The four variants of the Romberg's test altered visual input and auditory attention to the task

The nervous system must monitor the behaviour of muscles. It gets feedback information about the length of a muscle from

The stretch reflex is

Reflexes are automatic and nearly instantaneous responses elicited only by changes in muscle length

The stretch reflex

Motor units are activated in order of the number of motor neurons they contain.

Reflexes are automatic responses but can be modulated by input from higher brain levels

Motor units act as stretch receptors

Extending the leg by the maximum possible distance increased the size of the patellar reflex.

The motor units in a skeletal muscle are activated in the order from small motor units to large motor units.