A comparison of forefoot stiffness in running and running shoe bending stiffness

This study characterizes the stiffness of the human forefoot during running. The forefoot stiffness, defined as the ratio of ground reaction moment to angular deflection of the metatarsophalangeal joint, is measured for subjects running barefoot. The joint deflection is obtained from video data, while the ground reaction moment is obtained from force plate and video data. The experiments show that during push-off, the forefoot stiffness rises sharply and then decreases steadily, showing that the forefoot behaves not as a simple spring, but rather as an active mechanism that exhibits a highly time-dependent stiffness. The forefoot stiffness is compared with the bending stiffness of running shoes. For each of four shoes tested, the shoe stiffness is relatively constant and generally much lower than the mean human forefoot stiffness. Since forefoot stiffness and shoe bending stiffness act in parallel (i.e., are additive), the total forefoot stiffness of the shod foot is dominated by that of the human foot.     

Predicting changes in knee adduction moment due to load-altering interventions from pressure distribution at the foot in healthy subjects

The purpose of this pilot study of healthy subjects was to determine if changes in foot pressure patterns associated with a lateral wedge can predict the changes in the knee adduction moment. We tested two hypotheses: (1) increases or decreases in the knee adduction moment and ankle eversion moment due to load-altering footwear interventions can be predicted from foot pressure distribution and (2) changes in magnitude of the knee adduction moment and ankle eversion moment due to lateral wedges can be predicted from pressure distribution at the foot during walking. Fifteen healthy adults performed walking trials in three shoes: 0 degrees , 4 degrees , and 8 degrees laterally wedged. Maximum heel pressure ratio, first peak knee adduction moment, and peak ankle eversion moment were assessed using a pressure mat, motion capture system, and force plate. Increases or decreases in the knee adduction moment and ankle eversion moment were predicted well from foot pressure distribution. However, the magnitude of the pressure change did not predict the magnitude of the peak knee adduction moment change or peak ankle eversion moment change. Factors such as limb alignment or trunk motion may affect the knee adduction moment and override a direct relationship between the pressure distribution at the shoe-ground interface and the load distribution at the knee. However, changes (increases or decreases) in the peak knee adduction moment due to load-altering footwear interventions predicted from pressure distribution during walking can be important when evaluating these types of interventions from a clinical perspective.     

Unholey shoes: Experimental considerations when estimating ankle joint complex power during walking and running

For studies that aim to assess biological ankle function, calculating ankle joint complex (AJC) power between the calcaneus and shank is recommended over conventional inverse dynamics estimates between a rigid-body foot and shank. However, when designing a new experiment, it remains unclear whether holes should be cut in footwear to permit motion tracking via skin-mounted markers, or whether marker placement locations should be tightly controlled across conditions. Here we provide data to assist researchers in answering these questions. We performed a gait analysis study of walking (0.8, 1.2, 1.6 m·s⁻¹) and running (2.6, 2.8, 3.0 m·s⁻¹) while subjects (N = 10) wore custom-modified footwear, which allowed markers to be placed either on the shoe, or on the skin via cut-out windows in the shoes. First, we compared foot markers affixed to the skin vs. on the same locations on the shoe. Using statistical non-parametric mapping techniques, we discovered that skin vs. shoe markers had no statistically significant effect on net AJC power estimates throughout stance phase, for all walking and running speeds. Second, we compared calcaneal markers in the nominal shoe configuration vs. markers in a nearby location (∼27 mm below) on the shoe. We observed significant differences when marker placement on the shoe was varied, which may be relevant to repeated-measures study designs. The results suggest that when computing AJC power for walking and running, you may want to put down the scissors (i.e., forego cutting holes in your footwear), and instead pick up a Sharpie® (pen) or use a template, to maintain consistent marker placement across trials and conditions.     

Contributions of proximal and distal moments to axial tibial rotation during walking and running

The purpose of this study was to determine the cause and effect relationship between tibial internal rotation and pronation of the foot during walking and heel-toe running. This would allow predictions of orthotic effectiveness in reducing knee pain related to excessive internal tibial rotation. Kinematic and force plate data were collected from twenty subjects performing ten running and ten walking trials across a force plate. Using a least-squares algorithm, attitude matrices for each segment in each frame were obtained and the angular velocity vector of the tibia was calculated. The intersegmental moment at the ankle was calculated from ground reaction force and kinematic data, and the power flow from foot to tibia associated with axial tibial rotation was calculated. In walking, all subjects exhibited a clear power flow from tibia to foot during most of the stance phase, indicating that the foot was following the body. This suggests that the use of foot orthoses to reduce knee pain associated with tibial rotation during walking will not be successful. During running, power flow was also mainly proximal to distal, but there were brief periods of opposite power flow. There was more variability between subjects during running, with five subjects having large distal to proximal power flow peaks. These observations may explain and support previous work that has found variable clinical effects of orthoses between patients.     

