The Human Foot During Locomotion — Applied Research for Footwear

by Prof. Ewald M. Hennig

10 October 2002

Upright walking — or bipedalism — is the earliest human characteristic. The upright posture freed the hands of early humans for tool making and use. The use of tools created the need for intellectual capabilities and eventually resulted in a more and more complex brain structure. The human foot gradually changed from the grasping organ of early primate ancestors to the specialized weight bearing structure of modern man. The Laetoli Footprints, discovered by Mary Leakey in 1976, date back 3.6 millions years. These footprints are an early proof of bipedal locomotion. After a volcano eruption in what is now northern Tanzania in Africa, the landscape was covered by volcanic ash. With rain fall this ash took on properties similar to plaster, preserving any traces on the ground. At least two members of the species Australopithecus afarensis walked across the wet volcanic ash, leaving their footprints in the ash which, after drying, turned into a hard cement. These fossils demonstrate that 3.6 million years ago, early humans were walking upright on two legs. Their big toes hardly diverged from the rest of the foot, and thus were quite different from chimpanzee foot prints. In 1974, researcher Donald Johanson discovered the skeleton of Lucy, the oldest and most complete remains of a human ancestor. The skeleton demonstrated load bearing features as it is observed in humans, but from the skull structure it could be concluded that the brain capacity of Lucy was closer to that of a chimpanzee than of a human. Originally, in line with Darwin's theory, it was thought that intelligence had come before bipedalism. It was assumed that the increase in brain capacity drove early humanoids to use tools and get up on two legs. However, with the fossil remains of Lucy, anthropologists could prove that tool use and hunting arrived much later than bipedalism itself. The human foot has a dual role during locomotion. During ground contact it is a mobile structure that adapts to uneven surfaces. At push-off during walking and running the foot becomes a rigid and efficient lever to propel the body forward.

Foot Anatomy — Gender and Ethnic Differences

Leonardo da Vinci characterized the human foot as a masterpiece of engineering and a work of art. The foot consists of 26 bones and is characterized by three arches: a medial and a lateral longitudinal arch, as well as a transverse arch. The arches of the foot are maintained by the shapes of the bones and ligaments. Muscles and tendons also help in supporting the arches. The muscles of the foot are classified as either intrinsic or extrinsic. The intrinsic muscles are located within the foot and the extrinsic muscles have their origin in the lower leg. The foot is divided into forefoot, midfoot, and rearfoot. The main joint in the rearfoot is the subtalar joint. The axis of this hinge joint runs from the posterio-lateral plantar aspect of the foot antero-medially to its dorsum. In combination with the ankle joint, the subtalar joint allows pronation (coupled dorsiflexion, abduction, and eversion) and supination (coupled plantar flexion, adduction, and inversion) of the foot.

Gender and ethnic related differences in foot anatomy do not only have consequences for the construction of footwear but may also be the cause of specific foot complaints and injury prevalence. In addition to the obvious overall body dimensions, body composition is the major factor in differentiating women and men anatomically. Skeletal muscle mass as a percentage of total body weight is approximately 40 per cent for men and only 23 per cent for women. Sexual dimorphism in the pelvis is the major difference in the skeletal structure between the sexes. Adult foot size is attained at about 14 years of age in females and at about 16 in males, reflecting an earlier maturation in women. Women's feet have a considerably increased incidence of orthopaedic problems. Podiatrists blame the high incidence of women's feet problems primarily on the fashion shoes that they wear. However, foot structure may also play a role in the higher incidence of lower extremity problems in women. If foot length is normalized, women have a more slender foot than men. Furthermore, women's heels are narrower in proportion to the forefoot. There are also differences in foot morphology in different parts of the world. When foot length is identical, Asian feet have a wider forefoot width, a lower longitudinal arch, and more pronated feet.

