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.     

A biomechanical analysis of an instrumented spinal fixator under torsional loads

The effect of design features of an internal spinal fixator on the loading of its individual components is paramount to the understanding of the interaction between the fixator and the instrumented spine. Using a corpectomy injury model, a strain gauge instrumented spinal fixation device was employed to investigate the role of clamp tightening torque and the inclusion of transverse bars on the distribution of bending and torsional moments acting on the fixator under torsional loading. The increase in clamp torque from 5 to 10 Nm caused a marked decrease (40%) in torsional moments acting on the vertical rods, an increase of 24% in torsional moments acting on the screws and an increase of 44% in bending moments acting on the rods along the sagittal plane of the fixator. The inclusion of transverse elements significantly increased (132%) the bending moment acting on the rods and decreased (92%) the bending moments acting on the screws along the sagittal plane. The change in both design parameters significantly reduced the response hysteresis and decreased the asymmetry of loading. A theoretical model, developed to elucidate the load path mechanisms underlying this response, successfully predicted the external response of the fixator. This model suggested both design parameters would affect the internal force and moment distribution across the fixator and the relative role of each load response mechanism in effecting this response. The changes in load patterns across the fixator will influence both its ability to augment the process of spinal fusion and the long-term performance of its components.     

The contribution of the cruciate ligaments to the load-displacement characteristics of the human knee joint

Human knee specimens were subjected to anterior-posterior, medial-lateral, varus-valgus, and torsional displacement tests. Loads were recorded for the intact joint and for the joint with all soft tissues cut except for the cruciate ligaments. The effect of condylar interference was determined for anterior-posterior, medial-lateral, and torsional displacements. The variation in load with flexion angle was considerable for medial-lateral (0-90-deg flexion) displacements, and less for varus-valgus (0-45-deg flexion) displacements. The cruciates were found to carry almost the entire anterior-posterior load; they carried a significant percentage of the medial-lateral load which varied considerably with flexion angle. A small, but not insignificant percentage of the varus-valgus load was carried by the cruciates and the variations with flexion angle were small. In torsion, the cruciates resisted only internal rotation. In the tested displacement ranges, condylar interference had a small effect on the medial-lateral load but did not affect anterior-posterior or torsional loads.     

The effect of ankle constraint on the torsional laxity of the knee during internal-external rotation of the foot

The in vivo torsional laxity and stiffness of the knee joint are usually determined by rotating the foot and measuring the torque generated at the knee. However, when rotation is applied to the foot, significant three-dimensional forces and moments are produced at the knee. These forces and moments depend upon the external constraint of the ankle complex, and as a result, the observed laxity of the knee also depends on the ankle constraint. Tests are conducted with the foot of a subject in a shoe, with and without the ankle taped, and in a buckled and unbuckled (ski) boot that can effectively constrain ankle rotation. The average laxity of the primary (linear) region of the axial moment vs internal-external rotation is 30% greater when the ankle is constrained by the buckled boot than it is in three other cases of lesser ankle constraint.     

Walking on high heels changes muscle activity and the dynamics of human walking significantly

The aim of the study was to investigate the distribution of net joint moments in the lower extremities during walking on high-heeled shoes compared with barefooted walking at identical speed. Fourteen female subjects walked at 4 km/h across three force platforms while they were filmed by five digital video cameras operating at 50 frames/second. Both barefooted walking and walking on high-heeled shoes (heel height: 9 cm) were recorded. Net joint moments were calculated by 3D inverse dynamics. EMG was recorded from eight leg muscles. The knee extensor moment peak in the first half of the stance phase was doubled when walking on high heels. The knee joint angle showed that high-heeled walking caused the subjects to flex the knee joint significantly more in the first half of the stance phase. In the frontal plane a significant increase was observed in the knee joint abductor moment and the hip joint abductor moment. Several EMG parameters increased significantly when walking on high-heels. The results indicate a large increase in bone-on-bone forces in the knee joint directly caused by the increased knee joint extensor moment during high-heeled walking, which may explain the observed higher incidence of osteoarthritis in the knee joint in women as compared with men.     

