The ability to maintain balance is a critical factor for performing many activities of daily life. Humans maintain their balance by controlling the center of mass (COM) position relative to the base of support (BOS). However, because the BOS (described as the area inside the outline of the two feet) is relatively small and the COM is located well above the ground at approximately the S2 level (approximately 55% of body height), human vertical posture is inherently unstable.
The central nervous system uses sensory information to produce the appropriate muscle forces to control the COM position, so that the balance is maintained. Sensory systems contributing to balance include the visual, vestibular, and somatosensory systems.1 Although the role of each of these three systems in control of posture are important, there are indications that the somatosensory system is the biggest contributor of feedback for postural control.2 Receptors in the muscles, tendons, and joint capsules provide information about muscle length, contractile speed, muscle tension, and joint position: this information is commonly referred to as proprioception.3 The importance of proprioception in the performance of daily activities, including maintenance of posture, reaching and grasping tasks, and locomotion, has been documented in multiple studies.2,4 Concurrently, literature data suggest that loss of proprioception in the lower limbs contributes significantly to instability and falls.2,5
In particular, the elderly and individuals with diabetic neuropathy frequently experience proprioceptive deficits such as the loss of position, vibration and light touch sense, and sensory ataxia with the loss of ankle reflexes.6 The loss of sensation secondary to diabetic distal sensory neuropathy has a markedly detrimental effect on postural stability7 and contributes to altered gait patterns, feeling less safe while standing and walking, increased risk of falling,8,9 and decreased quality of life.10 Proprioceptive deficit was also reported in individuals who had knee ligament injuries,11 osteoarthritis,12,13 and ankle sprains.14,15
Several approaches have been developed to address these problems. Among them are therapeutic shoes, foot orthoses, and ankle-foot orthoses (AFOs), each of which influence the tactile and proprioceptive mechanisms in the lower legs, resulting in improvements to the sense of position15 and balance and a reduced risk for falling.16 Thus, it was reported that improvements in balance while using these devices is associated with the increased feedback from cutaneous receptors in the foot and ankle.16,17
It has also been shown that subsensory mechanical noise applied to the soles of the feet through vibrating insoles leads to reduced postural sway in healthy, young, and elderly individuals18 and individuals with diabetic neuropathy.19 In addition, reports indicate that wearing shoes with high collars are associated with better balance than when wearing shoes with low collars.20 Furthermore, it has been suggested that the application of an AFO increases the afferent feedback from cutaneous receptors in the foot and shank, which may in turn lead to an improved ankle joint position sense.21
It has been also reported that the utilization of additional somatosensory cues could help in balance maintenance.22 For example, the contact of the index finger against a stationary surface at a nonsupportive force level while standing has been associated with the reduction of postural sway in healthy individuals22,23 and individuals with peripheral neuropathy.24
Several reports suggest that the ability of humans to obtain early, directionally sensitive proprioceptive information is not limited to finger touch. Postural sway has also been reduced when contact with a stable surface is provided against the forehead, nose, or leg at nonsupportive force levels.25 Similarly, proprioceptive input from more proximal muscles, such as gluteus medius and paraspinals, can provide early, directionally sensitive proprioceptive information.26
Accordingly, in the presence of compromised distal sensation and proprioception, the provision of somatosensory cues to the lower limbs may constitute one approach to assist individuals better control their posture while standing and walking (Figure 1).
Along these lines, it has been shown that AFOs, which provide auxiliary sensory cues to the intact tissues of the lower limbs, improve automatic postural responses in individuals with peripheral neuropathy because of diabetes.27,28 However, as suggested in our previous study,28 the observed improvements in automatic postural responses could have been a result of either the auxiliary sensory cues or the additional mechanical support provided by the AFOs. Thus, the aim of this study was to clarify whether specially modified AFOs, which provide somatosensory information without additional stabilization of the ankle joints, could be responsible for the improvement of automatic postural responses.
A group of individuals with sensory neuropathy because of diabetes (N = 12) participated in static and dynamic balance tests with and without specially modified AFOs. Each of the AFOs was designed with the shank of the brace (1) connected to the foot plate (2) via a semi-rigid element (3). Velcro calf straps (4) were used to secure the shank of the brace to the leg (Figure 2). The AFOs provided sensory information to the calf through the shank of the brace and to the middle tibia through the calf straps, but because of the inclusion of the semirigid element, they did not provide any ankle stabilization in a sagittal plane. During the test, subjects were required to stand on a computer-controlled platform that was alternately stationary or moving with their eyes alternately open or closed. The efficiency of the subject's proprioception in balance control was assessed by measuring the sensory organization test scores and latency.29 The sensory organization test score is a measure of postural stability based on the observed degrees of the subject's offset from the centered position in the anterior-posterior plane. The latency is the time between the onset of the force platform translation and the start of a mechanical response initiated by the subject.30
For all tests, equilibrium scores were significant with the AFOs and their associated auxiliary sensory cues in comparison with conditions without cues (repeated measures analysis of variance, p < 0.05). Similarly, smaller latency scores were recorded in the AFO conditions (Figure 3). The results indicate that auxiliary sensory cues provided to the intact tissues of the lower limbs could improve automatic postural responses in individuals with diabetic peripheral neuropathy.
Proprioception plays an important role in the control of balance,3 occupational tasks, activities of daily living, and in sports.31 Because the ankle joint is located in close proximity to the body's BOS, the ankle plays an integral role in maintaining balance.32 As such, ankle proprioception is critical to balance during functional activities such as standing, walking, and running.33,34
It was previously suggested that ankle orthoses can improve postural stability or proprioceptive thresholds in young or athletic individuals.35–37 The results of this study demonstrate that using AFOs, which allow sensory information to bypass the disrupted pathways in the lower legs, can substitute for the lack of proprioceptive feedback and, therefore, help individuals with peripheral neuropathy to generate faster and more sensitively scaled postural responses while standing.
These findings are consistent with previous studies on the effect of external vibrotactile biofeedback on stance38,39 and gait.38 Conversely, it was recently shown that wearing AFOs that provided mediolateral support did not influence ankle inversion or eversion proprioceptive thresholds in older persons with diabetic peripheral neuropathy.40 However, in that study, AFOs were limited in their contact to the area of the anatomic ankle joints as their calf section did not extend proximally to the neck of the fibular head. The design of the AFOs used in this study was such that the shanks of the braces were in contact with the upper parts of the lower legs, allowing more sensory information to be used in balance control. Hence, it should be observed that the ability of AFOs to augment sensory information for postural control and substitute for the lack of proprioceptive feedback seems to depend on their design.
The observed improvements of automatic postural responses suggest that the AFOs providing auxiliary sensory information could potentially benefit the performance of other dynamic activities such as compensatory stepping in response to an unexpected platform perturbation and walking. Indeed, if the AFOs act as a “balance prosthesis,” helping individuals with peripheral neuropathy to generate faster and more sensitively scaled postural responses during the gait cycle, its use could help such individuals to walk with less fear of falling and, thus, contribute toward improvements in their quality of life.
The results revealed that automatic postural responses in individuals with diabetic peripheral neuropathy could be improved by using AFOs, which allow sensory information to bypass the disrupted pathways in the lower legs. The study outcome also highlights the need for further research in this exciting and relatively unexplored area of designing assistive means that could improve the balance and the performance of activities of daily living in individuals with proprioceptive deficits.
We thank the patients for their exceptional cooperation, Dan Hasso, CPO, for his help in designing AFOs, and Cheryl Carlson, PT, for her help in testing.
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