What is ‘normal’?

Reflecting on symmetry 

In reflecting back on various conversations about symmetry, or really asymmetry of the human body, I started to think about the comparisons we make in our patients to some pre-conceived ‘norm’. Often when there is a unilateral issue we use the patient’s body to help identify what is ‘normal’ for them, but we also compare what we are seeing or feeling to our own internal understanding of what we expect to see when things are normal and healthy. Our expectations are driven by many things, including our personal and clinical experiences, as well as the literature, when available. When it comes to neurodynamic testing, this expectation of normal, healthy tissue responses often includes an expectation for some degree of symmetry between sides. But we also consider what the common responses are to structural differentiation when the nervous system is in a healthy state.

To the literature

A number of recent research studies have influenced my sense of the “norm” or at least the “common” response for neurodynamic testing in healthy, asymptomatic individuals.

For one, the difference in mobility between limbs is quite commonly asymmetrical. This was not a surprise as I know we are not truly symmetrical creatures and our daily activities and habitual use of our bodies are likely what creates and reinforces these asymmetries. What did stand out to me was the magnitude of this typical asymmetry and the consistency of this finding across studies. For example, one study found that the average between limb difference within participants was over 7º for the ULNT (Boyd 2012) and over 4-5º for the SLR depending on the variation of the test performed (Boyd and Villa 2012). Interestingly, side dominance for writing or kicking did not correlate with which side was more or less mobile. Using calculations from these studies, the authors found that 90% of healthy, asymptomatic people can be expected to have (with 95% certainty) up to 18.4º difference between limbs on the ULNT and 10.9º on the SLR (Boyd 2012). Subsequent publications have further supported this asymmetry trend with ULNT testing for both median and radial nerve in asymptomatic individuals (Stalioraitis et al, 2014). This is a remarkable amount of asymmetry within the “normal” healthy population. The greater asymmetry noted in the upper limbs over the lower limbs might be explained by the tendency to use our upper limbs in a more asymmetrical pattern for every day movements compared to the lower limbs.

Another aspect of this ‘norm’ construct is responsiveness to structural differentiation. One study found that structural differentiation (ipsilateral cervical sidebend, contralateral cervical sidebend, release of scapular block) caused an alteration in the sensory response felt somewhere in the arm, forearm or hand provoked by the ULNT in over 90% of healthy, asymptomatic participants (Boyd 2012). In a similar study (Coppieters, 2002), different pre-positioning of the neck influenced the amount of elbow motion at the end of the ULNT sequence. Similarly, another study found that pre-positioning the ankle altered the hip flexion range of motion and provoked different levels of muscle activation during SLR neurodynamic testing in healthy, asymptomatic participants (Boyd et al 2009).

Back to the clinic

So what does this all mean for our clinical decision making? Our expectation of what we are anticipating clinically when a patient’s nervous system is sensitized is informed by what we are comparing that presentation to. Our concept of ‘norm’ – our concept of the common responses of a healthy nervous system, makes for an important comparison point. Our determination of a ‘positive’ test changes when we are expecting a certain degree of asymmetry even when the nervous system is healthy. Merely the presence of an asymmetry is rarely sufficient to determine relevance. Conversely, symmetry does not imply everything is ‘normal’ either.

One can have a bilateral sensitivity of the nervous system that presents as symmetrical on neurodynamic testing that may be quite relevant. This determination of  a ‘positive’, relevant finding is also influenced by the common response to structural differentiation as many asymptomatic, healthy individuals can present with some neurogenic component to their limitation of movement. The presence of structural differentiation indicating that the limitation to movement has a neurogenic component is rarely sufficient to determine relevance on its own. Asymmetries and structural differentiation should be taken in context with the patient’s symptomatic complaints and their available symptom free mobility compared to the motions necessary for their functional activities – work, sports, ADLs, and so on.

The literature can help us reconceptualize or deepen our understanding of ‘normal’, or at least the common response of the nervous system to neurodynamic testing in healthy, asymptomatic people. This allows us to have an evidence informed framework for our daily comparisons of our patient presentations to some concept of ‘norm’.

-Ben Boyd


Ben Boyd (PT, DPTSC, OCS) joined the NOI US teaching faculty in 2013. He is currently an Assistant Professor at Samuel Merritt University in Oakland, California, where he primarily teaches in the Musculoskeletal Patient/Client Management and Case Studies course series. He has been teaching courses on peripheral nervous system anatomy, biomechanics, clinical evaluation and intervention since 2004.

Ben’s research involves examination of the peripheral nervous system and intervention strategies for neuropathic pain, most recently focusing on the effects of Diabetes Mellitus on the mechanosensitivity of the nervous system and the impact on movement.

Ben is heading down under to help out at the new Mobilisation of the Neuroimmune System courses in Sydney (sold-out) and Adelaide (only 2 places left). 



Boyd BS, Wanek L, Gray AT and Topp KS (2009). Mechanosensitivity of the lower extremity nervous system during straight leg raise neurodynamic testing in healthy individuals. J Orthop Sports Phys Ther. 39(11):780-790.

Boyd BS (2012). Common inter-limb asymmetries and neurogenic responses during upper limb neurodynamic testing: implications for test interpretation. J Hand Ther 25(1):56-64.

Boyd BS, & Villa PS (2012). Normal inter-limb differences during the straight leg raise neurodynamic test: a cross sectional study. BMC Musculoskeletal Disorders, 13: 245. 

Boyd BS, Gray AT, Dilley A, Wanek L, Topp KS (2012). The pattern of tibial nerve excursion with active ankle dorsiflexion is different in older people with diabetes mellitus. Clin Biomech. 27(9):967-971.

Coppieters M, Stappaerts K, Janssens K and Jull G (2002)  Reliability of detecting onset of pain and submaximal pain during neural provocation testing of the upper quadrant. Physiotherapy Research International 7(3).

Stalioraitis V, Robinson K and Hall T (2014). Side-to-side range of movement variability in variants of the median and radial neurodynamic test sequences in asymptomatic people. Manual Therapy 19(4) 338-342. 

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