Complex sentence: The majority of emergency medicine deals with diagnosis of injury on-site; recovery and re-abilitation is dealt with upon arrival to a medical institution, and is generally not done in a situation where reaction time is a crucial factor in injury care.
Split into simpler sentences: The majority of emergency medicine deals with diagnosis of injury on-site. Recovery and rehabilitation is dealt with on arrival at a medical institution, and is generally not performed where reaction time is crucial to injury care.