Why Mobility is the 6th Vital Sign

Walking is something most of us take for granted. That belies the fact that it is an incredibly complex activity, requiring dynamic interactions between multiple systems throughout the body. With all systems fully functional, the brain takes in visual information on your surroundings, while the vestibular system helps keep the body in balance. The somatosensory system takes in information on touch, temperature and pain, and will let you know very quickly if you step on a Lego brick without any shoes on!

Outside of the brain, the cardiovascular system pumps blood around the body to generate the energy required for movement, while the lungs take in additional oxygen to help fuel it. That movement is provided by the musculoskeletal system, with strong muscles and joints propelling the body forward. The motion occurs in a rhythmic and coordinated manner that is controlled by the central nervous system. 

The 6th Vital Sign

With so many systems involved, it is then easy to see why deficiencies in any of these systems can be manifested in the speed and smoothness at which you walk. This is what makes mobility extremely valuable as a means of assessing overall health – so much so that it has been referred to as 'the 6th' or 'functional' vital sign (1).

Mobility is defined as the ability to move freely and easily (2), and underpins many of the normal activities of daily life. It should be no surprise then that mobility has been shown to be strongly related to both functional capacity and quality of life. One study of 655 seniors found that walking speed was associated with presence of self-reported difficulty on 3 gait related activities of daily living and 5 out of 6 motor-based, but gait independent, activities (3).

Data from Verghese et al. (3). Estimates derived from logistic regression analysis adjusted for age, gender, education, illness index, falls, fear of falling, and Blessed test scores.

It is also important to recognise that gait speed can be informative in midlife as well as later life. A large cohort study from New Zealand found that gait speed in adults aged 45 was associated with midlife aging and lifelong brain health (4). 

Limitations brought about by mobility loss can have a significant impact on quality of life. For example, both walking speed and results of timed up-and-go (TUG) tests have been found to be related to quality of life in a study of 155 individuals with a history of osteoporosis-related fractures (5). These studies illustrate the value of mobility as an overall health check, and on brain health in particular. We now look at this aspect in more detail.

Predicting Neurological Decline

Mobility assessment has been shown to be very valuable in highlighting neurological issues, but also in predicting them. Several studies have shown that walking speed can predict cognitive decline (6–8). For example, one study from Sweden found that slowing gait preceded cognitive decline detectable by the Clinical Dementia Rating (CDR) by up to 9 years (6).

The graph below shows the mean walking speed for those experiencing a decline in CDR during the course of the study versus those with no change. We can see that the group with a worse CDR score had a lower initial walking speed, and that it declined faster (whether it was ‘normal’ walking speed or ‘maximum’ speed). The study also found that baseline gait speed could significantly predict CDR score change, with the ‘normal’ walking speed measure working a little better in this regard.

Data from Skillbäck et al. (6)

Another study looked at conversion to Parkinson’s disease amongst a cohort of 696 healthy individuals who were tracked over a 6-year period (9). A total of 16 participants were diagnosed with Parkinson’s on average 4.5 years after first visit. Step time variability and asymmetry of gait characteristics were associated with a shorter time to PD diagnosis, while random effects linear mixed models indicated that lower walking speed deviates from that of non-Parkison’s converters approximately 4 years prior to diagnosis.

Mobility and Mortality

The value of mobility as a general indicator of health can be seen in the now well-established relationship between mobility and mortality (10–14).  One recent systematic review sought to untangle some of the associations involved, and found that several gait domains (including walking speed) were associated with mortality (11). It also found that the association between walking speed and mortality persisted after an extensive adjustment for covariates, confirming the efficacy of gait as a marker for overall health. Preliminary analysis from the Mobilise-D study confirms that a number of the Mobilise-D DMOs are better predictors of mortality than currently accepted mortality indicators such as age, quality of life and frailty. 

Conclusion

How well you walk is an excellent indicator of general health, and slower walking speeds can provide early indications of neurological decline and future quality of life issues. At Enoda we believe that mobility data can play an important role in clinical care by promoting more individualised care, provide greater scope for early intervention, enhancing remote care and empowering patients with better and more timely data on their health. Email info@enoda-health.com to find out more.

1.   Middleton A, Fritz SL, Lusardi M. Walking speed: The functional vital sign. J Aging Phys Act. 2015;23(2):314-322. doi:10.1123/japa.2013-0236

2.   Viceconti M, Tome M, Dartee W, et al. On the use of wearable sensors as mobility biomarkers in the marketing authorization of new drugs: A regulatory perspective. Front Med (Lausanne). 2022;9:996903. doi:10.3389/FMED.2022.996903/BIBTEX

3.   Verghese J, Wang C, Holtzer R. Relationship of clinic-based gait speed measurement to limitations in community-based activities in older adults. Arch Phys Med Rehabil. 2011;92(5):844-846. doi:10.1016/j.apmr.2010.12.030

4.   Jee Hartmann Rasmussen L, Caspi A, Ambler A, et al. Association of Neurocognitive and Physical Function With Gait Speed in Midlife. doi:10.1001/jamanetworkopen.2019.13123

5.   Ekström H, Dahlin-Ivanoff S, Elmståhl S. Effects of walking speed and results of timed get-up-and-go tests on quality of life and social participation in elderly individuals with a history of osteoporosis-related fractures. J Aging Health. 2011;23(8):1379-1399. doi:10.1177/0898264311418504

6.   Skillbäck T, Blennow K, Zetterberg H, et al. Slowing gait speed precedes cognitive decline by several years. Alzheimer’s and Dementia. 2022;18(9):1667-1676. doi:10.1002/alz.12537

7.   Seo K, Takayanagi N, Sudo M, et al. Association between daily gait speed patterns and cognitive impairment in community-dwelling older adults. Sci Rep. 2023;13(1). doi:10.1038/s41598-023-29805-4

8.   Mielke MM, Roberts RO, Savica R, et al. Assessing the temporal relationship between cognition and gait: Slow gait predicts cognitive decline in the mayo clinic study of aging. Journals of Gerontology - Series A Biological Sciences and Medical Sciences. 2013;68(8):929-937. doi:10.1093/gerona/gls256

9.    Del Din S, Elshehabi M, Galna B, et al. Gait analysis with wearables predicts conversion to Parkinson disease. Ann Neurol. 2019;86(3):357-367. doi:10.1002/ana.25548

10.  Studenski S, Perera S, Patel K, et al. Gait speed and survival in older adults. JAMA. 2011;305(1):50-58. doi:10.1001/jama.2010.1923

11.  Dommershuijsen LJ, Isik BM, Darweesh SKL, van der Geest JN, Kamran Ikram M, Arfan Ikram M. Unraveling the association between gait and mortality-one step at a time. Journals of Gerontology - Series A Biological Sciences and Medical Sciences. 2020;75(6):1184-1190. doi:10.1093/GERONA/GLZ282

12.  Rosmaninho I, Ribeirinho-Soares P, Nunes JPL. Walking Speed and Mortality: An Updated Systematic Review. South Med J. 2021;114(11):697-702. doi:10.14423/SMJ.0000000000001318

13.  Saint-Maurice PF, Troiano RP, Bassett DR, et al. Association of Daily Step Count and Step Intensity With Mortality Among US Adults. JAMA. 2020;323(12):1151-1160. doi:10.1001/JAMA.2020.1382

14.  Ekelund U, Tarp J, Steene-Johannessen J, et al. Dose-response associations between accelerometry measured physical activity and sedentary time and all cause mortality: systematic review and harmonised meta-analysis. BMJ. 2019;366. doi:10.1136/BMJ.L4570