EMG's

Electromyography (EMG) is a technique for evaluating and recording the activation signal of muscles. EMG is performed using an instrument called an electromyograph, to produce a record called an electromyogram. An electromyograph detects the electrical potential generated by muscle cells when these cells contract, and also when the cells are at rest.

There are two kinds of EMG in widespread use: surface EMG and needle (intramuscular) EMG. Needle EMG requires the insertion of needles into the muscle. This is invasive and for this reason we use surface EMG where gel pads are placed on the skin only, similar to an EKG which is done to investigate cardiac problems. If a satisfactory result can not be obtained from the surface EMG, then a needle EMG may be indicated.

There are many applications for the use of EMG. EMG is used clinically for the diagnosis of neurological and neuromuscular problems. It is used diagnostically by gait laboratories and by clinicians trained in the use of biofeedback or ergonomic assessment. EMG is also used in many types of research laboratories, including those involved in biomechanics, motor control, neuromuscular physiology, movement disorders, postural control, and physical therapy. At The Spine Center, we use EMG to evaluate radiculopathy which is pain, numbness or weakness in the arms or legs and may be caused by a pinched nerve.

EMG is used to diagnose two general categories of disease: neuropathies and myopathies. Neuropathic disease has the following defining EMG characteristics:

• An action potential amplitude that is twice normal due to the increased number of fibres per motor unit because of reinnervation of denervated fibres.
• An increase in duration of the action potential
• A decrease in the number of motor units in the muscle (as found using motor unit number estimation techniques)

Myopathic disease has these defining EMG characteristics:

• A decrease in duration of the action potential
• A reduction in the area to amplitude ratio of the action potential
• A decrease in the number of motor units in the muscle (in extremely severe cases only)

Because of the individuality of each patient and disease, some of these characteristics may not appear in every case. Abnormal results may be caused by the following medical conditions (please note this is nowhere near an exhaustive list of conditions that can result in abnormal EMG studies):

• Alcoholic neuropathy
• Axillary nerve dysfunction
• Becker's muscular dystrophy
• Brachial plexopathy
• Carpal tunnel syndrome
• Centronuclear myopathy
• Cervical spondylosis
• Charcot-Marie-Tooth disease
• Common peroneal nerve dysfunction
• Denervation (reduced nervous stimulation)
• Dermatomyositis
• Distal median nerve dysfunction
• Duchenne muscular dystrophy
• Facioscapulohumeral muscular dystrophy (Landouzy-Dejerine)
• Familial periodic paralysis
• Femoral nerve dysfunction
• Fields condition [1]
• Friedreich's ataxia
• Guillain-Barre
• Lambert-Eaton Syndrome
• Mononeuritis multiplex
• Mononeuropathy
• Motor neurone disease
• Myasthenia gravis
• Myopathy (muscle degeneration, which may be caused by a number of  disorders, including muscular dystrophy)
• Myotubular myopathy
• Neuromyotonia
• Peripheral neuropathy
• Poliomyelitis
• Polymyositis
• Radial nerve dysfunction
• Sciatic nerve dysfunction
• Sensorimotor polyneuropathy
• Shy-Drager syndrome
• Sleep bruxism
• Spinal stenosis
• Thyrotoxic periodic paralysis
• Tibial nerve dysfunction
• Ulnar nerve dysfunction