Hello Wela,
BMI160 had many features(any motion, significant motion, step counter, tap, etc...) because of internal algorithm inside, internal algorithm was not open out, but you could use the output result of algorithm.
In BMI160, it had step detector feature. For you application, if step event received you would know current activity status was not rest. If no step event, you could think current activity status was rest. This is my suggestion.
You could refer more information from BMI160 data sheet.
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