Cardio vs Strength Training: What Each One Actually Does to Your Body

Endurance training makes your body better at delivering and using oxygen. Resistance training makes it better at producing force. These are largely separate adaptations in separate tissues, they are driven by different signals, and neither one substitutes for the other. The framing of cardio against strength implies a choice between them, which is the wrong shape for the question, because the mortality data and the functional data both point the same way: people who do some of each are better off than people who do a lot of one.

That does not make the comparison pointless. The two modalities differ sharply in what they cost, what they deliver, how quickly they deliver it, and how they interfere with each other when stacked. What follows is what each one measurably changes, where the interference effect is real and where it is exaggerated, and how to allocate a finite number of weekly hours.

What endurance training changes

The headline adaptation is central. Sustained aerobic work expands plasma volume within days, and across weeks to months it increases left ventricular chamber size and improves ventricular filling, so stroke volume rises. Since maximal cardiac output is stroke volume multiplied by a maximum heart rate that barely moves, this is essentially the whole mechanism by which aerobic capacity improves. The details of that ceiling, and how stubbornly genetic its trainability is, sit in VO2 max and how much of it you can change.

Peripheral changes run alongside. Capillary density around each muscle fibre increases, mitochondrial content rises, and the enzymes of fat oxidation become more abundant, so a trained person burns proportionally more fat at any given absolute workload and spares glycogen. Resting heart rate falls. Blood pressure falls: Cornelissen and Smart's meta-analysis in the Journal of the American Heart Association (2:e004473, 2013) pooled controlled trials and found endurance training reduced systolic pressure by several millimetres of mercury on average, with substantially larger reductions in hypertensive participants.

The epidemiology is where endurance work has its strongest hand. Kodama and colleagues, pooling 33 cohorts in JAMA (301:2024-2035, 2009), found each one-MET increment of cardiorespiratory fitness associated with roughly 13 percent lower all-cause mortality. Nothing in the resistance training literature has that weight of evidence behind it, though as we will see, the resistance data are catching up.

What endurance training does not do is build much muscle or much strength. It can add a small amount in genuinely untrained people, particularly in cycling, and then it stops. Sustained high-volume endurance training in an energy deficit will lose you muscle, not gain it.

What resistance training changes

Resistance training's primary adaptations are neural first and structural second. Early strength gains are dominated by improved motor unit recruitment, firing rate and inter-muscular coordination, which is why a beginner adds meaningful weight to a lift within weeks without visible change in size. Hypertrophy accumulates more slowly, driven by mechanical tension and requiring adequate protein and energy, and the mechanics of driving it are covered in progressive overload.

Beyond muscle, three effects deserve more attention than they get.

Bone responds to load in a way it does not respond to oxygen. Watson and colleagues reported the LIFTMOR trial in the Journal of Bone and Mineral Research (33:211-220, 2018), in which postmenopausal women with low bone mass performed supervised high-intensity resistance and impact training twice weekly for eight months and improved bone mineral density at the lumbar spine and femoral neck. Walking and cycling do not produce that.

Connective tissue and joint tolerance improve with load in ways that make other activity more survivable. And lean mass is metabolically consequential over decades: age-related loss of muscle predicts frailty, falls and loss of independence, and resistance training is the only intervention that reliably reverses it.

The mortality evidence has now arrived too. Momma and colleagues, in the British Journal of Sports Medicine (56:755-763, 2022), pooled cohort studies on muscle-strengthening activity and found roughly 10 to 17 percent lower risk of all-cause mortality, cardiovascular disease, cancer and diabetes, with the association appearing to peak somewhere around 30 to 60 minutes a week and flattening or reversing beyond that. The dose that seems to matter is smaller than most lifters assume.

Setting the two side by side

The pattern is that they overlap on metabolic health and diverge on nearly everything structural. Insulin sensitivity, blood pressure and blood lipids respond to both. Oxygen transport responds only to one. Force production and bone respond mainly to the other.

Energy cost, and why it matters less than it looks

For fat loss, the naive comparison favours cardio, and the engine behind our calories burned calculator shows why. Using MET values from Ainsworth and colleagues' Compendium of Physical Activities (Medicine and Science in Sports and Exercise, 2011), for a 70 kg person across 45 minutes:

Running costs nearly three times what a light lifting session does for the same clock time. Those MET values are population averages, though, and the caveat is heavier for lifting than for running: a session's MET rating cannot distinguish between someone taking 90-second rests and someone taking five minutes, and the real spread around 6.0 is wide.

