BURNFIELD, JUDITH M.1,2; JORDE, AMY G.1,2; AUGUSTIN, TANNER R.1,2; AUGUSTIN, TATE A.1,2; BASHFORD, GREGORY R.2
Although cardiovascular exercise is widely encouraged as an essential aspect of health-promotion programs (9,21,27), one area of potential concern is the repetitive and sometimes prolonged pressures applied to the plantar aspect of the feet during specific exercises (e.g., running, stair climbing, elliptical training). Recurring exposure to high-impact forces has been implicated as one factor that may contribute to orthopedic conditions such as metatarsal stress fractures (29). High pressures are also deleterious for persons with diabetic sensory neuropathy (14), because these individuals are frequently unable to detect dangerous pressures that could lead to foot ulcers (10). The extent to which the foot pressures that arise during cardiovascular exercise could be injurious to susceptible populations is not well understood.
Exercise on the diverse types of cardiovascular equipment available in homes, medical facilities, and community fitness centers would be expected to lead to substantial differences in plantar pressures attributable to variations in the forces generated at the foot-support surface interface as well as the contact area between the body and supporting surface. For example, running on a treadmill incorporates periods of single-limb support when all of the body weight is concentrated through a single foot, and periods when neither limb is in contact with the ground (i.e., the double float period of swing). In contrast, the extended stance period of walking results in periods of double-limb support, when body weight is shared between the two limbs in addition to the periods of single-limb support and swing. During elliptical training and stair climbing, both feet remain in contact with the support surface (double-limb support) throughout the majority of the activity, and the contact area also may extend to include the hands. Recumbent biking incorporates the buttocks as an additional surface for distributing forces.
Age-related variations in ground-reaction force patterns and foot structure also could make certain populations more likely to experience increased pressures during exercise. For example, the elevated ground-reaction forces documented under the feet of younger compared with older individuals during level walking (25) and stair ascent (25) suggest that younger persons might experience higher pressures than their older counterparts during activities incorporating similar movement patterns (e.g., stair climbing). In contrast, the reduced heel pad elasticity documented in persons over the age of 40 (20) could compromise the ability to distribute forces under the heel, whereas flattening of the longitudinal arch (12) and forefoot deformities (18) would alter the contact area available for distributing forces. Because some medical conditions affecting the feet (e.g., diabetes mellitus) are more common in middle-aged versus younger adults (19), a better understanding of age-specific differences in pressure patterns seems warranted.
To date, no studies have been published that have systematically explored pressure-distribution patterns across the foot during exercise on equipment commonly available for use in community and home settings. Also lacking, thus far, are publications comparing plantar pressure variations between young and middle-aged adults when performing these common cardiovascular exercises. The purpose of this study was to quantify plantar pressure variations in healthy adults across five common cardiovascular exercises: walking and running on a treadmill, elliptical training, stair climbing, and recumbent biking. These data could be used in the clinical setting for comparison with values recorded from persons with foot pathology.
We hypothesized that peak foot pressures would be highest during upright exercises that included periods of supporting body mass on a single limb (i.e., walking and running) because of increased forces beneath a single, weight-bearing foot. Our second hypothesis was that peak foot pressures would be lowest during recumbent exercise (i.e., biking) because of reduced forces experienced at the foot-pedal interface. Finally, it was hypothesized that pressures would be higher in the younger versus the older group because of higher forces exerted by the younger group. Because prolonged and repetitive exposure to elevated plantar pressures can lead to pain and tissue injury in persons susceptible to orthopedic and neuropathic foot disorders, a better understanding of the impact of different types of exercise on foot pressures seems warranted.
Twenty individuals between the ages of 19 and 60 yr participated in this study. Subjects were recruited from the staff and student population at Madonna Rehabilitation Hospital (Lincoln, NE) and the local Lincoln area by word of mouth and flyers to serve in one of two groups: young (19-35 yr old) and middle-aged (45-60 yr old). Each group consisted of five males and five females. Only subjects capable of independent ambulation and exercise without assistive devices were included (Table 1).