Pressure Distribution Technology

Researchers have been interested in the distribution of loads under the human foot during various activities for more than 100 years. Early methods estimated plantar forces from impressions of the foot in Plaster-of-Paris and clay. Later techniques included optical methods with cinematographic recording. Instantaneous pressure distributions were recorded for the first time by Elftman (1934). He used a mat with rubber pyramids on one side, which was laid down on a glass plate. By stepping on the rubber mat, he increased the contact area of the pyramids with the glass plate. Film recording (72 frames/sec) of the illuminated glass plate enabled him to record instantaneous foot pressures. Only in recent years, the availability of inexpensive force transducers and modern data acquisition systems have made the construction of various pressure distribution measuring systems possible. The basic principle for the now commercially available NOVEL© capacitance pressure distribution instrumentation was introduced and patented by Nicol and Hennig (1976, 1978). An array of 256 capacitors were formed by the overlap of two sets of 16 conducting strips laid in orthogonal directions on opposite sides of the centre layer of a three-layer sandwich. Hennig et al. (1982) also described the usefulness of inexpensive piezoceramic materials for foot pressure measurements. Today, the major transducer technologies for pressure distribution devices are based on capacitive, piezoelectric, and resistive principles. All methods are based on the effect of changes in the electrical properties of sensors, caused by the mechanical deformation of its material. Therefore, the elastic response of material deformation plays a major role in the quality of a transducer. Data acquisition, processing, and storage are important issues for pressure distribution measurements. For instance, recording durations of one second at a sampling rate of 50 Hz for a pressure mat that comprises 2,000 elements will result in 100,000 pressure data points. With this high volume of information, visual presentation and data reduction techniques become important. Graphical representation of pressure distribution can be achieved through wire frame diagrams (figure 1). These pressure maps can be obtained for each sampling interval or at specific instants during the foot-ground contact. A peak pressure graphical representation can be used to illustrate individual foot contact behaviour with the ground. This image is created by presenting the highest pressures under the foot, as they have occurred at any time during the ground contact (figure 1).

Figure 1: Peak pressure wire frame diagram for a subject during walking

Using a capacitance pressure distribution mat (1 sensor/cm²), Cavanagh, P.R. et al. (1987) investigated the plantar pressures under the symptom-free feet of 107 subjects during bipedal standing. They found approximately 2.6 times higher heel (139 kPa) against forefoot pressures (53 kPa). The highest forefoot pressures were located under the second and third metatarsal heads. There was almost no load sharing contribution of the toes during standing. Furthermore, peak plantar pressures did not show a significant relationship to the body weight of subjects. Using a capacitive pressure distribution platform EMED F01 (NOVEL Inc.) with a higher resolution (2 sensors/cm²), Hennig et al. (1993) measured the pressures during bipedal standing and walking (approximately 1 m/s) under the symptom-free feet of 49 women and 62 men. During both standing and walking the highest pressures under the forefoot were found under the third metatarsal head (figure 2). A 'tripod' loading of the human foot, as it is mentioned in some textbooks, was not found in a pressure distribution study with the 111 adults. For bipedal standing as well as walking, peak pressures beneath the third metatarsal head were substantially higher than under the metatarsal heads I and V. Correlation analyses demonstrated that the pressure distribution during standing reveals only little information about the dynamic loads under the foot during gait. No differences in plantar pressures were found between the women and men except for the pressures under the longitudinal arch. Under the midfoot the women showed highly significantly (p < 0.01) reduced peak pressures during standing. Similar to the findings of Cavanagh et al. (1987), relationship between peak pressures and body weight was not found across all subjects under any of the investigated anatomical regions. When grouping the subjects according to gender, however, a highly significant relationship (r = + 0.65, p < 0.01) was found for the comparison of body weight against the peak pressures under the longitudinal arch of the women's feet in the walking condition. Because this phenomenon was only present for the women during walking, the authors speculated that a weaker ligamentous structure in women's feet results in a more significant collapse of the longitudinal arch during weight bearing. This hypothesis has recently been supported by a study comparing foot function of women and men during running (Hennig, 2001). Midfoot loading as well as the amount of rearfoot pronation were significantly increased for the women.

Pressures Under the Feet of Children

At birth the mechanical structure of the foot is predominantly soft tissue. On radiographs the infant foot skeleton appears as a loose assembly of ossified diaphyses of the phalangeal and metatarsal bones and the nuclei of the talus and calcaneus. The connection of the bones to a skeletal structure takes place with the proceeding transformation of cartilage to bone. Only by age six are the major structural changes of the foot completed and its appearance is similar to an adult foot. A plantar pressure study with 15 infants (7 male, 8 female) between the ages of 14 and 32 months was conducted by Hennig & Rosenbaum in 1991. The results were compared with data on plantar pressures from 111 adults (Hennig & Milani, 1993). Considerably reduced peak pressures in the infant group could be attributed to a softer foot structure and a lower body-weight to foot-contact area ratio. An almost three times higher relative load under the midfoot of the infant foot shows that the longitudinal foot arch is still a weak structure in this age group. Across the age range of the children it was observed that with increasing age the relative load under the midfoot decreases while it become more pronounced under the third and fifth metatarsals. Within only a few months a rapid development of the growing foot towards an adult loading pattern was observed. In a later study, using the same instrumentation, peak pressures and relative loads under the feet of 125 children (64 boys, 61 girls) between 6 and 10 years of age were determined (Hennig et al., 1994). These results were also compared to the data from 111 adults (Hennig & Milani, 1993). The school children showed considerably lower peak pressures under all anatomical structures (figure 2).

