My circuits class goes like this:
5 minute warm-up of running step, star-jumps, mountain climbers, burpees and press-ups and some jogging.
then onto the circuits where we do about 1 min 30 secs on each station. Stations vary between strength training and cardio so todays was: tricep dips, boxing sit-ups, spin bike sprint, running high knees, up and downs, boxing jabs, medicine ball sprints, squat presses, wall-sits, punchbag and medicine ball lunges.
In between each circuit we do some cardio like step-running, burpees, jump squats etc.
About half way through our instructor takes us outside and makes us sprint 400m round the block and then do sprint training in the car park............................cos he is a total bastard!
We then do a 5-6 minute cooldown of core work (sit-ups, planks etc) and stretches.
It sounds tough, and tbh it is. The first time i went I was really unfit and nearly died!
But, you take each station at your own pace and if you can get through the warm-up then you are probably ok to continue.
You WILL hurt after you first few classes as the muscles you didn't know about start to become awakened, so give yourself a day off in between classes. This will fade after a few weeks and when you feel brave enough you can do classes on consecutive days and also back-to-back classes (I do body pump followed by body combat which i find a good combo).
Body pump is weight training to music, lots of reps. Its good fun and a really sociable class. As its not cardio you do not become a sweaty mess if you start with quite low weights. You are in control of your weights so you do as much/little as you feel like (remember to put more weight on for bigger muscles like your legs and back).
More tips for not making a fool of yourself:
invest on decent length tops that cover your belly and don't ride up.
invest in good sports bra.
dri-fit or clothes that wick the sweat away are good for preventing sweat patches.
take a towel and water.
Have some decent trainers to support your joints.
If you have long hair find some way of securing it away from your face/body as hair gets really, really sweaty and will drip sweat all over you and everyone else if you leave it flapping around.
Take this from somebody who made all of the above errors in her first few weeks
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Finally, enjoy!!!! I started going in february of this year and have lost 2.5 stone through exercise alone (carried on eating like a pig all the way through!), got back to pre-preg clothes and a size 10-12 finally!
Good luck, you can do it!