Neurological · Patient education from the Pegasus Peak physiotherapy team.
Reviewed by the Pegasus Peak physiotherapy team — AHPRA-registered physiotherapists led by principal physiotherapist Kosta Logothetis. Last reviewed 2025-10-15.
Learn what myotomes and dermatomes are, how they’re tested, and why they matter for assessing nerve health and muscle strength in the upper limb.
How physios pinpoint which nerve root might be behind your weakness or numbness — and why pattern recognition matters.
If you’ve ever wondered how clinicians figure out which nerve might be behind muscle weakness or altered sensation, the answer often lies in understanding myotomes and dermatomes. These patterns form the basis of neurological assessment and give valuable clues about which spinal nerve roots are involved when something isn’t working quite right.
Let’s break them down simply and clearly.
Myotomes aren’t specific muscles or structures, they’re patterns of movement controlled by individual spinal nerve roots.
Each spinal nerve sends signals to certain groups of muscles. So, when you perform a specific movement, it’s those nerves that are firing. If that nerve root becomes irritated or compressed, the signal it sends weakens, and so does the corresponding muscle action.
In other words, myotomes help us understand which nerve is responsible for which movement. When we test strength in certain motions and notice weakness, we can begin to trace it back to a specific spinal level that might be affected.
For example, when assessing the upper limb, clinicians typically start at C4 and go through to T1, these are the main cervical and thoracic nerve roots that control shoulder, arm, and hand movements.
While myotomes relate to movement, dermatomes relate to sensation.
The worddermameans “skin,” and dermatomes are simply skin regions supplied by a single spinal nerve root. When sensation changes, like numbness, tingling, or hypersensitivity, it often follows one of these dermatome patterns.
Testing dermatomes is straightforward: a clinician lightly touches the skin on each area and compares the feeling between sides. If one side feels dull or different, it could indicate irritation or damage to the corresponding nerve root.
So while myotomes look at output (movement), dermatomes look at input (sensation), together giving a full picture of how the nervous system is functioning.
When it comes to the upper limb, movements can be linked to specific nerve roots. The following table outlines the main myotomes and their associated actions.
Note: Theboldednerves are considered thedominantcontributors.
Shoulder Elevation
C4
Glenohumeral External rotation
C5
Glenohumeral Internal rotation
C6, C7, C8
Shoulder Abduction
C5
Shoulder Adduction
C6, C7, C8
Shoulder Flexion
C5
Shoulder Extension
C6, C7, C8
Elbow Flexion
C5,C6
Elbow Extension
C6,C7
Wrist flexion
C6,C7
Wrist Extension
C6, C7
Wrist Supination
C6
Wrist Pronation
C7, C8
Digit Flexion
C7,C8
Digit Extension
C7, C8
Finger Abduction/Adduction
T1
(Gest, 2020).
This pattern helps clinicians quickly identify where weakness is coming from. For instance, if elbow flexion (C5, C6) is weak but elbow extension (C6, C7) is strong, C5 might be the main culprit.
Yes, and that’s actually a good thing.
The body is built with redundancy, meaning multiple nerves can supply similar regions or actions. This overlap is due to the brachial plexus, the network of nerves that branches out from the cervical spine to the shoulder, arm, and hand.
Because of this overlap, you can’t always pinpoint a problem from one movement alone. Instead, clinicians compare several movements.
For example:
It’s a process of ruling in and ruling out, combining movement tests and sensory checks to narrow down the affected nerve root.
Dermatomes follow a much simpler and more sequential pattern than myotomes.
Starting with C4, which supplies sensation around the shoulder and lateral deltoid region, each subsequent nerve root wraps around the arm in order until T2, which supplies sensation around the armpit and upper inner arm.
This predictable pattern means that if a patient reports numbness or tingling along, say, the thumb and index finger (C6), a clinician can quickly suspect involvement of that specific nerve root.
Visual diagrams often help here, but in essence, dermatomes form a map of sensation that runs down the arm in a spiraling, organized way.
(Tyrrell, 2018).
Understanding myotomes and dermatomes is like having a roadmap to the nervous system. They don’t exist as tangible structures but as functional patterns that help us make sense of how the body’s electrical wiring connects to movement and sensation.
By testing both strength and touch, clinicians can localize where nerve irritation or compression may be occurring, whether from a disc injury, inflammation, or another cause.
Although there’s some overlap between nerves, the combination of pattern recognition, systematic testing, and comparison between sides allows for accurate identification of the problem area.
Ultimately, knowing your myotomes and dermatomes doesn’t just help with diagnosis, it deepens the understanding of how the nervous system coordinates every move and feeling in the upper limb.
References:Tank, P. W., Gest, T. R., & Burkel, W. (2009).Lippincott Williams & Wilkins atlas of anatomy. Lippincott Williams & Wilkins.
Tyrrell, R. (2018, May 14).Dermatomes and myotomes. Geeky Medics.https://geekymedics.com/dermatomes-and-myotomes/
Questions about upper limb myotomes and dermatomes? Our physiotherapists in Horningsea Park and Gregory Hills can assess and treat you — no referral needed.
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