Insole research explores postural control effects
A growing body of evidence suggests that foot orthoses may be a helpful addition to other therapies for improving balance and potentially reducing the risk of falls. The findings have been mixed, however, and clinical enthusiasm for this type of insole intervention also varies.
By Hank Black
As the earliest humans began to stand, they also began to fall, having trouble staying upright in stance. Balance and postural control, they learned, required myriad body systems to work together in parallel. Thousands of years later, modern humans are learning that foot orthoses may help facilitate that complex process.
Humans—those who are active and young, sedentary and aged, or those who have diabetic neuropathy1, multiple sclerosis,2 Parkinson disease,3,4 or another pathology—are still losing their balance and falling in great numbers. Particularly among the elderly, fall prevention has become a global concern. Worldwide, falls are the second leading cause of accidental or unintentional injury deaths, and 37.3 million falls that require medical attention occur annually.5
Maintenance of balance and the control of body sway is largely dependent on ankle joint proprioception—the complex interplay of coordinated visual, vestibular, and somatosensory information, together with signals to the central nervous system from ankle ligaments and surrounding muscles, to communicate a sense of the body’s position in space.6-9 Proprioception, in other words, involves all sensory mechanisms used by the body for ankle joint stability.10
Growing awareness of plantar mechano- receptors and their processing centers has prompted an upsurge in research related to balance-facilitating foot orthoses.
When that joint’s stability is diminished—due to an ankle sprain, disease, or the progressive laxity of ligaments and other tissues from aging—healthcare professionals enter the picture with a variety of treatments ranging from ankle taping to ankle surgery.
Evidence is now accumulating that foot orthoses may be a helpful addition to other therapies for improving balance. Some studies have failed to confirm this, and among practitioners, the degree of enthusiasm for it varies.
Multiple clinical trials have found custom foot insoles beneficial for postural control.11-14 Some investigators have found unmolded prefabricated foot insoles can improve postural stability,15,16 but others found no benefit from wearing noncustom orthoses for this purpose.17
“Custom foot orthoses can give increased balance and proprioception, and it’s not wrong to use them as a starting point for balance,” according to Matthew D. Sorensen, DPM, FACFAS, a foot and ankle surgeon who practices at Weil Foot and Ankle Institute in Chicago. “They won’t cure everything, but it can be helpful in an instability situation. For severe balance issues, I think it’s an underpowered modality.”
Sorensen considers prefabricated insoles hit-or-miss in treating ankle instability.
“Most off-the-shelf insoles are accommodative devices without a lot of internal structure,” he said. “They’re quite flexible and don’t really control the mechanics of the foot. They’re fine for cushioning, but don’t hold a candle to the true, functional, custom-made device.”
Robert A. Weil, DPM, in private practice in Aurora, IL, is quick to prescribe foot orthoses to enhance stability and balance.
“For youngsters, superstar athletes, and your grandma, proper foot and ankle alignment and support is always helpful for balance, and custom orthotics can provide an advantage,” he said.
Weil sees top athletes in many sports.
“You can’t show me a sport that requires the ultimate in precision and balance more than figure skating,” Weil said. “I’ve been prescribing custom orthotics for decades in skating, including for 2010 Olympic gold medalist Evan Lysacek, who was 10 when we first put them in his skates. And the improvement in everything, including balance and stability, is quite remarkable.”
The same improvement is often seen in seniors, he said, when fall prevention is being stressed.
Orthoses are not prescribed only to support the foot, Weil noted.
“Many foot types like high arches don’t need support. Orthotics help position the foot joints properly so supination for push-off is timed with optimum stability,” he said. “Often, repetitive ankle instability is directly associated with pushing off on a hyperpronated foot.”
Patrick A. DeHeer, DPM, of Hoosier Foot and Ankle Podiatry in Indianapolis, IN, prescribes custom foot orthoses for improving balance among his patients with ankle instability and other pathologies.
“They can be beneficial if they have a biomechanical association, such as rearfoot varus,” DeHeer said. “The key is understanding the underlying biomechanical etiology.”
Thomas W. Kaminski, PhD, ATC, professor of kinesiology and applied physiology and director of athletic training at the University of Delaware in Newark, recommends custom orthoses for improving balance, particularly in individuals with chronic ankle instability.
“Custom orthoses are key,” Kaminski said. “They get the foot back to its more anatomically correct position; that may be of particular benefit to those whose ankles tend to sprain.”
Like DeHeer, Kaminiski noted custom foot orthoses designed to reposition the rearfoot can be particularly useful for improving balance.
“Excessive rearfoot supination at heel strike may be leading to episodes of ankle instability, because as the patient lands they’re already in a position of vulnerability with the ankle wanting to turn inward or give way,” he said.
