"Eye exercises" is commonly used to describe non-prescribed, non-doctor-directed activities for vision correction. "Vision training" is commonly used to describe prescribed, doctor-directed activities for vision correction. The thought by some is that muscles of the eye are responsible for the development of myopia and that by either strengthening, relaxing or better coordinating these muscles that myopia can be prevented, controlled or eliminated.
Because of the confusion over what eye exercises means, one has to be specific about the actual type of treatment. The shortest answer is that there is no credible evidence that exercises work to control myopia but there are valid reasons to suspect that prescribed vision training for one type of myopia holds some promise of benefit. This type of myopia is where an individual has a tendency to cross their eyes slightly when reading (they don't actually cross) AND where the eyes don't fully focus on the book but rather further away, behind the book, something called a lag of accommodation. You can read about the research in the Research menu under the "Accommodation (focusing)" topic. Your doctor is the only one who can determine if you meet these criteria and might benefit from directed vision training.
Because "exercises" often seems to imply a non-prescription treatment with no doctor involvement, the idea of exercises to treat myopia has a large following in the alternative lifestyle press. There are volumes of testimonials available from people who say their eyes got better or from people offering to sell programs of exercises designed to do the same. Read more about these ideas under Pseudo Science in the Treatments menu.
People can get used to blurred vision and they can become better at guessing at blurred letters. For some that is enough "proof" that their eyes improved. Scientists measure myopia by where the light focuses in the eye and the length of the eyeball. Identifying letters on an eye chart is a secondary method that is quite variable between different patients with normal vision.
The eye has six large external muscles that control where the eye is aimed. Coordination of the muscles between the two eyes is important to avoid double vision and eyestrain. You don't want your left eye looking left while your right eye looks up. The eye also has a smaller internal muscle that controls the distance at which the eye is focused. This ciliary muscle can either activate (perform work) and focus the eye on objects nearby, or it can relax to focus on objects further away. The muscle is attached to and changes the shape of the lens in the eye to change the eye's focus. As a person ages, the lens loses elasticity (just like skin which ages and loses elasticity resulting in wrinkles) and the now older, more rigid lens will not reshape itself for different distances. At that point, usually in a person's forties, reading glasses become necessary.
It helps to understand how the focusing muscle works. The muscle is attached to the lens by fibers, essentially strings. The ciliary muscle is a circular muscle with the lens in the middle of the circle. Imagine a water balloon with a circle of strings attached to an outer ring, the ciliary muscle. If the ring of muscle activates, it makes the opening smaller. This relaxes the strings attached to the lens in the center and the lens is free to bulge large under its own elasticity, just like a water balloon. If the muscle relaxes, making the opening larger, the strings are pulled tighter and the lens is pulled against its elasticity into a flatter shape that allows for distance vision.
Most "exercises" described as helping myopia in the alternative press are actually relaxation techniques, or perhaps flexibility techniques. A common exercise would be to roll your eyes in a circle. There is no problem with that by itself. Some people do have restricted muscle actions, but it is generally the more elderly population, not the age group concerned about myopic progression. Other exercises work to relax the real focusing muscle, the ciliary muscle. The idea is that if the ciliary muscle can be relaxed enough, the eye should return to distance focus as the lens is pulled flatter. Thus directions for myopia control in the popular press will be along the lines of "Close your eyes, hold your warm hands over your eyes and imagine you can see pure black." The only problem is that myopia is caused by the eye being too long for the relaxed muscle. If relaxing the muscle creates an improvement in vision, the person wasn't that nearsighted to begin with. That is why doctors use various techniques to determine if the ciliary body is not releasing its full focus, often including the use of special eye drops that numb the muscle (a procedure called cycloplegia) so a determination of the true amount of myopia can be determined without interference from the muscle.
It is possible to cramp the muscle by overworking it for near vision tasks, such as long periods of time focused on objects very close. This is called pseudo myopia and is sometimes noticeable at the end of the day when distance vision is more blurred but is clear by morning or perhaps just blurred all the time. Relaxation techniques, vision therapy or eye exercises might work in these situations to relax the muscle but there has been no study to show that pseudo-myopia, if not treated, leads to more myopia. Sometimes glasses are prescribed to try and prevent the condition by wearing a different prescription (reading glasses) with the goal of preventing the cramping from occurring. No study has been done to show any decrease in myopic progression when prescribing glasses for pseudo myopia. At the same time, there is no harm in relaxation. Studies would suggest that prescribing directly for pseudo-myopia, in effect over prescribing of power, should lead to more myopic progression, but no study has tested that theory either.
Several studies have shown that something called "lag of accommodation" is greater for people with myopia. Accommodation is the term used to indicate muscle activated focusing of the eye for near distances. It is possible to measure how accurately a person is doing this focus. The most common situation is that a person will "lag" or under-focus for the distance being viewed. Thus if the book is 18 inches away, a person may lag their focus so that their eyes are actually in focus at perhaps 22 inches, close enough to read the text but not the sharpest focus. Some exercises are aimed to make this activity more accurate with the goal of reducing feelings of eyestrain or even to prevent the progression of nearsightedness. Studies differ on the meaning of this lag. Some indicate that treating the lag will slow progression and some have shown that the lag does not seem to be predictive of future myopia but rather a result of myopia that has already developed. It may be that the lag is an indicator who will actually benefit from treatment, not who will eventually develop myopia.
The main risk is that after significant investments of time and money you find yourself still nearsighted and continuing to become more nearsighted.