Shoe inserts and orthotics are frequently used to relieve a wide variety of lower extremity ailments, including ankle or knee pain or, more specifically, plantar fasciitis, posterior tibial syndrome, Achilles tendonitis, patellar femoral pain syndrome, or osteoarthritis (9). These pathologies are particularly frequent in populations that perform highly demanding physical activities such as running or military training (11,25). The effect of shoe inserts and orthotics is still debated. Many studies claim that 50–90% of patients who were treated with orthotics reported complete relief or great improvement of their condition (9,12).
Shoe inserts and orthotics can be characterized by their shape and material properties, which are assumed to be essential for their effect on lower extremity ailments (15,24). It has been suggested that shoe inserts and orthotics reduce the symptoms of injuries by realigning the foot or by the cushioning effect of the insert material (9). The realigning effect of shoe inserts and orthotics is still controversial (5,28). However, prospective studies showed that shock-absorbing or viscoelastic shoe inserts reduce the frequency of overuse injuries of the foot and tibial stress syndrome in a military population (7,8,18,26). Shock-absorbing shoe inserts also reduced the soreness in the lower extremities and back for soccer referees (6). These results imply that the material characteristics of shoe inserts are important features for injury prevention.
One main requirement for shoe inserts and orthotics is that they are comfortable, yet little is known about footwear comfort. Comfort of shoe inserts and orthotics has been associated with various factors, including plantar pressure, foot sensitivity, and foot and leg alignment (2,20). The material characteristics of shoe inserts affect the frequency and amplitude characteristics of the input signal into the body, which is detected by the body’s sensory system (20). Thus, it is speculated that the sensitivity of the plantar surface of the foot is related to comfort perception. Furthermore, several foot and leg geometry parameters such as Q-angle, static Achilles tendon angle, and foot arch height have been associated with injuries (3,9,27,30) and, thus, may also be related to comfort perception. Plantar pressure and foot sensitivity, as well as foot and leg alignment, are individually different, and, consequently, footwear comfort must be different for different individuals. Thus, a footwear condition that is comfortable to some people may be uncomfortable to others (19). Therefore, it is speculated that functional groups of subjects with similar characteristics and same preferences of shoe inserts exist.
Footwear comfort is not well understood, and the quantification of footwear comfort is not well defined. However, comfort seems to be very important. It has been shown that a preferred footwear condition is the condition with the lowest level of lower extremity or back pain for standing tasks (1). Comfortable footwear requires the absence of pain and discomfort, which was reported to occur before the onset of injuries (29). To date, the relationship between injury frequency and footwear comfort in a dynamic situation has not been investigated. It is proposed that the comfort of shoe inserts and orthotics is a prerequisite for the reduction of injury-related complaints in a physically active population. Therefore, the purposes of this study were (a) to determine lower extremity anthropometric and sensory factors that are related to differences in comfort perception of inserts with varying shape and material and (b) to investigate whether the comfort of shoe inserts is related to injury frequency. It was hypothesized that shoe inserts that improve footwear comfort decrease injury frequency.
A total of 206 military personnel (10 female, 196 male, age: 28.5 ± 6.6 yr, mass: 81.7 ± 10.4 kg, height: 1.78 ± 0.07 m) volunteered for this study. All subjects participated in regular military training at the time of the study. Each subject gave informed written consent. Only subjects without any current musculoskeletal or lower extremity disorders were included. The subjects were randomly assigned to an insert group (N = 103, age: 28.9 ± 7.0 yr, mass: 80.8 ± 11.1 kg, height: 1.78 ± 0.09 m) or a control group (N = 103, age: 28.1 ± 7.1 yr, mass: 82.6 ± 10.6 kg, height: 1.78 ± 0.07 m).
First, measurements of foot shape, foot and leg alignment, and foot sensitivity were taken from the left foot and leg of all subjects. Subjects in the insert group then rated six pairs of boot inserts different in material and shape with regards to specific and overall comfort. After the comfort assessment, each subject in the insert group was provided with the pair of inserts that he/she had given the highest rating and wore these inserts for a period of 4 months. During these 4 months, subjects in the insert and control group were asked to report any pain or injuries they had experienced. Finally, each subject provided a final injury assessment at the end of the testing period.
Foot and leg geometry.
