Muscle weakness

Muscle weakness is reduced ability to generate force with a muscle or group of muscles. It can arise from problems in muscle tissue, nerves, the neuromuscular junction, or the brain and spinal cord.

Motor unit: A motor neuron and all the muscle fibers it activates. Damage to a motor unit can reduce the force a muscle produces.
Manual muscle testing: A clinical examination that grades muscle strength against gravity and applied resistance. It provides a standard bedside estimate of weakness.
Myasthenia gravis: An autoimmune disorder that disrupts signaling at the neuromuscular junction and causes fluctuating weakness. Its weakness often worsens with repeated activity and improves with rest.
Asthenia: A subjective sense of tiredness or lack of energy without necessarily having measurable loss of muscle force. People may describe asthenia as weakness even when strength testing is normal.
Neuromuscular junction: The synapse where a motor neuron communicates with a muscle fiber. Impaired transmission here can prevent a muscle from responding fully to nerve signals.
Dynamometry: Measurement of force or torque using an instrument called a dynamometer. It quantifies strength when an examiner’s resistance is not precise enough.
Duchenne muscular dystrophy: An inherited muscle disease caused by loss of functional dystrophin, with progressive childhood weakness. It illustrates weakness caused by structural damage within muscle fibers.
Paresis: Partial loss of voluntary movement or strength, often caused by nervous-system dysfunction. Clinical use sometimes distinguishes partial paralysis from broader muscle weakness.
Muscle contraction: The process by which muscle fibers develop tension and generate force. Weakness occurs when fibers cannot develop normal force.
Neurological examination: A clinical assessment of nervous-system function, including strength, reflexes, sensation, and coordination. Its findings help localize the source of weakness.

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