Spatial-dependent mechanical properties of the heel pad by shear wave elastography

The heel pad plays an important role in gait, and its biomechanical behavior and functionality are determined by its specialized architecture and mechanical properties. The purpose of this study was to apply supersonic shear wave elastography, an ultrasound-based noninvasive modality that can quantitatively estimate the shear stiffness of the tissue, to investigate the spatial-dependent mechanical properties of the heel pad. Measurements were conducted in 40 heel pads of 20 normal participants aged between 20 and 30 years by shear wave elastography. The continuous change in local shear stiffness of the heel pad along the depth direction of the heel pad was measured. The result showed that the mechanical properties of the heel pad were highly heterogeneous but followed a simple and specific pattern that local heel pad shear stiffness was highest beneath the plantar skin and decreased continuously with increasing depth. This finding provides a better understanding of the heel pad biomechanics and basis for further investigation of the heterogeneous properties of the heel pad.     

The effect of muscle stiffness and damping on simulated impact force peaks during running.

It has been frequently reported that vertical impact force peaks during running change only minimally when changing the midsole hardness of running shoes. However, the underlying mechanism for these experimental observations is not well understood. An athlete has various possibilities to influence external and internal forces during ground contact (e.g. landing velocity, geometrical alignment, muscle tuning, etc.). The purpose of this study was to discuss one possible strategy to influence external impact forces acting on the athlete's body during running, the strategy to change muscle activity (muscle tuning). The human body was modeled as a simplified mass-spring-damper system. The model included masses of the upper and the lower bodies with each part of the body represented by a rigid and a non-rigid wobbling mass. The influence of mechanical properties of the human body on the vertical impact force peak was examined by varying the spring constants and damping coefficients of the spring-damper units that connected the various masses. Two types of shoe soles were modeled using a non-linear force deformation model with two sets of parameters based on the force-deformation curves of pendulum impact experiments. The simulated results showed that the regulation of the mechanical coupling of rigid and wobbling masses of the human body had an influence on the magnitude of the vertical impact force, but not on its loading rate. It was possible to produce the same impact force peaks altering specific mechanical properties of the system for a soft and a hard shoe sole. This regulation can be achieved through changes of joint angles, changes in joint angular velocities and/or changes in muscle activation levels in the lower extremity. Therefore, it has been concluded that changes in muscle activity (muscle tuning) can be used as a possible strategy to affect vertical impact force peaks during running.     

Image-based midsole insert design and the material effects on heel plantar pressure distribution during simulated walking loads

The primary objective of this paper is to study the use of medical image-based finite element (FE) modelling in subject-specific midsole design and optimisation for heel pressure reduction using a midsole plug under the calcaneus area (UCA). Plugs with different relative dimensions to the size of the calcaneus of the subject have been incorporated in the heel region of the midsole. The FE foot model was validated by comparing the numerically predicted plantar pressure with biomechanical tests conducted on the same subject. For each UCA midsole plug design, the effect of material properties and plug thicknesses on the plantar pressure distribution and peak pressure level during the heel strike phase of normal walking was systematically studied. The results showed that the UCA midsole insert could effectively modify the pressure distribution, and its effect is directly associated with the ratio of the plug dimension to the size of the calcaneus bone of the subject. A medium hardness plug with a size of 95% of the calcaneus has achieved the best performance for relieving the peak pressure in comparison with the pressure level for a solid midsole without a plug, whereas a smaller plug with a size of 65% of the calcaneus insert with a very soft material showed minimum beneficial effect for the pressure relief.     

Constitutive formulation and numerical analysis of the heel pad region

The aim of this work is to provide a numerical approach for the investigation of the mechanical behaviour of the heel pad region. A visco-hyperelastic model is formulated with regard to fat pad tissue, while a fibre-reinforced hyperelastic model is considered for the heel skin tissue. Bone components are defined by means of an orthotropic linear elastic model. Particular attention is paid to the evaluation of constitutive parameters within different models adopted in consideration of experimental tests data. Preliminarily, indentation tests on a skinless cadaveric foot are considered with regard to fat pad tissue. Indentation tests on an intact heel pad of a cadaveric foot are subsequently adopted for the final identification of constitutive parameters of fat pad and skin tissues. A numerical model of the rear foot is defined and different loading conditions are assumed according to experimental data. A comparison between experimental and numerical data leads to the evaluation of the real capability of the procedure to interpret the actual response of the rear foot.     

