The Journals of Gerontology Series A: Biological Sciences and Medical Sciences 58:B680-B686 (2003)
© 2003 The Gerontological Society of America
Regional Variation and Changes With Ageing in Vibrotactile Sensitivity in the Human Footsole
Cari Wells1,
Lawrence M. Ward2,
Romeo Chua1,3 and
J. Timothy Inglis1,3
1 School of Human Kinetics
2 Department of Psychology
3 International Collaboration on Repair Discoveries (ICORD), University of British Columbia, Vancouver, Canada.
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Abstract
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Recently there has been indirect evidence suggesting that age-related elevation in footsole vibration detection may be associated with balance and gait dysfunction. As a first step in investigating this dysfunction, the current study determined by how much plantar vibration sensation decreases as a function of age, and if change is dependent on frequency and location of vibration application. Vibration thresholds were assessed at 4 frequencies (25–400 Hz), at 55 locations, and in young and older participants. Results showed there were 3 regions of sensitivity on the footsole: the ball/medial arch, the lateral border of the foot and heel, and the toes. Thresholds for fast-adapting type I receptor (FAI)-mediated frequencies were age invariant; however, thresholds for fast-adapting type II receptor (FAII)-mediated frequencies increased with age. These changes may be one of many factors contributing to age-related changes in gait.
IT has been hypothesized that vibrotactile information from the glabrous cutaneous receptors of the human footsole may play an important role in locomotion, specifically, in regulating stepping reactions (1), in gait (2), and in gait termination (3). To establish the role of footsole vibrotactile information, it is important first to determine absolute vibrotactile acuity, since this determines the range of cutaneous information available to control balance and equilibrium. While, to date, there is no direct evidence, it has been hypothesized that age-related decline in vibrotactile acuity may be a factor in dysfunctional balance (1) and dysfunctional gait as well.
In humans, vibrotactile sensation in the glabrous skin of the hand is encoded by mechanoreceptors that are associated with 4 different types of afferent nerve fiber (4). Slowly adapting type II afferents (SAIIs) preferentially encode frequencies below 8 Hz and have receptive fields about 10 mm in diameter. Slowly adapting type I afferents (SAIs) are superficial receptors with small receptive fields (
2–3 mm) that encode 2–32 Hz. Fast-adapting type I afferents (FAIs) encode 8–64 Hz and have receptive fields of 2–3 mm. Finally, fast-adapting type II afferents (FAIIs) mediate frequencies above 64 Hz, and have receptive fields greater than 10 cm (5). Recently, it was determined that all 4 types of receptors and afferents also exist in the human footsole (6), although their distribution and thresholds of activation were different from those described for the hand (5).
Plantar vibrotactile thresholds have been investigated, however, not comprehensively. Kenshalo (7) measured vibration sensitivity at 2 frequencies (40 Hz and 250 Hz) in young and older participants. Kekoni, Hämäläinen, and Rautio (8) measured vibration thresholds at 7 different locations on the plantar surface of the foot (heel, lateral midfoot, medial midfoot, lateral ball, midball, medial ball, and toes) and at 3 different frequencies (20, 80, and 240 Hz). Nurse and Nigg (9) looked at plantar sensitivity at 4 different locations on the plantar surface of the foot (heel, lateral arch, first metatarsal head, and hallux) at 30 and 125 Hz vibration. These studies established normative vibration threshold values for the frequencies and locations tested, however, only in a young population [n = 6, age = 23–36 years (8); n = 15, age = 26.2 years, SD (standard deviation) = 6.28 years (9)], or, only at 1 location on the footsole [young: aged 19–31 years; older: aged 55–84 years; tested on the thenar eminence and an unspecified location on the footsole (7)]. As a result, no conclusions could be made about how plantar sensitivity across the whole foot changes with age, or about the implications for control of balance in standing and equilibrium in walking.
