If you stop exercising, your muscle turns into fat?

1. If you stop exercising, your muscle turns into fat: 
• Once you've got your workout routine down, you'll be surprised at how toned you feel. But something like a vacation or sickness can set your regimen back, sometimes leading to weight gain. While many people believe the weight gain is from muscles turning into fat, both tissues are completely different and can't convert from one to the other (similarly, there's no way to make muscles leaner, since they are already fat-free). Instead, building muscle helps burn fat, so when you have less of it, your metabolism rate will be lower.
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2. Stretching muscles before a workout helps: 
• Whether it's indoctrinated in you from years of high school sports or just part of your routine, many people think that cold stretching before a workout helps them perform better. In fact, stretching before exercising has been shown to be a waste of time and detrimental before certain workouts, like lifting weights. Instead, save the hamstring stretches for after your workout, and warm up muscles before with a few minutes of low-intensity cardio or one of these dynamic stretches.
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3. You shouldn't exercise with sore muscles: 
• You took a butt-burning barre class yesterday, and now you can barely walk without wincing. You may think that you shouldn't work out again so soon afterward, but in reality, the right kind of exercise can not only make your sore muscles feel better, but also help you recover faster. Just make sure you aren't focusing too much on the affected area. If yesterday's workout included lower-body work, spend today focusing on your upper body, do a less-intense cardio session, or give weary muscles a good stretch in yoga class. Find out what else you should do when exercising with sore muscles here.
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4. Heavy weights make you bulk up:
•  It's a common belief: lifting heavy weights will have you looking more bodybuilder than long and lean. But in reality, your muscles won't get Ms. Olympia-sized from lifting a 20-pound kettlebell; the size of your muscles is related to your genes and strength-training routine, not the size of your weights. Using heavier weights actually saves you time — studies show that you will get the same results when lifting heavier weights for fewer reps as you do with lifting lighter weights for longer. But no matter what size weight you use, make sure you choose one that is challenging your body the right way. The American Council on Exercise recommends that you choose a weight that fatigues your muscles within 90 seconds (aka makes you unable to perform another rep correctly), since that's within the limit of your muscles' supply of anaerobic energy.
5. Soreness comes from lactic acid buildup: 
• It's an often-quoted principle that the delayed onset muscle soreness (DOMS) you feel in the days after your workout is from lactic acid in your body. In fact, DOMS is a symptom of micro tears in the muscles that happen when you work out. Lactic acid does play a part in your workout, however, since it is the cause of that burning sensation you feel when working your muscles. It actually fuels muscles to help you work out longer, so pushing past that burning sensation will help you increase your strength and endurance.