The effect of foot orthotics on myoelectric fatigue in the vastus lateralis during a simulated skier’s squat

Fatigue in the legs is a problem experienced by skiers. It has been suggested that optimal orthotics may reduce muscle fatigue for a given movement task by minimising muscle activity (Med. Sci. Sports Exerc. 31 (1999) S421). The aims were to determine whether EMG would provide an independent method of analysing myoelectric fatigue in the vastus lateralis (VL) during a skier’s squat and whether orthotics could affect this fatigue response. Six skiers performed skier’s squats for as long as possible with no orthotic, low volume orthotics and high volume orthotics in their ski boots. Bipolar, active surface electrodes recorded EMG activity in the VL throughout each squat. Results for the EMG median frequency showed a significant shift in the power density spectrum towards the lower frequencies (P<0.05) at the end of the contraction, suggesting that myoelectric fatigue was occurring and was measurable using EMG. All conditions displayed a significant decrease in median frequency at the end of the contraction (P=0.001). The high volume orthotic showed a significant reduction in myoelectric fatigue, however, there was no difference in the duration of squats across the three conditions (P>0.05). Subjective and objective findings support the use of the high volume foot orthotic for skiers.     

Kinematics and kinetics of the shoe during human slips

This paper quantified the heel kinematics and kinetics during human slips with the goal of guiding available coefficient of friction (ACOF) testing methods for footwear and flooring. These values were then compared to the testing parameters recommended for measuring shoe-floor ACOF. Kinematic and kinetic data of thirty-nine subjects who experienced a slip incident were pooled from four similar human slipping studies for this secondary analysis. Vertical ground reaction force (VGRF), center of pressure (COP), shoe-floor angle, side-slip angle, sliding speed and contact time were quantified at slip start (SS) and at the time of peak sliding speed (PSS). Statistical comparisons were used to test if any discrepancies exist between the state of slipping foot and current ACOF testing parameters. The main findings were that the VGRF (26.7 %BW, 179.4 N), shoe-floor angle (22.1°) and contact time (0.02 s) at SS were significantly different from the recommended ACOF testing parameters. Instead, the testing parameters are mostly consistent with the state of the shoe at PSS. We argue that changing the footwear testing parameters to conditions at SS is more appropriate for relating ACOF to conditions of actual slips, including lower vertical forces, larger shoe-floor angles and shorter contact duration.     

Material properties of the human calcaneal fat pad in compression: experiment and theory

The structural behaviour of the human heel pad has been studied extensively due to its ability to absorb shock, protect against excessive local stress, and reduce plantar pressures. However, the material properties of the tissue have not been adequately measured. These must be known in order to perform a finite element analysis of the effect of factors such as foot geometry and shoe/surface construction on heel pad function. Therefore, the purposes of this study were to (a) measure the viscoelastic behaviour of the fat pad in compression, and (b) to determine an appropriate constitutive equation to model the tissue. A series of unconfined compression tests were performed on 8 mm diameter cylinders of fat pad tissue, consisting of quasi-static, 175, 350 mm/s and stress-relaxation tests to 50% deformation. The tissue exhibited nonlinear, viscoelastic behaviour. No significant difference was found in the quasi-static behaviour between samples from different locations and orientations in the heel. The stress-relaxation tests were used to determine the time constant (τ₁=0.5 s), the 175 mm/s test to determine the relaxation coefficient (g₁=28), and the 350 mm/s compression test to determine the material constants (C₁₀₀=C₀₁₀=0.01, C₂₀₀=C₀₂₀=0.1 Pa) of a single-phase, hyperelastic, linear viscoelastic strain energy function (r²=0.98).     

