Like it or not, as we age, most of our sensory systems begin to break down: our hearing worsens, our vision gets blurry, and our reflexes slow down. An under-appreciated sensory system that also worsens with age is the ability of cutaneous receptors located along the bottom of our feet to accurately quantify pressure. This is a big deal because information supplied by these tiny receptors plays a huge role in keeping us upright and balanced, as they supply a rich source of sensory information that allows us to constantly track the location of our balance point (a.k.a. center of pressure). If the center of pressure veers off too far in one direction, a reflexive muscular response brings us back to a safer balance point (Fig. 1). These cutaneous receptors, which are illustrated in figure 2, are unlike other sensory receptors associated with balance in that they interact with a wide range of interneurons, allowing them to favorably modify activity in all limbs, not just the stimulated limb. This comes in handy for preventing falls, as it allows you to quickly move the opposite limb and/or adjust arm position in order to reestablish balance. Unfortunately, as we age, the receptors in the bottom of our feet become less sensitive.

Even without underlying neuropathy, it takes 20% more pressure to stimulate cutaneous receptors in the soles of the feet by age 50, and 75% more pressure to stimulate the same receptors by age 80 (1). The reduced sensory input from the soles of our feet makes it extremely difficult to maintain balance and adds to the high fall rate present in senior citizens. In any given year, nearly 40% of seniors aged 70 and over will fall at least once (2), and the resultant injuries often begin a downward spiral of weakness and frailty.

For more than 50 years, researchers have attempted to improve balance and prevent falls by incorporating textured insoles with specific elevations designed to stimulate cutaneous receptors. These textures range from small triangles and pyramids, to round tubes and/or circular nodules. Some are made from hard plastics, and others from soft foams. In theory, textured insoles improve balance by increasing output from cutaneous receptors that are becoming increasingly insensitive with age. The problem with textured insoles is that the research regarding efficacy has never been that impressive. Some research shows these insoles reduce fall risk (3,4), while other studies show these insoles produce little change (5,6), or worse, they can impair balance making you more vulnerable to a fall (7,8).

In my opinion, the problem with textured insoles is that the irritating textures are applied uniformly over the entire foot. Almost 30 years ago, Robbins et al. (9) showed that this can have negative consequences as they convincingly prove that the muscular response to pressure along the bottom of the foot varies depending upon the location of the stimuli. For example, stimulating the skin under the inner forefoot produces a reflex downward contraction of the toe muscles, which distributes force away from the metatarsal heads into the toes. In contrast, stimulating the skin beneath the arch has the opposite effect in that it causes the toes to move upward, shifting pressure away from the toes and onto the metatarsal heads. The inability of the toes to push downward greatly increases our risk of falling forward (10) (refer back to figure 1).

New research supports the concept of site-specific cutaneous specificity along the bottom of our feet. Using advanced EMG technologies to stimulate specific cutaneous receptors in different locations along the bottom of the foot, Zehr et al. (11) note that when cutaneous receptors beneath the heel are stimulated, there is an immediate increase in activity of the soleus and gastroc muscles (Fig. 3A), with inhibition of tibialis anterior. Stimulating the lateral midfoot increased activity in peroneus brevis (Fig. 3B), while stimulating the lateral forefoot (Fig. 3C) increased activity in tibialis anterior. The authors specifically state that stimulating the lateral side of the midfoot is an important location of transition that provides “greater resolution in the fine sculpting of motor output.” The authors make the interesting statement that site-specific stimulation provides “a kind of guided tuning” that the authors refer to as “sensory steering.” 

In the most detailed study of cutaneous receptors to date, researchers from Canada used microneurography to determine the exact location of each type of receptor in the bottom of the foot (12). Figure 4 illustrates the results of their research, which for the first time ever proves that there is a gradual increase in sensory receptors when moving from the back of the heel to the forefoot, and when moving from the medial to the lateral side of the foot. The outer side of the midfoot is especially well innervated, possessing both slow and fast adapting cutaneous receptors.

The abundance of sensory receptors along the lateral foot makes perfect sense when you look at the progression of the center of mass while walking (refer back to Fig. 1). Because we spend so much time with our weight distributed over a small section of our foot, it is important that that section have the ability to provide the greatest amount of information in order to allow us to respond immediately to even minor perturbations that may affect balance. The plethora of cutaneous receptors located along the lateral foot is also key for preventing the most dangerous type of falls: lateral falls. While forward falls can be prevented by firmly pressing down with your toes, and medial falls are rarely a problem as the opposite leg is there for support, lateral falls require a complex cross-over movement from the opposite leg, which is difficult to perform and nearly impossible to recover from once initiated. In fact, a recent paper published in the Journal of Biomechanics demonstrated that seniors who are most likely to fall have greater lateral displacement of their center of mass while initiating their first step while walking (13). Lateral falls have also been proven to result in greater fracture rates (14,15).

