The most common motor impairment seen after stroke is paresis.
Paresis is a decreased ability to volitionally activate motor units and is caused by damage to the corticospinal system (the primary motor cortex, nonprimary cortical motor areas, and the corticospinal tract).
Poor or absent volitional control of motor units means that muscles and sets of muscles can neither be activated in a timely, coordinated manner nor activated with sufficient force.
Clinically, paresis appears as weakness and results in slower, less accurate, and less efficient movements compared with those in neurologically intact individuals.
A stroke will cause paresis on one side of the body, contralateral to the lesioned brain, that is, hemiparesis.
Individuals with mild paresis will have movements that appear to be normal or near normal, whereas those with severe paresis or plegia, may not be able to move at all. Contrary to common perception, it has been shown that the severity of paresis is similar across all segments of the UE and is not worse at the distal segments compared with the proximal ones.