Paresis is the medical term for partial or mild paralysis, usually described as muscle weakness. This is a common symptom of multiple sclerosis.
Although MS is a disease of the central nervous system and not of the muscles, lesions along the motor nerve pathways give rise to effective muscle weakness.
Lack of use of muscles over an extended time often leads to muscle atrophy - a wasting away of muscles. Physical therapy (physiotherapy) and appropriate exercise can help to protect against this.
Paresis in one leg is known as monoparesis, in both legs as paraparesis, in one half of the body as hemiparesis and as quadraparesis when it affects all four limbs. Paresis may often be compensated for by appropriate braces, canes, walkers, scooters, wheelchairs or other assistive devices.
Paresis can be caused by cerebellar, brainstem or spinal lesions giving rise to a loss of muscular strength, either during voluntary muscle contraction or in holding posture.
Paresis should not be confused with ataxia which is the inability to properly coordinate muscles, although both can affect walking and other activities. Paresis results from damage to the motor nerve pathways whereas ataxia results from problems coordinating sensory signals with motor ones.