What is a reasonable estimate of damages for pain and suffering with a severe double fracture of humerus bone in arm near shoulder?

If the treadmill was defective, you would have separate claims of negligence and strict liability against the manufacturer of the treadmill.  If the treadmill was defective, you would also have claims of negligence and strict liability against the gym.  If the treadmill was not defective, your only claim would be for negligence against the gym.  Aside from the mechanical problem with the treadmill, you may also be able to argue that the treadmill was defective because there weren't any markings which would have served as warnings on the moving path.

Negligence is the failure to exercise due care (that degree of care that in this case a reasonable gym and/or a reasonable manufacturer would have exercised under the same or similar circumstances to prevent foreseeable harm.  With regard to the manufacturer, the standard of care would be that degree of care to produce a product that is not defective.

Strict liability imposes liability whether or not due care was exercised.

When you complete your medical treatment and are released by the doctor or are declared by the doctor to be permanent and stationary which means you have reached a point where no further improvement is anticipated, obtain your medical bills, medical reports and documentation of any wage loss.  Your personal injury claim will consist of these items.  File your personal injury claim with the insurance carriers for the manufacturer of the treadmill and with the gym.   Compensation for the medical bills is straight reimbursement.  Compensation for wage loss is straight reimbursement.  The medical reports will document the nature and extent of your injury and will be used to determine compensation for pain and suffering.  Compensation for pain and suffering is an amount in addition to the medical bills.

There isn't any mathematical formula for determining compensation for pain and suffering.  I would normally ask for quadruple the medical bills as compensation for pain and suffering, but NOT expecting to get that.  That would be a starting point in negotiations with the insurance company or in this case insurance companies.  The insurance companies will respond with much lower offers and you can continue negotiations,  If you are dissatisfied with settlement offers from the insurance companies, reject the offers and file your lawsuit for negligence and strict liability against the defendants.  If the case is settled with the insurance company/companies, NO lawsuit is filed.  If the case is NOT settled, you will need to file your lawsuit prior to the expiration of the applicable statute of limitations or you will lose your rights forever in the matter.  If the case is settled with both defendants' insurance companies (gym and treadmill manufacturer) NO lawsuit is filed.  If the case is settled with one of the defendants, only name the other defendant in your lawsuit.  If the case is NOT settled with either insurance company, name both defendants in your lawsuit.  If the case is NOT settled, you will need to file your lawsuit prior to the expiration of the applicable statute of limitations or you will lose your rights forever in the matter.  It would be advisable to speak with a personal injury attorney.