Rehabilitative footwear: Importance of comfort

A growing body of research suggests that footwear comfort can improve movement performance and, in particular, neuromus­cular control of balance. These findings could have significant implications for rehabilitation of older adults and patients with lower extremity disorders.

By M. Owen Papuga, PhD, and Jeanmarie R. Burke, PhD

Fit and comfort are perhaps the most important aspects of any footwear purchase. Most of the time customers and patients try on the footwear and then are instructed to walk around to “see how it feels.” Clinicians even use comfort as a criterion to rate the appropriateness of orthotic interventions. Is footwear comfort also an important factor for movement performance and, perhaps, balance control?

What is comfort?

Comfort is difficult to quantify due to its inherently subjective nature. At the root of the problem are defining, and then measuring, specific aspects of footwear comfort and how each aspect affects movement performance. Nigg and colleagues performed a series of experiments that attempted to provide scientific evidence for the relationships among footwear comfort, shoe characteristics, and subject-specific characteristics.^1,2 Foot arch height, foot and leg alignment, and foot sensitivity were found to contribute to the perception of footwear comfort.^2,3 Mills et al reported that a combination of heel cushioning and support, forefoot cushioning, and arch cushioning explained 69% of the overall comfort rating given for a jogging shoe, with heel cushioning having the strongest correlation with overall perception of footwear comfort.⁴

Measuring comfort

The two primary methodologies for measuring footwear comfort are ordinal (ranking) scales and visual analog scales (VAS). Ordinal scales require subjects to rank footwear comfort from “worst” to “best.”Although ordinal scales are reliable they cannot quantify the magnitudes of differences between footwear conditions.⁵ Ordinal scales merely indicate that one type of footwear is more or less comfortable than another type of footwear. An ordinal scale that includes discrete ranking values, such as on a scale of one to 10 from “least comfortable” to “most comfortable,” is an improvement but still does not provide sufficient discrimination, especially in cases in which there may be subtle differences among footwear conditions.⁵

A VAS is a reliable indicator of comfort when footwear conditions are compared with a control condition during a physical activity such as running.⁶ Research has demonstrated enhanced reliability of the VAS to measure footwear comfort when the scale is represented by extremes in comfort, from “not comfortable at all” to “most comfortable condition imaginable” on a 150-mm scale and when uniform standard instructions are given prior to use.⁶

It would be a mistake, however, to indicate that because investigators can measure reliable differences in comfort percep­tion, every measureable difference is therefore important. Mills et al estimated that minimal clinically important differences (MCID) for subject-derived (questionnaire) and data-derived standard error of the measurement (SEM) methods using a 100-mm VAS were 10.2 mm and 9.59 mm, respectively.⁴ They also found that, though there were statistically significant differences in reported comfort levels between a subject’s normal jogging shoe and four commercially available prefabricated insoles, the differences did not meet thresholds for MCIDs and therefore were not clinically relevant.⁴ Similar studies have estimated an MCID between 20 mm and 25 mm on a 150-mm VAS for rating footwear comfort.^1,2,6

Some studies have emphasized measuring the distribution of forces across the plantar surface of the foot to address footwear comfort.^7-10 This focus is intuitive, as a majority of the force applied to the foot is passed through its plantar surface. Increased peak plantar pressure, decreased contact area, and increased per­centage of force acting on the forefoot have been associated with footwear conditions that have been rated least comfortable.^7,8,10

Comfort and movement performance

The skeletal aligning effect of footwear remains controversial.^11,12 From a biomechanical perspective, changes to foot and ankle alignment during movements while wearing orthoses seem to be small and subject-specific, and there are also no systematic relationships between changes in skeletal alignment and incidence of movement-related injuries.^13,14 Nigg et al have suggested that the widely held theory of optimal musculoskeletal biomechanics, in which footwear corrects biomechanics to achieve some consistently optimal movement pattern, is unlikely, and introduced the paradigm of a “preferred movement path.”^15,16 Footwear conditions that support the preferred movement path will be perceived as more comfortable and enhance movement economy while footwear conditions that counteract the preferred movement path will be perceived as less comfortable and decrease movement economy.^14-16

This theory is supported by findings of subtle but consistent changes of 1% to 2% in steady-state oxygen consumption for a given running velocity that occurred as a function of increasing midsole longitudinal bending stiffness (arch height support) or increasing the elasticity of heel material characteristics (heel cushioning).^14,17 Arch height support and heel cushioning also are positively related to comfort perception.^1,2,5,6 Measuring movement economy and comfort perception as a function of footwear condi­tions may provide further insights into the clinical effectiveness of orthotic interventions within the “preferred movement path” paradigm.

