“Cardio kills gains” is one of the most stubborn gym myths. The research shows something more nuanced: very large amounts of endurance training can blunt strength and power gains, but sensible amounts barely affect muscle growth and bring major benefits for the heart, recovery and fat loss. Here is how to get the benefits without the cost.
- The “interference effect” is real but modest, and mainly appears with high volumes of endurance work.
- Two to three moderate cardio sessions a week have little impact on muscle growth for most lifters.
- Cycling and incline walking interfere less with leg training than lots of running.
- For anyone using anabolic steroids, cardio is one of the best tools for protecting the heart and blood pressure.
What the interference effect is
Combining strength and endurance training can produce slightly smaller strength and power gains than strength training alone, especially when endurance volume is high, sessions are back-to-back and the cardio involves lots of running. Analyses of studies suggest effects on muscle size are small, and many people see no meaningful loss at moderate amounts.
HIIT or steady state?
| HIIT | Steady state | |
|---|---|---|
| What it is | Short hard intervals with rest | Continuous moderate effort |
| Time needed | Short sessions | Longer sessions |
| Recovery cost | Higher; can tire legs for lifting | Low |
| Fitness gains | Strong for aerobic capacity | Strong for base fitness |
| Best for | Time-poor, well-recovered lifters | Recovery days, fat loss, heart health |
Both improve cardiovascular fitness. A mix, such as one interval session and one or two longer, easy sessions a week, works well for most lifters.
Scheduling to limit interference
- Separate cardio and leg training by at least six hours, or do them on different days.
- If on the same day, lift first and do cardio after.
- Prefer cycling, rowing or incline walking over high-volume running when strength is the priority.
- Eat enough to cover the extra energy you burn.
Why lifters on steroids need cardio
Anabolic steroids raise blood pressure, lower HDL cholesterol and can thicken the heart wall. Regular aerobic exercise lowers resting blood pressure, modestly raises HDL and improves the heart’s efficiency. It is one of the few things that works directly against those risks. See steroids and your heart.
Cardio and fat loss
Fat loss is driven by a calorie deficit. Cardio helps by increasing energy burned and improving how the body handles fuel, but diet does most of the work. Walking is underrated: it burns meaningful calories with almost no recovery cost. See building a cutting diet that keeps muscle.
How much is enough?
Canadian physical activity guidelines recommend at least 150 minutes of moderate-to-vigorous aerobic activity a week for adults, alongside muscle-strengthening activity at least twice a week. For most lifters that means two or three cardio sessions of 30 to 45 minutes, plus daily walking.
For easy cardio, you should be able to talk in full sentences. For intervals, the hard parts should feel genuinely hard but repeatable.
Warning signs during cardio
Unusual breathlessness, chest pain, a racing or irregular heartbeat or dizziness during exercise are reasons to stop and see a doctor, particularly for anyone using anabolic steroids or stimulants.
Health and bloodworkAnabolic steroids and your heartRead guide
Frequently asked questions
Will cardio make me lose muscle?
Moderate cardio has little effect on muscle for most lifters, especially with enough food and protein.
Is HIIT better than steady cardio for fat loss?
Both work. Total calories burned and diet matter more than the style.
Should I do cardio before or after lifting?
After, or on separate days, if strength is your priority.
Educational information only. Nothing here is medical advice or a recommendation to use any product. Anabolic steroids are Schedule IV under the Controlled Drugs and Substances Act. Talk to a doctor and get bloodwork before and after using any compound.
