Later, together with his brothers Bosio, Francesco and Bartolo and two cousins, he joined the company of Alberico da Barbiano, who nicknamed him "Sforza" ("Strong") for his staunchness and his abilities to suddenly reverse the fortunes of battles. In 1398 he was at the service of Perugia against the Milanese troops of Gian Galeazzo Visconti, to whom Muzio soon switched his loyalty following the typical behavior of mercenary chieftains of the time. Later he fought for Florence against Visconti but in 1402, at the battle of Casalecchio, was defeated by his former master Alberico da Barbiano. In 1406 he captured Pisa and was subsequently hired by Niccolò III d'Este of Ferrara, who was being menaced by Ottobono Terzi of Parma.