The influence of whole body vibration on the plantarflexors during heel raise exercise

Whole body vibration (WBV) during exercise offers potential to augment the effects of basic exercises. However, to date there is limited information on the basic physiological and biomechanical effects of WBV on skeletal muscles. The aim of this study was to determine the effects of WBV (40 Hz, 1.9 mm synchronous vertical displacement) on the myoelectrical activity of selected plantarflexors during heel raise exercise. 3D motion capture of the ankle, synchronised with sEMG of the lateral gastrocnemius and soleus, was obtained during repetitive heel raises carried out at 0.5 Hz on 10 healthy male subjects (age 27 ± 5 years, height 1.78 ± 0.04 m, weight 75.75 ± 11.9 kg). During both vibration and non vibration the soleus activation peaked earlier than that of the lateral gastrocnemius. The results indicate that WBV has no effect on the timing of exercise completion or the amplitude of the lateral gastrocnemius activity, however significant increases in amplitudes of the soleus muscle activity (77.5–90.4% MVC P < 0.05). WBV had no significant effect on median frequencies of either muscle. The results indicate that the greatest effect of WBV during heel raise activity is in the soleus muscles during the early phases of heel raise.     

Muscle mechanism: The acceleration of the load

The load (force/cross-section) determines the response of muscle power output, force and speed of contraction). The force is the product of the mass by the acceleration, thus the same force is generated by an infinite number of mass and acceleration couples and each one of these couples displays different physical and biological effects. Therefore, the load must be defined both by the mass and by the acceleration. Early muscle investigators were well aware of this situation as it is indicated by the work of Hill on the flexion of the arm against the “heavy fly-wheel”. By making use of a model of sarcomere contraction we show here that the acceleration of the load is the first determinant of the time course of the process of generation of the isometric tension. We also propose that, in order to reproduce the rapid release, it is not necessary to invoke the presence of a distinct elastic element in the contractile machinery. It is sufficient to assume that the stiffness of the same machinery increases with the contractile force.     

3D propagation of the shock-induced vibrations through the whole lower-limb during running

Shock-induced vibrations to the feet have been related to the feel of comfort, the biomechanical control of performance, and the risk of fatigue or injury. Up to recently, the complexity of measuring the human biodynamic response to vibration exposure implied to focus most of the research on the axial acceleration at the tibia. Using wireless three-dimensional accelerometers, this paper investigates the propagation of shock-induced vibrations through the whole lower-limb during running in the temporal and the spectral domains. Results indicated that the vibrations were not consistent across the lower-limb, showing various spatial and spectral distributions of energy. The amount of energy was not constantly decreasing from the distal to the proximal extremity of the runner’s lower-limb, especially regarding the lateral epicondyle of the femur. Vibrations in the transversal plane of the segments were substantial compared to the longitudinal axis regarding the distal extremity of the tibia, and the lateral epicondyle of the femur. Further, the spectral content was wider at the distal than at the proximal end of the lower-limb. Finally, to get a thorough understanding of the risks incurred by the runners, the need to account for shock-induced vibrations up to 50 Hz has been stressed when investigating three-dimensional vibrations. The overall study raises attention on the substantial importance of the transverse components of the acceleration, and their potential relation to shear fatigue and injury during running.     

Diverse speed response properties of motion sensitive neurons in the fly’s optic lobe

Speed and acceleration are fundamental components of visual motion that animals can use to interpret the world. Behavioral studies have established that insects discriminate speed largely independently of contrast and spatial frequency, and physiological recordings suggest that a subset of premotor descending neurons is in this sense speed-selective. Neural substrates and mechanisms of speed selectivity in insects, however, are unknown. Using blow flies Phaenicia sericata, intracellular recordings and dye-fills were obtained from medulla and lobula complex neurons which, though not necessarily speed-selective themselves, are positioned to participate in circuits that produce speed-selectivity in descending neurons. Stimulation with sinusoidally varied grating motion (0–200°/s) provided a range of instantaneous velocities and accelerations. The resulting speed response profiles are indicative of four distinct speed ranges, supporting the hypothesis that the spatiotemporal tuning of mid-level neurons contains sufficient diversity to account for the emergence of speed selectivity at the descending neuron level. This type of mechanism has been proposed to explain speed discrimination in both insects and mammals, but has seemed less likely for insects due to possible constraints on small brains. Two additional recordings are suggestive of acceleration-selectivity, a potentially useful visual capability that is of uncertain functional significance for arthropods.     

Suppression of radiation damping during selective excitation of the water signal: The WANTED sequence

Summary A new pulse sequence is presented allowing the use of long selective pulses at the water frequency using standard equipment. Radiation damping is suppressed during the pulse by the use of gradient echoes programmed between the single pulses of a DANTE train. This WANTED (water-selective DANTE using gradients) sequence thus allows the observation of interactions with water without the use of special probe heads or filtering of undesired resonances. By combining the WANTED sequence with NOESY, ROESY and NOESY-GSQC experiments, we obtain selective 1D and 2D spectra fit to the observation of chemical exchange and dipolar interactions between water and protein protons.     

