I’ve been working out for about 4 months now (HIIT on a spin bike and a bit of weightlifting ~20 mins.) three times a week. I’m seeing good improvement in health and physique despite having no idea what I’m doing.
My motivator was calories, but now when I work out I burn way less calories because my body is getting used to the stress, I think. What’s happening? Is my workout less useful than before? Can I not eat as much or is my body burning more fuel now?
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If you’re doing the same exercise you’ll burn the same amount of calories*. If your fitness tracker is reporting fewer calories burnt, that may be just be an error. Most methods of estimating calories are inaccurate to some degree, and some activities are harder to get good estimates for than others. If your fitness tracker uses heart rate to calculate calories, that’s a likely source of error. If you’re in better aerobic condition then your lungs, heart and vascular system are now better at delivering the oxygen you need to your muscles, which means your heart doesn’t have to beat as fast as before, but that doesn’t mean the exercise magically requires less energy now. The basic physics don’t change.
*There is a caveat to this statement, which is that some types of exercise involve moving your own body weight around, and so these require less energy if you lose weight. For instance, a 200-lbs person walking 10 miles will burn more calories than someone walking beside them who weighs 150 lbs. However, calories burnt on a stationary bike do not depend on weight since you’re pushing against a fixed external resistance (unless you’re doing spin class-type exercises where you get up out of the saddle and engage in a lot of vertical motion, but that’s not something the bike can track). Weightlifting *can* depend on your weight, e.g. when doing squats, but your weightlifting likely only accounts for a small portion of your calorie burn anyway, and also 20 lbs of weight loss isn’t that much relative to the combined weight of your body and the weights you’re using so it shouldn’t cause a big difference.
Also, there may be slight improvements in your body’s efficiency at doing certain things, which mean that you waste less energy doing the same exercise, but again that should be minor and definitely not something a fitness tracker can account for.
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