The Physical Activity IG is based on US Core 3.1.1 where Simple Observation does not yet exist, which makes derivation impossible.  The Physical Activity IG defines a large number of observation profiles capturing information related to physical activity (e.g. step count, heart rate, minutes per day of exercise, etc.)

All of these observations largely comply with the US Core R6 "Simple Observation" profile, which is not available in the 3.1.1 release used by this IG.  All are fully aligned with the Simple Observation profile with one exception:  
The physical activity profiles do not have the 'derivedFrom' element marked as mustSupport.  In general, these observations will always be recorded directly and not derived.  The high-level observations that capture things like "minutes per day" could potentially be derived from a QuestionnaireResponse, but this is not supported in US Core 6.0.0, and even if it were, the physical activity use-case doesn't care about this relationship enough to impose it as a mustSupport on implementers.

Please approve an exception for US Core derivation requirements here.