Circuit training has been around for years and will be around for many more and is the very basis of Interval training. If you're unsure what a circuit is (where have you been?), then in my experience usually I really like the following format (e.g with 4 exercises)
Lower Body Exercise
Upper Body Exercise
Full Body Exercise
All repeated for a set amount of time with a set amount of time for rest between stations
So you might do it in the following format using these exercises as an example
1. 20 secs on bodyweight sqautting with 20 sec rest
2. 20 secs on bodyweight press ups with 20 sec rest
3. 20 secs russian twist with 20 sec rest
4. 20 secs star jumps with 20 sec rest
That would be one circuit of 20 on 20 off
Your next circuit could be done using the same format but with different exercises.
Once you have built up fitness you could introduce more circuits and increase the length of the set or reduce the rest period, even working upto doing active recovery between sets. There is no limit to your options with circuit training.
Assuming that you use 1:1 ratio for work and rest you could use a similar format as this one below (progression)
Then start adding in active rest bewteen circuits.
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