Is it possible to move your arms above your head when handcuffed in the back?
This is not normally possible or would be extremely painful if done. But there are special cases.


  
  Atraumatic dislocation. 
  This occurs when the shoulder dislocates with minimal force such as reaching up for an object or turning over in bed. Usually it will 'pop' back in itself or with a little help. Normally this type of dislocation does not need reducing in A&amp;E. It can occur regularly throughout the day and will be associated with certain positions the arm is placed into. This type of dislocation is associated with people that have 'lax' joints, for example people who hyper-extend their knees and elbows and can get the palms of both hands onto the floor with ease.  This joint laxity is normal for these people and the onset of dislocation can be associated with a change in how the muscles around the shoulder are interacting with each other or a change in posture/ position of the arm. This can produce an imbalance in the control of the joint. Referral for appropriate physiotherapy is the initial form of management. The physiotherapist should look at the way in which the muscles and shoulder joint is moving and posture aiming to restore the balance. Treatment can 'cure' the problem as long as the exercises and advice is continued, but in some cases there is only minimal or nil benefit. At this point surgical intervention is indicated. 
  Positional Non-traumatic dislocations. 
  This group of people can dislocate their shoulders without any form or history of trauma. Some may have started out dislocating their shoulder as a party trick; others may have always had shoulders that just 'fall' out of joint. This type of dislocation is usually painless and can be put back in easily. Both shoulders are typically involved. The cause of this type of dislocation is usually a result of what we call 'abnormal muscle patterning' which means the strong muscles around the shoulder joint are not working in the correct order causing them to pull the shoulder out of joint with active movement in the particular direction such as lifting the arm forward above the head or out to the side and above the head.  The main treatment for this is physiotherapy that looks at re-sequencing the muscles in order to prevent further dislocations.  Occasionaly surgery in the form of thermal capsular shrinkage or plication may be neccessary.
  


Taken from here such people would be able to dislocate then get their hands in front and relocate.

The body can be trained to be quite flexible through training like gymnastics etc...