining cardiovascular fitness
Will’s typical client results with this schedule:
- Squat: +5-10kg every 4-6 weeks
- Deadlift: +7.5-12.5kg every 4-6 weeks
- Body composition: Visible muscle development, improved definition
Schedule 2: Balanced (Equal Strength and Cardiovascular Fitness)
Monday: Strength (45 min) + Cardio (20 min) Tuesday: Cardio only (40 min moderate) Wednesday: Strength (45 min) + Cardio (20 min) Thursday: Cardio only (40 min moderate) Friday: Strength (45 min) + Cardio (20 min)Saturday: Light activity—walk, yoga Sunday: Rest
Weekly totals:
- Strength: 3 sessions, 135 minutes
- Cardio: 5 sessions, 140 minutes
- Result: Develops both strength and cardiovascular endurance
Will’s observation: Works for women who enjoy both equally. Strength gains are slightly slower than Schedule 1 but still consistent. Cardiovascular fitness improves noticeably—resting heart rate drops, daily activities feel easier.
Schedule 3: Time-Constrained (Maximum Efficiency)
Monday: Strength full body (50 min) + Cardio (15 min) Wednesday: Strength full body (50 min) + Cardio (15 min)Friday: Strength full body (50 min) + Cardio (15 min) Saturday: Cardio only (30-40 min)
Weekly totals:
- Strength: 3 sessions, 150 minutes
- Cardio: 4 sessions, 75 minutes
- Result: Minimal time investment (4 days weekly) with balanced benefits
Will’s application: Popular with busy professionals, parents. Despite only 4 days weekly, they make solid progress. Example: Client increased squat 40kg → 70kg, deadlift 50kg → 95kg in 12 months whilst maintaining cardiovascular fitness.
Common Mistakes When Combining Cardio and Strength
Mistake 1: Doing Long Cardio Before Strength Training
The problem: 45-60 minute run before attempting heavy squats leaves you fatigued, unable to lift usual weights, missing progressive overload.
The fix: Strength first, always. Or cardio on separate day.
Will’s rule: If combining same session, maximum 5-10 minutes light cardio for warm-up, strength training, then cardio after if desired.
Mistake 2: Excessive Cardio Volume
The problem: 5-6 long cardio sessions weekly whilst trying to build muscle overloads recovery capacity.
What I see: Women running 5× weekly (40-60 min per run) plus 3-4 strength sessions. They plateau on lifts, feel constantly fatigued, don’t build muscle.
The fix: Reduce cardio to 2-3× weekly or reduce duration to 20-30 minutes.
Will’s experience: When we cut cardio from 5 days to 2-3 days (whilst maintaining strength training), strength gains resume within 2-3 weeks. They’re often surprised how quickly progress returns.
Mistake 3: Undereating for Combined Training Volume
The problem: Trying to build muscle whilst doing significant cardio and eating in large deficit.
Example: 1,400 calories daily with 3 strength sessions + 4 cardio sessions weekly.
Why this fails: Insufficient energy for muscle protein synthesis, poor recovery, compromised performance in both modalities.
The fix: Increase calories to maintenance or slight surplus if muscle gain is goal.
Will’s guideline: For 3 strength + 2-3 cardio sessions weekly, most women need 1,800-2,200 calories minimum (varies by size/activity). Less than this, and recovery suffers.
Mistake 4: Neglecting Recovery
The problem: Training 6-7 days weekly with no rest days.
Will’s observation: Women feel guilty taking rest days, thinking “more is better.” Then they wonder why they’re not getting stronger.
The reality: Muscle grows during recovery, not during training. Without adequate rest, neither strength nor cardio performance improves optimally.
The fix: Minimum 1-2 complete rest days weekly. Consider active recovery (walking, yoga, stretching) instead of intense training.
How 12REPS Balances Strength and Cardio
Primary Focus: Strength Training
12REPS programmes focus on progressive strength training because that’s what builds muscle, bone density, and metabolic health long-term.