In a psychophysical investigation of vibrotactile acuity as a function of age, Verrillo measured detection thresholds in 4 groups of participants, each group having a different mean age (10, 21, 50, and 65 years) (10). While testing was done in the thenar eminence, the results likely have homologues in the foot. The first result showed that low-frequency thresholds (hypothesized by Verrillo to be FAI mediated) were age invariant. Second, high frequency thresholds (hypothesized FAII mediated by Verrillo) increased as a function of age. Third, the "break-point frequency," that is, the frequency at which mediation switched from FAI to FAII, also increased as a function of age. A similar pattern of frequency mediation has been demonstrated for 2 age groups (young: aged 19–31 years; older: aged 55–84 years); however, at only 1 unspecified location on the plantar surface of the foot, and at only 2 frequencies (40 and 250 Hz) (7). These limitations make it difficult to speculate on the role of declining vibrotactile sensitivity in the control of equilibrium during locomotion.
To investigate the effect of diminished vibrotactile sensation on stepping reactions used to maintain balance, Perry and colleagues increased detection thresholds by cooling the footsole, and observed the ensuing changes on recovery from unexpected balance perturbations (1). The researchers reasoned that using cooling as an analog to aging (in order to diminish or ablate cutaneous vibration information) may cause dysfunction similar to that seen in aging. As a result of the cooling (vibrotactile threshold on the first metatarsal–phalangeal joint at 100 Hz increased to 6.9 ± 4.0 from 4.1 ± 1.1 µm, precooled condition), the authors speculated that participants were less able to detect foot contact and control the subsequent weight transfer, resulting in the multiple step recoveries. This recovery strategy was similar to that used by elderly individuals (average age = 69 years) with measurable loss of vibration sensation at 100 Hz (11).
In summary, these studies (1,11) suggest that plantar vibration sensation is an important input influencing gait. It has also been hypothesized that cutaneous input from specific areas of the footsole contributes to the control of specific aspects of gait (9). Further studies show that vibration sensation becomes decrepit with age, with a possible effect on gait (7,10). Motivated by these different findings, it was the purpose of the present study to determine by how much plantar vibration sensation decreases as a function of age, and if this change is also dependent on frequency and location of vibration application. The results of this are a first step toward inferring how the gait-related functions of different areas of the footsole may be affected by age-induced vibration insensitivity.
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MATERIALS AND METHODS
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Subjects
A total of 12 participants were involved in this study: 6 in a young age group (mean age = 26 years; range: 22 years 11 months–29 years 5 months) and 6 in an older age group (mean age = 88 years 8 months; range: 83 years 4 months–94 years 0 months). The number of participants was the same as 2 previous vibration–age studies (8,10). The age groups were counter-balanced for gender. Participants were self-reported free of neurological disease, were nonsmokers, and were moderate drinkers. While some participants in both age groups were taking medication regularly, none of the medications have been shown to affect sense of touch. All research was done using methods approved by the University of British Columbia Office of Research Services and Administration Behavioral Research Ethics board.
Experimental Protocol
Vibrotactile thresholds were determined for 4 different frequencies at 55 locations on the footsole (see Figure 1) in both age groups. The 55 locations were spread across the footsole in an 11 x 5 point grid in order to assess thresholds in all areas of the foot. The 4 frequencies, 25, 50, 250, and 400 Hz, were chosen for four reasons. First, if frequency mediation changes as a function of age as suggested by Verrillo, then an age x frequency x threshold level interaction will be apparent between 25 Hz and 50 Hz. Secondly, 25 and 50 Hz capture the major frequency content at heel strike during walking (12–14). Third, work by Verrillo (10) shows that 250 Hz is the frequency with the lowest threshold, an important parameter of acuity to test. Finally, this range of frequencies fits the range of physiological response in human cutaneous receptors (4) and allows comparison of sensitivity between the glabrous skin of the hand and foot.
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Figure 1. A, Footsole test locations. B, Equipment set-up. Upper panel: Lever arm applied to footsole. Lower panel: Motor and lever arm, with 6 in/15 cm ruler
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Equipment, Instrumentation, and Stimuli
The participants lay face down on a portable massage table with their right foot supported in a padded restraint to expose the footsole (see Figure 1). The foot was strapped to the restraint using soft webbing.
Sinusoid signals were generated using Matlab 5.1 (The MathWorks, Inc., Natick, MA) and LabView 4.2 (National Instruments Corporation, Austin, TX), and sent as a voltage value via a PCI-MIO XE-10 multi-input–output (National Instruments Corporation) to a BNC 2090 output box (National Instruments Corporation). The voltage waveform was supplied to an ASI model 300B dual-mode lever arm motor system (Aurora Scientific, Inc., Aurora, ON, Canada). The diameter of the lever arm head was 1.5 mm. The excursion amplitude, measured in µm, was the dependent variable of interest.
