The effects of railroad ballast surface and slope on rearfoot motion in walking

The purpose of this study was to investigate the effects of transversely sloped ballasted walking surface on gait and rearfoot motion (RFM) parameters. Motion analysis was performed with 20 healthy participants (15 male and 5 female) walking in six surface-slope conditions: two surfaces (solid and ballasted) by three slopes (0, 5, and 10 degrees). The gait parameters (walking velocity, step length, step rate, step width, stance time, and toe-out angle) showed significant surface effect (p = .004) and surface-slope interaction (p = .017). The RFM motion parameters (peak everted/inverted position, eversion/inversion velocity, and acceleration) revealed significant surface (p = .004) and slope (p = .024) effects. The ballasted conditions showed more cautious gait patterns with lower walk velocity, step length, and step rate and longer stance time. In the RFM parameters, the slope effect was more notable in the solid conditions due to the gait adaptations in the ballasted conditions. Ballast conditions showed reduced inversion and increased eversion and RFM range. The RFM data were comparable to other typical walking conditions but smaller than those from running.     

Effect of the degree of hill slope on acute downhill running velocity and acceleration

This study analyzes the effects of hill slope on acute overspeed running. This study considers both acceleration and supramaximal velocity. Forty-four athletes ran 40-yard sprints, on 5 different hill slopes, ranging from 2.1 degrees to 6.9 degrees . Forty-yard sprint times and 10-yard split times were recorded using the Brower Timing System Speedtrap II. Analysis reveals that 40-yard and 10-yard sprints performed on hill slopes of approximately 5.8 degrees were optimal compared to flatland running and the other slopes assessed. Sprinting on a 5.8 degrees slope increased the subjects' maximal speed by 7.09% +/- 3.66% and increased the subjects' acceleration by 6.54% +/- 1.56%. Strength and conditioning professionals who train athletes for speed should develop and use overspeed hills or platforms with slopes of approximately 5.8 degrees in order to maximize acute sprinting velocity and acceleration.     

A comparative collision-based analysis of human gait

This study compares human walking and running, and places them within the context of other mammalian gaits. We use a collision-based approach to analyse the fundamental dynamics of the centre of mass (CoM) according to three angles derived from the instantaneous force and velocity vectors. These dimensionless angles permit comparisons across gait, species and size. The collision angle Φ, which is equivalent to the dimensionless mechanical cost of transport CoT_mech, is found to be three times greater during running than walking of humans. This threefold difference is consistent with previous studies of walking versus trotting of quadrupeds, albeit tends to be greater in the gaits of humans and hopping bipeds than in quadrupeds. Plotting the collision angle Φ together with the angles of the CoM force vector Θ and velocity vector Λ results in the functional grouping of bipedal and quadrupedal gaits according to their CoM dynamics—walking, galloping and ambling are distinguished as separate gaits that employ collision reduction, whereas trotting, running and hopping employ little collision reduction and represent more of a continuum that is influenced by dimensionless speed. Comparable with quadrupedal mammals, collision fraction (the ratio of actual to potential collision) is 0.51 during walking and 0.89 during running, indicating substantial collision reduction during walking, but not running, of humans.     

Acceleration slope of exercise-induced impacts is a determinant of changes in bone density

High acceleration levels (>4g) seen during impact exercises have been shown to increase bone mineral density (BMD) in premenopausal women. The aim of this study was to examine how the other acceleration signal characteristics, i.e. the slope, area and energy of the signal are related to changes in bone density, using long-term quantification of physical activity. Daily physical activity was continuously assessed with a waist-worn accelerometer-based body movement monitor in 64 premenopausal women participating in a 12-month population-based exercise trial. The daily number of exercise-induced impacts at different slope, area and energy levels of the acceleration signal was analyzed. Physical activity inducing slopes 1000 m/s(3), acceleration peak areas 2m/s or signal energies 75 m(2)/s(3) was associated with BMD change in the hip (p<0.05). Impacts with the smallest slopes (<1000 m/s(3)) were positively associated with changes in calcaneal speed of ultrasound, while impacts with slopes 1500 m/s(3) or areas 4m/s were positively correlated with broadband ultrasound attenuation changes (p<0.05). We conclude that the acceleration slope of exercise-induced impacts is an important determinant of bone density. The slope threshold for improving BMD at the hip is 1000 m/s(3), which can be achieved during normal exercise including fast movements such as running and jumping.     

