I'm ashamed to say I'm surfing the net for information and found something
interesting. My optician tells me that 'muscle imbalance' causes eyestrain. I
don't understand how but was interested to see this when doing some research,
this is from the Indian Journal of Opthalmology which says:

Ideal treatment of myopia is aimed­
1. To give optimal visual correction.
2. To halt the progression of myopia.
3. To cause regression in myopia
4. To treat associated muscle imbalance.

What exactly about myopia causes a muscle imbalance? I've had myopia for
years but only recently the so-called muscle imbalance.  Can anyone describe
for me what causes it and what happens?
Many thanks