To begin a trial, the head of the lever arm was placed on the footsole perpendicular to the test site. The lever arm head was placed against the foot so that the head was in contact with the footsole, but so that no force offset was produced. This was done to ensure that mechanoreceptors were not activated before the stimulus started and would not bias the threshold. While the loads applied to the plantar surface of the foot are generally quite high during the stance phase of gait, our testing scheme is analogous to the footsole being unloaded during the swing phase of gait (no preloading of stimulus), and then loaded during heel-strike (onset of vibration stimulus). In addition, the footsole was tested in this position (i.e., unloaded) because it gave the experimenter access to the entire footsole, making it possible to perform the protocol in a timely and accurate way. When the lever arm head came in contact with the footsole, force control was used so that the force applied would be similar to that experienced during standing. Sinusoidal vibrotactile stimuli were delivered in 1-second pulses, with a 1-second interpulse time. The stimuli were sampled at 10 times the current test frequency.
Thresholds at each location were determined using the up-and-down staircase method with a step size of 0.25 µm (0.05 µm for young participants at frequencies above 25 Hz, to accommodate lower thresholds) (15). On the first iteration of the staircase, the amplitude of the stimulus sinusoid was set at threshold values determined in by Kekoni and colleagues (8) for similar footsole areas for young participants, and by pilot data for older participants. The "staircase" was repeated 10 times. The last 8 of 10 up-and-down flights were averaged to determine the threshold value. The 55 sites were tested in serial order; however, order was counterbalanced between participants to eliminate serial effects. To decrease testing time, all 4 frequencies were tested at 1 site before moving to a new site. Trials were blocked by frequency; however, the order of presentation of the 4 blocks was randomized. The total experiment time was 3–4 hours. Participants rested on an as-needed basis.
Two data sets were collected for each trial: a time series representing the force exerted by the motor arm on the participant's footsole, and a voltage time series representing the excursion of the lever arm against the footsole. The force data were used to monitor the force with which the stimulus was applied. Any trials where the force application was irregular were discarded, e.g., trials where the participant's foot had moved away from the lever arm. The range of unusable trials across participants was between 0 (minimum) and 4 (maximum). The voltage time series was converted to the dependent measure µm using a conversion factor supplied by the equipment manufacturer. The data were collected and recorded using a personal computer and analyzed off-line.
Data Analysis
For the sake of comparison, thresholds from the 55 locations were grouped into 7 anatomical regions that were similar to those used by Kekoni and colleagues (8). Next, the thresholds from the 55 data points were clustered in two ways [clustering is a data technique where objects are grouped together based on similar characteristics (16)]: first, threshold values from the 55 data points were clustered within each frequency and age group. This was done using a hierarchical tree cluster algorithm, where the data points were assigned to distinct groups in successive steps based on the points having similar threshold values. This preliminary clustering showed a trend of 3 clusters: toes, arch/ball, and lateral border of the foot/heel. Secondly, k-means clustering was run on all participants at each frequency. In k-means clustering, the number of clusters is set a priori (in this case, 3, as determined by the tree analysis). The algorithm then allocates the data points into 3 clusters so that intercluster variability is maximized, and intracluster variability is minimized, like a reverse analysis of variance (ANOVA) (16). The distinction of the clusters was confirmed by running a 2 age x 4 frequency x 3 region ANOVA on the clusters generated by 3-means clustering. ANOVA results were considered significant at p <.01. To ensure that the clusters were not an artifact of the analysis technique, ANOVAs were run on the results of 2-means and 4-means clustering. Of the different k-means clustering results, 3-means clustering produced the lowest intracluster variability and highest intercluster variability.
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RESULTS
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There were two interesting results from this experiment: first, there were 3 anatomical regions of sensitivity found in the human footsole skin. Second, there was acuity loss in elderly persons at vibration frequencies of 50 Hz and higher.
