Hi RR, I think it depends on what you are doing. Weight stuff adds more muscle more quickly. I am no expert though but I think a couple of weeks for it to settle? Interestingly I read that top athletes weigh loads!!
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This is a really tricky question to answer, but I'll try and explain some basics (I trained as a PE teacher and did lots of sports psychology and anatomy).
Muscle only weighs 18% more for the same MASS/VOLUME of fat. I'm sad to say that a few weeks of exercise or even weight training will NOT convert lots of fat to muscle or put on weight. It takes several MONTHS of specific exercise do do this.
Yes, athletes can't use BMI as a measure of their body fat as it will probably class most Olympians obese - especially field eventers, boxers, wrestlers etc - all people who use weight training to build the muscle for their event. But on any good BMI sites it does state that this measuremnt is not suitable for athletes etc for the reason above.
But also remember that long distance runners can end up being down in the UNDERWEIGHT category. Look at Paula Radcliffe! All skin and bone, no bulgy muscles just highly trained and a natural build for a long distance runner.
So sorry to say that unless you are doing SERIOUS exercise of 5 or 6 hours a day then this is not to blame for a STS or gain.
2 questions. Are you:
a) drinking enough after your exercise? If you're running / exercising for 1 hour you need to get
at least 2 pints of water back inside you within the next hour, on top of what you may have drunk whilst exercising.
b) getting calories back inside you immediately after you exercise? Your body burns more calories
after exercise than it does whilst you are exercising (don't ask - I don't actually understand why) and it does it for about another 24-36 hours. If you aren't eating enough immediatiately after exercising then your body will go into semi-starvation mode and actually grab calories for the next few days and not let go of them!! Hence you will not lose weight.
Diet and exercise is a VERY complicated subject and one which I find fascinating so yell if you want any more info. Pommette's guide to weight loss and exercise could be mad available!!! LOL