The influence of footwear on foot motion during walking and running

There are evidences to suggest that wearing footwear constrains the natural barefoot motion during locomotion. Unlike prior studies that deduced foot motions from shoe sole displacement parameters, the aim of this study was to examine the effect of footwear motion on forefoot to rearfoot relative motion during walking and running. The use of a multi-segment foot model allowed accurate both shoe sole and foot motions (barefoot and shod) to be quantified. Two pairs of identical sandals with different midsole hardness were used. Ten healthy male subjects walked and ran in each of the shod condition.The results showed that for barefoot locomotion there was more eversion of the forefoot and it occurred faster than for shod locomotion. In this later condition, the range of eversion was reduced by 20% and the rate of eversion in late stance by 60% in comparison to the barefoot condition. The sole constrained both the torsional (eversion/inversion) and adduction range of motion of the foot. Interestingly, during the push-off phase of barefoot locomotion the rate and direction of forefoot torsion varied between individuals. However, most subjects displayed a forefoot inversion direction of motion while shod. Therefore, this experiment showed that the shoes not only restricted the natural motion of the barefoot but also appeared to impose a specific foot motion pattern on individuals during the push-off phase. These findings have implications for the matching of footwear design characteristics to individual natural foot function.     

The effects of squatting footwear on three-dimensional lower limb and spine kinetics

Altering footwear worn during performance of the barbell back squat has been shown to change motion patterns, but it is not completely understood how this affects biomechanical loading demands. The primary objective was to compare lower back and extremity net joint moments in 24 experienced weightlifters (12M, 12F) who performed 80% one-repetition maximum back squats under three different footwear conditions (barefoot, running shoes, weightlifting shoes). Results showed that there was a significant main effect of footwear condition on the knee extension moment (p = 0.001), where the running and weightlifting shoes produced significantly larger moments than the barefoot condition. There was also a main effect of footwear condition on knee external rotation moments (p = 0.002), where the weightlifting shoe produced significantly larger moments than both other conditions. At the hip, there was also a main effect of footwear condition on the extension moment (p = 0.004), where the barefoot condition produced significantly larger moments than either the running shoe or weightlifting shoe condition. Lastly, there was also a significant main effect of footwear condition on both hip external (p = 0.005) and internal (p = 0.003) rotation moments, where the barefoot condition produced greater internal rotation and less external rotation moments than either shod condition. This study indicates that altering footwear conditions while performing the barbell back squat may redistribute the internal biomechanical loading patterns amongst the lower extremity joints and perhaps alter the musculoskeletal adaptations elicited.     

Towards a footwear design tool: Influence of shoe midsole properties and ground stiffness on the impact force during running

Several spring–damper–mass models of the human body have been developed in order to reproduce the measured ground vertical reaction forces during human running (McMahon and Cheng, 1990; Ferris et al., 1999; Liu and Nigg, 2000). In particular, Liu and Nigg introduced at the lower level of their model, i.e. at the interface between the human body and the ground, a nonlinear element representing simultaneously the shoe midsoles and the ground flexibility. The ground reaction force is modelled as the force supported by this nonlinear element, whose parameters are identified from several sets of experimental data. This approach proved to be robust and quite accurate. However, it does not explicitly take into account the shoe and the ground properties. It turns out to be impossible to study the influence of shoe materials on the impact force, for instance for footwear design purposes. In this paper, a modification of the Liu and Nigg's model is suggested, where the original nonlinear element is replaced with a bi-layered spring–damper–mass model: the first layer represents the shoe midsole and the second layer is associated with the ground.Ground is modelled as an infinite elastic half-space. We have assumed a viscoelastic behaviour of the shoe material, so the damping of shoe material is taken into account. A methodology for the shoe-soles characterization is proposed and used together with the proposed model. A parametric study is then conducted and the influence of the shoe properties on the impact force is quantified. Moreover, it is shown that impact forces are strongly affected by the ground stiffness, which should therefore be considered as an essential parameter in the footwear design.     

