Face masks, vision, and risk of falls

Dear Editor,

Kal et al. (1) present an interesting discussion of issues related to face mask wear, vision and risk of falling. They suggest that people at an increased risk of falls may reduce their risk while wearing a face mask by decreasing their walking speed and that advice to those people might better take the form “take your time” rather than “look down”. One potential benefit of decreasing walking speed not mentioned by the authors is an increase in anterior mechanical stability. The mechanical stability of one’s body configuration during walking in the anterior direction is increased when forward centre of mass velocity is reduced (2, 3). Therefore, everything else being equal, reducing walking speed will reduce the likelihood of a forward fall due to a trip. Pavol et al. (4) have made a similar argument from a biomechanical perspective previously.

The authors acknowledge that slowing down may come with disadvantages and mention increased gait variability. The response by Callisaya et al. (5) also claim that decreasing walking speed may increase risk of falls. However, definitions of stability are important here. Mechanical stability of the body configuration at a given moment (i.e. onset of a trip) and variability of the unperturbed gait pattern (measured by linear or non-linear methods) do not measure the same thing and do not necessarily relate to each other (6-9). The references provided by Callisaya et al. (5) refer to head, pelvis and trunk accelerations during unperturbed walking (and various linear and non-linear measures of variability thereof), none of which specifically relate to the ability to attenuate or recover from a trip, which is an acute disturbance of mechanical stability. I would suggest that a slight increase in gait variability (which will not necessarily directly increase falls risk – most studies on this relationship are associative, not causal) in exchange for an increase in anterior mechanical stability (which directly reduces the risk of a forward fall) is favourable when the risk of trips and forward-directed falls is specifically increased, as may be the case with an obscured lower visual field.

References
1. Kal EC, Young WR, Ellmers TJ. Face masks, vision, and risk of falls. BMJ. 2020;371:m4133.
2. Suptitz F, Karamanidis K, Moreno Catala M, Bruggemann GP. Symmetry and reproducibility of the components of dynamic stability in young adults at different walking velocities on the treadmill. J Electromyogr Kinesiol. 2012;22(2):301-7.
3. McCrum C, Willems P, Karamanidis K, Meijer K. Stability-normalised walking speed: A new approach for human gait perturbation research. J Biomech. 2019;87:48-53.
4. Pavol MJ, Owings TM, Foley KT, Grabiner MD. Gait characteristics as risk factors for falling from trips induced in older adults. J Gerontol A Biol Sci Med Sci. 1999;54(11):M583-90.
5. Callisaya M, Hill K, Hill AM, Mackintosh S, Batchelor F, Said CM, et al. Rapid Response: Face masks and risk of falls – a vision for personalised advice and timing? 2020 [Available from: https://www.bmj.com/content/371/bmj.m4133/rr.
6. Perry JA, Srinivasan M. Walking with wider steps changes foot placement control, increases kinematic variability and does not improve linear stability. Royal Society open science. 2017;4(9):160627.
7. Dingwell JB, Cusumano JP, Cavanagh PR, Sternad D. Local dynamic stability versus kinematic variability of continuous overground and treadmill walking. J Biomech Eng. 2001;123(1):27-32.
8. Stergiou N, Decker LM. Human movement variability, nonlinear dynamics, and pathology: is there a connection? Human movement science. 2011;30(5):869-88.
9. Granata KP, England SA. Reply to the Letter to the Editor. Gait Posture. 2007;26(2):329-30.