• muscle cannot exert normal force
• can be total loss of contraction (paralysis or plegia) or partial (pareis); also can be hemiplegic (weakness of arm and leg on same side), monoplegia (one limb) or paraplegia (both legs)
• most important causes are – upper motor neuron lesion (eg brainstem lesion); lower motor neuron lesion; extrapyramidal lesions (eg Parkinsonism, chorea); cerebellar lesions; primary muscular disorder (eg the myopathies); hysteria (eg ‘hysterical muscular weakness’).
• lower limb weakness is a common cause of gait disorders
• Weakness patterns:
• Hemiparesis – weakness of arm and leg on same side; usually upper motor neuron lesion (most commonly a CVA);
• Paraparesis – weakness of both legs; spasticity often present; most common cause is multiple sclerosis (spastic paraparesis)
• Monomelic paresis – weakness of one leg or arm; may indicate a compressive lesion (eg compression from herniated nucleus pulposus; nerve entrapment);