After seeing research by Strzalkowski et al. (12), I looked at every paper ever published on textured insoles and made a series of prototypes, each possessing a different pattern of ridges and elevations. The final version was made with a series of rounded elevations that get progressively larger when moving laterally (Fig. 5). I deliberately placed the smaller elevations on the inner side of the Balance Buttons to stimulate slow adapting receptors, which provide constant information regarding the location of the center of mass. I then placed the larger elevations far enough laterally that the individual could avoid stimulating them by walking with a slightly wider base of gait. Walking with your feet farther apart greatly diminishes the risk of lateral falls as your center pressure is maintained in a more central position. Should the individual accidentally shift his or her weight so far to the side that they press on the larger nodules, it is likely to initiate an immediate response from the fast-adapting Meissner’s corpuscles, which have extensive branches to specific muscles in the lower extremity responsible for fall prevention (16). Last but not least, the entire pad acts as a wedge lifting the outer side of the foot, which shifts the center of pressure to a safer midline position.

Keep in mind that Balance Buttons, like all textured insoles, only work while you are wearing them as there is an almost immediate return to baseline poor balance when discontinued (17). As a result, it is extremely important to strengthen your feet and legs, especially your toes, as foot strength plays a huge role in fall prevention. Research from Australia shows that every 1% increase in toe strength decreases a seniors’ risk of falling by 7% (10).

It is easy to measure toe strength using a toe strength dynamometer and ideally individuals will be able to generate 10% of their body weight beneath their big toe, and 7% beneath the lesser toes (Fig. 6). My favorite way to strengthen the legs and feet is with the ToePro exercise platform, which is easy to do and extremely effective. A pilot study at Temple University showed that in addition to increasing toe strength by more than 20%, using the ToePro for just 6 weeks appreciably improved balance, as measured with the Y-excursion balance test (18).

The easiest way to determine if you might benefit from using Balance Buttons is to get into a safe location and stand on one foot with your eyes open and try to balance for a full 10 seconds. In a 12-year study of more than 1,700 older adults, the ability to successfully balance on one foot with eyes open was strongly correlated with longevity (19). In fact, individuals who were unable to balance for 10 seconds after 3 tries had an 84% higher risk of all-cause mortality, even when adjusting for other risk factors, such as heart disease, hypertension, and obesity. This study emphasizes the disastrous consequences associated with impaired balance. It also proves that it is imperative that people have their balance evaluated regularly, just like they have their cholesterol, blood pressure, and eyes examined annually. If impairment is found, preventive measures should be initiated as soon as possible, and inexpensive Balance Buttons coupled with a few simple home exercises can make a tremendous difference.

References:

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2. Petridou E, et al. What works better for community dwelling older people at risk to fall? A meta-analysis of multifactorial versus physical exercise alone interventions. J Aging. Vol 21, Issue 5, 2009.
3. Palluel E, et al. Do spike insoles enhance postural stability and plantar-surface cutaneous sensitivity in the elderly? AGE. 2008, 30: 3 – 61.
4. Clark D, et al.  Enhanced somatosensory feedback reduces prefrontal cortical activity during walking in older adults. J Gerontol Biol Sci Med Sci. 2014;69:1422–1428.
5. Hatton A, et al. The effect of textured surfaces on postural stability and lower limb muscle activity. J Electromyogr Kinesiol. 2009,19: 957-964.
6. Wilson M, et al. Effect of textured foot orthotics on static and dynamic postural stability in middle-aged females. Gait Posture. 2008;27:36-42.
7. McKeon P, et al. Altered plantar receptor stimulation impairs postural control in those with chronic ankle instability. J Sport Rehabil. 2012,21:1–6.
8. Hatton A, et al. Altering gait by way of stimulation of the plantar surface of the foot: the immediate effect of wearing textured insoles in older fallers. J Foot Ankle Res. 2012, 5:11.
9. Robbins S, et al. Running-related injury prevention through innate impact moderating behavior. Med Sci Sports Exerc. 1989;21:130-139.
10. Mickle K, et al. Efficacy of a progressive resistance exercise program to increase toe flexor strength in older people. Clinical Biomechanics. December 2016;40:14-19.
11. Zehr EP, et al. Cutaneous stimulation of discrete regions of the sole during locomotion produces “sensory steering” of the foot. BMC Sports Science, Medicine, and Rehabilitation 2014, 6 :33.
12. Strzalkowski N, et al. Cutaneous affer et al. ent innervation of the human foot sole: what can we learn from single-unit recordings? J Neurophysiol 120: 1233–1246, 2018.
13. Yoshida K, et al. Detecting differences in gait initiation between older adult fallers and non-fallers through multivariate functional principal component analysis. J Biomech. October 2022.
14. Hayes W, et al. Etiology and prevention of age-related hip fractures. Bone. 1996;18:77S–86S. 
15. Maki B, et al. Age-related differences in laterally directed compensatory stepping behavior. J Gerontol Med Sci. 2000;55A:M270–M277.
16. Fallon J, et al. Evidence for strong synaptic coupling between single tactile afferents from the sole of the foot and motoneurons supplying leg muscles. J Neurophysiol. 2005;94: 3795–3804.
17. Palluel E, Nougier V. The lasting effects of spike insoles on postural control in the elderly. Behav Neurosci. 2009;123:1141-1147.
18. Song J, et al. Effects of eccentric exercises on foot structure, balance, and dynamic plantar loading. Gait Study Center, Temple University School of Podiatric Medicine. In press 2023.
19. Araujo et al. Successful 10-second one-legged stance performance predicts survival in middle-aged and older individuals. British Journal of Sports Medicine 2022;56:975-980.
20. Johnson KO. 2001. The roles and functions of cutaneous mechanoreceptors. Curr Opin Neurobiol. 11:455 – 461.