Froude number fractions to increase walking pattern dynamic similarities: application to plantar pressure study in healthy subjects.

The purpose of this study was to determine if using similar walking velocities obtained from fractions of the Froude number (N(Fr)) and leg length can lead to kinematic and kinetic similarities and lower variability. Fifteen male subjects walked on a treadmill at 0.83 (VS(1)) and 1.16ms(-1) (VS(2)) and then at two similar velocities (V(Sim27) and V(Sim37)) determined from two fractions of the N(Fr) (0.27 and 0.37) so that the average group velocity remained unchanged in both conditions (VS(1)=V (Sim27)andVS(2)=V (Sim37)). N(Fr) can theoretically be used to determine walking velocities proportional to leg lengths and to establish dynamic similarities between subjects. This study represents the first attempt at using this approach to examine plantar pressure. The ankle and knee joint angles were studied in the sagittal plane and the plantar pressure distribution was assessed with an in-shoe measurement device. The similarity ratios were computed from anthropometric parameters and plantar pressure peaks. Dynamically similar conditions caused a 25% reduction in leg joint angles variation and a 10% significant decrease in dimensionless pressure peak variability on average of five footprint locations. It also lead to heel and under-midfoot pressure peaks proportional to body mass and to an increase in the number of under-forefoot plantar pressure peaks proportional to body mass and/or leg length. The use of walking velocities derived from N(Fr) allows kinematic and plantar pressure similarities between subjects to be observed and leads to a lower inter-subject variability. In-shoe pressure measurements have proven to be valuable for the understanding of lower extremity function. Set walking velocities used for clinical assessment mask the effects of body size and individual gait mechanics. The anthropometric scaling of walking velocities (fraction of N(Fr)) should improve identification of unique walking strategies and pathological foot functions.

Mechanical inputs related to perception of lower extremity impact loading severity.

PURPOSE: Human responses to repetitive locomotor loadings are likely dependent upon the perceived severity of the impact. Few researchers have attempted to identify the mechanical variables upon which perception of impact severity is based. This study examined the relationship of selected impact loading variables to the perception of impact severity by employing an established psychophysical test procedure. METHODS: A human pendulum apparatus was used to administer and measure impact loadings similar to those encountered during running. Nineteen subjects experienced over 100 right foot impacts which comprised nine different impact conditions presented in a random manner. The conditions represented combinations of three impact velocities and three interface materials covering a force platform. RESULTS: Group mean subjective ratings of impact severity were highly related to all measured biomechanical descriptors of impact severity. The variables of impact force rate of loading (FRA) and peak shank acceleration had correlation coefficients of 0.99 with perceived severity. When all individual results were combined to determine the relationship of impact loading variables to perception, correlations were generally 0.7 or above with FRA alone explaining 64% of the perceptual rating variability. CONCLUSIONS: These results indicate that impact perception was highly associated with the mechanical input variables commonly measured and that midsole materials such as those typically found in athletic footwear do not remove our ability to perceive the severity of impact loads.

Dominant role of interface over knee angle for cushioning impact loading and regulating initial leg stiffness.

For in vivo impact loadings administered under controlled initial conditions, it was hypothesized that larger initial knee angles (IKA) and softer impacting interfaces would reduce impact loading and initial leg stiffness. A human pendulum was used to deliver controlled impacts to the right foot of 21 subjects for three IKA (0, 20 and 40 degrees) and three interfaces (barefoot, soft and hard EVA foams). The external impact force and the shock experienced by the subjects' shank were measured simultaneously with a wall mounted force platform and a skin mounted accelerometer, respectively. Stiffness of the leg was derived using impact velocity and wall reaction force data. The results disproved the role of the knee joint in regulating initial leg stiffness and provided only partial support for the hypothesized improved cushioning. Larger knee flexion at contact reduced impact force but increased the shock travelling throughout the shank. Conversely, softer interfaces produced sizable reductions in both initial leg stiffness and severity of the impact experienced by the lower limb. Force rate of loading was found to be highly correlated (r = 0.95) to limb stiffness that was defined by the heel fat pad and interface deformations. These results would suggest that interface interventions are more likely to protect the locomotor system against impact loading than knee angle strategies.