Before participation, each subject was fully informed of the nature of the study and signed a written informed consent to participate in this study according to a protocol approved by the institutional review board at Madonna Rehabilitation Hospital. After obtaining informed consent and HIPPA authorization, each subject completed a medical history questionnaire and physical activity readiness questionnaire (PAR-Q) to determine whether they could safely participate in the study (26). Subjects with known neurologic or orthopedic conditions that would interfere with gait or exercise were excluded from the study. Additionally, subjects with previous back injuries, recent fractures, muscle strains, joint sprains, or potential pregnancy were excluded.
Plantar foot pressure data were recorded using the Pedar system (Novel Electronics Inc., Munich, Germany). Each pressure insole consists of a 2-mm-thick array of 99 capacitive pressure sensors. Before commencement of data collection, the insoles were calibrated according to the manufacturer's guidelines through a measurement range of 0-600 kPa using the Trublu calibration device. The sensors were sampled at a rate of 60 Hz. Fourteen different sizes of insoles ensured an appropriate fit and comfortable wear in the shoes of all study participants. Plantar pressure data were edited and evaluated using Emedlink and Multimask Evaluation software (Novel Electronics, Inc., Munich, Germany), respectively.
Walking and running were performed on a 97 Ti Treadmill (LifeFitness, Franklin Park, IL; Fig. 1A and B, respectively). The 95 Xi Elliptical Cross-Trainer (LifeFitness; Fig. 1C) was used for all elliptical training trials. The 95 Si Stairclimber (LifeFitness; Fig. 1D) was used for stair climbing. Recumbent biking was completed using the 95 Ri Recumbent Exercise Bike (LifeFitness; Fig. 1E).
FIGURE 1-Pedar press...Image Tools
The footwear used during the exercise trials were the subjects' preferred shoes and socks that they "would choose for working out" on cardiovascular equipment. In the young group, subjects elected to wear either running (N = 7) or cross-training (N = 3) footwear. For the middle-aged group, footwear selections included cross-training (N = 4), walking (N = 3), running (N = 2), and casual (N = 1) shoes.
All testing occurred in the Movement Sciences Center located in the Institute for Rehabilitation Science and Engineering at Madonna Rehabilitation Hospital. Each individual participated in four sessions: three for familiarization purposes, and one for biomechanical assessment.
During the first three sessions, participants exercised on the equipment to ensure familiarity with operation of each device and to determine appropriate self-selected settings (e.g., seat position, speed range). This included walking, running, elliptical training, stair climbing, and recumbent biking for a minimum of 5 min on each respective piece of equipment during every session. The participants were instructed to perform each exercise at a comfortable pace that they "would be able to maintain during a typical 30-min workout." After each exercise, the participants indicated their perceived level of exertion on a Borg scale of 6-20 to ensure comfort with using the scale (4). Additionally, basic anthropometric data (e.g., height and weight) were recorded as well as each participant's date of birth, gender, and dominant foot during the first session. To identify the dominant foot, participants were instructed to kick a ball placed in front of them. The foot used to kick the ball was deemed the dominant foot.
All pressure mapping occurred during the fourth session. Initially, the pair of Pedar pressure insoles that most closely matched the size of their respective shoe insole was selected from an inventory of 14 sizes. The pressure sensors were placed inside the shoes between the insole and sock-covered foot. Participants then tied their shoes to a comfortable tautness. Next, the pressure insoles were zeroed according to the manufacturer's guidelines. This involved the participant successively lifting the left and then the right foot from the ground to define the zero baselines for the unloaded insoles. The zeroing process was performed immediately before each exercise activity.
During testing, participants performed the five exercises in a manner similar to that used during the familiarization sessions. In particular, activities were performed for 5 min apiece at a self-selected pace. The order of activities was randomized using a computer-program written in MATLAB (version 188.8.131.52, The MathWorks, Inc., Natick, MA). During the final minute of each exercise, plantar pressure data were recorded. Additionally, subjects were asked to rate their levels of perceived exertion using the Borg scale. To reduce the potential cumulative impact of exercise on fatigue, participants were permitted to rest for a minimum of 5 min between each exercise, and to drink water for hydration.
