Please help, bringing back into work

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Intermittent lameness, nerve blocks, xrays and remedial shoeing and it is now believed it is a shoulder injury that has caused the problem. Loss of muscle on his left side and I have been told to bring the pony back into work, paying particular attention to increasing muscle tone in his shoulder. What exercises can you suggest to develop more all round muscle tone?
Many thanks for your help.

 

[spike123]

once you are allowed to start trotting under saddle lots of hillwork in a really slow trot and raised trotting poles.