as human beings our bodies and minds adapt to changes in our lifestyles over time, so yes you can see a decrease in fitness levels. my suggestion is to not work in the peak zones. never overtax yourself but maintain a consistant growth pattern when you train. I'm sure you've heard the saying attainable goals.
Best AnswerIt might be that you're you're not getting enough rest between intense workouts. You should not exercise in peak mode on consecutive days. Take at least a day or two of easier, less strenuous workouts between high intensity workouts to let your body recover. And get plenty of sleep.
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Sleep and nutrition. Think of it as a healthy triangle, Nutrition, Sleep, Movement. All need to be in balance for your body to recover from workouts, lose weight, grow muscle, improve repertory and cardiac performance, and so forth.
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Like any muscle in your body you have to let your heart rest. I would for sure follow all the advice here to maintain a good balance. Also, if you haven't had a chance to read about it, here is a link that can explain the fitness level feature and help you figure out how to properly measure yours... https://help.fitbit.com/articles/en_US/Help_article/2096#measure
Elena | Pennsylvania
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Also, regardless of your exercise output, a rising resting heart rate can throw off the score temporarily. Once the heart rate returns to its normal state, the score should go up again.
If you're spending a bunch of time in peak zone, I would recommend looking into HIIT training, which will make you take a break to let the heart get out of peak for a few.
Also, taking a run of at least 10 minutes, assuming the trackers has true or connected gps feature, will update the score. So if you feel it should be higher, this is one way to test it.