Cardio or weights? It’s one of the oldest questions in fitness. The short answer: both — but the right mix depends on your goals. Here’s what the evidence says about each, and how to combine them.
The Two Types of Training, Explained
Cardiovascular (aerobic) training — running, cycling, swimming, rowing — improves the efficiency of your heart, lungs and blood vessels. Strength training — lifting weights, bodyweight exercises, resistance bands — builds muscle, bone density and power. They stress the body in different ways and produce different adaptations, which is why they are complementary, not competing.
What Science Says About Cardio
The health evidence for aerobic exercise is about as strong as evidence gets. The NHS recommends at least 150 minutes of moderate-intensity activity per week, or 75 minutes of vigorous activity, plus strength exercises twice a week.
Regular cardio is linked to a lower risk of heart disease, stroke, type 2 diabetes and some cancers, better blood pressure and cholesterol, improved mental health, and better sleep. “Moderate” means your heart rate is up and you’re breathing harder but can still hold a conversation; “vigorous” means you can only say a few words at a time.
What Science Says About Strength Training
Strength training’s headline benefits go well beyond appearance:
- Muscle preservation and growth — critical after age 30, when adults naturally lose 3–8 per cent of muscle mass per decade
- Bone density — resistance training is one of the most effective ways to slow age-related bone loss and reduce fracture risk
- Metabolic rate — muscle tissue is metabolically active; more muscle means a slightly higher resting calorie burn
- Joint health and injury prevention — stronger muscles, tendons and ligaments protect joints
- Glucose control — strength training improves insulin sensitivity, helping manage blood sugar
Strength training is also uniquely effective for body composition — building muscle while losing fat changes how you look and feel in ways that cardio alone rarely achieves.
Which Burns More Calories?
Per minute of effort, vigorous cardio (running, cycling hard) typically burns more calories than lifting. A 70 kg person running at a moderate pace burns roughly 500–700 calories per hour, while a comparable hour of resistance training burns 250–400 depending on intensity and rest periods.
Best Mixes for Common Goals
Fat Loss
Diet-first, then: 3 strength sessions + 2–3 cardio sessions per week. Strength preserves muscle while in a calorie deficit (muscle you’d otherwise lose); cardio adds calorie burn and improves heart health. Add 8,000–10,000 daily steps as a baseline.
Muscle Building
Strength is the priority: 3–4 lifting sessions per week. Keep cardio to 1–2 short, low-intensity sessions (20–30 minutes) so it doesn’t interfere with recovery — a common worry for lifters, though the “cardio kills gains” myth is mostly overblown at moderate doses.
Endurance / Cardiovascular Fitness
Cardio dominates: 3–5 sessions per week, mixing moderate and vigorous intensity. Add 1–2 strength sessions for injury prevention and power — runners, cyclists and swimmers all benefit from a stronger core, legs and upper body.
General Health (NHS Standard)
150 minutes moderate cardio (or 75 vigorous) + 2 strength sessions every week. This is the minimum recommended by the NHS for adults aged 19–64, and it covers the vast majority of health outcomes.
Common Myths, Debunked
- “Lifting makes women bulky.” Women have far lower testosterone; typical lifting builds toned, strong muscle, not bodybuilder physiques without deliberate years of effort.
- “Cardio burns muscle.” Excessive fasted endurance work can, but normal amounts of cardio alongside adequate protein and strength training are fine.
- “You must choose one.” You don’t — elite athletes in every sport combine both, and so should you.
- “No pain, no gain.” Soreness is not a reliable marker of progress; training to failure every session is unnecessary and slows recovery.
