Leida, I think doing them on your own in a gym is not the same as doing them in a class with a trainer watching your form and telling when to try more weight/move up.
I think the key is a good instructor who can advise you when to push and when not to. Whether that exists everywhere is arguable. I was told flat out "don't try double unders today, this is a tough day to learn them on".
It's been very effective for me, more effective than anything I have ever tried and I see a lot of women getting similar if not better results.
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I did not add any muscular mass with Cross-fit. I immediately start gaining muscular mass when I lift in the 5x5 set up. Neither I dropped any fat. I like the idea of a daily workout (or whatever the schedule is, 5/6 or something) and the variety of the exercises on the plate. It's cool, and I normally finished the workouts in about 20-30 min - I did as many sets on average as people with higher athletic ability did and posted.
For me a trainer or a group class is not an advantage, it is the least preferred option, since I like training early in the day, and dislike working in a group (I keep waving between sparring and boxing against the bag for example). It takes a lot of gall for me to attend a weekly hap-ki-do class already, and I dunno if it will be more beneficial for me to simply box my own combos. I generally know myself, and should I have listened to my inner voice. I do not know how Cross-fit classes are run, really, but I know in a group class setting a trainer simply cannot monitor everyone anyway. I like a PT for my custom goals.
All that aside, once again, if I have seen the muscular gain or a drop in fat in that month I did Cross-fit like workouts, I would have continued it once the knee has healed. But lifting heavy is simply more effective for me.
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