Usually when people complain of eye strain, it's due to one of two reasons.

Poor eye teaming

One reason is poor eye teaming mechanisms, such as overconvergence or convergence. It is the ability of the eyes to move together as one. In the brain, there is a nerve center that commands the eyes to make this motion or converge.

Poor accommodative system

The other reason is a poor accommodative system. Basically, the accommodative system controls the lens inside the eye to either become flat and release focus as we look away, or to become bulbous and focus closer. When we're young, we have this ability all the time and the system is working without us even knowing it. The muscles controlling that system are smooth muscles. The smooth muscles work subconsciously. As we age, we lose the ability to flex the lens as easily. And most of us end up needing bifocal glasses, or progressive glasses or reading glasses at a certain point in our lives.

Treatment with vision therapy

The reason why kids have this problem is usually because the system did not develop properly. This is something which can be trained through vision therapy. The objective here is to work these muscles in contraction and expansion through plus and minus lenses, and then move the lens to flat and bulbous. Thus, we're creating a pathway from our brains to our eyes that says focus at this distance or focus in this manner. Thus, the kid can build focusing power while also having the ability to focus in a flexible way. It is important when the child needs to see up close and far away at school. Flexibility skills are essential for copying the board quickly enough to get all the notes down.