Two corrections cut against the naive reading. Excess post-exercise oxygen consumption (the afterburn) is genuinely larger after intense resistance work than after moderate cardio, but LaForgia, Withers and Gore's review in the Journal of Sports Sciences (24:1247-1264, 2006) put the magnitude in the tens of kilocalories for typical sessions, not the hundreds that marketing implies. It does not close a 350-kilocalorie gap.

The second correction is more important. Willis and colleagues, reporting the STRRIDE-AT/RT trial in the Journal of Applied Physiology (113:1831-1837, 2012), randomised overweight adults to eight months of aerobic training, resistance training, or both. Aerobic training reduced body mass and fat mass; resistance training did not reduce either but added lean mass; the combination reduced fat while preserving lean tissue. What actually determines fat loss is energy balance across weeks, which is why how a calorie deficit works matters more than the modality of any session, and why resistance training earns its place in a fat-loss phase by protecting muscle rather than by burning calories. The composition question is developed further in body recomposition. For total daily needs, start with the TDEE calculator.

One further wrinkle: sessions that leave you exhausted tend to reduce spontaneous movement for the rest of the day, and that unlogged activity is a large and variable slice of daily expenditure. A hard session that costs 500 kilocalories and takes 300 off your afternoon fidgeting is a smaller net win than the watch reports.

The interference effect, honestly sized

Hickson demonstrated in the European Journal of Applied Physiology (45:255-263, 1980) that training for strength and endurance simultaneously blunted strength development compared with strength training alone, while endurance gains were unaffected. That asymmetry has held up: endurance interferes with strength more than strength interferes with endurance.

Wilson and colleagues quantified it in the Journal of Strength and Conditioning Research (26:2293-2307, 2012), pooling concurrent training studies. Hypertrophy and strength were attenuated when endurance work was added, and the size of the attenuation scaled with endurance volume, with frequency, and most strongly with modality. Running produced more interference than cycling, plausibly because of the eccentric loading and muscle damage involved. More recent analyses have narrowed the effect further, and the current picture is that interference is real, mode-dependent, dose-dependent, and modest at the volumes most people actually train.

For a recreational lifter doing two or three cardio sessions a week, interference is close to a non-issue. It becomes a genuine programming problem for people chasing maximal strength alongside high endurance volume, and for anyone in a substantial energy deficit, where recovery capacity is the limiting resource rather than the training itself.

Practical mitigation is unglamorous. Separate the sessions by several hours where possible, or put them on different days. If both must happen in one session, do the quality first, so the adaptation you care most about gets the fresh nervous system. Prefer cycling or rowing over running when strength is the priority. And eat enough, because most attributed interference is under-recovery.

Allocating the week

The public health guidance is a reasonable floor rather than a ceiling: 150 to 300 minutes of moderate aerobic activity weekly, or 75 to 150 minutes of vigorous, plus muscle-strengthening activity on two or more days. The ACSM Position Stand on quantity and quality of exercise (2011) sets out the same structure with more detail on progression.

Beyond the floor, allocate by goal, and be honest that both cannot be maximised at once.

If health span is the objective, two resistance sessions and two or three aerobic sessions weekly covers the ground and neither component is large enough to interfere with the other. If strength or muscle is the priority, three or four resistance sessions with aerobic work kept to low-intensity maintenance protects the adaptation you are chasing. If endurance performance is the priority, endurance volume dominates and resistance training becomes a supporting element, one or two sessions weekly aimed at injury resilience and economy rather than size.

For anyone who has been sedentary, the most useful observation is that the first few weeks of either modality deliver disproportionate returns, and the largest health gains in the entire fitness distribution occur between doing nothing and doing something modest. Tracking a working strength number with the one-rep max calculator from an ordinary hard set, rather than from a maximal attempt, is enough to know whether the resistance side is progressing.

The comparison that actually matters

Cardio and strength answer different questions about the same body. One asks how much oxygen you can move and use; the other asks how much force you can produce and how much tissue you have to produce it with. Both predict how well you will function in your seventies, through different mechanisms, and the association between each and mortality survives adjustment for the other.

Two clinical notes belong here. Anyone with known cardiovascular disease, uncontrolled hypertension, or symptoms such as chest pain, unusual breathlessness or palpitations during exertion needs individual medical advice before starting or intensifying either modality, and heavy resistance training with a Valsalva manoeuvre raises blood pressure sharply in a way that is specifically relevant to that conversation. Return to exercise after injury, surgery or pregnancy is likewise a matter for a clinician who can assess you in person.

For everyone else, the useful reframing is that the two modalities are not competing for the same outcome. They are competing only for your time, and the evidence suggests dividing it rather than choosing.