Mechanical energy contribution of the metatarsophalangeal joint to running and sprinting

Despite the fact that a number of studies have investigated lower extremity energy generation during locomotion, the influence of the metatarsophalangeal (MP) joint remained unknown. The purpose of this study was to determine the relative contribution of the MP joint to the total mechanical energy in running and sprinting. A sagittal plane analysis was performed on data collected from 10 trained male athletes (five runners and five sprinters). The MP moment was assumed to be negligible until the ground reaction force acted distal to the joint. During running, once the ground reaction force crossed the MP joint, the MP moment was plantarflexor for the remainder of ground contact with average peak values of 59.9 Nm. The MP joint moment was plantarflexor throughout the stance phase for sprinting with average peak values of 112.4 Nm. Since the MP joint was dorsiflexing throughout the majority of the stance phase the joint absorbed large amounts of energy, on average 20.9 J during running and 47.8 J during sprinting. A lack of plantarflexion of the MP joint resulted in a lack of energy generation during take-off. Thus, the energy that was absorbed at the joint was dissipated in the shoe and foot structures.     

Comparison of Minimalist Footwear Strategies for Simulating Barefoot Running: A Randomized Crossover Study

Possible benefits of barefoot running have been widely discussed in recent years. Uncertainty exists about which footwear strategy adequately simulates barefoot running kinematics. The objective of this study was to investigate the effects of athletic footwear with different minimalist strategies on running kinematics. Thirty-five distance runners (22 males, 13 females, 27.9 ± 6.2 years, 179.2 ± 8.4 cm, 73.4 ± 12.1 kg, 24.9 ± 10.9 km.week^-1) performed a treadmill protocol at three running velocities (2.22, 2.78 and 3.33 m.s^-1) using four footwear conditions: barefoot, uncushioned minimalist shoes, cushioned minimalist shoes, and standard running shoes. 3D kinematic analysis was performed to determine ankle and knee angles at initial foot-ground contact, rate of rear-foot strikes, stride frequency and step length. Ankle angle at foot strike, step length and stride frequency were significantly influenced by footwear conditions (p<0.001) at all running velocities. Posthoc pairwise comparisons showed significant differences (p<0.001) between running barefoot and all shod situations as well as between the uncushioned minimalistic shoe and both cushioned shoe conditions. The rate of rear-foot strikes was lowest during barefoot running (58.6% at 3.33 m.s^-1), followed by running with uncushioned minimalist shoes (62.9%), cushioned minimalist (88.6%) and standard shoes (94.3%). Aside from showing the influence of shod conditions on running kinematics, this study helps to elucidate differences between footwear marked as minimalist shoes and their ability to mimic barefoot running adequately. These findings have implications on the use of footwear applied in future research debating the topic of barefoot or minimalist shoe running.     

Effects of ramp negotiation, paving type and shoe sole geometry on toe clearance in young adults

Trips are a major cause of falls and result from involuntary contact of the foot with the ground during the swing phase of gait. Adequate toe clearance during swing is therefore crucial for safe locomotion. To date, little is known about the effects of environmental factors and footwear on toe clearance. This study reports on modulation of toe clearance and toe clearance variability in response to changes in ground inclination, paving type, and shoe sole geometry. Toe clearance and toe clearance variability for ten healthy young adults were calculated two-fold: a) for the commonly-used position on the foremost part of the sole of the shoe and b) for the lowest of a total of 7 sole positions, located between the metatarsals and the toe tip across the entire width of the sole. Utilizing a full-factorial design we found that toe clearance was affected by ground inclination, paving type, and sole geometry regardless of the computational method used (with p-values<0.01) but the use of the foremost part of the sole for toe clearance calculation results is an overestimation of this value. Our findings highlight the importance of considering footwear and environmental factors when assessing the risk of tripping. Future work needs to investigate to which extent the same factors affect toe clearance in more vulnerable parts of the population.     

Mechanical analysis of the landing phase in heel-toe running.

Results of mechanical analyses of running may be helpful in the search for the etiology of running injuries. In this study a mechanical analysis was made of the landing phase of three trained heel-toe runners, running at their preferred speed and style. The body was modeled as a system of seven linked rigid segments, and the positions of markers defining these segments were monitored using 200 Hz video analysis. Information about the ground reaction force vector was collected using a force plate. Segment kinematics were combined with ground reaction force data for calculation of the net intersegmental forces and moments. The vertical component of the ground reaction force vector Fz was found to reach a first peak approximately 25 ms after touch-down. This peak occurs because, in the support leg, the vertical acceleration of the knee joint is not reduced relative to that of the ankle joint by rotation of the lower leg, so that the support leg segments collide with the floor. Rotation of the support upper leg, however, reduces the vertical acceleration of the hip joint relative to that of the knee joint, and thereby plays an important role in limiting the vertical forces during the first 40 ms. Between 40 and 100 ms after touch-down, the vertical forces are mainly limited by rotation of the support lower leg. At the instant that Fz reaches its first peak, net moments about ankle, knee and hip joints of the support leg are virtually zero. The net moment about the knee joint changed from -100 Nm (flexion) at touch-down to +200 Nm (extension) 50 ms after touch-down. These changes are too rapid to be explained by variations in the muscle activation levels and were ascribed to spring-like behavior of pre-activated knee flexor and knee extensor muscles. These results imply that the runners investigated had no opportunity to control the rotations of body segments during the first part of the contact phase, other than by selecting a certain geometry of the body and muscular (co-)activation levels prior to touch-down.     

