The expansion of the Islamic rule during early Islamic era was quite impressive. Before 629, Muhammad's rule was limited to one city Medina. 15 years later, at the end of Umar's rule, the Caliphate more or less conquered the whole Persian Empire, and took over most of the Byzantium except probably Anatolia and Greece.
Perhaps there are many possible factors for this, including leadership, zeal, unity, the exhausting of the Persian and Byzantine empires, or some people would claim divine intervention.
But my question is more about the Muslim military manpower at that time. What kind of military training was available for the Hijaz (or Arabian) population at that time? Did they have compulsory training in combat, tactics, etc? I imagine, as powerful and mature empires during that time, Persia and Byzantium must have an advanced military knowledge and training for its armies. How about the Muslims?