One of the questions on the entry forms for my office is, "Which hand is your dominant hand?" This question may seem odd to some, but handedness is significant when it comes to neuromusculoskeletal conditions and how they develop and heal.
Clinical Evaluation: Right-Handed, Left-Handed ... or Ambidextrous?
Some individuals claim ambidexterity because they can use a fork or drink with either hand. For some, their primary skills are divided between their hands. An example is a person who writes right-handed, but throws left-handed. This characteristic is termed mixed handedness, a trait that is only slightly more common than ambidexterity.
A more realistic version of ambidexterity is a person capable of writing or throwing with either hand. I refer to mixed handedness as pseudo-ambidexterity. True ambidexterity is a rare trait.
More than 40 genes contribute to the determination of handedness. However, the use of a non-dominant extremity can be developed. Until recent decades, left-handed children were forced to use their right hand because being left-handed was considered sinister or "of the devil."
Ambidextrous and mixed-handed skills can be developed out of necessity when the naturally dominant hand is dysfunctional. They can also develop because of activities of daily living, job requirements or the lack of available tools designed for left-handers.
For example, the placement of controls on automobiles, scissors, can openers, levers on folding knives, and the side of hot and cold faucets on a sink are designed for right-handed individuals.
4 Reasons Handedness Is Vital in Neuromusculoskeletal Care
Posture evaluation is the first reason. The shoulder of the dominant extremity is often lower than the shoulder of the non-dominant extremity. During postural assessment, a discrepancy of shoulder height should be investigated to determine if handedness is responsible or if another reason, such as scoliosis, exists.
Motor testing of the upper extremities / grip strength and impairment rating are the second and third reasons, respectively, for knowing handedness. Basic motor testing can be easily accomplished by shaking the patient's hands. Handshaking requires proper function of the nerve roots emanating from the cervical and upper thoracic spinal regions into the upper extremities, C5 through T1.
The anterior deltoid and biceps must be flexed to extend the arm when reaching for the other person's hand. These actions require C5 and C6 nerve root function. Fingers must be extended and spread slightly to receive the other person's hand. These actions require C7 and T1 nerve root function.
To grasp the other person's hand, fingers must flex and pull together. These actions require C8 and T1 nerve root function. To stabilize the wrist during shaking, wrist flexors and extensors must hold firm, actions that need C6 and C7 nerve root function. Alternating flexion and extension of the elbow for shaking requires C5, C6 and C7 nerve root function.
Bilateral handshaking is recommended for initial motor testing. Patients should be asked during the test if it bothers them and if they are experiencing any difficulty with grip strength. Handshaking is proficient in testing the upper extremities, especially when no upper-extremity signs and symptoms are reported or observed.
If handshaking reveals any weakness or upper-extremity symptoms, testing should be supplemented with testing each upper-extremity muscle / nerve root group. Grip strength testing using a dynamometer may also be employed.
The dominant hand is typically 5-10 percent stronger than the non-dominant hand. If a dynamometer is used to test grip strength and strength is equal, differential diagnosis is necessary. The patient may be ambidextrous, use the non-dominant hand more than the average person or have a 5-10 percent loss of strength in the dominant hand.
In years past, the final rating for upper extremity impairment was not assigned until the side of handedness was considered. The non-dominant hand was rated lower than the dominant hand.
I speak of non-dominant hand ratings in the past tense, as I cannot find the rule in the newest impairment rating guides. In a recent discussion with a hand surgeon who performs a substantial number of impairment ratings, I asked him about the rule. He was unaware of it. My takeaway was the rule is no longer utilized.
While the rule is no longer applicable to impairment rating, handedness should be a factor in disability rating. As a reminder, impairment rating and disability rating are not the same. Impairment rating determines how much of a patient's body function has been lost. Disability rating determines how lost body function will influence the patient's ability to perform the tasks required for his / her vocation.
Side-posture manipulation of the lumbar spine is a fourth reason for knowing handedness. Medical researcher Harry Farfan views disk injuries of the lumbar spine as either compression or torsion injuries.
Compression injuries cause herniations of the nucleus toward the periphery or into the endplates of the vertebral bodies. Torsion injuries involve tearing of the disk's annular fibers. It is tears of the annular layers that relate to lumbar side-posture manipulation.
The annulus is composed of multiple layers of obliquely oriented fibers. Each layer is oriented in the opposite direction of its adjacent layers. The orientation of alternating layers helps the disk withstand torsion to the right and the left.
In theory, a person stresses fibers that resist torsion to one side more than another due to handedness. A right-handed individual usually twists to the left when throwing a ball, or swinging a bat, racket or club. Twisting to the left places most of the stress on the annular fibers that resist left rotation and minimal stress on the fibers that resist right rotation.
The opposite is true for left-handed individuals. Tears from repeated twisting to one side can produce what Farfan terms a torsion injury.
Farfan's theory mandates that right-handed patients receive side-posture manipulation with their right side up. Left-handed patients receive side-posture manipulation with their left side up. This way, side-posture manipulations are performed with any resulting rotation occurring in the opposite direction of the direction that attributed to the injury. In this theory, adjustments are predictably only performed on one side.
An exception occurs for ambidextrous or mixed-handed patients; side-posture manipulation may be necessary on both sides. Side-posture manipulation may be contraindicated for patients exhibiting both left and right torsion injury and instability.
General Resources
- Magee DJ. Orthopaedic Physical Assessment, 5th Edition. Canada: Saunders-Elsevier, 2009.
- Farfan H. The scientific basis of manipulative procedures. Clinics Rheumatol Dis, 1980;6(1):159-173.
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