Mechanics of toe and heel landing in stepping down in ongoing gait

When stepping down from a height difference in ongoing gait, subjects are known to use a heel landing at small height differences and switch to toe landing for larger height differences. We hypothesized that in toe landing, the leading leg can perform more negative work, to control the momentum gained during the descent, than in heel landing. Ten young male participants walked over a 10-m walkway at 5km/h to step down a height difference of 10cm halfway, using a heel or toe landing in five trials each. Kinematic data and ground reaction forces under the leading and trailing legs were recorded. Inverse dynamical analysis of both strategies showed that the leading leg performed more negative work in toe landing, while the vertical velocity at ground contact was lower. In addition, the impact forces were lower in toe landing than in heel landing. Toe landing was found to reduce gait velocity in the first step on the lower level and required higher moments and negative power around the ankle joint than heel landing. Our results indicate that heel landing may be preferred when stepping down small height differences, because it is less demanding especially for the plantar flexor muscles, while toe landing may be preferred for stepping down larger height differences, because it improves control over the momentum gained during the descent.     

Predictions of knee and ankle moments of force in walking from EMG and kinematic data

A deterministic model was developed and validated to calculate instantaneous ankle and knee moments during walking using processed EMG from representative muscles, instantaneous joint angle as a correlate of muscle length and angular velocity as a correlate of muscle velocity, and having available total instantaneous joint moments for derivation of certain model parameters. A linear regression of the moment on specifically processed EMG, recorded while each subject performed cycled isometric calibration contractions, yielded the constants for a basic moment-EMG relationship. Using the resultant moment for optimization, the predicted moment was proportionally augmented for longer muscle lengths and reduced for shorter lengths. Similarly, the predicted moment was reduced for shortening velocities and increased if the muscle was lengthening. The plots of moments predicted using the full model and those calculated from link segment mechanics followed each other quite closely. The range of root mean square errors were: 3.2-9.5 Nm for the ankle and 4.7-13.0 Nm for the knee.     

Identification of knee joint models for varus-valgus and internal-external rotations: snow skiing experiments

Sprains at the knee are the most frequent of the severe injuries occurring during alpine snow skiing. This paper discusses the development of analytical models describing rotations across the knee joint caused by varus-valgus and internal-external moments applied at the foot during skiing. Identification of an ARMAX model requires simultaneous measurements of the rotations across the knee and the moments at the foot during skiing. As the models only relate the measured input (moment) and output (rotation) data, they also identify components of apparent rotation resulting from imperfect fixation of the rotation measuring instrument on the test subject and resulting from other inputs. The models identified for all subjects are of order four or five for both varus-valgus and internal-external rotation, and they describe modes with oscillatory and exponentially decaying components. Application of the models to prediction of rotation across the knee from the measured moment at the foot is illustrated by example. A new, and virtually mechanically uncoupled, six degrees-of-freedom, strain gauge dynamometer is developed to record the moments at the foot during skiing. The concept of the dynamometer design has general application.     

Effect of ski boot settings on tibio-femoral abduction and rotation during standing and simulated skiing

Ski boots are designed to transfer high forces from the skier to the ski. For this purpose they are made of stiff materials and constrain the leg of the skier to an unnatural position. To overcome the problem of unnatural knee posture, the ski boots can be adjusted in the frontal plane as well as in the horizontal plane by the canting mechanism and the "v-position", respectively. Canting enables lateral and medial orientation of the shaft with respect to the base of the boot. The "v-position" is a pronounced outward rotation of the boot's base with respect to the ski's long axis. The purpose of this study is to investigate the effect of different foot rotations and ski boot canting settings on knee kinematics during standing and simulated skiing. Knee kinematics was measured by means of motion analysis and with the help of skin-mounted markers on 20 subjects. The ski boots in their standard settings significantly constrained the skier to an unnatural valgus position. Ski boot base rotation had a significant effect on internal external knee rotation, whereas canting had an effect on varus-valgus angles during standing. However, for the simulated skiing position no effects were observed. The study suggests that the constraints of the ski boots result in a clinically relevant valgus misalignment. Canting settings reduced the misalignment but only by about 10%. Increased ski boot canting settings would therefore be desirable. Knee kinematics showed that rotational misalignment could not be linked to any significant increase in injury risk.     

