Niki M. Stolwijk

A new method to normalize plantar pressure measurements for foot size and foot progression angle

Plantar pressure measurement provides important information about the structure and function of the foot and is a helpful tool to evaluate patients with foot complaints. In general, average and maximum plantar pressure of 6-11 areas under the foot are used to compare groups of subjects. However, masking the foot means a loss of important information about the plantar pressure distribution pattern. Therefore, the purpose of this study was to develop and test a simple method that normalizes the plantar pressure pattern for foot size, foot progression angle, and total plantar pressure. Moreover, scaling the plantar pressure to a standard foot opens the door for more sophisticated analysis techniques such as pattern recognition and machine learning. Twelve subjects walked at preferred and half of the preferred walking speed over a pressure plate. To test the method, subjects walked in a straight line and in an approaching angle of approximately 40 degrees . To calculate the normalized foot, the plantar pressure pattern was rotated over the foot progression angle and normalized for foot size. After normalization, the mean shortest distance between the contour lines of straight walking and walking at an angle had a mean of 0.22 cm (SD: 0.06 cm) for the forefoot and 0.14 cm (SD: 0.06 cm) for the heel. In addition, the contour lines of normalized feet for the various subjects were almost identical. The proposed method appeared to be successful in aligning plantar pressure of various feet without losing information.

Plantar pressure changes after long-distance walking.

PURPOSE The popularity of long-distance walking (LDW) has increased in the last decades. However, the effects of LDW on plantar pressure distribution and foot complaints, in particular, after several days of walking, have not been studied. METHODS We obtained the plantar pressure data of 62 subjects who had no history of foot complaints and who walked a total distance of 199.8 km for men (n = 30) and 161.5 km for women (n = 32) during four consecutive days. Plantar pressure was measured each day after the finish (posttests I–IV) and compared with the baseline plantar pressure data, which was obtained 1 or 2 d before the march (pretest). Mean, peak, and pressure–time integral per pixel as well as the center of pressure (COP) trajectory of each foot per measurement day were calculated using the normalization method of Keijsers et al. A paired t-test with an adjusted P value was used to detect significant differences between pretest and posttest. RESULTS Short-term adjustment to LDW resulted in a significant decreased loading on the toes accompanied with an increased loading on the metatarsal head III–V (P < 0.001). At all stages, particularly at later stages, there was significantly more heel loading (P < 0.001). Furthermore, the COP significantly displaced in the posterior direction but not in the mediolateral direction after marching. Contact time increased slightly from 638.5 +/- 24.2 to 675.4 +/- 22.5 ms (P < 0.001). CONCLUSIONS The increased heel loading and decreased function of the toes found after marching indicate a change of walking pattern with less roll-off. It is argued that these changes reflect the effect of fatigue of the lower leg muscles and to avoid loading of the most vulnerable parts of the foot.

Plantar pressure with and without custom insoles in patients with common foot complaints.

Background: Although many patients with foot complaints receive customized insoles, the choice for an insole design can vary largely among foot experts. To investigate the variety of insole designs used in daily practice, the insole design and its effect on plantar pressure distribution were investigated in a large group of patients. Materials and Methods: Mean, peak, and pressure-time-integral per sensor for 204 subjects with common foot complaints for walking with and without insoles was measured with the footscan® insole system (RSscan International). Each insole was scanned twice (precision3D), after which the insole height along the longitudinal and transversal cross section was calculated. Subjects were assigned to subgroups based on complaint and medial arch height. Data were analyzed for the total group and for the separate subgroups (forefoot or heel pain group and flat, normal or high medial arch group). Results: The mean pressure significantly decreased under the metatarsal heads II-V and the calcaneus and significantly increased under the metatarsal bones and the lateral foot (p < 0.0045) due to the insoles. However, similar redistribution patterns were found for the different foot complaints and arch heights. There was a slight difference in insole design between the subgroups; the heel cup was significantly higher and the midfoot support lower for the heel pain group compared to the forefoot pain group. The midfoot support was lowest in the flat arch group compared to the high and normal arch group (p < 0.05). Conclusion: Although the insole shape was specific for the kind of foot complaint and arch height, the differences in shape were very small and the plantar pressure redistribution was similar for all groups. Clinical Relevance: This study indicates that it might be sufficient to create basic insoles for particular patient groups.