An electromagnetic digitizing device (Polhemus Inc., Colchester, VT, accuracy: 0.7 mm) was used to digitize a total of 26 points on the surface of the foot and leg while the subject was standing upright on a testing table with the measured foot loaded with body weight (14). The unloaded foot rested on a stand 25 cm high. The testing table was a wooden board raised 44 cm above the floor to minimize the influence from any metal materials under the floor. From the digitized three-dimensional coordinates of the 26 points, 21 variables were calculated to represent the characteristics of the shape of the foot and the alignment of the foot and leg (Table 1, Figs. 1 and 2). All variables were measured during single leg support as the other leg rested on a platform to maintain balance except for the Q-angle, which was measured during bipedal standing. <
The sensitivity of the plantar surface of the foot was evaluated by a pressure threshold test and by a vibration sensitivity threshold test (21). Pressure thresholds were determined using a set of Semmes-Weinstein monofilaments (North Coast Medical Inc., San Jose, CA) (13). During the test, the subject lay on his/her back in a quiet room with a curtain preventing any view of his/her foot and the examiner. A modified 4–2–1 stepping algorithm (4) was used to determine the pressure threshold at three locations on the left foot of each subject (heel, medial arch, and hallux). Three trials were performed for each tested location.
Vibration sensitivity thresholds were determined using a vibration exciter (Brüel and Kjær, Decatur, GA, type 4809) powered by an oscillator (Brüel and Kjær, type 1022) (21). A metal probe of 8 mm in diameter was screwed into the vibration exciter and protruded 2 mm through a hole of a wooden footrest in contact with the plantar surface of the foot. An accelerometer (Brüel and Kjær, type 4367) was mounted beside the metal probe on the exciter and was attached to a RMS multimeter (Fluke 8060A, Everett, WA) through a signal conditioner. The use of an accelerometer allowed for accurate measurement of the probe’s movement and compensated for the various damping effects of different locations on the foot. The three tested locations were the heel, first metatarsal, and hallux. The test was performed using the Békésy-method (22) of increasing and decreasing amplitude at 30 Hz and at 125 Hz. Displacements were obtained by integrating the accelerations based on the assumption of a sinusoidal waveform.
Seven insert conditions were tested. They included a no insert condition (control condition) and six conditions with inserts. Because custom-made orthotics usually differ in both material and shape and, thus, it is unclear which are the key features of an optimal orthotic insert, six different pairs of shoe inserts (Marketmall Shoe Repair, Calgary, AB) were used in this study (Fig. 3). The inserts were commercially available inserts that were fit to the Combat Boot Mark III (Department of National Defense, Canada) and modified such that the inserts varied in shape, hardness, and elasticity (Table 2). The material and shape characteristics of the inserts were selected to provide insert combinations with only one difference in either material or shape. For example, insert 1 and insert 2 differed in hardness but were identical in shape and elasticity. The thickness of all shoe inserts was approximately 5 mm. Subjects wore their own combat boots to eliminate the effect of fit on footwear comfort.
Subjects in the insert group marched with each insert in their boots at their preferred speed on an outdoor course consisting of different surfaces including grass, gravel, concrete, and pavement for approximately 500 m. For each insert condition, they evaluated the subjective short-term comfort by using a comfort questionnaire (Fig. 4). The first and control condition for all subjects was the military boots without any inserts. The subjects completed the course and then rated the comfort of this condition. This procedure was then repeated for each of the six insert conditions, tested in random order. A visual analog scale (VAS) was used to assess footwear comfort because, to date, a standard scale for footwear comfort has not been established and VAS has been proven as a reliable measure to assess subjective pain (23). The left end of the scale was labeled “not comfortable” and corresponded to a comfort rating of 0, the right end of the scale was labeled “very comfortable” (comfort rating 10). To test the repeatability of the subjective comfort rating, each subject in fact evaluated seven insert conditions (six inserts plus one that was repeated). Thus, each of the six inserts was evaluated repeatedly by at least 16 subjects. The order of the repeated insert within the six tested inserts was randomly selected. The average difference between comfort ratings for repeated insert conditions was 0.53 comfort points.
Subjects in both the insert and the control groups were asked to record physical activities and any injuries or pain in a personal logbook, which was provided by the investigators. Four months after the initial assessment, subjects were given an exit questionnaire to determine the occurrence, frequency, and severity of injuries and pain experienced. Subjects were asked if they had any pain or injury during the last 4 months, if they had seen a physician, and where applicable the kind of the injury, its location, duration, and frequency.