Data management and analysis.
Plantar pressure data were initially screened and divided into cycles or steps for each exercise using Emedlink software (Novel Electronics, Inc.). Novel Multimask Evaluation software (Novel Electronics, Inc.) was then used to divide the insole data into three anatomical areas using a manufacturer-provided masking routine (Percent Mask Insole-3). The anatomical regions delineated by the masks were the forefoot (distal 40% of longitudinal foot length), arch (intermediate 30% of longitudinal foot length), and heel (proximal 30% of longitudinal foot length; Fig. 2). A regional analysis of plantar pressure variables was then performed. Although data were recorded bilaterally, only data recorded for the dominant limb during the final minute of each exercise were used for subsequent analysis and hypotheses testing. The majority of participants demonstrated right-limb dominance in both the young (N = 9 out of 10) and middle-aged (N = 9 out of 10) groups. Only steps with data available for the entire cycle were used for analysis of the pressure variables.
FIGURE 2-Three anato...Image Tools
For each subject, four pressure variables were calculated within each anatomical mask region (heel, arch, forefoot) across the five conditions (walking, running, elliptical training, stair climbing, recumbent biking). Mean maximum peak pressure (PP) was the average of the peak pressures recorded during the series of movement cycles. Mean maximum force (MF) identified the average of the maximum force calculated across the cycles. Contact area (CA) was determined by summing the area of all loaded sensors within a mask region. Mean normalized contact time (CT) identified the average percentage of each movement cycle (expressed as % cycle) that a foot region was in contact with the ground across the cycles of each activity.
Descriptive statistics were performed to describe all key variables. Separate one-way analyses of variance with repeated measures (5 × 2 ANOVA) were used to identify significant differences in each pressure variable across the five exercise activities and between the two age groups (young vs middle aged) for each anatomical region (forefoot, arch, heel). Bonferroni adjustments were applied to account for multiple comparisons within each variable type. An alpha level of 0.0167 (i.e., 0.05/3 foot regions) tested for significance. All data were analyzed using SPSS 15.0 statistical software (SPSS Inc., Chicago, IL).
Perceptions of Fatigue While Exercising at a Self-Selected Speed
Subject's perceptions of exertion varied moderately across the different exercises (Fig. 3), with the self-selected pace for most activities being performed at the "light to moderate" level (Borg RPE = 10-13), and some performed at the lower end of the "hard to very hard" (Borg RPE = 14-18) level (4,21). Running was perceived as most challenging for the young group (Borg = 14.0/20), whereas middle-aged subjects identified that stair climbing required the greatest exertion (Borg = 14.8/20). In contrast, walking was the least challenging exercise for both the young (Borg = 10.7/20) and middle-aged (Borg = 9.8/20) groups.
FIGURE 3-Borg Percei...Image Tools
Influence of Exercise on Plantar Pressures Variables
Mean peak pressures under the forefoot were significantly higher during walking, running, and elliptical training (all exceeding 210 kPa) than during stair climbing and recumbent biking (P ≤ 0.001). Biking peak pressures were lower than all other conditions (P < 0.001) (Table 2).
Forces under the forefoot were highest during running (P < 0.001) and were lowest during recumbent biking (P ≤ 0.001) compared with the other activities. The intermediate-level forces recorded under the forefoot during walking and elliptical training significantly exceeded those recorded during stair climbing (P ≤ 0.001).
The contact area beneath the forefoot was largest during running and walking compared with elliptical training, stair climbing, and biking (P ≤ 0.002). The smallest contact area occurred during biking (P ≤ 0.001).
The percentage of time the forefoot remained in contact with the ground throughout the movement cycle was longer during elliptical training, stair climbing, recumbent biking, and walking compared with running (P ≤ 0.003). The forefoot also remained in contact with the ground for a longer period of time during elliptical training and stair climbing compared with walking (P < 0.001).