Kinematics and kinetics of the lower extremities of young and elder women during stairs ascent while wearing low and high-heeled shoes

The effect of the heel height on the temporal, kinematic and kinetic parameters was investigated in 16 young and 11 elderly females. Kinematic and kinetic data were collected when the subjects ascended stairs with their preferred speed in two conditions: wearing low-heeled shoes (LHS), and high-heeled shoes (HHS). The younger adults showed more adjustments in forces and moments at the knee and hip in frontal and transverse planes. Besides a few significantly changes in joint forces and moments, the elder group demonstrated longer cycle duration and double stance phase, larger trunk sideflexion and hip internal rotation, less hip adduction while wearing HHS. Most differences in joint motions between two groups were found at the hip and knee either in LHS or HHS condition. Instead, the differences in moment occurred at the hip joint and only in HHS. The interaction of the heel height and age showed the influences of heel height on trunk rotation, hip abduction/adduction, and knee and hip force and moment at the frontal plane depended on age. These phenomena suggest that younger and elderly women adapt their gait and postural control differently during stair ascent (SA) while wearing HHS.     

Reduced knee joint moment in ACL deficient patients at a cost of dynamic stability during landing

The current study aimed to examine the effect of anterior cruciate ligament deficiency (ACLd) on joint kinetics and dynamic stability control after a single leg hop test (SLHT). Twelve unilateral ACLd patients and a control subject group (n=13) performed a SLHT over a given distance with both legs. The calculation of joint kinetics was done by means of a soft-tissue artifact optimized rigid full-body model. Margin of stability (MoS) was quantified by the difference between the base of support and the extrapolated center of mass. During landing, the ACLd leg showed lower external knee flexion moments but demonstrated higher moments at the ankle and hip compared to controls (p<0.05). The main reason for the joint moment redistribution in the ACLd leg was a more anterior position of the ground reaction force (GRF) vector, which affected the moment arms of the GRF acting about the joints (p<0.05). For the ACLd leg, trunk angle was more flexed over the entire landing phase compared to controls (p<0.05) and we found a significant correlation between moment arms at the knee joint and trunk angle (r2 = 0.48;p<0.01). The consequence of this altered landing strategy in ACLd legs was a more anterior position of the center of mass reducing the MoS (p<0.05). The results illustrate the interaction between trunk angle, joint kinetics and dynamic stability during landing maneuvers and provide evidence of a feedforward adaptive adjustment in ACLd patients (i.e. more flexed trunk angle) aimed at reducing knee joint moments at the cost of dynamic stability control.     

Muscular resistance to varus and valgus loads at the elbow.

Although the contributions of passive structures to stability of the elbow have been well documented, the role of active muscular resistance of varus and valgus loads at the elbow remains unclear. We hypothesized that muscles: (1) can produce substantial varus and valgus moments about the elbow, and (2) are activated in response to sustained varus and valgus loading of the elbow. To test the first hypothesis, we developed a detailed musculoskeletal model to estimate the varus and valgus moment-generating capacity of the muscles about the elbow. To test the second hypothesis, we measured EMGs from 11 muscles in four subjects during a series of isometric tasks that included flexion, extension, varus, and valgus moments about the elbow. The EMG recordings were used as inputs to the elbow model to estimate the contributions of individual muscles to flexion-extension and varus-valgus moments. Analysis of the model revealed that nearly all of the muscles that cross the elbow are capable of producing varus or valgus moments; the capacity of the muscles to produce varus moment (34 Nm) and valgus moment (35 Nm) is roughly half of the maximum flexion moment (70 Nm). Analysis of the measured EMGs showed that the anconeus was the most significant contributor to valgus moments and the pronator teres was the largest contributor to varus moments. Although our results show that muscles were activated in response to static varus and valgus loads, their activations were modest and were not sufficient to balance the applied load.     