Analysis of variance (ANOVA) was used to detect a significant difference between the overall comfort ratings for the six pairs of inserts. Paired t-tests were conducted to identify significant differences between insert combinations (α = 0.05). Significant relationships between subject characteristics and comfort perception were revealed using a two-step analysis. First, for each subject characteristic, subjects with values higher than mean plus one standard deviation formed the high group, and subjects with values lower than mean minus one standard deviation formed the low group (Fig. 5). Significant differences in overall and specific comfort ratings for insert combinations between the high and the low group were then detected using multivariate analysis of variance (MANOVA) and Student’s t-tests. Second, differences between the comfort ratings for the inserts of each insert combination were calculated. Subjects with values higher than mean plus one standard deviation formed the high group, and subjects with values lower than mean minus one standard deviation formed the low group (Fig. 5). Again, significant differences in subject characteristics between the high and the low group were detected using MANOVA and Student’s t-tests. For a significant relationship between a subject characteristic and comfort perception, both tests were required to be significant. This two-way analysis was assumed to be stronger than a simple test for significance, and, thus, the significance level for the individual tests was set at α = 0.10. χ2 tests (α = 0.05) were conducted to detect significant differences of the number and the severity of injuries and their location between the insert and the control group.
The difference in age, mass, and height between the insert and the control group were not significant (P > 0.50).
Shoe inserts and footwear comfort.
All shoe inserts improved footwear comfort compared with the no insert condition (Fig. 6) (P < 0.001). The average comfort ratings for all insert conditions were at least 2.3 comfort points higher than the average comfort rating for the control condition. The insert conditions with the lowest average comfort rating were insert 1, which was the hard insert, and insert 3, which had viscous material in the forefoot region. The average comfort ratings for these two inserts were significantly lower than the average comfort ratings for inserts 2, 4, 5 and 6, which had soft material in the heel and forefoot region and elastic material in the forefoot region (P < 0.001). These four inserts were able to improve footwear comfort for 90.3% of the tested military population (Table 2), i.e., were rated at least two comfort points higher than the control condition. Fig. 6 also shows the number of subjects that rated each particular footwear condition highest, listed in the bottom line in parentheses.
Shoe inserts and injury frequency.
A total of 79 subjects returned a completed injury assessment at the completion of the 4-month wear period, 45 in the control group and 34 in the insert group, and, thus, the following results are based on these 79 assessments. The reported injuries were categorized into blisters, pain, and stress fractures. Blisters mostly coincided with long marches and are assumed to be related to the fit of the boots and, thus, were not included in further analyses. The main location for stress fractures and pain for the insert and control group were the foot, knee, and lower back regions (Fig. 7). The number of stress fractures and pain at any location was between 1.5% and 13.4% lower for the insert compared with the control group (P = 0.053). The greatest difference in stress fractures or pain incidence occurred at the foot. Ten subjects in the control group (22.2%) suffered a stress fracture or pain at the foot as opposed to only three subjects in the insert group (8.8%).
Subject characteristics and comfort perception of insert shape and material.
Foot arch height, foot and leg alignment, and foot sensitivity were significantly related to comfort perception. Subjects with a low foot arch rated viscous and hard insert material higher than elastic and soft insert material, whereas subjects with a high foot arch rated elastic and soft insert material higher than viscous and hard insert material (P < 0.040). Subjects with a high sensitivity to vibration stimuli favored soft and viscous insert material and the high-arched insert. Subjects with low vibration sensitivity favored elastic insert material and a low-arched insert (P < 0.100). Individuals with a well-aligned skeleton, i.e., small Q-angle and small forefoot flexion angle, rated soft and elastic insert material higher than hard and viscous insert material compared with subjects with a poorly aligned skeleton who rated viscous material higher than elastic material and did not distinguish between soft and hard material (P < 0.071).
Shoe inserts and footwear comfort.
The majority of subjects rated all insert conditions higher than the control condition, which is in agreement with an earlier study (1). The fact that the control condition and the hard insert condition were rated lower than all soft insert conditions suggests that the comfort ratings for the different inserts are related to the hardness of the insert material. This supports the findings of an earlier study where individuals were able to perceive differences in the hardness of footwear (10). These results indicate that hardness of a shoe insert is a dominant factor for comfort perception.
Within the soft insert conditions, the shoe insert with viscous material in the forefoot region was rated significantly lower than the shoe insert with elastic material in the forefoot region. However, these two insert conditions were rated highest by approximately the same number of individuals (viscous forefoot insert: 15.9%; elastic forefoot insert: 17.8%). Thus, an insert that is rated in average significantly lower in comfort can still be the most comfortable insert for a considerable number of subjects. This result shows that subgroups of individuals exist and that the evaluation of individual results can reveal important information that may not be obtained by the analysis of group means. Therefore, in order to satisfy the majority of a given population, more than one footwear condition is crucial.
