Hi, dear
I have gone through your question. I can understand your concern.
First of all
masturbation has no relation with penis size,
sperm count or motility. So no need to worry about masturbation.
Your sperm count is at low normal range but your
sperm motility is low. If you want to conceive then you need high motility.
Check for any infection and if needed take antibiotics for infection.
You can go for semen wash to increase motility.
Artificial insemination is the other option to conceive.
For knee and
back pain you need some investigations like X ray or MRI. Consult your doctor and take treatment accordingly for that.
Hope I have answered your question, if you have any doubts then contact me at bit.ly/Drsanghvihardik, I will be happy to answer you.
Thanks for using health care magic.
Wish you a very good health.