Peak pressures beneath the arch were highest during running compared with all other conditions (P ≤ 0.001) (Table 3). Intermediate-level pressures recorded during walking and elliptical training exceeded those recorded during stair climbing (P ≤ 0.002). Pressures were lowest during recumbent biking (P < 0.001).
Similar to the pattern observed for pressures, the peak forces under the arch were significantly higher during running compared with all other activities, intermediate for elliptical training, walking, and stair climbing, and significantly lower for recumbent biking (P < 0.001).
Arch contact area was significantly larger during running than all other conditions (P ≤ 0.011). The smallest contact area occurred during recumbent exercise (P < 0.001).
The arch remained in contact with the ground through a longer percentage of the movement cycle during elliptical training, stair climbing, walking, and recumbent biking compared with running (P ≤ 0.010). Contact time was also longer during elliptical training compared with walking and recumbent biking (P ≤ 0.011).
Peak pressures under the heel varied nearly ninefold across activities (Table 4). The highest pressures occurred during walking and running, significantly exceeding those recorded during all other activities (P < 0.001). Moderate pressures during elliptical training significantly exceeded stair climbing values (P = 0.014). The lowest pressures occurred during recumbent biking, differing significantly from all other activities (P < 0.001).
Maximum force values under the heel varied greatly across activities (more than 25-fold). Similar to the pattern observed for pressures, the peak forces under the heel were significantly higher during walking and running compared with all other activities, intermediate for elliptical training and stair climbing, and significantly lower for recumbent biking (P < 0.001).
Average contact area under the heel was largest during walking and running (~45 cm2), significantly exceeding values recorded during stair climbing and recumbent biking (P < 0.001). During recumbent biking, the contact area was smallest compared with all other activities (10 cm2; P < 0.001).
The heel remained in contact with the ground for a shorter percentage of the movement cycle during running compared with all other activities (P ≤ 0.008).
Influence of age group on plantar pressure variables.
Contact area under the arch was significantly larger in the young compared with middle-aged group, when averaged across exercise activities (52.5 vs 42.8 cm2; P = 0.011). Beyond this finding, no other significant differences were identified between age groups among the variables analyzed.
Cardiovascular exercise is a cornerstone of initiatives aimed at improving the health and well-being of individuals across the age spectrum (9,21). Its value is underscored by the inclusion of specific objectives targeting cardiorespiratory fitness in Healthy People 2010, the national health-promotion and disease-prevention effort aimed at improving the life expectancy and quality of life of persons living in the United States. Exercise goals include engaging in vigorous activity three or more times a week for at least 20 min per session and participating in moderate physical activity for at least 30 min daily (27).
The potential risk that different cardiovascular activities pose to the feet of exercisers has not been explored previously and, hence, served as the focus of this research. Pressures differed substantially among exercises, with PP varying greater than sixfold under the forefoot (walking = 253 kPa vs recumbent biking = 41 kPa), fourfold beneath the arch (running = 144 kPa vs recumbent biking = 33 kPa), and eightfold under the heel (running = 215 kPa vs recumbent biking = 25 kPa) across activities. The primary cause of these differences was the change in maximum force exerted through the reference limb (and foot) when performing tasks, particularly those containing periods of single-limb support compared with activities incorporating primarily double-limb or buttock support. Whereas modest gains in contact area between the support surface and foot occurred during upright activities compared with recumbent biking, these gains were inadequate to offset the accompanying increases in force through the same regions of the foot (i.e., forefoot, arch, heel). For example, contact area under the forefoot increased approximately 230% during walking compared with recumbent biking (walking = 79.2 cm2 vs recumbent biking = 33.3 cm2); however, the forces increased approximately 970% (walking = 818 N vs recumbent biking = 84 N). The resulting peak pressure during walking was approximately 620% of that occurring during recumbent biking (walking = 253 kPa vs recumbent biking = 41 kPa).
