How much cardio should you do during contest prep?

Short answer

There is no set number. The least cardio that keeps fat coming off on schedule is enough. Adding cardio made no difference to muscle size, but the more often and the longer it was done, the smaller the lifting gains, and it was lifting that protected muscle.

How much cardio should you do during contest prep?
'More cardio, sharper look' — does that hold?
As a show gets closer, many people keep adding cardio time. But that comes with a worry: lots of cardio eats into muscle. We checked how much cardio actually goes up and what happens when there is a lot of it.
Cardio really does go up before a show
It did in surveys of 127 and 235 bodybuilders
A survey of 127 bodybuilders found that from 6 weeks out, cardio went up while lifting sets and loads went down. A separate survey of 235 bodybuilders also found a clear rise in cardio during contest prep.
26 weeks of rising cardio: the result?
About 15 kg of body weight and 10 points of body fat came off
In a case study of 1 bodybuilder, he cut calories and added cardio for 26 weeks while trying to keep lifting loads. Body weight went from 88.6 kg to 73.3 kg, and body fat from 17.5% to 7.4%. Resting heart rate fell from 71 to 44 beats per minute.
So does cardio eat muscle?
There was no difference in muscle size
An analysis that pooled 43 studies found that adding cardio to lifting for 4 weeks or more made no difference to muscle size or maximal strength versus lifting alone. The only thing blunted was explosive power. A 2026 analysis of 144 studies and 1,492 people also found no difference in muscle growth.
But the dose changes the story
The more often and the longer, the smaller the lifting gains
An analysis that pooled 21 studies found that the more often cardio was done, and the longer each session, the smaller the gains in muscle, strength and power. The problem was not cardio itself but how much of it.
Lifting, not cardio, protected muscle
Cardio alone gave no clear muscle protection
A 2026 analysis of 34 trials and 1,455 people found that adding exercise to dieting kept on average 0.87 kg more lean mass (muscle and everything except fat). Mixed lifting and cardio kept 1.20 kg and lifting 0.83 kg, while cardio alone, at 0.51 kg, was not a clear difference.
Late in the cut, muscle goes too
9 people lost 0.9 kg in the final 8 weeks
9 natural bodybuilders cut fat from 8.8 kg to 5.3 kg starting 16 weeks out. In the final 8 weeks, lean mass also dropped, from 71.8 kg to 70.9 kg. It came back to 72.5 kg 4 weeks after the show.
Bottom line: the least that keeps fat coming off
Cardio is about 'enough', not 'more'
Adding cardio did not shrink muscle, but doing it more often and longer shrank lifting gains, and it was lifting that protected muscle. So the contest-prep advice is the fewest sessions and minutes that still take off enough fat.