The bending stiffness of shoes is beneficial to running energetics if it does not disturb the natural MTP joint flexion

A local minimum for running energetics has been reported for a specific bending stiffness, implying that shoe stiffness assists in running propulsion. However, the determinant of the metabolic optimum remains unknown. Highly stiff shoes significantly increase the moment arm of the ground reaction force (GRF) and reduce the leverage effect of joint torque at ground push-off. Inspired by previous findings, we hypothesized that the restriction of the natural metatarsophalangeal (MTP) flexion caused by stiffened shoes and the corresponding joint torque changes may reduce the benefit of shoe bending stiffness to running energetics. We proposed the critical stiffness, k_cr, which is defined as the ratio of the MTP joint (MTPJ) torque to the maximal MTPJ flexion angle, as a possible threshold of the elastic benefit of shoe stiffness. 19 subjects participated in a running test while wearing insoles with five different bending stiffness levels. Joint angles, GRFs, and metabolic costs were measured and analyzed as functions of the shoe stiffness. No significant changes were found in the take-off velocity of the center of mass (CoM), but the horizontal ground push-offs were significantly reduced at different shoe stiffness levels, indicating that complementary changes in the lower-limb joint torques were introduced to maintain steady running. Slight increases in the ankle, knee, and hip joint angular impulses were observed at stiffness levels exceeding the critical stiffness, whereas the angular impulse at the MTPJ was significantly reduced. These results indicate that the shoe bending stiffness is beneficial to running energetics if it does not disturb the natural MTPJ flexion.     

A study of the viability of obtaining a generic animation of the foot while walking for the virtual testing of footwear using dorsal pressures

Establishing the appropriate pressure exerted by the shoe upper over the foot surface is fundamental for the design of specific footwear, although measuring the dorsal pressures can also provide important additional information. In previous works, a virtual simulator to perform studies of comfort and functionality in CAD footwear design was presented. This paper describes the procedure carried out to obtain the foot animations used in this simulator. The virtual feet used in the simulator are feet without a standard form scanned in a static way. Their movements are rebuilt from the register of movements of several foot anatomical points during a complete step. The dorsal pressures exerted by some shoe uppers on these anatomical points were measured for several subjects and used to establish the viability of the use of these animations in a virtual simulator for footwear.     

Resultant lower extremity joint moments in below-knee amputees during running stance

Resultant flexion/extension lower extremity joint moments of four below-knee amputees running between 2.5 and 5.7 m s-1 were computed during stance on their intact and prosthetic limbs. All subjects wore patellar tendon-bearing prostheses with either a SACH or Greissinger foot component. During stance on the prosthesis, the resultant hip extensor moment on the amputated side was greater in magnitude and duration than its counterpart on the intact limb during its corresponding stance period. Since the artificial foot was planted on the ground, such a moment may help control knee flexion and promote knee extension of the residual limb. For the three subjects whose knees continued to flex at the beginning of stance, there was a dominant extensor moment about the knee joint during stance on the prosthesis. By contrast, for the fourth subject whose knee remained straight or hyperextended throughout stance on the prosthesis, a flexor moment was dominant.     

Computer simulation of the effects of shoe cushioning on internal and external loading during running impacts

Biomechanical aspects of running injuries are often inferred from external loading measurements. However, previous research has suggested that relationships between external loading and potential injury-inducing internal loads can be complex and nonintuitive. Further, the loading response to training interventions can vary widely between subjects. In this study, we use a subject-specific computer simulation approach to estimate internal and external loading of the distal tibia during the impact phase for two runners when running in shoes with different midsole cushioning parameters. The results suggest that: (1) changes in tibial loading induced by footwear are not reflected by changes in ground reaction force (GRF) magnitudes; (2) the GRF loading rate is a better surrogate measure of tibial loading and stress fracture risk than the GRF magnitude; and (3) averaging results across groups may potentially mask differential responses to training interventions between individuals.     