The increased pressures documented under the forefoot when walking, running, and elliptical training have important clinical implications for adults who may be at risk for diabetic foot ulcers. A position statement jointly developed by the American College of Sports Medicine and the American Diabetes Association outlined key guidelines for cardiovascular exercise that included a recommendation for individuals to train up to 60 min per session, 3-7 d·wk−1, to achieve targeted cardiorespiratory, body composition, and blood glucose goals (2,3). Yet, individuals with diabetic sensory neuropathy often lack the ability to sense potentially damaging pressures that could lead to foot ulcers (5). As higher pressures increase the risk for foot ulceration (6,8) and amputation (1,8) in individuals with diabetes mellitus, data from the current work suggest that consideration should be given to the types of cardiovascular activities incorporated into diabetic therapeutic exercise programs. When protection of the forefoot is of importance (e.g., with diabetic foot neuropathies), biking and stair climbing seem to offer the optimal reduction in peak pressures. Further research in persons with diabetes mellitus is required to determine whether the pressure patterns identified in healthy young and middle-aged adults also occur in the presence of foot deformities/amputations (which could reduce contact area) and neuropathic muscle weakness (which would limit forces at the foot-support surface interface during exercise).
The significantly higher pressures that occurred under the heel during walking and running present important implications for persons with select orthopedic conditions. Exercises that repetitively load the heel could exacerbate heel pain in persons with subcalcaneal spurs, particularly in light of the associated reduction in heel pad elasticity (20). Data from the current study suggest that when protecting the heel from high pressures is of importance, activities such as recumbent biking and stair climbing should be considered.
Relatively low pressures also can lead to tissue damage if applied for a sufficiently long period of time (16,22). Linder-Ganz and colleagues (16) report that pressures as low as 40-70 kPa applied continuously for periods as short as 30 min could lead to tissue damage in underlying skeletal muscle tissue of the albino rat. In humans, Reswick and Rogers (22) have identified an inverse relationship between the maximum pressure experienced at the skin-surface interface and the time over which the pressure could be applied without developing a pressure sore. Collectively, these studies highlight the interrelationship between the magnitude and duration of pressure in the development of ulcers.
In the current study, the contact time between different foot regions and the support surface varied significantly across activities. Contact times were consistently longer during elliptical training (forefoot = 88% movement cycle; arch = 74% cycle; heel = 65% cycle) and shorter during running (forefoot = 42% movement cycle; arch = 39% cycle; heel = 32% cycle). Given that the forefoot remains in contact with the ground for 88% of each elliptical training movement cycle, then, theoretically, the forefoot region would experience pressure for nearly 53 min out of a typical 60-min workout (i.e., 88% of 60 min). In contrast, during running the forefoot would incur pressures for only 25 min during a 60-min exercise routine (i.e., 42% of 60 min). During walking, contact times were intermediate under the forefoot (58% cycle) and heel (52% cycle); however, pressures were highest in these regions (forefoot = 253 kPa; heel = 215 kPa) compared with all other exercises. The extent to which the extended period of contact during specific activities poses a risk for tissue injury in susceptible populations warrants further study.
Contrary to our third hypothesis, we did not identify significantly higher pressures in the young versus middle-aged group. It is possible that middle-aged subjects who agreed to participate in this study did not vary significantly in their force-producing capacity from subjects in the young group. The emphasis on, and requirement to, exercise may have resulted in only relatively physically fit individuals participating in the study. Additionally, no effort was made to control for subjects' choice of footwear, so this also may have served as a confounding factor between the young and middle-aged groups.
Despite a lack of significant differences in peak pressure values between the subject groups, contact area under the arch did vary significantly. When averaged across activities, contact area under the arch was 22% more expansive for the young compared with the middle-aged group (52.5 vs 42.8 cm2). The maximum forces also were consistently higher under the arch for the young versus middle-aged group for each activity; however, this difference did not rise to the level of statistical significance. Footwear may have contributed to the variation in contact area between groups. Subjects in the young group wore running (N = 7) or cross-training (N = 3) footwear, which may have provided better arch support. For the middle-aged group, selections included walking (N = 3) and casual (N = 1) shoes in addition to running (N = 2) and cross-training (N = 4) footwear. Further work exploring the impact of footwear on surface area variables may be warranted.