Figure 2: Peak plantar pressures (kPa) from 111 adults and 125 children during walking

These lower pressures could mainly be attributed to the larger foot dimensions per kg body mass for the children. No differences were found for the foot pressures between boys and girls. With increasing age a medial load shift in the forefoot was observed for the older children. Reduced loading of the first metatarsal head in the younger children was attributed to a valgus knee condition with hyperpronation of the foot and a reduced stability of the first ray. No reduction in pressures under the longitudinal arch with an increase of age suggests that foot arch development is almost complete before the age of six. Contrary to the findings in adults, body weight was identified to be of major influence on the magnitude of the pressures under the feet of school children. Between boys and girls no differences in the peak pressure or relative load patterns were present.

Overweight and obesity are major health problems in many parts of the world and the incidence of these conditions is increasing. Among numerous other medical conditions, a high incidence of osteoarthrits, painful feet, and symptomatic complaints in the joints of the lower extremities are frequently reported for overweight persons. Plantar pressure analysis may provide additional insight into the aetiology of pain and lower extremity complaints. Hills et al. (2001) investigated the plantar pressure differences between obese and non-obese adults during standing and walking. Thirty-five males (67-179 kg) and 35 females (46-150 kg) were divided into an obese (body mass index (BMI) 38.8 kg/m²) and a non-obese (BMI 24.3 kg/m²) group. A BMI of greater than or equal to 30 kg/m² was used to classify individuals as obese. Using this criterion, 17 men and 18 women were categorized as obese. All subjects were otherwise healthy with no locomotor limitation such as symptomatic osteoarthritis. During standing and walking data were collected with a capacitive pressure distribution platform (Emed F01). Pressures were evaluated for eight anatomical sites under the feet. The obese subjects showed an increase in the forefoot width to foot length ratio, suggesting a broadening of the forefoot under increased weight loading conditions. In spite of the increased load bearing contact area of the foot with the ground, the obese men and women had substantially higher pressures under the heel, midfoot, and forefoot during standing. For both obese groups the dynamic peak pressures during walking were also significantly increased under most foot areas, especially under the midfoot and across the metatarsal heads (figure 3).

Figure 3: Peak pressures (kPa) under the foot of obese and non-obese women during walking

In the static as well as the dynamic loading situations, the highest pressure increases for the non-obese group were found under the longitudinal arch of the foot. Increases in pressure under the mid-foot were higher during standing for the obese women as compared to the obese men. A strong relationship between BMI and the peak pressures (r² = 0.66) under the longitudinal foot arch is apparent across the 35 women. This relationship is much weaker for the 35 men (r² = 0.26). The clear gender related influence of body weight on the flattening of the arch may be the consequence of a reduced strength of the ligaments in women's feet. The authors concluded that their findings may have implications for lower extremity pain and discomfort in the obese, the choice of footwear and predisposition to participation in activities of daily living such as walking.

In a recent study, Dowling et al. (2001) compared the effects of obesity on plantar pressure distributions in prepubescent children. Thirteen obese children with a body mass index of 25.5 were compared to 13 age- and height-matched non-obese children (BMI 16.9). Footprint structural differences were observed between the groups, identifying a lower longitudinal arch, a flatter cavity, and a broader midfoot area for the obese children. For walking the forefoot pressures as well as the forefoot contact area were significantly increased for the obese group. The authors hypothesized that increased forefoot plantar pressures may lead to foot discomfort and may hinder obese children from participating in physical activity.