To investigate the potential relationship between comfort per­cep­tion and movement performance, Mündermann et al recruited 21 volunteers to perform overground running trials at 4 m/s while wearing three different types of orthotic devices and one control insole.¹ Data recordings included measurements of lower extremity kinematics, kinetics, and muscle activity by means of electro­myographic (EMG) recordings. Collapsed across nine experimental sessions, there were 108 running trials per participant for each of the four footwear conditions. Although 15 kinematic, kinetic, and EMG variables could only account for 34.9% of the variance in comfort perception, these same 15 variables correctly classified 75% of running trials to the corresponding footwear condition.

Recent evidence also demonstrates small but systematic improvements in rearfoot motion control and vertical impact loading during overground running when participants wear foot orthoses.^18-20 It is possible that increased comfort resulting from the use of orthoses results in improved function. The use of individually molded foot orthoses addressed possible subject-specific reactions to footwear.¹⁸ In addition, from a practical perspective, it has been observed that footwear that wearers feel is uncomfortable is simply not worn.^21,22

Footwear comfort has also been tested as a prognostic indicator of orthosis effectiveness based on functional bio­mechanics. A change of midfoot width from weightbearing to nonweightbearing has been shown to be a predictor of those individuals with anterior knee pain who would benefit from the use of orthoses.^23,24 Large changes in midfoot width are believed to be an indicator of increased functional midfoot mobility, which is theorized to decrease comfort and perhaps lead to an increase in the incidence of overuse injury.²⁵ Mills et al concede that their findings of small kinematic and EMG differences in those with increased midfoot mobility, while statistically significant and outside the standard error of measurement, are small and their clinical meaning is unknown.²⁵However, recent evidence indicates that the impact of footwear on gait kinematics is small, but detectable using mathematical and statistical techniques that address movement variability.²¹ Moreover, the detection of subtle movement pattern variations may be important, as the cumulative effect of these small kinematic and neuromuscular changes may lead to clinically meaningful treatment of overuse syndromes²² and provide a basis for the development of rehabilitative footwear.²¹

Implications for balance control

The research of Nigg and colleagues suggest that altering sensory feedback signals from the plantar surface of the foot by optimizing comfort perception across shoe conditions may allow for systematic changes in neuromuscular control.^{1-3,6,14,15,26,27} Sensory feedback signals from cutaneous receptors on the plantar surface of the foot regulate the distribution of plantar pressures during walking. Systematic shifts of plantar pressures from areas of desensitization to areas of normal sensitivity occurred when individuals performed walking trials prior to and following ice exposure to the forefoot, rearfoot, and whole foot.²⁷Similarly, Wu and Chiang addressed the effects of altered foot sensation on plantar pressure and muscle activity during static and dynamic balance tasks.^28,29 A foam padding intervention increased the compliance of the supporting surface, thereby decreasing the reliability of sensory feedback signals for postural control. The relationship between center of pressure displacements and plantar pressure measurements was evident when individuals balanced on hard surfaces (correlation coefficient r = -.76), but there was no meaningful relationship between these variables when individuals balanced on compliant surfaces (r = .14).²⁹ These data suggest that the postural control system compensates for the decreased reliability of sensory feedback signals from plantar cutaneous receptors by shifting to a proprio­ceptive hip strategy or increasing reliance on visual and/or vestibular sensory feedback mechanisms.²⁹

It may be concluded that plantar cutaneous reflexes play an important role in regulating the plantar pressure distributions and lower limb muscle responses that are necessary for posture and balance control.^28,30,31 Studies offer varying levels of clinical evidence to support the use of foot orthoses as a biomechanical intervention modality for the prevention and treatment of musculoskeletal disorders involving the low back, hip, knees, and ankles.^13,32-35 As reviewed by Salavati et al, there is also evidence to indicate balance impairments in patients with musculoskeletal disorders involving the low back, hip, knees, and ankles.³⁶

Evoking tibial nerve H-reflexes during single leg balance tasks provides an indirect but valid assessment of the contributions of Ia afferent-α motoneuron pathway to the neuromuscular control of balance.³⁷ Emerging evidence suggests the ability to decrease the size of the tibial nerve H-reflex reduces postural sway during balance and postural tasks.^37-40 Investigators recently evaluated the effects of footwear comfort perception on the neuromuscular control of balance using the tibial H-reflex methodology in 11 asymptomatic individuals and 13 individuals with lower extremity musculoskeletal disorders.⁴¹ Researchers altered the sensory feedback signals by having participants perform single-leg balance tasks while standing on the floor (stable surface) or on a mini-trampoline (unstable surface). These investigators assessed participants using the VAS methodology of Mündermann et al,⁶ and all participants with musculoskeletal disorders rated improvements of at least 20 mm in the overall comfort and/or arch height support of their preferred aerobic sneaker with the insertion of the custom-molded foot orthoses. The inclusion of individuals with musculo­skeletal disorders allowed for the use of pain reduction as another index of footwear comfort perception; preferred footwear conditions are those in which the lowest level of leg pain or back pain is experienced during standing tasks.⁴² The score that those participants assigned for their most severe pain was significantly lower when they were wearing custom-molded foot orthoses (37.3 ± 26.06 mm) than when they were not wearing the orthoses (62.8 ± 17.65 mm). Comfort perception ratings of the 11 asymp­tomatic individuals for their preferred aerobic sneaker were similar to values reported in the literature.^2,6

