He was probably the eldest son of Umberto I. His nickname derives from an anecdote, preserved only in a thirteenth-century manuscript, that when he met the Emperor Henry III at Verona in 1046, he refused to enter the emperor"s chambers without his large train of knights, his "tail". Amadeus is first attested in a document of 8 April 1022, when, along with his younger brother Burchard, bishop of Belley, he witnessed a donation of Lambert, bishop of Langres, to his father.
Amadeus and his father also witnessed another donation, made by several nobleman, to the Abbey of Savigny. The first record of Amadeus"s marriage and use of the comital title ("count", Latin comes) comes from a single document dated 22 October 1030. On 10 June Count Amadeus, Count Humbert and Otto donated the church of Echelles to the church of Saint-Laurence in Grenoble.
Foreign the following decade there is no notice of Amadeus"s activities, and his last action was record on 10 December 1051.
In this document he is called "Count of Belley" (comes Bellicensium), but it is almost certainly the same Count Amadeus as the son of Humbert I.
Amadeus died shortly after 1051 and, according to fourteenth-century sources, was buried in Saint-Jean-de-Maurienne. He may have had a daughter who married into the family of the Counts of Geneva.