Classification of forefoot pain based on plantar pressure measurements

BACKGROUND Plantar pressure is widely used to evaluate foot complaints. However, most plantar pressure studies focus on the symptomatic foot with foot deformities. The purposes of this study were to investigate subjects without clear foot deformities and to identify differences in plantar pressure pattern between subjects with and without forefoot pain. The second aim was to discriminate between subjects with and without forefoot pain based on plantar pressure measurements using neural networks. METHODS In total, 297 subjects without foot deformities of whom almost 50% had forefoot pain walked barefoot over a pressure plate. Foot complaints and subject characteristics were assessed with a questionnaire and a clinical evaluation. Plantar pressure was analyzed using a recently developed method, which produced pressure images of the time integral, peak pressure, mean pressure, time of activation and deactivation, and total contact time per pixel. After pre-processing the pressure images with principal component analysis, a forward selection procedure with neural networks was used to classify forefoot pain. FINDINGS The pressure-time integral and mean pressure were significantly larger under the metatarsals II and III for subjects with forefoot pain. A neural network with 14 input parameters correctly classified forefoot pain in 70.4% of the test feet. INTERPRETATION The differences in plantar pressure parameters between subjects with and without forefoot pain were small. The reasonable performance of forefoot pain classification by neural networks suggests that forefoot pain is related more to the distribution of the pressure under the foot than to the absolute values of the pressure at fixed locations.

Flat Feet, Happy Feet? Comparison of the Dynamic Plantar Pressure Distribution and Static Medial Foot Geometry between Malawian and Dutch Adults

In contrast to western countries, foot complaints are rare in Africa. This is remarkable, as many African adults walk many hours each day, often barefoot or with worn-out shoes. The reason why Africans can withstand such loading without developing foot complaints might be related to the way the foot is loaded. Therefore, static foot geometry and dynamic plantar pressure distribution of 77 adults from Malawi were compared to 77 adults from the Netherlands. None of the subjects had a history of foot complaints. The plantar pressure pattern as well as the Arch Index (AI) and the trajectory of the center of pressure during the stance phase were calculated and compared between both groups. Standardized pictures were taken from the feet to assess the height of the Medial Longitudinal Arch (MLA). We found that Malawian adults: (1) loaded the midfoot for a longer and the forefoot for a shorter period during roll off, (2) had significantly lower plantar pressures under the heel and a part of the forefoot, and (3) had a larger AI and a lower MLA compared to the Dutch. These findings demonstrate that differences in static foot geometry, foot loading, and roll off technique exist between the two groups. The advantage of the foot loading pattern as shown by the Malawian group is that the plantar pressure is distributed more equally over the foot. This might prevent foot complaints.

Effect of a metatarsal pad on the forefoot during gait

BACKGROUND Metatarsal pads are frequently prescribed for patients with metatarsalgia to reduce pain under the distal metatarsal heads. Several studies showed reduced pain and reduced plantar pressure just distal to the metatarsal pad. However, only part of the pain reduction could be explained by the decrease in plantar pressure under the forefoot. Therefore, an alternative hypothesis is proposed that pain relief is related to a widening of the foot and the creation of extra space between the metatarsal heads. This study focused on the effect of a metatarsal pad on the geometry of the forefoot by studying forefoot width and the height of the second metatarsal head. METHODS Using a motion analysis system, 16 primary metatarsalgia feet and 12 control feet were measured when walking with and without a metatarsal pad. RESULTS A significant mean increase of 0.60 mm in forefoot width during the stance phase was found when a metatarsal pad was worn. During midstance, the mean increase in forefoot width was 0.74 mm. In addition, walking with a metatarsal pad revealed an increase in the height of the second metatarsal head (mean, 0.62 mm). No differences were found between patients and controls. CONCLUSIONS The combination of increased forefoot width and the height of the second metatarsal head produced by the metatarsal pad results in an increase in space between the metatarsal heads. This extra space could play a role in pain reduction produced by a metatarsal pad.