The finding that peak pressure under the forefoot region did not differ significantly between running and walking (251 vs 253 kPa, respectively) presents an apparent paradox in light of the significantly higher maximum force during running compared with walking (999.4 vs 818.0 N, respectively) yet equal contact area (79.2 cm2 for both). The reason for this discrepancy likely arises from the algorithms used to calculate each of the pressure variables. Specifically, peak pressure was calculated as the average of the maximum pressures experienced by any one sensor in the forefoot region across all cycles of a trial (for our study, the forefoot region contained approximately 38 sensors). In contrast, contact area and peak force were determined by summing the respective values recorded in all forefoot sensors loaded during a cycle. The summed value for each cycle was then averaged across all cycles in a trial. Additionally, because of the dynamic nature of each activity and the extended period of time spent on the forefoot region, it is possible that the peak force during walking was not synchronized temporally with the instant of greatest contact area.
The peak pressures recorded under our subjects' heel and forefoot regions during treadmill walking were notably higher than some values previously reported in the literature for shod walking. Grampp and colleagues (11) evaluated plantar pressures while young adults walked on a treadmill wearing their own self-selected athletic shoes. During level walking, peak pressures under the heel region (137 kPa) were approximately 40% lower than the average peak pressures recorded for the young group in the current study (226 kPa). Similarly, under the forefoot, the average peak pressure (175 kPa, second and third metatarsal head region) reported by Grampp et al. (11) was approximately 30% lower than those identified for the young group in the current study (251 kPa). One reason for these differences may have been the influence of walking speed on plantar pressure values. Subjects in the Grampp et al. study walked at a constant speed (58 m·min−1), whereas subjects in the current study walked at a self-selected pace, averaging 91 m·min−1 across subjects. Previous researchers have identified a significant increase in pressure under the heel (7,15) and forefoot (7) associated with faster walking speeds. Burnfield and colleagues (7) report significantly higher pressures values under the heel and forefoot regions during shod walking at both the fast speed (97 m·min−1) and the medium velocity (80 m·min−1) compared with a slow "stroll" pace (57 m·min−1), reinforcing the influence of walking velocity on pressure values (7). Differences in the compliance of the walking surface also may have contributed to pressure-value variations between the two brands of treadmills (Woodway vs LifeFitness 97 Ti Treadmill).
The peak pressure values recorded during treadmill running for our subjects demonstrate some notable similarities with values previously reported in the literature, despite variations in the average running speed of subjects in the two studies. Willson and Kernozek (30) studied the impact of fatigue on plantar pressure values recorded during treadmill running in 20- to 30-yr-old active adults. After 5 min of running at a self-selected speed (209 m·min−1, males; 161 m·min−1, females), subjects in the Willson and Kernozek (30) study experienced peak pressures in the heel region (210 kPa) that were similar to those experienced by our participants (215 kPa) while running at an average speed of 150 m·min−1. Pressures in the region with the highest values (270 kPa, great toe) slightly exceeded our peak pressures in the forefoot region (253 kPa). Differences in footwear, age of subjects, and compliance of the walking surface may have contributed to pressure-value variations between the two studies. With fatigue, a factor not explored in the current study, Willson and Kernozek (30) document a significant reduction in peak pressures under only the heel region during treadmill running (rested, 210 kPa vs fatigued, 189 kPa).
Running shoe use is another factor that also seems to influence peak pressure values during running. Verdejo and Mills (28) recorded heel pressures at specific intervals during treadmill running and identified that values increased with running shoe use. When the shoes were new, the peak pressure was approximately 210 kPa at the end of 5 min of running. After 300 km of use on tracks and roads, peak pressure had increased to about 230 kPa during treadmill running, and by 500 km of use, the peak pressure values were approximately 280 kPa at the end of 5 min of treadmill running (28). These data highlight the influence of shoe use on pressure values, a factor not controlled for in the current study.