Certified athletic trainer Chris J. Hamlyn, MS, LAT, ATC, however, uses off-the-shelf foot insoles in his practice at Anderson University in Anderson, IN, where he is director of athletic training education in the Department of Kinesiology.
Hamlyn was the lead author of a 2010 study15 that found prefabricated orthoses were associated with improved postural stability in individuals with functional ankle instability. He said some prefabricated insoles have the presumed advantage of cupping the heel better or providing increased arch contact (vs no insole), “similar to what a custom orthotic would have to provide more proprioceptive feedback, allowing that individual to be more aware of the position of their foot and ankle.”
Hamlyn said the use of foot orthoses for better balance has not yet become part of the normal clinical practice or the standard of care for ankle instability.
“The relatively small amount of research in this field, while growing, has yet to provoke widespread adoption,” he said. “The price of a custom orthotic is prohibitive for most individuals or athletic training programs.”
For the study, Hamlyn and colleagues recruited 40 individuals with unilateral functional ankle instability, as confirmed by the Cumberland Ankle Instability Test. Postural stability was determined using a force plate to measure center of pressure. In the first of three sessions, participants were asked to shift from double-leg stance to single-leg stance and maintain balance with eyes closed for 20 seconds while wearing their own athletic shoes.
In session two, the 20 participants in the experimental group were tested with prefabricated insoles in both shoes. The insoles were full-length semirigid orthotic devices with a urethane base and an ethylene vinyl acetate top cover, designed to support the medial longitudinal arch and stabilize the rearfoot. The participants were instructed to wear the inserts daily for two weeks, then returned for the same test in session three. (A control group of 20 participants repeated the testing protocol from the first session for all three visits.)
In the experimental group, postural stability was significantly better in sessions two and three than in session one. There were no differences across sessions for the control group.
Among the general population, ankle sprain is the lower limb injury that most often results in an emergency department visit.18 The National Electronic Injury Surveillance System reports an estimated incidence of 2.06 ankle sprains per 1000 people a year. Ankle joint sprain is the most common sports injury, accounting for 15% of athletic injuries,19,20 and can lead to chronic ankle instability (CAI).21,22
“CAI has an impact on activities of daily life, lowering overall quality of life. They may never again run or move confidently because of this,” said Kaminski, lead author of the National Athletic Trainers’ Association position statement on Conservative Management and Prevention of Ankle Sprains in Athletes23—which does not include a role for foot orthoses.
The instability likely will not go away, Sorensen agreed.
“The joint becomes mechanically unstable, usually with the lateral ligament actually stretching out or being partially torn or ruptured,” he said. “And it generally does not completely recover. Whenever you stretch a ligament badly, there’s always a little plastic deformation, and the ligament attenuates and becomes a little longer. It’s difficult for the ligament to reestablish its normal physiologic tension. That occurs even if the ankle joint is immediately immobilized.”
While the number of clinical studies of orthoses and balance for ankle instability have increased in recent years, Kaminski said, more research is needed.
“This is not yet a mainstream intervention,” he said.
The somatosensory system is under increasing investigation as researchers try to suss out how each component works and integrates with other components. Multiple types of mechanoreceptors are found on the plantar surface, for example, which receive sensory information such as touch, temperature, and pain.24 Signals they send to the central nervous system result in the activation of muscles to control posture, though the role of each individual sensory system in balance regulation is unclear.3,25,26
Research suggests neurophysiological factors can affect the biomechanical alignment of the lower limb.27,28 Orthoses placed under the midfoot and forefoot may enhance afferent feedback from cutaneous receptors and reduce eversion due to contraction of inverting muscles.29
In part, it’s the existence of these plantar mechanoreceptors and their processing centers that has prompted an upsurge in balance-facilitating foot orthoses in recent years. One research group found a custom insole with a compliant raised ridge located around the posterior perimeter of the rearfoot significantly increased postural stability for older adults who had mild age-related loss of foot sensation and for people with Parkinson disease.30-32
Texture and vibration
Others have explored the effects on balance and postural stability of textured insoles and even noise-enhanced orthoses. Mechanoreceptors are seen to respond to mechanical stimuli, including recesses and stretching of the skin to provide information on the spacing, roughness, and direction of the texture pattern.33
In several clinical trials, textured insoles have produced such improvements. McKeon and colleagues assessed the effects of textured shoe insoles that provided no mechanical support on postural control. Twenty healthy physically active participants with self-reported CAI each balanced in shod single-limb stance with eyes open and eyes closed while wearing textured insoles, sham insoles, or no insole. They found the textured insoles were associated with significant reductions in time-to-boundary measures for the mediolateral (ML) direction.34
In a study of 25 patients with relapsing-remitting multiple sclerosis, Kalron et al found textured insoles altered static postural control parameters such as center of pressure when participants were examined with eyes closed, but not with eyes open.35 The textured insoles did not have significant effects on the spatiotemporal parameters of gait.