Differential shock transmission response of the human body to impact severity and lower limb posture.

The shocks imparted to the foot during locomotion may lead to joint-degenerative diseases and jeopardize the visual-vestibular functions. The body relies upon several mechanisms and structures that have unique viscoelastic properties for shock attenuation. The purpose of the present study was to determine whether impact severity and initial knee angle (IKA) could alter the shock transmission characteristics of the body. Impacts were administered to the right foot of 38 subjects with a human pendulum device. Combinations of velocities (0.9, 1.05 and 1.2 m s-1) and surfaces (soft and hard foams) served to manipulate impact severity in the first experiment. Three IKA (0, 20 and 40 degrees) were examined in the second experiment. Transmission between shank and head was characterized by measuring the shock at these sites with miniature accelerometers. Velocity and surface had no effect on the frequency profile of shock transmission suggesting a consistent response of the body to impact severity. Shank shock power spectrum features accounted for the lower shock ratio (head/shank) measured under the hard surface condition. IKA flexion caused considerable reduction in effective axial stiffness of the body (EASB), 28.7-7.9 kNm-1, which improved shock attenuation. The high correlation (r = 0.97) between EASB and shock ratio underscored the importance of EASB to shock attenuation. The present findings provide valuable information for the development of strategies aimed at protecting the joints, articular cartilage, spine and head against locomotor shock.

Human pendulum approach to simulate and quantify locomotor impact loading.

The understanding of impact mechanics during locomotion is important for research within the fields of injury prevention and footwear design. Instrumented missiles offer a worthy solution to the lack of control inherent in in vivo activities and to the isolated nature of tissue studies. However, missiles cannot mimic the magnitude and temporal characteristics of locomotion impacts. A human pendulum approach employed the subject's own body as the missile to impart controlled impacts to the lower extremity. The subject is swung toward a force platform instrumented wall while lying supine on a suspended lightweight bed. The ability of the pendulum to reproduce locomotor impact loading was assessed for heel-toe running. Axial reaction force and shank acceleration patterns recorded during pendulum tests in ten subjects were found to closely resemble running patterns and they were obtained without discomfort to the subjects. This new approach relies upon one's own body to impart impacts representative of locomotion. It should prove useful to study human impact loading in a controlled manner.

Tibial shock measured with bone and skin mounted transducers.

The purpose of this study was to assess the value of superficial transducer mounting to measure tibial shock during locomotion. Surface (SMT) and bone mounted transducers (BMT) simultaneously recorded axial tibial acceleration in five subjects who ran at 4.5 m s-1. SMT produced inconsistent recording across the subjects both in the time and frequency domains. In two subjects, SMT signals provided close approximation of BMT signals, some distortion occurred in one subject while severe distortions were observed in the other two subjects. The present results established that SMT could not be used directly to quantify the shock transmitted through the tibia during running. However, frequency transformation of SMT recordings produced encouraging results; the transformed SMT signals mimicked the signals recorded with the bone mounted transducer.

Influences of inversion/eversion of the foot upon impact loading during locomotion.

Pronation of the foot is believed to be one of the mechanisms used during locomotion to attenuate the loading experienced by the body at ground contact. The purpose of this study was to quantify the changes in loading induced by modifications to the normal pronation of the foot during walking and running. Impact loading in 10 subjects was determined using ground reaction force and tibial acceleration. The results indicated that impact loading was increased when normal pronation was prevented during running. However, there was no reduction in impact loading when normal pronation was exaggerated. RELEVANCE: Orthotic corrections are commonly prescribed to control excessive foot pronation in patients who experience knee pain. The findings of the present paper demonstrate that pronation modifications can affect the magnitude of the impact experienced by the body during locomotion. It was also found that the loading responses to pronation modifications were not consistent between walking and running. Thus care should be exercised when orthotics that will be used for both walking and running are prescribed. Furthermore, patients should be informed that their orthotics may be activity specific.

Transfer function between tibial acceleration and ground reaction force.

