Neuromuscular electrical stimulation (NMES) and functional electrical stimulation (FES). Physiological principles, stimulation parameters, therapeutic goals, and clinical applications

01Physiological principles

Electrotherapy can make a muscle work when the patient cannot, or not enough. Neuromuscular electrical stimulation (NMES) does it for exercise; functional electrical stimulation (FES) does it for a task. Both use the patient's own nerve as the wire: the current fires the motor axons under the electrodes, and the nerve fires the muscle. The tetanic (faradic) current of the professor's course, 50 Hz rectangular pulses in trains, is the classic form; modern devices use charge-balanced biphasic pulses.

From one pulse to a smooth contraction

1. 1

   One pulse, one twitch

   A pulse above motor threshold fires each reached motor unit once: a twitch.
2. 2

   Summation

   From about 10 Hz each twitch starts before the last has relaxed, and the force adds up into a trembling contraction.
3. 3

   Tetanus

   From about 30 Hz, fully by 50 Hz, the twitches fuse into a smooth contraction. Higher frequencies add little force and bring fatigue sooner.

Force rises in two ways: with intensity and pulse duration (more units reached, deeper) and with frequency (each unit pulling harder, up to fusion).

Why electrical contractions tire muscle

In a voluntary contraction the brain follows the size principle: small, fatigue-resistant units are recruited first, large fast ones last, and units take turns resting. A current does it differently:

• It reaches the axons nearest the electrodes, and among them the thickest first, which supply large, fast, fatigable units.
• It fires the same units every time, all together, at the set rate: no rotation.

So the muscle tires quickly. The answers are long off times (often three to five times the on time at first), ramps, the lowest frequency that still gives a smooth contraction, and moving electrodes a little between sessions. This also means NMES trains fast fibres well, which is useful in disuse, where they waste first.

How NMES makes muscle stronger

Muscle grows stronger when it is overloaded. A strong enough electrical contraction overloads it like a heavy weight would, and it also overcomes reflex inhibition, as in the quadriceps after a knee operation, where pain and swelling switch the muscle off however hard the patient tries. Weak, comfortable stimulation does not strengthen: the force of the contraction is the dose.

The motor point

The motor point is where the motor nerve enters the muscle, the spot on the skin where the least current gives the strongest contraction. It is found by moving a small electrode over the muscle at low intensity. One electrode goes there, the other further along the muscle.

When the nerve is dead

NMES and FES need an intact lower motor neuron: anterior horn cell, root, peripheral nerve. After a complete peripheral nerve lesion, the axon beyond the injury degenerates within days, leaving only muscle fibres. These have a long chronaxie and need pulses of tens to hundreds of milliseconds, best with a slow rise so that healthy skin nerves are not fired: the exponential current of low-frequency pulsed currents. Upper motor neuron lesions (stroke, spinal cord injury above the conus, multiple sclerosis, cerebral palsy) leave the lower motor neuron intact, so NMES and FES work.

02Stimulation parameters

pulse duration for most NMES; longer for large muscles

200–400µspulse duration for most NMES; longer for large muscles

for a fused contraction; 50 Hz the classic tetanic current

30–50Hzfor a fused contraction; 50 Hz the classic tetanic current

of maximal voluntary force, for strengthening where tolerated

at least 50per centof maximal voluntary force, for strengthening where tolerated

Placement

• Both electrodes over the muscle, along its fibres, one over the motor point: bipolar placement localises the contraction.
• A small active electrode and a large dispersive one: concentrates the effect under the small one.
• For FES, over the nerve as it passes near the skin: the common peroneal nerve behind and below the fibular head for dorsiflexion, the median and radial nerves for the hand.
• Never on both sides of the trunk at once in a way that sends current across the chest; the lecture also advises stimulating the abdominal muscles on one side at a time.

Ways of triggering

03Therapeutic goals

04Clinical applications

Orthopaedics and sports

After knee surgery (anterior cruciate ligament reconstruction, knee replacement) the quadriceps switches off and wastes. NMES started early, at high intensity, alongside exercise, speeds strength recovery. It is also used during immobilisation in a cast or brace, and after hip fracture. See rehabilitation after joint replacement.

Advanced disease and intensive care

People with severe COPD, chronic heart failure or cancer often cannot do whole-body exercise: they become breathless before their legs are trained. NMES of the quadriceps loads the legs without loading the heart and lungs much. A Cochrane review found it increases quadriceps strength (low-quality evidence) with no serious adverse events, and suggests it can be considered as an exercise treatment within rehabilitation, especially for patients who find ordinary exercise hard to take part in. In intensive care, trials of NMES against weakness show mixed results.

Stroke and other central lesions

• Foot drop: FES of the common peroneal nerve, switched on by a heel switch as the heel lifts at the end of stance and off when it strikes the ground again, lifts the foot through swing and makes walking faster and safer. NICE supports it for drop foot of central neurological origin (stroke, multiple sclerosis, incomplete spinal cord injury, cerebral palsy). Implanted versions exist.
• Shoulder subluxation: NMES of supraspinatus and posterior deltoid in the flaccid stage reduces subluxation; a Cochrane review found no clear effect on shoulder pain.
• Upper limb: NMES and FES of wrist and finger extensors, especially EMG-triggered and combined with task practice, help motor recovery in some patients.
• Swallowing: NMES of the neck muscles for dysphagia, with mixed evidence, by trained teams.

See stroke rehabilitation.

Spinal cord injury

FES lets paralysed muscles work: FES cycling and rowing for fitness, bone and circulation; standing and short-distance walking systems; hand grasp devices in tetraplegia; implanted sacral anterior root stimulators for bladder emptying; phrenic nerve or diaphragm pacing in high tetraplegia. In injuries at T6 or above, stimulation below the lesion can trigger autonomic dysreflexia (a dangerous surge in blood pressure), so blood pressure is watched. See spinal cord rehabilitation.

Other uses

• Pelvic floor stimulation for stress and urge urinary incontinence.
• Calf stimulation to prevent venous thrombosis when compression cannot be used.
• Denervated muscle after peripheral nerve injury, with long pulses, while the nerve regrows (see neuropathies).
• The diadynamic RS form and sinusoidal modulated currents in the old school, for innervated muscle.

05Contraindications and safety

Expected side effects are muscle soreness after strong sessions and skin redness under the electrodes. Very intense stimulation of large muscles can, rarely, damage muscle. Electrodes are checked for full contact, because a peeling electrode concentrates current and can burn; the intensity is raised from zero and lowered to zero before removal.