Performing any one sport all the time will produce muscle imbalances if not complimented with other activities. Cycle involves many repeated movements in a position that is not ‘natural’. As already mention you use your front muscles a lot more than your upper back muscles (lower back muscle are used quite a lot), apart from any pulling action. If you have poor posture on or of the bike your shoulder complex will not work as well as it could regardless how much exercise you do. Shoulder blade position is vital for prober shoulder mechanics.
ventanarider make sure your physio assesses your thoracic spine as most people are to stiff in this area which also effects shoulder mechanics, any decent physio should be able to manip this area if it is stiff to help start the process of gaining improved mobility.
Every cyclist road or off road would benefit from a prober constructed strength / condition program.
check out ‘Bikejames’ you might not agree with his opinion on flat pedals but he does have some really good stuff on his site.