The purpose of the present study was to capture the relationship between ground reaction force (GRF) and tibial axial acceleration. Tibia acceleration and GRF were simultaneously recorded from five subjects during running. The acceleration of the bone was measured with a transducer mounted onto an intracortical pin. The signals were analyzed in the frequency domain to characterize the relationship between GRF and tibial acceleration. The results confirmed that for each subject this relationship could be represented by a frequency transfer function. The existence of a more general relationship for all five subjects was also confirmed by the results. The transfer functions provided information about transient shock transmissibility for the entire impact phase of running.

Foot inversion-eversion and knee kinematics during walking.

The purpose of this study was to monitor selected aspects of the three-dimensional kinematics of the knee during walking with regular shoes and with modified shoes that induced either pronation or supination of the foot. Steinmann traction pins were inserted into the right tibia and femur of five adult men who had apparently normal lower extremities. Target clusters mounted onto the pins were filmed by four cine cameras operating at 100 frames/sec. Two trials per subject were analyzed for each of the three experimental conditions: regular running shoes, running shoes with a 10 degree valgus wedge, and running shoes with a 10 degree varus wedge. The different types of footwear induced only minor kinematic changes at the knee during the stance phase of walking. The angular patterns of the tibiofemoral joint were modified by less than 1 degree, whereas the translatory patterns were altered by 2 mm. Immediately following foot-strike, the valgus-wedge shoes caused the tibia to rotate internally 4 degrees more than the varus-wedge shoes, but at the tibiofemoral joint no consistent differences in the pattern of internal-external rotation between normal and modified footwear were measureable. These findings suggest that, in the healthy lower extremity, increased internal and external tibial rotation is resolved at the hip joint, with changes at the tibiofemoral joint that barely are detectable with the techniques used in this study.

Three-dimensional kinematics of the human knee during walking.

Three-dimensional kinematics of the tibiofemoral joint were studied during normal walking. Target markers were fixed to tibia and femur by means of intra-cortical traction pins. Radiographs of the lower limb were obtained to compute the position of the target markers relative to internal anatomical structures. High-speed cine cameras were used to measure three-dimensional coordinates of the target markers in five subjects walking at a speed of 1.2 m s-1. Relative motion between tibia and femur was resolved according to a joint coordinate system (JCS). The measurements have identified that substantial angular and linear motions occur about and along each of the JCS axes during walking. The results do not, however, support the traditional view that the so-called 'screw home' mechanism of the knee joint operates during gait.

Cushioning properties of footwear during walking: accelerometer and force platform measurements.

Repetitive impact loadings of the musculoskeletal system have been linked to the development of osteoarthritis and low back pain. An important function of footwear is to attenuate foot-ground impacts. The purpose of this study was to measure the effects of footwear types upon the impact ground reaction forces and the transient stress waves transmitted up the lower limb. The results have shown that both transient stress waves and ground reaction forces are affected by footwear during walking. Furthermore, with harder midsoles footwear, higher shock was transmitted to the lower extremities. This paper confirms the importance of using footwear to cushion the impact generated at heelstrike during walking. It also reveals that both shock and force measurements are required to evaluate and prescribe footwear to patients suffering from impact-related chronic diseases.

Contribution of angular motion and gravity to tibial acceleration.

A bone-mounted accelerometer and high-speed cinematography were used to compare the axial tibial acceleration caused by ground impact with the total tibial axial acceleration as measured by a transducer. Due to the effects of gravity and tibial angular motion, the magnitude of the peak acceleration at foot strike was 43% below and 18% above the peak axial acceleration due to impact for running and walking, respectively. Depending on the distance of the accelerometer from the tibial center of rotation which is located at the ankle joint, different axial acceleration signals should be expected during comparable locomotor activities.

Three-dimensional acceleration of the tibia during walking and running.

Measurements of tibial acceleration during walking and running were obtained by means of a triaxial accelerometer. The accelerometer was fixed to the free end of a Steinmann pin inserted into the right tibia of one volunteer subject. The patterns of tibial acceleration showed little step-to-step variation within each experimental condition. Following foot strike and depending upon footwear, the resultant tibial acceleration reached between 2.7 and 3.7 g during walking. The tibia experienced maximal accelerations of 10.6 g during running. The high values of tibial acceleration recorded in the antero-posterior (AP) and medio-lateral (ML) directions clearly revealed the importance of measuring all three components of acceleration to quantify the magnitude of the shock experienced by the lower limbs during locomotor activities.