In vivo biomechanical behavior of the human heel pad during the stance phase of gait

A technique is introduced for simultaneous measurements of the heel pad tissue deformation and the heel–ground contact stresses developing during the stance phase of gait. Subjects walked upon a gait platform integrating the contact pressure display optical method for plantar pressure measurements and a digital radiographic fluoroscopy system for skeletal and soft tissue motion recording. Clear images of the posterior-plantar aspect of the calcaneus and enveloping soft tissues were obtained simultaneously with the pressure distribution under the heel region throughout the stance phase of gait. The heel pad was shown to undergo a rapid compression during initial contact and heel strike, reaching a strain of 0.39±0.05 in about 150 ms. The stress–strain relation of the heel pad was shown to be highly non-linear, with a compression modulus of 105±11 kPa initially and 306±16 kPa at 30% strain. The energy dissipation during heel strike was evaluated to be 17.8±0.8%. The present technique is useful for biomechanical as well as clinical evaluation of the stress–strain and energy absorption characteristics of the heel pad in vivo, during natural gait.     

Analysis of the human and ape foot during bipedal standing with implications for the evolution of the foot

The ratio of the power arm (the distance from the heel to the talocrural joint) to the load arm (that from the talocrural joint to the distal head of the metatarsals), or RPL, differs markedly between the human and ape foot. The arches are relatively higher in the human foot in comparison with those in apes. This study evaluates the effect of these two differences on biomechanical effectiveness during bipedal standing, estimating the forces acting across the talocrural and tarsometatarsal joints, and attempts to identify which type of foot is optimal for bipedal standing. A simple model of the foot musculoskeletal system was built to represent the geometric and force relationships in the foot during bipedal standing, and measurements for a variety of human and ape feet applied. The results show that: (1) an RPL of around 40% (as is the case in the human foot) minimizes required muscle force at the talocrural joint; (2) the presence of an high arch in the human foot reduces forces in the plantar musculature and aponeurosis; and (3) the human foot has a lower total of force in joints and muscles than do the ape feet. These results indicate that the proportions of the human foot, and the height of the medial arch are indeed better optimized for bipedal standing than those of apes, further suggesting that their current state is to some extent the product of positive selection for enhanced bipedal standing during the evolution of the foot.     

The anticoagulant properties of the endothelium studied by the standard venous occlusion test]

Venous occlusion (VO) during which thrombin (Th) is postulated to be generated is routinely used for evaluation of fibrinolytic potential of endothelium (E). This study was performed to find out whether VO can also be used for assessment of anticoagulant function of E. VO was performed in 98 male patients (pts) with ischemic heart disease. Levels of protein C (PrC) which is related to Th binding by thrombomodulin and fibrinopeptide A (FpA)--a marker of presence of free Th--were determined together with some other factors of coagulation and fibrinolysis. Differences between pre- and postVO PrC levels fluctuated from -54.8% to +57.3%. According to reaction of PrC to VO pts were divided into 2 groups: 13 pts with increase or no change and 17 pts with decrease (consumption) of PrC. In pts without PrC consumption there was a significant increase in FpA. In pts with PrC consumption FpA was unchanged. In pts with PrC consumption exceeding its median value for this group (14%) PAI-1 antigen level fell significantly (-8.4 + 4%) during VO. Thus PrC consumption after VO indicates that TH is effectively removed from blood stream by endothelial factors. Absence of consumption of PrC is a sign of ineffective anticoagulant function of E. Increase in PrC level during VO in some pts may be due to its escape from tissue depot.     