Shoe inserts and injury frequency.
The number of injuries that occurred with and without inserts certainly best describes the effect of an insert during physical activities such as military training. The inserts used in this study reduced the occurrence of lower extremity pain and injuries although the inserts were not custom-made and only varied in arch height, heel cup shape, and hardness and elasticity in the heel and forefoot regions. Due to the small number of completed questionnaires, the results of the injury assessment have to be interpreted with caution. However, the results of this study are in agreement with several earlier studies (9). The largest reduction in stress fractures and pain occurred at the foot. Similar results for modified basketball shoes were found by Milgrom et al. (18), who associated the pathogenesis of metatarsal fractures with vertical acceleration (17). According to the former study, shock-absorbing footwear reduces the frequency of metatarsal fractures, metatarsalgia, and heel and arch pain by reducing the vertical acceleration. However, the fact that in the current study inserts of different shapes and materials reduced injury frequency suggests that shock absorption may not be the major aspect of injury reducing inserts.
This study has shown that shoe inserts of different material and shape that are comfortable are able to decrease injury frequency. The next step will be to detect the mechanism underlying comfort perception. It has been speculated that an optimal footwear condition is comfortable, decreases injury frequency, which has been shown in this study, and reduces the amount of muscle activation (20). Possible lines of future research could be to study the relationship between comfort perception of shoe inserts and orthotics and muscle activation patterns. Because a change in kinematic or kinetic parameters will be related to muscle activation patterns, these parameters should be included in future studies. It is speculated that kinematic and kinetic parameters and muscle activation patterns will characterize group specific effects of footwear more distinctly.
Subject characteristics and comfort perception of insert shape and material.
The subject characteristics that were related to differences in comfort perception and that specify functional groups of subjects included foot arch height, foot and leg alignment, and foot sensitivity. This study was phenomenological in nature, and, thus, the causes for subject specific comfort perception given in the following are speculative. Individuals with a high foot arch preferred soft insert material and thus may require an insert that provides cushioning. In comparison, subjects with a low foot arch preferred hard insert material and therefore may require less cushioning. Vibration sensitivity of the plantar surface of the foot was related to differences in comfort perception of soft/hard and viscous/elastic insert combinations. A shoe insert filters the ground reaction force signal. The amplitude of the filtered signal is higher for elastic than for viscous material and the frequency of the filtered signal is higher for hard than for soft insert material. Individuals with low vibration sensitivity may require a signal that is slightly filtered to obtain sufficient feedback information compared with highly sensitive subjects who may perceive a highly filtered input signal as comfortable. This speculation is supported by results of a previous study, which suggested that the body’s sensory system differentiates between impacts of different frequency contents (16).
This study provides evidence for the relationship between short-term and long-term comfort because short-term comfort, which was measured in this investigation, was related to injury frequency. It is speculated that comfort assessment is a possible prognostic indicator for the success rate of shoe inserts and orthotics. Thus, further research should be conducted to increase the understanding of footwear comfort and its relationship to dynamic parameters. With the combat boot, a very basic shoe was used in this study. Although the thickness of the shoe inserts was only 5 mm, the results of this study showed clear indications for the relationship between comfort of shoe insert shape and material and anthropometric and sensory factors. It is speculated that these results are applicable to running shoes where the thickness of material ranges from 15 to 20 mm, and, thus, the effect of differences in material may be even more distinct.
This study was supported financially by the Da Vinci Foundation (Engineered Air), Calgary, AB, the Canadian Department of Defense (Defense and Civil Institute of Environmental Medicine, Downsview, ON) under contract no. W7711–8-7498/001/SRV and NSERC (Canada).
The contributions of M.A. Nurse, D.L. Chow, R. Frank, L. Strudsholm, and L. Busnello in the data collection are appreciated.
Current address of authors: Human Performance Laboratory, Faculty of Kinesiology, University of Calgary, 2500 University Drive N.W., Calgary, Alberta T2N 1N4, Canada.
Address for correspondence: Anne Mündermann, Human Performance Laboratory, Faculty of Kinesiology, The University of Calgary, 2500 University Drive N.W., Calgary, Alberta T2N 1N4, Canada; E-mail: firstname.lastname@example.org.
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Keywords:© 2001 Lippincott Williams & Wilkins, Inc.
INJURIES; FUNCTIONAL GROUPS; FOOT AND LEG ALIGNMENT; FOOT SHAPE; FOOT SENSITIVITY