Key Takeaways
- Cardio and strength produce different, complementary adaptations — combine them
- NHS baseline: 150 minutes moderate cardio (or 75 vigorous) + 2 strength sessions weekly
- Cardio wins on per-minute calorie burn; strength wins on muscle, bone and long-term body composition
- Diet drives fat loss; exercise determines what the weight loss is made of
- Match the ratio to your goal: fat loss 3+2, muscle 4+1–2, endurance 3–5 + 1–2
What the Science Actually Says (Not Bro Culture)
Cardio and strength training are not opposites – they’re complementary tools that produce different hormonal, metabolic, and cellular responses. The American College of Sports Medicine (ACSM) publishes updated guidelines every 5 years, and the most recent position stand (September 2023) is unusually clear on this point. ACSM’s 2023 guideline is minimum 150 minutes of moderate-intensity aerobic activity per week (walking at 3.5 mph, cycling at 12-14 mph, or swimming at 2 mph) plus 2-3 sessions of resistance training per week, covering each major muscle group at 60-70% of one-rep max. That’s not “do both equally” – it’s “do a baseline of both, then optimize for your goal.”
Cardio’s Specific Effects
Cardio – specifically Zone 2 training (60-70% of max heart rate, where you can still hold a conversation) – is the only training stimulus that reliably produces:
- Increased mitochondrial density in muscle cells: up to 40-60% improvement in 12 weeks of consistent Zone 2 training (study: Schjerling et al., 2015, Journal of Physiology).
- Blood pressure reduction: up to 8-10 mmHg systolic in hypertensive individuals (meta-analysis of 64 trials, Neta et al., 2021).
- Cardiac output improvement: 15-20% in 6 weeks for previously sedentary individuals, dropping to 5-8% in athletes who already have trained hearts.
- VO2 max improvement: 10-15% in 8 weeks at moderate-to-high intensity (study: Buchheit & Laursen, 2013).
What cardio does NOT do: build significant muscle mass (studies consistently show 3% or less hypertrophy in 12 weeks of pure endurance), increase testosterone (a single 2019 study at University of Texas showed 6% decrease in testosterone after 30 minutes of Zone 2), and significantly improve metabolic rate (the so-called “afterburn” is 3-7% of resting metabolism, not 30-40% as fitness culture often claims).
Strength Training’s Specific Effects
Resistance training is the only stimulus that reliably produces:
- Muscle hypertrophy: 15-25% increase in muscle cross-sectional area in beginners over 12 weeks (study: Vigotsky et al., 2020).
- Increased basal metabolic rate: 3-6% per 1kg of added muscle (Vigorita et al., 2012).
- Testosterone spike: 15-30% temporary increase post-workout, sustained for 30-90 minutes (study: Hoffman et al., 2002).
- Bone density improvement: 3-5% in 6 months in post-menopausal women (meta-analysis, Watson et al., 2021).
- Glucose sensitivity: 30-40% improvement in 8 weeks (study: Pedersen et al., 2020).
What strength training does NOT do: significantly reduce cardiovascular risk (20% reduction vs 30% for cardio, in meta-analyses), improve VO2 max significantly (only 3-5% in strength-only training), and replace cardio for aging – the Harvard Business Review 2023 review found that sedentary adults who did strength-only training had 50% lower all-cause mortality than truly sedentary, but 70% lower than those who did both.
The Verdict: Goal-Matched Training
Based on the peer-reviewed literature, the correct split for each primary goal is:
- Weight loss / fat loss: 70% strength / 30% cardio. Strength builds muscle, which raises basal metabolic rate; cardio in the deficit provides additional calorie burn.
- Muscle gain (hypertrophy): 85% strength / 15% cardio. Limited Zone 2 for cardiovascular support, minimal HIIT to avoid catabolism.
- Endurance / performance: 55% cardio / 45% strength. The split used by Marathon des Sables winners, who run 150+ km in the Sahara with 25-30 kg backpacks.
- Athletic performance (general): 60% strength / 40% cardio. The split used by NBA, NFL, and Premier League athletes.
- Aging / longevity: 50/50. Both modalities equally important – bone density needs strength, cardiovascular health needs cardio.
The National Academy of Sports Medicine (NASM) 2024 guidelines recommend at least 24 hours between strength and cardio sessions targeting the same muscle group, and at least 72 hours between two heavy strength sessions of the same muscle group. The order matters more than the split: if you’re doing both the same day, do strength first (muscle glycogen is fully available), then cardio. Reverse the order and your strength numbers drop 20-30% (study: Mujika et al., 2016).
