Original Editor - Your name will be added here if you created the original content for this page.
The anatomical term myotome refers to the muscles served by a spinal nerve root. A myotomes is therefore a set of muscles innervated by a specific, single spinal nerve.The term is also used in embryology to describe that part of the somite which develops into the muscles.
There are 31 spinal nerves.Each vertebrae has a spinal nerve. The nerves are categorized by the vertebra which house them. There are:
8 cervical nerves, 12 thoracic nerves, 5 lumbar nerves, 5 sacral nerves,1 coccygeal nerve.
16 of these 31 nerves has a specific myotome that controls voluntary muscle movement. 
Most muscles in the limbs receive innervation from more than one spinal nerve root, and are hence comprised of multiple myotomes. For example, the biceps brachii muscle flexes the elbow. It is innervated by the musculocutaneous nerve, which is innervated by C5, C6 and C7 nerve roots. All three of these spinal nerve roots can be said to be associated with elbow flexion.
The list below details which movement(s) has the strongest association with each myotome:
C5- shoulder abduction
C6– Elbow flexion Wrist extension
C7 – Elbow extension
C8 – Finger flexion
T1 – Finger abduction
L2 – Hip flexion
L3 – Knee extension
L4 – Ankle dorsiflexion
L5 – Great toe extension
S1 – Ankle plantarflexion
Myotome testing is an essential part of neurological examination when suspecting radiculopathy. Muscle strength in a particular myotome may help in identifying at which level a nerve root compromised. Testing of myotomes, in the form of isometric resisted muscle testing, gives information about the level in the spine where a lesion may be present. During myotome testing, you are looking for muscle weakness of a particular group of muscles. Results may indicate lesion to the spinal cord nerve root, or intervertebral disc herniation pressing on the spinal nerve roots.
Begin by asking the client to perform a movement as per instructions and hold an isometric contraction against therapist resistance for a count of 5.
C5- Shoulder abduction Ask the patient to raise both their arms to the side of them simultaneously as strongly as then can while the examiner provides resistance to this movement. Compare the strength of each arm.
C6- Elbow flexion Test the strength of lower arm flexion by holding the patient's wrist from above and instructing them to "flex their hand up to their shoulder". Provide resistance at the wrist. Repeat and compare to the opposite arm. This tests the biceps muscle. Test the strength of wrist extension by asking the patient to extend their wrist while the examiner resists the movement. This tests the forearm extensors. Repeat with the other arm.
C7- Elbow extension Ask the patient to extend their forearm against the examiner's resistance. Begin their extension from a fully flexed position because this part of the movement is most sensitive to a loss in strength. This tests the triceps. Note any asymmetry in the other arm.
C8- Finger Flexion Examine the patient's hands. Look for intrinsic hand, thenar and hypothenar muscle wasting. Test the patient's grip by having the patient hold the examiner's fingers in their fist tightly and instructing them not to let go while the examiner attempts to remove them. Normally the examiner cannot remove their fingers. This tests the forearm flexors and the intrinsic hand muscles. Compare the hands for strength asymmetry. Finger flexion is innervated by the C8 nerve root via the median nerve.
C8- Finger abduction & adduction Test the intrinsic hand muscles once again by having the patient abduct or "fan out" all of their fingers. Instruct the patient to not allow the examiner to compress them back in. Normally, one can resist the examiner from replacing the fingers. Finger abduction or "fanning" is innervated by the T1 nerve root via the ulnar nerve.
C8 & T1- Thumb Opposition To complete the motor examination of the upper extremities, test the strength of the thumb opposition by telling the patient to touch the tip of their thumb to the tip of their pinky finger. Apply resistance to the thumb with your index finger. Repeat with the other thumb and compare. Thumb opposition is innervated by the C8 and T1 nerve roots via the median nerve.
L1 & L2 : Hip Flexion Proceeding to the lower extremities, first test the flexion of the hip by asking the patient to lie down and raise each leg separately while the examiner resists. Repeat and compare with the other leg. This tests the iliopsoas muscles.
L3 Test extension at the knee by placing one hand under the knee and the other on top of the lower leg to provide resistance. Ask the patient to "kick out" or extend the lower leg at the knee. Repeat and compare to the other leg. This tests the quadriceps muscle.
L4: Ankle Dorsiflexion Test dorsiflexion of the ankle by holding the top of the ankle and have the patient pull their foot up towards their face as hard as possible. Repeat with the other foot. This tests the muscles in the anterior compartment of the lower leg.
L5: Great toe extension Ask the patient to move the large toe against the examiner's resistance "up towards the patient's face". This tests the extensor halucis longus muscle.
S1: Ankle plantar flexion and eversion/knee flexion Holding the bottom of the foot, ask the patient to press down as hard as possible. Or in standing rise up onto the ball of their foot. Repeat with the other foot and compare. This tests the gastrocnemius and soleus muscles in the posterior compartment of the lower leg.
S2: Test flexion at the knee by holding the knee from the side and applying resistance under the ankle and instructing the patient to pull the lower leg towards their buttock as hard as possible. Repeat with the other leg. This tests the hamstrings. 
The comprehensive videos below on Myotomes also report on the clinical significance of these tests. From studies and a systematic review (2017) they report the finding attribute a weak clinical significance to these tests but, they point out, they are the best tools we have. 
- Wikipedia. Myotome. Available from: https://en.wikipedia.org/wiki/Myotome (last accessed 22.4.2019)
- Study.com. What are myotomes. Available from: https://study.com/academy/lesson/what-are-myotomes-definition-testing.html (last accessed 22.4.2019)
- Teach me anatomy. Myotomes. Available from: https://teachmeanatomy.info/the-basics/embryology/myotomes/ (last accessed 22.4.2019)
- Slide share. Dermatomes and myotomes. Available from: https://www.slideshare.net/TafzzSailo/special-test-for-dermatomes-and-myotomes (last accessed 22.4.2019)
- Physiotutors. Myotomes upper limb. Available from: https://www.youtube.com/watch?v=kPuQPqBMGj0&t=92s (last accessed 23.4.2019)
- Physiotutors. Myotomes of the lower limb. Available from: https://www.youtube.com/watch?v=SzAyUsA25MQ (last accessed 23.4.2019)