Balance is not a single ability but the output of three sensory systems being combined and corrected continuously. Balance training works by loading that integration process rather than by strengthening any particular muscle.
Three systems supply the information
Vision reports the position of the body relative to the surroundings. It is the channel most people rely on most heavily.
The vestibular organs in the inner ear detect head rotation and linear acceleration, providing a reference for which way is down independent of what can be seen.
Proprioceptors in muscles, tendons and the soles of the feet report joint position and pressure distribution, which is the fastest of the three channels.
The brain weights the inputs and resolves conflicts
The three streams are combined continuously, with the brain assigning more weight to whichever is currently most reliable.
When they disagree, the resulting conflict produces the disorientation felt on a moving boat or in a room with sloping walls.
Training that removes one channel, such as closing the eyes, forces reweighting towards the remaining two, which is why it feels disproportionately hard.
Corrections happen faster than conscious control
Standing still is not static. The body sways continuously and is corrected by small adjustments at the ankle and hip.
These corrections operate through reflex loops at spinal and brainstem level, because a route through conscious processing would be too slow.
Attempting to control balance deliberately usually makes it worse, which is the mechanism behind the observation that concentrating on not falling increases sway.
The system degrades with age and disuse
Vestibular sensory cells decline in number with age, and sensation in the feet reduces, particularly where circulation or nerve function is affected.
Vision is often compensating for both, which is why balance problems become obvious in dim light or on uneven ground.
Reliance on a single channel is fragile, since removing it leaves nothing adequate in reserve.
Why practice produces specific rather than general gains
Improvements from balance training tend to be specific to the conditions practised, including the surface, the stance and whether the eyes were open.
Varying those conditions produces broader transfer, which is the reasoning behind practising on different surfaces rather than perfecting one position.
Anyone with recurrent dizziness, a history of falls or an inner ear condition should be assessed clinically, since several treatable causes present the same way.