In agreement with the literature,^43,44 the size of the tibial nerve H-reflex in the asymptomatic individuals was smaller when balancing on an unstable surface than when balancing on a stable surface.Similarly, the unstable surface was associated with a smaller tibial nerve H-reflex than the stable surface in participants with musculoskeletal disorders when they were wearing custom-molded orthoses. However, when wearing flat foam insoles, the individuals with musculoskeletal disorders adopted an inappropriate neuromuscular control strategy in which the tibial nerve H-reflex was greater when balancing on the unstable surface than when on the stable surface.

It is plausible that by increasing the detection of plantar pressures under the foot (e.g., improved arch support), the custom-molded foot orthoses would evoke cutaneous reflexes to enhance the resolution of sensory feedback signals contributing to the neuromuscular control of balance.^28,30,31 Decreases in footwear comfort perception may negatively impact neuromuscular control of balance, and a custom-molded orthotic intervention may improve the resolution of sensory feedback signals that contribute to neuromuscular control of balance.

The influence of shoe material characteristics on balance control has also been addressed from the perspective of functional biomechanics. Measuring instantaneous center of pressure (COP) with an instrumented force plate allows for the calculation of an average sway velocity, a widely used and validated assessment of balance and posture control. Increased sway velocity is associated with decreased static and dynamic postural stability.^45-50 The effects of textured insoles on static and dynamic postural stability indicate improved postural sway parameters in older adults, especially when the complexity of the balance/postural task is increased by manipulating vision (eyes open vs eyes closed) and/or standing surface conditions (force platform without foam vs with foam).⁴⁵ When wearing thinner, less resilient insoles, older adults generate lower sway velocities than when wearing thicker, more resilient insoles.^46,47 Work by Robbins et al and Ring et al indicates that increasing shoe insole thickness increases sway velocities in both younger and older adults.^48-50 However, when a texture (granulations or “dimples”) is added to the surface of the insole, postural sway areas progressively decrease from barefoot to hard insoles to soft insoles.⁴⁵ In general, sway areas have a positive correlation with sway velocities, and this implies decreased sway velocities in soft textured insoles. The increased comfort associated with softer insoles must therefore be weighed against the possible decreases in stability due to proprioceptive deficits.

Clinical implications

Increasing the resolution of sensory feedback signals with rehabilitative footwear is a critical component of improving movement performance and balance control. Prospective studies on neuromuscular control of balance that include custom foot orthotic interventions, textured insoles, assessment of comfort perception, and instrumented static and dynamic posturography will provide insights about the potential of cost-effective rehabilitative footwear for improving stability in older adults and patients with lower extremity musculoskeletal disorders. These prospective studies may help healthcare professionals develop clinical protocols for the prescription of rehabilitative footwear based on supporting the preferred movement pattern of the individual.

Future directions

Designing laboratory-based studies on gait and posture that address the classification of preferred movement patterns is a critical next step. What are the key kinematic, kinetic, and neuromuscular variables that reveal a particular movement pattern? Refining and developing mathematical and statistical techniques that address movement variability may be necessary to detect preferred movement patterns.^21,51 Other questions involve understanding the impacts of footwear and perceived comfort on these preferred movement patterns. How can these preferred movement patterns be recognized in a clinical setting, and how is footwear best utilized to relieve pain and reduce injuries based on these preferred movement patterns? From these future studies general guidelines for the proper prescription of orthoses and recommendation of footwear can be developed based on kinematic, kinetic, and neuromuscular data underlying preferred movement patterns rather than on anecdotal evidence. Evidence-based diagnosis and treatment of lower extremity musculoskeletal disorders is key to demonstrating the clinical efficacy of rehabilitative footwear.

M. Owen Papuga, PhD, is assistant professor at New York Chiropractic College in Seneca Falls, NY, and adjunct assistant professor at the Center for Musculoskeletal Research, University of Rochester, NY. Jeanmarie R. Burke, PhD, is dean of research at New York Chiropractic College.