Influence of landing on the supination and pronation in the foot joint

The purpose of this study was to measure the changes of supination and pronation in the ankle joint at landing to quantify the influence of shock attenuation during landing. The subjects did two different motions, jumping down on the force platform from posterior and lateral views. The rear view of single foot contact in a jump from height of 30 and 60 cm showed a landing on the inside of the rear part of the foot (pronation) followed after about 0.03 sec by a rolling outward of the foot (supination). The variables describing changes in three angles of the ankle joint indicated that the standing position was more sensitive on the pronation and supination during ground contact.     

Effect of heel height on in-shoe localized triaxial stresses

Abnormal and excessive plantar pressure and shear are potential risk factors for high-heeled related foot problems, such as forefoot pain, hallux valgus deformity and calluses. Plantar shear stresses could be of particular importance with an inclined supporting surface of high-heeled shoe. This study aimed to investigate the contact pressures and shear stresses simultaneously between plantar foot and high-heeled shoe over five major weightbearing regions: hallux, heel, first, second and fourth metatarsal heads, using in-shoe triaxial force transducers. During both standing and walking, peak pressure and shear stress shifted from the lateral to the medial forefoot as the heel height increased from 30 to 70mm. Heel height elevation had a greater influence on peak shear than peak pressure. The increase in peak shear was up to 119% during walking, which was about five times that of peak pressure. With increasing heel height, peak posterolateral shear over the hallux at midstance increased, whereas peak pressure at push-off decreased. The increased posterolateral shear could be a contributing factor to hallux deformity. It was found that there were differences in the location and time of occurrence between in-shoe peak pressure and peak shear. In addition, there were significant differences in time of occurrence for the double-peak loading pattern between the resultant horizontal ground reaction force peaks and in-shoe localized peak shears. The abnormal and drastic increase of in-shoe shear stresses might be a critical risk factor for shoe-related foot disorders. In-shoe triaxial stresses should therefore be considered to help in designing proper footwear.     

Estimation of sex from the dimensions of foot, footprints, and shoe

This study intends to determine if the sex of an individual can be identified by foot lengths, shoe lengths, and/or footprints. For this purpose, foot length, foot breadth, and foot heel breadth of 506 subjects, comprising 253 females and 253 males ranging from 17.56 to 82.92 years of age, were taken. In addition, the footprints (length, breadth, and heel breadth) and footwear (length and breadth) of the same subjects were measured. Finally, the shoe size of the subjects was recorded. Univariate and multivariate discriminant function models were developed for sex allocations. Statistical analyses indicated that univariate models correctly assign approximately 67-94% of individuals to their correct sex groups. Among univariate models the most reliable measurement was shoe length. The results of multivariate models were better than those of univariate ones, with an approximately 82-96% correct assignment. The best multivariate model was comprised of four variables: foot length, shoe length, shoe breadth and shoe size. It could be suggested that these discriminant functions can provide useful clues to establish personal identity whenever complete or partial feet, footprints, or footwear are recovered.     

Response of able-bodied persons to changes in shoe rocker radius during walking: Changes in ankle kinematics to maintain a consistent roll-over shape

Recent studies have determined a seemingly consistent feature of able-bodied level ground walking, termed the roll-over shape, which is the effective rocker (cam) shape that the lower limb system conforms to between heel contact and contralateral heel contact during walking (first half of the gait cycle). The roll-over shape has been found to be largely unaffected by changes in walking speed, load carriage, and shoe heel height. However, it is unclear from previous studies whether persons are controlling their lower limb systems to maintain a consistent roll-over shape or whether this finding is a byproduct of their attempt to keep ankle kinematic patterns similar during the first half of the gait cycle. We measured the ankle–foot roll-over shapes and ankle kinematics of eleven able-bodied subjects while walking on rocker shoes of different radii. We hypothesized that the ankle flexion patterns during single support would change to maintain a similar roll-over shape. We also hypothesized that with decrease in rocker shoe radii, the difference in ankle flexion between the end and beginning of single support would decrease. Our results supported these hypotheses. Ankle kinematics were changed significantly during walking with the different rocker shoe radii (p<0.001), while ankle–foot roll-over shape radii (p=0.146) and fore–aft position (p=0.132) were not significantly affected. The results of this study have direct implications for designers of ankle–foot prostheses, orthoses, walking casts/boots, and rocker shoes. The results may also be relevant to researchers studying control of human movements.