The 3 regions generated from the 55 data points are illustrated in Figure 1, and their means presented in Table 1 and Table 2. The clusters consisted of the toes, which consistently had the highest threshold; the lateral border of the foot and the heel; and the ball of the foot and medial arch, which consistently had the lowest threshold. Interestingly, this ordinal pattern of regional sensitivity was maintained across age and frequency (see Figure 2); however, the specific values of thresholds for the clusters changed as a function of these factors. ANOVA revealed a three-way interaction of frequency, cluster, and age on threshold [Greenhouse-Geiser correction, F(2.917, 29.174) = 3.00, p =.048]. Tukey's post hoc analysis showed that, at FAI-mediated frequencies [young = 25 Hz; old = 25 Hz, 50 Hz (10)] all 3 anatomical clusters were significantly different from one another (Tukey's Honestly Significant Difference; critical difference = 9.54 µm. At the lowest FAII frequency, however [young = 50 Hz, old = 250 Hz (10)], the sole clusters were not distinct from one other, but were different from the toes. At 250 and 400 Hz in young participants, all 3 clusters were similar. At 400 Hz in old participants, the sole clusters were again not distinct from each other, but distinct from the toes (Figure 2).
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Figure 2. Thresholds for the 3 clusters for each of the age groups at the 4 frequencies are shown. Thresholds are measured in µm
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Acuity loss in elderly people is summarized in Figure 3 and quantified in Table 3. Figure 3 shows threshold levels averaged across all 55 data points, and shows the global changes in the vibrotactile sense that occur with age. At 25 Hz, both young and old groups have similar thresholds. As the frequency increases, the curve representing the younger group is V-shaped, while the older group's curve is flat until 50 Hz, and then becomes V-shaped. The distance between the 3 curves represents acuity loss. When expressed as a percent ([old threshold value/young threshold value] * 100; see Table 3), loss increased as a function of frequency starting at 50 Hz. At frequencies that are hypothesized to be mediated by FAII receptors in both the young and old (250 and 400 Hz), loss of acuity was greatest in the heel/lateral border area, followed by the ball/arch area, and finally the toes. At all frequencies, the variance of thresholds in the old group was greater than that of the young group, consistent with larger individual differences in elderly people.
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Figure 3. Vibrotactile sensitivity in the footsole plotted as a function of age and frequency. All 55 data points are averaged. The x-axis represents frequency (Hz) and the y-axis represents detection threshold (µm). Bars represent standard deviations
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DISCUSSION
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The 2 results of the study are that there are 3 distinct clusters of different sensitivities, and that elderly people experience acuity loss. We present possible causes of acuity loss, along with a comparison of vibrotactile thresholds in the hand and foot. Finally, since it has been suggested that foot region and gait function may be related (9), we speculate that the cause of decline in elderly gait function may be vibration acuity loss.
Differences in Vibration Sensitivity Between the Hand and Foot
With the 55 locations grouped into the 7 anatomical regions used by Kekoni and colleagues (8), thresholds for young participants were comparable at similar frequencies (our 25 Hz with Kekoni and colleagues 20 Hz; our 250 Hz with Kekoni and colleagues 240 Hz). Data from Kenshalo (7) shows plantar vibrotactile thresholds to be, in general, lower when compared with our results, especially at 40 Hz for young participants (young participants, 40 Hz = 4.85 µm, 250 Hz = 0.57 µm; older participants, 40 Hz = 34.59 µm, 250 Hz = 28.25 µm). There are several possible reasons for this. First, the mean age of Kenshalo's participants is not specified. Depending on the age of his participants, 40 Hz could be an FAII-mediated frequency, making thresholds dependent on age. Thus, his participants would possibly have lower thresholds. Second, the data we present in Table 1 and Table 2 show averages for an entire cluster. Since these values are averaged from between 10 data points (toes) and 23 data points (lateral border/heel cluster), and since each data point is in turn averaged from 16 values determined from the up-and-down staircase method, it is reasonable to expect that our values will be different from Kenshalo's (7). For the sake of comparison, Table 1 and Table 2 include the lowest and highest values of data points from each cluster and at each frequency. The values supplied by Kenshalo are within the range of our low and high values. Finally, Kenshalo never specifies where the testing on the plantar surface is done. It could be that his values are from an extremely sensitive area of the foot. These differences in data collection make it difficult to draw conclusions from comparisons of our data with Kenshalo's.