In a 2015 systematic review, a Brazilian group affirmed the support in the literature for positive effects on postural control associated with textured insoles.33 Four clinical studies reported these insoles helped activate the tibialis anterior muscle and reduce postural oscillations, especially in the ML direction.3,29,36,37 The insoles studied differed with regard to materials, thickness, and Shore value—one featured pyramid peaks or a convex circular pattern, another had rigid discs and spikes.
This review also considered three studies of balance-enhancing effects of insoles that vibrate due to piezoelectric elements in the insole.38-40 Based on the phenomenon of stochastic resonance, the vibration provides a mechanical noise that allows auditory feedback, with a resultant improvement in postural stability. The three studies detected improvements in balance and oscillation velocity in the anteroposterior direction associated with stochastic resonance insoles.38-40 Priplata et al found vibrating insoles provided a significant reduction in sway parameters in patients with diabetic neuropathy, stroke patients, and healthy older adults.40
A more recent study of postural responses to this vibrotactile noise found the vibration stimulus modulates the impact of other environmental sensory demands, such as visual field rotations and mental calculations. Improvement of postural stabilization was seen when multiple sensory demands were present, though with less effect than without the other sensory input.41
Not all studies of textured insoles have produced positive results for balance and postural sway. A review published this year by an Iranian team concluded vibratory or magnetic insoles were associated with an increase in balance control, but textured insoles were not.42
Muscle activation onset
Faster neuromuscular responses to a potential ankle inversion may prevent a fall. Dingenen et al found43 that shoes and foot orthoses produce significant neuromuscular effects in participants with CAI who previously used foot orthoses.
Using a single force plate and surface electromyography of nine lower extremity muscles, the group measured changes when transitioning from double-leg to single-leg stance with eyes open and closed. Participants were tested barefoot and with shoes only, shoes with standard foot orthoses, and shoes with custom foot orthoses.
Compared with wearing shoes plus standard foot orthoses, wearing shoes with custom foot orthoses was associated with quicker muscle activation onset times at the ankle for the peroneal longus and tibialis anterior muscles and at the knee for the vastus medialis oblique and vastus lateralis muscles.
However, the neuromuscular effects of foot orthoses appeared to vary between individuals, which is a consistent theme in the foot orthoses literature.44
“Future research should try to further identify the underlying mechanisms for these in order to finally better define the group of individuals who may benefit most from this intervention,” said Bart Dingenen, PhD, PT, a physical therapist and postdoctoral researcher on the Faculty of Kinesiology and Rehabilitation Sciences at the University of Leuven in Belgium.
“We had only one measurement at one specific point in time and participants were already adapted to their foot orthoses. Other researchers may be interested in how these neuromuscular adaptations may develop over time,” Dingenen said. “The potential interaction of this intervention with others, such as exercise therapy, remains unknown.”
Weil incorporates orthoses along with training for balance, proprioception, and strengthening exercises for the foot, ankle, and lower extremities using rubber bands and surgical tubing.
“Creating imbalance on wobble boards, mini trampolines, or Bosu ball demands all the body’s stabilizer muscle groups to work to strengthen all areas that balance and stabilize the joints, and, importantly, strengthening of the body core,” he said. “That’s why you should include both custom orthotics and instability training when you’re looking for optimum balance and stability.”
DeHeer is among those who lean heavily on a physical examination and a detailed history that includes determining under what circumstances balance become problematic. He sends patients to a physical therapist for evaluation and to introduce fall-prevention strategies. His team looks at an individual’s need for muscle strengthening and stretching, as well as biomechanics, gait, and foot structure.
“But it all comes back to the underlying reason for the problem, possibly including vision and inner ear, and we treat accordingly,” he said.
Sorensen does not make use of formal balance questionnaires.
“It’s not for me,” he said. “I conduct a fairly straightforward evaluation. A lot has to do with what the patient tells me in their history—when do they not trust their steps, when must they go slowly and hold on to something as they go. If someone is walking down the street and turns an ankle, that’s clearly instability. If they have trouble going down the stairs while holding on, that’s a whole different thing that may require a more comprehensive evaluation for additional pathologies before settling on a specific therapy or orthotics.”
The manual examination of the foot and ankle is important to gauge overall strength and pain, Kaminski said. He and most practitioners interviewed for this article said they assess balance by looking at the patient’s single-leg stance over 20 seconds with and without eyes open.
“We look at range of motion, flexibility, and other tangible measures. But equally important is his or her answer to questions designed to determine when the ankle gives way,” Kaminski said. “What’s the situation when that happens—is it stable when walking but unstable when jogging? Is going up and down stairs a problem? That helps us focus on how to design our interventions.”
Hank Black is a freelance writer in Birmingham, AL.