An evaluation of anatomical and functional knee axis definition in the context of side-cutting

Side-cutting is commonly used to evaluate knee joint kinematics and kinetics in the context of anterior cruciate ligament injury risk. Many existing side-cutting studies fail to clearly define the orientation of the femoral frame and the knee axis, making comparisons between studies difficult. A femoral frame constructed using the ISB or existing functional methods does not necessarily have a medial-lateral axis that is aligned with the axis of the knee. A functional frame that directly aligns with the medial-lateral knee axis was compared to the ISB anatomical frame and the Besier functional frame (Besier et al., 2003) to determine whether the chosen frame would affect the interpretation of side-cutting data. Kinematic and kinetic variables were calculated during three side-cutting manoeuvres of 28 subjects. Differences in mean frame orientation were correlated with the differences in mean knee angle during side-cutting. The differences between the ISB anatomical frame and the functional frames were significantly correlated with the differences in superior-inferior and medial-lateral axis orientations. Coefficients of multiple correlation showed a good to high (CMCs≥0.74) similarity between frames for knee angles and moments. Using a femoral anatomical frame rather than a functional frame most significantly affected offset rather than cross talk in knee angles and moments measured during side-cutting. There were no significant differences in offset or cross talk between the two functional methods. Maximum differences of <4° for frontal plane knee angle requires cautious interpretation but differences <8Nm for knee joint moment were not thought to affect the interpretation of side-cutting data when comparing between studies.     

Validation of method for analysing mechanics of unloader brace for medial knee osteoarthritis

Unloader braces are one non-invasive treatment of knee osteoarthritis, which primarily function by applying an external abduction moment to the joint to reduce loads in the medial compartment of the knee. We developed a novel method using brace deflection to estimate the mechanical effect of valgus braces and validated this model using strain gauge instrumentation.Three subjects performed static and walking trials, in which the moment applied by an instrumented brace was calculated using the deflection and strain methods. The deflection method predicted average brace moments of 8.7 Nm across static trials; mean error between the deflection model predictions and the gold-standard strain gauge measurements was 0.32 Nm. Mean brace moment predictions throughout gait ranged from 7.1 to 8.7 Nm using the deflection model. Maximum differences (MAE) over the gait cycle in mean and peak brace moments between methods were 1.50 Nm (0.96) and 0.60 Nm (0.42).Our proposed method enables quantification of brace abduction moments without the use of custom instrumentation. While the deflection-based method is similar to that implemented by Schmalz et al. (2010), the proposed method isolates abduction deflection from the 3 DOF angular changes that occur within the brace. Though the model should be viewed with more caution during swing (MAE = 1.16 Nm), it was shown that the accuracy is influenced by the uncertainty in angle measurement due to cluster spacing. In conclusion, the results demonstrate that the deflection-based method developed can predict comparable brace moments to those of the previously established strain method.     