Prediction of walking speed using single stance force or pressure measurements in healthy subjects.

Walking speed is one of the best measures of overall walking capacity. In plantar pressure measurements, walking speed can be assessed using contact time, but it is only moderately correlated with walking speed. The center of pressure might be of more value to indicate walking speed since walking speed alters foot loading. Therefore, the purpose of this study is to assess walking speed using the velocity of the center of pressure (VCOP). Thirty-three subjects walked over a Footscan pressure plate at three speed conditions; slow, preferred, and fast. Walking speed was measured by a motion analysis system. (Multiple) linear regression analysis was used to indicate the relation between walking speed and independent variables derived from the pressure plate such as mean VCOP and stance time for all walking conditions separately and together. The mean VCOP had the highest correlation coefficient value with walking speed for all walking conditions combined (0.94) and for the preferred walking condition (0.80). The multiple regression analysis, based on a number of additional parameters, revealed a small to modest increase in the performance of predicting walking speed (r=0.98 for combined and r=0.93 for preferred). The mean VCOP was the best predictor for walking speed when using a plantar pressure plate. The mean VCOP predicts the walking speed with a 95% accuracy of 0.20m/s when healthy subjects walk at their preferred walking speed.

Foot lengthening and shortening during gait: A parameter to investigate foot function?

INTRODUCTION Based on the windlass mechanism theory of Hicks, the medial longitudinal arch (MLA) flattens during weight bearing. Simultaneously, foot lengthening is expected. However, changes in foot length during gait and the influence of walking speed has not been investigated yet. METHODS The foot length and MLA angle of 34 healthy subjects (18 males, 16 females) at 3 velocities (preferred, low (preferred -0.4 m/s) and fast (preferred +0.4 m/s) speed were investigated with a 3D motion analysis system (VICON(®)). The MLA angle was calculated as the angle between the second metatarsal head, the navicular tuberculum and the heel in the local sagittal plane. Foot length was calculated as the distance between the marker at the heel and the 2nd metatarsal head. A General Linear Model for repeated measures was used to indicate significant differences in MLA angle and foot length between different walking speeds. RESULTS The foot lengthened during the weight acceptance phase of gait and shortened during propulsion. With increased walking speed, the foot elongated less after heel strike and shortened more during push off. The MLA angle and foot length curve were similar, except between 50% and 80% of the stance phase in which the MLA increases whereas the foot length showed a slight decrease. CONCLUSION Foot length seems to represent the Hicks mechanism in the foot and the ability of the foot to bear weight. At higher speeds, the foot becomes relatively stiffer, presumably to act as a lever arm to provide extra propulsion.

Velocity of centre of pressure as a predictor of walking speed

Background Walking speed is one of the best measures of overall walking ability. In plantar pressure measurements, walking speed is usually assessed using a stopwatch, photocells, or camera systems. As a simple alternative, contact time can be used but contact time and walking speed are only moderately correlated. Because walking speeds alters foot loading, the centre of pressure might be of more value to indicate walking speed. Therefore, the purpose of this study is to assess walking speed using the velocity of the centre of pressure (VCOP).

The effect of various subject characteristics on plantar pressure pattern

Background Plantar pressure is highly influenced by many factors such as walking velocity, body weight, and age. The impact of these subject characteristics on plantar pressure is usually studied separately. However, many of these factors are interact with each other; for example walking velocity is negatively correlated with body weight and age. The purpose of this study is to investigate the effect of several subject characteristics in relation to plantar pressure pattern for a large group of subjects.