In order to improve the condition of your muscular system you could aerobic activity at least a 3 minutes 5 times a week b 10 minutes 4 times a week c 30 minutes 3 times a week d 50 minutes 3 times a week

The answer to this should be 30 minutes 3 times a week because if you do less than 30 minutes for  aerobic activity then it wont have effect. 
The ideal way to do it is to perform activities that are enjoyable and interests you. Set it also on alternating days, this way your cardio-respiratory system is continuously challenged but still has enough time to recover.

So 30 minutes 3 times a week is the answer you are looking for

Hope this helped :)
Have a great day 

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