Optimised loads for the simulation of axial rotation in the lumbar spine

Simplified loading modes (pure moment, compressive force) are usually applied in the in vitro studies to simulate flexion-extension, lateral bending and axial rotation of the spine. The load magnitudes for axial rotation vary strongly in the literature. Therefore, the results of current investigations, e.g. intervertebral rotations, are hardly comparable and may involve unrealistic values. Thus, the question 'which in vitro applicable loading mode is the most realistic' remains open. A validated finite element model of the lumbar spine was employed in two sensitivity studies to estimate the ranges of results due to published load assumptions and to determine the input parameters (e.g. torsional moment), which mostly affect the spinal load and kinematics during axial rotation. In a subsequent optimisation study, the in vitro applicable loading mode was determined, which delivers results that fit best with available in vivo measurements. The calculated results varied widely for loads used in the literature with potential high deviations from in vivo measured values. The intradiscal pressure is mainly affected by the magnitude of the compressive force, while the torsional moment influences mainly the intervertebral rotations and facet joint forces. The best agreement with results measured in vivo were found for a compressive follower force of 720N and a pure moment of 5.5Nm applied to the unconstrained vertebra L1. The results reveal that in many studies the assumed loads do not realistically simulate axial rotation. The in vitro applicable simplified loads cannot perfectly mimic the in vivo situation. However, the optimised values lead to the best agreement with in vivo measured values. Their consequent application would lead to a better comparability of different investigations.     

Biomechanical strategies for successful obstacle crossing with the trailing limb in older adults with medial compartment knee osteoarthritis

To investigate the biomechanical strategy adopted by older adults with medial compartment knee osteoarthritis (OA) for successful obstacle crossing with the trailing limb, and to discuss its implications for fall-prevention, 15 older adults with bilateral medial compartment knee OA and 15 healthy controls were recruited to walk and cross obstacles of heights of 10%, 20%, and 30% of their leg lengths. Kinematic and kinetic data were obtained using a three-dimensional (3D) motion analysis system and forceplates. The OA group had higher trailing toe clearance than the controls. When the trailing toe was above the obstacle, the OA group showed greater swing hip abduction, yet smaller stance hip adduction, knee flexion, and ankle eversion. They showed greater pelvic anterior tilt and toe-out angle. They also exhibited greater peak knee abductor moments during early stance and at the instant when the swing toe was above the obstacle, while a greater peak hip abductor moment was found during late stance. Smaller knee extensor, yet greater hip extensor moments, were found in the OA group throughout the stance phase. In order to achieve higher toe clearance with knee OA, particular joint kinematic and kinetic strategies have been adopted by the OA group. Weakness in the hip abductors and extensors in individuals with OA may be risk factors for tripping owing to the greater demands on these muscle groups during obstacle crossing by these individuals.     

Loading of the knee joint during activities of daily living measured in vivo in five subjects

Detailed knowledge about loading of the knee joint is essential for preclinical testing of implants, validation of musculoskeletal models and biomechanical understanding of the knee joint. The contact forces and moments acting on the tibial component were therefore measured in 5 subjects in vivo by an instrumented knee implant during various activities of daily living.Average peak resultant forces, in percent of body weight, were highest during stair descending (346% BW), followed by stair ascending (316% BW), level walking (261% BW), one legged stance (259% BW), knee bending (253% BW), standing up (246% BW), sitting down (225% BW) and two legged stance (107% BW). Peak shear forces were about 10–20 times smaller than the axial force. Resultant forces acted almost vertically on the tibial plateau even during high flexion. Highest moments acted in the frontal plane with a typical peak to peak range ?2.91% BWm (adduction moment) to 1.61% BWm (abduction moment) throughout all activities. Peak flexion/extension moments ranged between ?0.44% BWm (extension moment) and 3.16% BWm (flexion moment). Peak external/internal torques lay between ?1.1% BWm (internal torque) and 0.53% BWm (external torque).The knee joint is highly loaded during daily life. In general, resultant contact forces during dynamic activities were lower than the ones predicted by many mathematical models, but lay in a similar range as measured in vivo by others. Some of the observed load components were much higher than those currently applied when testing knee implants.     