Strength programmes offered:
- 2-day full body
- 3-day full body
- 4-day upper/lower split
Why strength first: Research and real-world results show strength training provides the greatest return on investment for women’s health, body composition, and functional fitness.
Cardio Recommendations Included
12REPS provides cardio guidelines based on your training frequency and goals:
If training 2-3 days strength:
- Add 2-3 moderate cardio sessions (20-30 min) on non-strength days
- Total: 5-6 active days weekly
If training 4 days strength:
- Add 1-2 moderate cardio sessions OR do 15-20 min post-strength
- Total: 5-6 active days weekly
If prioritising maximum muscle gain:
- Limit cardio to 2× weekly, 20 minutes incline walking or cycling
- Focus energy on progressive overload
Flexibility Based on Goals
12REPS allows customisation:
- Muscle gain primary goal: Minimal cardio recommendations
- Fat loss goal: Moderate cardio recommendations (2-3× weekly)
- Cardiovascular health priority: Higher cardio recommendations (3-4× weekly)
Will’s philosophy embedded in 12REPS: Cardio complements strength training—it doesn’t replace it. Women get best results from structured strength progression + moderate, sustainable cardio.
The Bottom Line
The interference effect is real in extreme circumstances—but those circumstances don’t apply to most women.
What research shows:
✅ Moderate cardio (2-3 sessions weekly, 20-40 minutes) does NOT significantly interfere with strength or muscle gains
✅ Interference occurs primarily with excessive volumes (5-6+ cardio sessions weekly, 60+ minutes, high intensity)
✅ Beginners experience virtually no interference effect
✅ Separating cardio and strength (different days or 3+ hours apart) minimises any potential interference
What Will’s 10+ years of experience confirms:
✅ Women doing 3 strength sessions + 2-3 moderate cardio sessions weekly build muscle and strength effectively
✅ The real culprits are overtraining (excessive total volume) and undereating (insufficient calories/protein)
✅ Strength training first (when combining same day) maintains progressive overload
✅ Incline walking, cycling, moderate-pace running don’t interfere when volume is reasonable
✅ HIIT should be limited to 2× weekly maximum if strength is priority
How to combine effectively:
✅ Best: Separate days—strength Mon/Wed/Fri, cardio Tue/Thu/Sat
✅ Good: Same day, strength first, cardio after (strength full energy, cardio slightly fatigued is acceptable)
✅ Acceptable: Same day separated by 3+ hours with adequate nutrition between
✅ Avoid: Long cardio immediately before strength training
Cardio recommendations by type:
✅ Incline walking: 3-5× weekly, no interference
✅ Cycling: 3-4× weekly, minimal interference
✅ Moderate running: 2-3× weekly maximum if strength is priority
✅ HIIT: 2× weekly maximum
✅ Rowing: 2-3× weekly, separate from upper body training
Common mistakes to avoid:
❌ Long cardio before strength training (causes acute fatigue)
❌ Excessive cardio volume (5-6+ sessions weekly, 60+ minutes)
❌ Undereating for combined training volume
❌ No rest days (muscle grows during recovery, not training)
12REPS approach:
✅ Prioritises progressive strength training (2-4 days weekly)
✅ Includes moderate cardio recommendations (2-3× weekly, 20-30 min)
✅ Customisable based on goals (muscle gain, fat loss, cardiovascular health)
✅ Evidence-based balance between strength and cardio
The honest truth from 10+ years training women: You don’t have to choose between building muscle and cardiovascular fitness. The women with the best results do both—2-3 strength sessions building progressive overload, 2-3 moderate cardio sessions for heart health and calorie burn. They eat adequately to support both, prioritise recovery, and don’t overthink the “interference effect.”
Try 12REPS free for 7 days. Get structured strength programming designed to build muscle whilst leaving room for the cardio you enjoy—without obsessing over interference.
You can be strong AND have good cardiovascular fitness. Research proves it. My clients prove it. Stop choosing—start balancing.