The peak pressures documented during the stair climbing exercise for the subjects participating in the current study were notably lower than those previously reported for traditional stair negotiation. Rozema and colleagues (23) studied pressure variations under the heel and forefoot regions during stair ascent in a cohort of young adults (20-33 yr old). Subjects ascended 10 linoleum-covered concrete steps at a predetermined rate of 84 steps per minute without holding onto the rails. The mean peak pressure under the heel region (115 kPa) was approximately 75% higher than the mean peak heel pressure occurring in our subjects during stair climbing (66 kPa). The average peak pressure under the forefoot was nearly twofold higher during stair ascent (240 kPa, great toe region) in the Rozema et al. (23) study compared with the value identified during the stair climbing exercise for our subjects (130 kPa). Factors that may have contributed to the relatively higher heel pressures in the Rozema study (23), compared with the current investigation, include the relatively harder surface being negotiated (linoleum-covered concrete step versus a rubber-covered metal plate) and footwear (Oxford-style with no arch support and minimal cushioning vs self-selected footwear). Both walking surface (17) and footwear (7,17) can significantly impact plantar pressure values recorded under the feet of healthy adults. Additionally, subjects were not allowed to use their arms for support when negotiating the stairs in the Rozema et al. (23) study, whereas subjects consistently used their arms during the stair climbing exercise.
Although nonrecumbent cycling pressures have been evaluated in elite and recreational athletes (13,24), the authors are unaware of any published work focusing on pressures during recumbent biking. Jarboe and Quesada (13) evaluated the influence of footwear on peak plantar pressures while cycling on a bicycle mounted to an indoor magnetic resistance trainer; they report that the mean peak pressures under the forefoot were significantly higher in cycling shoes containing a carbon fiber composite sole (121 kPa) compared with those incorporating the more traditional plastic sole (103 kPa). The higher values reported by Jarboe and Quesada (13) may reflect the use of cycling shoes, which traditionally have a stiffer sole, as well as the difference between the upright posture of subjects in their study versus the recumbent biking posture in the current investigation. During upright biking on a Velodyne stationary trainer, Sanderson et al. (24) report a significant increase in peak pressure under the forefoot, midfoot, and heel when power output increased from 100 to 400 W. In contrast, peak pressure significantly decreased with faster cadences (i.e., 100 vs 60 rpm). In the current study, subjects exercised at their own self-selected speed and resistance, simulating levels they would use during a typical 30-min exercise routine.
One potential limitation of the current study was that subjects exercised at a comfortable pace for only 5 min on each piece of equipment. This design was employed to limit the cumulative impact of fatigue on movement patterns across the five exercises. Subjects were asked to exercise at a level that they would be able to "maintain during a typical 30-min workout." Further study, within each exercise activity, is required to define how pressure patterns vary with muscular fatigue, because this may also alter the risk of tissue injury. Fatiguing loading conditions have been implicated in the development of metatarsal stress fractures (29). Subsequent research, employing a design to control for exercise intensity, could also provide valuable insights into the impact of this variable on plantar pressures within an activity as well as across the exercises. This may have important implications for training program designs.
In conclusion, this study is the first, to the authors' knowledge, that systematically evaluated plantar pressure patterns across a variety of cardiovascular exercise activities commonly performed in fitness clubs, homes, and medical facilities. Collectively, the findings from this work have important clinical implications for protecting regions of the foot that may be susceptible to injury. In particular, when protection of the forefoot is of importance (e.g., with diabetic foot neuropathies), the findings from this study suggest that biking and stair climbing offer the optimal reduction in pressures. If protection of the heel from high pressures and forces is warranted (e.g., with subcalcaneal heel spurs or plantar fasciitis), then recumbent biking, stair climbing, and elliptical training provide greater relief compared with running and walking.
This work was supported, in part, by grants from the Daniels Fund, the UCARE program at the University of Nebraska-Lincoln, the Madonna Auxiliary, and the Gifford Swenson Estate. Additionally, the authors of this paper would like to gratefully acknowledge the contributions of Shannon Ogren, DPT and Thad Buster for their assistance with subject familiarization sessions and data collection.
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