An old myth laid to rest

Where did the idea come from that bluegrass bands have to stand still and look solemn? As a necessary corrective to this, as a counter to Monday blues, and as a favour to yourself, look at this clip of Flatt & Scruggs & the Foggy Mountain Boys, with 'Uncle Josh' Graves and 'Cousin Jake' Tullock duetting on 'Hand me down my walking cane.'