Plantar Pressure Analysis in Clinical Applications

Diabetes mellitus results from a failure of the endocrine system to control blood glucose levels within normal limits. Depending on the severity of the disease, at a later stage of the natural disease progression, neuropathic and vascular changes can occur in these patients. Sensory neuropathy will result in a progressive, distal to proximal, loss of sensation in the lower extremities, often resulting in ulceration at locations of high plantar pressures. Ulceration frequently results in partial or total amputation of the foot. Numerous studies have proven the usefulness of pressure distribution measurements for the prescription of therapeutic footwear. As soon as modern pressure distribution instrumentation became available, a number of research groups realized the potential of this technology for the diagnosis and treatment of the diabetic feet (Boulton et al., 1983; Cavanagh et al., 1985; Duckworth et al., 1985). In the last 10 years a number of publications have described the value of plantar pressure analysis for the understanding of diabetic foot function as well as the possibilities for therapeutic intervention. Many studies have investigated therapeutic footwear for the diabetic foot, especially various designs of rocker bottom shoes. A good review about the possibilities and limitations of plantar pressure measurements for the diagnosis and therapy of the diabetic foot has been published by Cavanagh et al. in 1993 and in 2000. Plantar pressure studies on the rheumatoid foot have also received special attention by a number of research groups, especially in recent years (Bitzan et al., 1997; Hodge et al., 1999; Orlin et al., 1997; Woodburn & Helliwell, 1996; Woodburn et al., 2000). Hallux valgus, a fairly common forefoot deformity at the metatarsophalangeal joint, is more frequently seen in women. Despite congenital factors, poorly fitting footwear and fashionable high-heeled shoes, worn by many women, are mentioned by many authors to be the etiologic factor in many cases of hallux valgus. A lateral forefoot load shift in hallux valgus feet has been described by Blomgren et al. (1991), the effect of high-heeled shoes by Nyska et al. in 1996, and several authors have reported the effect of various surgical hallux valgus treatments (Kernozek et al., 1997; Samnegard et al., 1991). Different orthotic materials were investigated by Brown et al. (1996) to demonstrate the beneficial effect of orthotics in relieving high pressures in shoes. However, as the authors point out, stress relieve can only occur at the cost of increasing pressure in other areas of the plantar surface. Using the centre of pressure path and other pressure variables, plantar pressure instrumentation is used more in the analysis of postural control (Nurse & Nigg, 2001; Tanaka et al., 1997, 1999) and the analysis of neurological disorders such as Hemiparesis and Parkinsonian gait (Kimmeskamp & Hennig, 2001; Nieuwboer et al., 1999).

It has been estimated that 65 per cent of runners sustain an overuse injury each year that will cause them to stop running. The etiologic factors generally associated with these injuries include anthropometric, training, rearfoot movement, kinetic, and strength variables. Based on medical knowledge and from biological experiments, biomechanical parameters are identified to judge the properties of athletic footwear in providing overuse injury protection and performance enhancement. Using technical tests, impacter devices may be used to quantify midsole material properties, thus providing information about production tolerances. Biomechanical measurements are essential in determining the influence of footwear construction on kinematic and kinetic variables of human performance. Using running shoes as an example, rearfoot pronation, foot pressure patterns, and shock absorption properties have to be identified for determining shoe characteristics which will reduce the risk of overuse injuries. Field tests with many subjects are necessary to evaluate shoe fit and comfort during running on varying terrain. Only the combination of a technical test, biomechanical evaluation and subjective ratings by the athletes provides a complete picture for the quality of athletic shoes. Applying this strategy, five major running shoe studies — including approximately 100 different shoe constructions — were performed during the last 10 years for a consumer product testing agency in Germany. Although a general trend towards better overall shoe quality (durability and biomechanical performance) was observed over the last decade, substantial differences between shoes were found in each test (Hennig & Milani, 1995). For some footwear models, these tests also revealed a surprisingly high deterioration of shoe properties after use (220 km). Therefore, the testing of athletic footwear has to be applied to new as well as used shoes. In-shoe pressure measurements during running provide information of the interaction between footwear and foot mechanics.

Figure 4: Peak in-shoe pressures for 22 subjects, running in two different types of footwear

The peak pressure image (figure 4) reveals substantial in-shoe pressure differences between footwear constructions. Shoe B shows substantially increased pressures in the heel area and under the first metatarsal head whereas the forefoot pressures under the third and fifth metatarsal heads and the hallux are only slightly increased. More details on the influence of footwear on biomechanical parameters are summarized in Hennig & Milani (1995).

Overpronation during running has been linked to many overuse injuries. The results from different studies suggest a high probability that restriction of excessive rearfoot motion and improved shock absorption will reduce the risk of running injuries. The determination of subtalar joint pronation is probably the most important measurement for the evaluation of running shoes.

Figure 5: Maximum pronation (Achilles tendon angle) for 20 subjects running in 19 types of running footwear

Figure 6: Peak tibial acceleration (in g) for 20 subjects running in 19 types of running footwear

Figures 5 and 6 depict the maximum pronation and peak acceleration differences as they were determined with the different footwear conditions. It may be concluded that the type of running footwear substantially influences the shock experienced by runners during each ground contact.

Upright walking and being able to use our hands for tool-making marked the origin of mankind. They were also necessary for the development of human intelligence. The human foot gradually changed from the grasping organ of early primate ancestors to the specialized weight bearing structure of modern man. The human foot guarantees a stable support of the body, attenuates potentially harmful impact shocks, and provides sensory information about the contact with the ground. Pressure distribution measurement techniques are useful in analysing and understanding the mechanical behaviour of the human foot during static and dynamic loading situations in adults, children, and patient groups. Plantar pressure distribution instrumentation has become a standard clinical tool for diagnostic and therapeutic interventions. The understanding of foot biomechanics and the specific demands of everyday and sports activities are essential in the design of adequate footwear. Biomechanical testing of athletic footwear has been proven to be valuable in determining injury preventive and performance related properties of commercially available sport shoes.

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