Building Muscle and Cardio At The Same Time...
One comment made to me by a coach I had years ago was “The slower you go, the bigger and faster you’ll grow”
He was referencing time under tension when lifting weights.
In my opinion it’s the thing that’s most often overlooked with group fitness and online programs these days. I feel it’s shifted this way because of the hype around high intensity interval type workouts and our busy lives making us want to get in and get out of the gym as fast as possible.
So while higher fitness is brought about by doing faster movements with lighter weights bigger muscles will come about when you perform slower movement with more weight and more time under tension.
If your goal is to build lean muscle, stay injury free and learn new movements the slower approach will work best.
If your goal is to simply build cardio fitness and not necessarily build muscle then increase the speed of your workouts, do them in a shorter space of time and you will definitely get just that.
If you want both which I feel is the holy grail of fitness then you need to have a mixture of both. Lift heavy weights slowly some days to build strength and muscle then on the other days move more quickly with lighter weights to build your cardio fitness.
Training for muscle and cardio together: what the research says
Doing strength work and conditioning in the same program is called concurrent training. The old worry was an "interference effect", the idea that endurance work blunts strength and muscle gains. More recent reviews, including a 2022 meta-analysis, suggest the effect is smaller than people feared. It shows up mostly for explosive power, and mostly when the amount of conditioning is high. For muscle size, the evidence suggests you can do both.
Where the "slower is better" idea fits
Controlling the lowering phase of a lift is a good habit, and a slow tempo is a useful tool for learning a movement or fixing a weak position (we cover this in our post on tempo training). But the research on rep speed suggests muscle growth does not depend on lifting slowly. What matters most is lifting weights that are heavy enough, doing enough hard sets, and taking those sets close to failure. Think of tempo as a way to improve quality, not as the thing that builds the muscle.
How to organise your week
Separate your hardest sessions. Where you can, leave several hours, and ideally a day, between a hard lifting session and a hard conditioning session.
Put your priority first. Do the session you care most about when you are fresh. If you do both in one day, do that one first.
Choose your conditioning wisely. Rowing, cycling and the ski erg tend to interfere less with leg strength than lots of running.
Keep most conditioning easy. Most of your aerobic work can sit at a conversational pace, with one or two harder sessions a week for intensity.
Keep your lifting heavy enough to matter. A handful of hard sets of low to moderate reps on the main lifts, with good technique, is a solid base.
A sample week for four training days
Monday: strength, lower body focus
Tuesday: easy conditioning, 30 to 40 minutes at a conversational pace
Thursday: strength, upper body focus
Saturday: harder conditioning, such as intervals on the rower or bike
Treat this as a starting point, not a prescription. The right mix depends on your goals, your history and how you recover, and a coach can help you adjust it.
Common questions
Can I build muscle and improve my fitness at the same time?
Yes, especially if you are newer to training. Gains in both can come together when the program is organised sensibly.
Should I do cardio before or after weights?
Do the one that matters most to you first. If strength is the priority, lift first.
How many days a week do I need?
Three days can work for both if each session is well planned. Four or five gives you more room to separate hard sessions.