In general, detection thresholds are higher on the foot than on the hand [Kekoni and colleagues: thresholds on the hallux are approximately 100 times that of thenar eminence at 240 Hz (8); Kenshalo: thresholds on the plantar are approximately 6.3 times that of the thenar eminence at 250 Hz in men (7)]. One reason for this may be that, while the glabrous skin of the footsole and hand contains similar receptors, footsole receptors have higher force activation thresholds [footsole: FAI: 11.8 mN, FAII: 4.0 mN (6); hand: FAI: 0.58 mN, FAII: 0.54 mN (17)]. Since there appear to be no differences in mechanoreceptor density in the footsole (6), possible reasons for regional differences in footsole sensitivity may be due to changes in mechanoreceptor function and morphology, and/or changes in skin mechanics (increased stiffness of the skin due to calluses).
Finally, it has been speculated that age-related changes in myelination may cause decreased nerved conduction velocity. Thus, the longer conduction distances to the feet would induce greater vibrotactile sense dysfunction and higher thresholds (7).
Possible Causes of Vibration Sensation Decline
Our results support the postulation that, as people age, progressively higher frequencies are mediated by FAI receptors, and that for frequencies that remain mediated by FAII receptors, the vibrotactile system becomes less sensitive (see Figure 3) (10). Because the activity of FAII receptors summates over skin area, a decrease in the number of receptors would result in a decreased sensitivity to high-frequency vibration (18). A similar decrease in the number of FAI receptors would not cause decreased sensitivity in FAI-modulated frequencies because these receptors do not summate over skin area (19,20).
Bolton, Winkelman, and Dyck (21) have investigated the mechanoreceptor network associated with touch in human glabrous skin and correlated changes in the network with age. With increased age, there is a loss in the total number of receptors, while the remaining receptors are irregularly distributed, and their receptive fields are varied in size and shape (22). In addition to receptor changes, the skin itself changes: the epidermis thins and there is a decrease in the amount of collagen and elastin in the skin (23,24), accompanied by the build-up of calluses. These changes may lead to decreased acuity because it has been shown that thicker skin impedes the transmission of high-frequency vibrations (25). In our experiment, those areas that were less callused, like the arch/ball cluster, had lower thresholds than the areas that were more clustered, like the heel/lateral border of the foot.
Speculation on the Role of Vibration Sensation in Gait
Nurse and Nigg (9) found that regional differences in sensitivity are correlated with peak forces and pressures during walking and running: the higher the sensitivity, the higher the peak force or pressure. These correlations of sensitivity and pressure, and sensitivity and force, along with the fact that different footsole regions have different sensitivities, suggest that whole regions within the footsole may play different roles during the gait cycle. For example, FAs located in the ball of the foot would play a significant role in detecting vibrations during foot-off, while FAs in the heel sense vibration at foot contact.
Nurse and Nigg (9) also suggested that the correlation of higher acuity and thresholds during running indicate that sensory feedback from the feet during locomotion could modify locomotion. From this, one could make the argument that if sensory feedback can modify walking, then degraded sensitivity may cause pathological locomotion (26). For example, elevated thresholds in the FAIIs in the heel of the foot would mean that degraded sensory feedback at foot contact would cause dysfunction in the aspects of gait reliant on foot contact sensation (1). In support of the hypothesis that degraded sensory input from the footsole causes pathological gait in elderly people, Murray and colleagues noted that walking in elderly people does not resemble that of someone with a nervous system pathology, but more closely resembles the gait of someone walking over ice or in the dark who thus has a reduced ability to use sensory information.
Conclusion
This work shows that vibration sensation thresholds increase with age. Since it has been speculated that vibration sensation is one factor in controlling gait, it may be that degraded vibration sensation could have a detrimental effect on gait control. A direct link between impoverished sensation and degraded gait should be investigated in the future.
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Acknowledgments
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We thank Professor James J. Collins of Boston University for use of the equipment. Research was supported by Natural Sciences and Engineering Research Council of Canada (NSERC) Operating and Equipment Grants to J. Timothy Inglis, and an NSERC Post-Graduate Scholarship for Cari Wells.
Address correspondence to J. Timothy Inglis, School of Human Kinetics, UBC, 210 War Memorial Gym, 6081 University Blvd., Vancouver, BC Canada V6T 1Z1. E-mail: tinglis{at}unixg.ubc.ca
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Footnotes
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Decision Editor: James R. Smith, PhD
Received December 16, 2002
Accepted May 1, 2003
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References
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Abstract
Materials and Methods
