Eye strain, or asthenopia, is fatigue of the ciliary muscles and extraocular muscles. Eye strain does not cause myopia, but it can lead to pseudomyopia. Eye strain can lead to discomfort, including soreness and redness. As a rule the goal of EndMyopia is to use stimulus without causing unnecessary strain.
The over-worked muscles
The ciliary muscles control focus by changing the shape of the lens. Looking at near objects for extended periods of time strains the ciliary muscle, leading to a ciliary spasm. This muscle is most relaxed when looking far in the distance.
Extraocular muscles control the movement of the eye and the eyelid. These muscles may tighten during intense visual work or when the body perceives a threat. When tightened, a person may appear to be scowling or squinting. Closing the eyes and massaging around the eye can temporarily relax these muscles, but they will quickly tighten again if the core problem is not addressed with better habits. In addition to causing discomfort, strained extraocular muscles will result in a wrinkly, unpleasant facial expression.
Causes and solutions
Blur from uncorrected vision, or severely under corrected vision, causes eye strain. This problem is solved by wearing the differential or normalized lenses that are most appropriate for the situation and distance. Good ambient lighting will also go a long way to help avoid eye strain. More light means better vision, and natural light is the best light.
Staring at a computer monitor or other electronic screen for extended periods of time may cause eye strain under certain conditions:
- The screen contrasts with the environment. The color temperature and brightness of the screen should be adjusted to match the surrounding environment. Or, the lighting in the room may need to be adjusted to better match the screen. Having the lights off in the room while staring at a screen is never a good idea.
- The screen is too close to eyes. The screen should be at least an arm's length away from the eyes. With tiny phone screens, this is impractical; the bad habit of doing everything on the phone should be replaced with a good habit of doing as much as possible on a desktop or laptop computer at a healthy distance from the eyes. If you use a smartphone, consider replacing it with a feature phone.
- The staring is being done without reasonable breaks. At the very least, the eyes should be given a short break every 20 minutes and a long break every 60 minutes.
- The screen is low-quality or uses old technology. The old CRT technology is rarely used now, but be aware that its inherent flicker strains the eyes. The human eye certainly struggles when viewing light that flickers on and off 60 times per second! Unfortunately, even the current display technology, LCD, can flicker. Low-quality LCDs use PWM dimming, where the backlight is switched on and off hundreds of times per second. High-quality LCDs are designed with DC dimming, where the light is constant.
Distance Vision Strain
Some participants report feeling strain after long sessions of distance active focus.
One suggestion for handling distance strain is to balance both near and far accommodation, which is referred sometimes as "dynamic vision" activity. 
Different theories which are still in contention were proposed to describe the mechanism of accommodation. Some of those theories are:
Helmholtz theory of accommodation (1855)– This theory is also called the capsular theory of accommodation . Helmholtz theorized that when the eyes are viewing a distant object, the ciliary muscle relaxes and the zonular fibers between the ciliary body 1 and the equator of the lens stay flattened but when the object of focus is close, the ciliary muscles contract and the zonular fibers loosen. In the Helmholtz theory, the lens equator slides away from the sclera during accommodation and closer to the sclera when accommodation ends. In this theory, the zonular fibers are relaxed during accommodation and the zonular fibers are under tension when there is no accommodation reflex.
Schacher theory of accommodation (2006)- This theory states that when the lens is in focus, there is increased tension on the lens through the equatorial zonular fibers and when there is contraction of the ciliary muscle, the zonular fibers located equatorially increase their tensile strength. This results in the steepness of the central surface of the lens, an increase in thickness of the lens and a flattening of the lens edges. As the tension on the equatorial zonular fibers increase during accommodation, the anterior and posterior zonular fibers relax. The anterior and posterior zonular fibers serve as passive support structures for the lens, but the equatorial zonular fibers determines the refractive power of the lens.
Possibly related to: Theory of Reciprocal Zonular Action: https://www.youtube.com/watch?v=1yIpyitm6eE
Catenary theory of accommodation (1970)– This theory is also called the Coleman theory of accommodation. It states that the lens and the zonula fibers form a diaphragm, which is held in a catenary (a curve formed by a wire, rope, or chain hanging freely from two points that are not in the same vertical line) shape due to the difference in pressure between the aqueous and vitreous bodies of the lens. A change in diameter of the ciliary body results in a change of the catenary shape. It means there is a continuous pressure difference on the lens. The strength of this pressure difference is approximately 2.3 cm of water column, with major changes occurring during the initial seconds of the accommodation phase. The anterior capsule and the zonular fibers form hammock shaped surface that is duplicable but depends on the diameter of the ciliary body. The ciliary body however makes a shape like the pillars of a suspension bridge, but does not need to support the force around the equator to flatten the lens.