The effect of weightbearing and external loading on anterior cruciate ligament strain

A force balance between the ligaments, articular contact, muscles and body weight maintains knee joint stability. Thus, it is important to study anterior cruciate ligament (ACL) biomechanics, in vivo, under weightbearing conditions. Our objective was to compare the ACL strain response under weightbearing and non-weightbearing conditions and in combination with three externally applied loadings: (1) anterior-posterior shear forces, (2) internal-external torques, and (3) varus-valgus moments. A strain transducer was implanted on the ACL of 11 subjects. All joint loadings were performed with the knee at 20 degrees of flexion. A significant increase in ACL strain was observed as the knee made the transition from non-weightbearing to weightbearing. During anterior shear loading, the strain values produced during weightbearing were greater than those of the non-weightbearing knee (shear loads <40N). At higher shear loads, the strain values became equal. During axial torsion, an internal torque of 10Nm strained the ACL when the knee was non-weightbearing while an equivalent external torque did not. Weightbearing significantly increased ACL strain values in comparison to non-weightbearing with the application of external torques and low internal torques (<3Nm). The strains became equal for higher internal torques. For V-V loading, the ACL was not strained in the non-weightbearing knee. However, weightbearing increased the ACL strain values over the range of moments tested. These data have important clinical ramifications in the development of rehabilitation protocols following ACL reconstruction since weightbearing has been previously thought to provide a protective mechanism to the healing graft.     

Patellofemoral joint compression forces in backward and forward running

Patellofemoral pain (PFP) is a common injury and increased patellofemoral joint compression forces (PFJCF) may aggravate symptoms. Backward running (BR) has been suggested for exercise with reduced PFJCF. The aims of this study were to (1) investigate if BR had reduced peak PFJCF compared to forward running (FR) at the same speed, and (2) if PFJCF was reduced in BR, to investigate which biomechanical parameters explained this. It was hypothesized that (1) PFJCF would be lower in BR, and (2) that this would coincide with a reduced peak knee moment caused by altered ground reaction forces (GRFs). Twenty healthy subjects ran in forward and backward directions at consistent speed. Kinematic and ground reaction force data were collected; inverse dynamic and PFJCF analyses were performed. PFJCF were higher in FR than BR (4.5±1.5; 3.4±1.4BW; p<0.01). The majority of this difference (93.1%) was predicted by increased knee moments in FR compared to BR (157±54; 124±51 Nm; p<0.01). 54.8% of differences in knee moments could be predicted by the magnitude of the GRF (2.3±0.3; 2.4±0.2BW), knee flexion angle (44±6; 41±7) and center of pressure location on the foot (25±11; 12±6%) at time of peak knee moment. Results were not consistent in all subjects. It was concluded that BR had reduced PFJCF compared to FR. This was caused by an increased knee moment, due to differences in magnitude and location of the GRF vector relative to the knee. BR can therefore be used to exercise with decreased PFJCF.     

Static and dynamic bending responses of the human cervical spine.

The quasi-static and dynamic bending responses of the human mid-lower cervical spine were determined using cadaver intervertebral joints fixed at the base to a six-axis load cell. Flexion bending moment was applied to the superior end of the specimen using an electrohydraulic piston. Each specimen was tested under three cycles of quasi-static load-unload and one high-speed dynamic load. A total of five specimens were included in this study. The maximum intervertebral rotation ranged from 11.0 to 15.4 deg for quasi-static tests and from 22.9 to 34.4 deg for dynamic tests. The resulting peak moments at the center of the intervertebral joint ranged from 3.8 to 6.9 Nm for quasi-static tests and from 14.0 to 31.8 Nm for dynamic tests. The quasi-static stiffness ranged from 0.80 to 1.35 Nm/deg with a mean of 1.03 Nm/deg (+/- 0.11 Nm/deg). The dynamic stiffness ranged from 1.08 to 2.00 Nm/deg with a mean of 1.50 Nm/deg (+/- 0.17 Nm/deg). The differences between the two stiffnesses were statistically significant (p < 0.01). Exponential functions were derived to describe the quasi-static and dynamic moment-rotation responses. These results provide input data for lumped-parameter models and validation data for finite element models to better investigate the biomechanics of the human cervical spine.