The role of the heel pad and shank soft tissue during impacts: a further resolution of a paradox

The aim of this study was to test the hypothesis that the motion of the soft tissue of the lower leg contributes significantly to the attenuation of the forces during heel impacts. To examine this, a two-dimensional model of the shank and heel pad was developed using DADS. The model contained a heel pad element and a rigid skeleton to which was connected soft tissue which could move relative to the bone. Simulations permitted estimation of heel pad properties directly from heel pad deformations, and from the kinematics of an impacting pendulum. These two approaches paralleled those used in vitro and in vivo, respectively. Measurements from the pendulum indicated that heel pad properties changed from those found in vitro to those found in vivo as relative motion of the bone and soft tissue was allowed. This would indicate that pendulum measures of the in vivo heel pad properties are also measuring the properties of the whole lower leg. The ability of the wobbling mass of the shank to dissipate energy during an impact was found to be significant. These results demonstrate the important role of both the heel pad and soft tissue of the shank to the dissipation of mechanical energy during impacts. These results provide a further clarification of the paradox between the measurements of heel pad properties made in vivo and in vitro.     

Comparative studies on the in vitro properties of phytases from various microbial origins

The physical and chemical properties of six crude phytase preparations were compared. Four of these enzymes (Aspergillus A, Aspergillus R, Peniophora and Aspergillus T) were produced at commercial scale for the use as feed additives while the other two (E. coli and Bacillus) were produced at laboratory scale. The encoding genes of the enzymes were from different microbial origins (4 of fungal origin and 2 of bacterial origin, i.e., E. coli and Bacillus phytases). One of the fungal phytases (Aspergillus R) was expressed in transgenic rape. The enzymes were studied for their pH behaviour, temperature optimum and stability and resistance to protease inactivation. The phytases were found to exhibit different properties depending on source of the phytase gene and the production organism. The pH profiles of the enzymes showed that the fungal phytases had their pH optima ranging from 4.5 to 5.5. The bacterial E. coli phytase had also its pH optimum in the acidic range at pH 4.5 while the pH optimum for the Bacillus enzyme was identified at pH 7.0. Temperature optima were at 50 and 60°C for the fungal and bacterial phytases, respectively. The Bacillus phytase was more thermostable in aqueous solutions than all other enzymes. In pelleting experiments performed at 60, 70 and 80°C in the conditioner, Aspergillus A, Peniophora (measurement at pH 5.5) and E. coli phytases were more heat stable compared to other enzymes (Bacillus enzyme was not included). At a temperature of 70°C in the conditioner, these enzymes maintained a residual activity of approximately 70% after pelleting compared to approximately 30% determined for the other enzymes. Incubation of enzyme preparations with porcine proteases revealed that only E. coli phytase was insensitive against pepsin and pancreatin. Incubation of the enzymes in digesta supernatants from various segments of the digestive tract of hens revealed that digesta from stomach inactivated the enzymes most efficiently except E. coli phytase which had a residual activity of 93% after 60 min incubation at 40°C. It can be concluded that phytases of various microbial origins behave differently with respect to their in vitro properties which could be of importance for future developments of phytase preparations. Especially bacterial phytases contain properties like high temperature stability (Bacillus phytase) and high proteolytic stability (E. coli phytase) which make them favourable for future applications as feed additives.     

The effects of simulated weightlessness on bone biomechanical and biochemical properties in the maturing rat

Histomorphometric and biomechanical changes in bone resulting from hypogravity (simulated weightlessness) were examined in this study. Using a head-down hindlimb suspension model, three groups of six male rats underwent simulated weightlessness for periods of one, two and three weeks while a fourth recovery group was suspended for two weeks followed by two weeks of normal activity. Biomechanical data were collected during static and dynamic bending and torsion tests on intact femora. Histomorphometric values were determined from midshaft bone cross sections and material properties were obtained using ash and calcium assays. The experimental groups exhibited significantly lower geometric and material properties than the controls, resulting in structural hypotrophy; geometric and material changes contributed equally to the structural changes. Recovery following a return to normal activity was indicated, although full recovery may take longer than the weightlessness period. In the rat, altered maturation and reduced bone strength were the sequelae of weightlessness.     

Aggregative properties of different cell types of the fresh-water spongeEphydatia fluviatilis isolated on ficoll gradients

Summary Cell suspensions of the fresh-watersponge Ephydatia fluviatilis have been fractionated by means ofFicoll gradient centrifugation. Three fractions were isolated. The densest contains archeocyte-like cells only; the intermediate fraction is very rich in choanocytes, and the lightest is a mixture of cell types. Earch fraction shows specificaggregative properties and potentialities to reconstitute functional sponges.It appears that the sequence of reconstitution events can be selectively altered by certain disequilibria in the cell populationThese preliminary results constitute a first approach to the analysis ofcell type specificity in sponges.