Enthalpy efficiency of the soleus muscle contributes to improvements in running economy

During human running, the soleus, as the main plantar flexor muscle, generates the majority of the mechanical work through active shortening. The fraction of chemical energy that is converted into muscular work (enthalpy efficiency) depends on the muscle shortening velocity. Here, we investigated the soleus muscle fascicle behaviour during running with respect to the enthalpy efficiency as a mechanism that could contribute to improvements in running economy after exercise-induced increases of plantar flexor strength and Achilles tendon (AT) stiffness. Using a controlled longitudinal study design (n = 23) featuring a specific 14-week muscle–tendon training, increases in muscle strength (10%) and tendon stiffness (31%) and reduced metabolic cost of running (4%) were found only in the intervention group (n = 13, p < 0.05). Following training, the soleus fascicles operated at higher enthalpy efficiency during the phase of muscle–tendon unit (MTU) lengthening (15%) and in average over stance (7%, p < 0.05). Thus, improvements in energetic cost following increases in plantar flexor strength and AT stiffness seem attributed to increased enthalpy efficiency of the operating soleus muscle. The results further imply that the soleus energy production in the first part of stance, when the MTU is lengthening, may be crucial for the overall metabolic energy cost of running.     

Torsion and Antero-Posterior Bending in the In Vivo Human Tibia Loading Regimes during Walking and Running

Bending, in addition to compression, is recognized to be a common loading pattern in long bones in animals. However, due to the technical difficulty of measuring bone deformation in humans, our current understanding of bone loading patterns in humans is very limited. In the present study, we hypothesized that bending and torsion are important loading regimes in the human tibia. In vivo tibia segment deformation in humans was assessed during walking and running utilizing a novel optical approach. Results suggest that the proximal tibia primarily bends to the posterior (bending angle: 0.15°–1.30°) and medial aspect (bending angle: 0.38°–0.90°) and that it twists externally (torsion angle: 0.67°–1.66°) in relation to the distal tibia during the stance phase of overground walking at a speed between 2.5 and 6.1 km/h. Peak posterior bending and peak torsion occurred during the first and second half of stance phase, respectively. The peak-to-peak antero-posterior (AP) bending angles increased linearly with vertical ground reaction force and speed. Similarly, peak-to-peak torsion angles increased with the vertical free moment in four of the five test subjects and with the speed in three of the test subjects. There was no correlation between peak-to-peak medio-lateral (ML) bending angles and ground reaction force or speed. On the treadmill, peak-to-peak AP bending angles increased with walking and running speed, but peak-to-peak torsion angles and peak-to-peak ML bending angles remained constant during walking. Peak-to-peak AP bending angle during treadmill running was speed-dependent and larger than that observed during walking. In contrast, peak-to-peak tibia torsion angle was smaller during treadmill running than during walking. To conclude, bending and torsion of substantial magnitude were observed in the human tibia during walking and running. A systematic distribution of peak amplitude was found during the first and second parts of the stance phase.     

Muscle fascicle and series elastic element length changes along the length of the human gastrocnemius during walking and running

Ultrasound imaging has recently been used to distinguish the length changes of muscle fascicles from those of the whole muscle tendon complex during real life movements. The complicated three-dimensional architecture of pennate muscles can however cause heterogeneity in the length changes along the length of a muscle. Here we use ultrasonography to examine muscle fascicle length and pennation angle changes at proximal, distal and midbelly sites of the human gastrocnemius medialis (GM) muscle during walking (4.5 km/h) and running (7.5 km/h) on a treadmill. The results of this study have shown that muscle fascicles perform the same actions along the length of the human GM muscle during locomotion. However the distal fascicles tend to shorten more and act at greater pennation angles than the more proximal fascicles. Muscle fascicles acted relatively isometrically during the stance phase during walking, however during running the fascicles shortened throughout the stance phase, which corresponded to an increase in the strain of the series elastic elements (SEEs) (consisting of the Achilles tendon and aponeurosis). Measurement of the fascicle length changes at the midbelly level provided a good approximation of the average fascicle length changes across the length of the muscle. The compliance of the SEE allows the muscle fascicles to shorten at a much slower speed, more concomitant with their optimal speed for maximal power output and efficiency, with high velocity shortening during take off in both walking and running achieved by recoil of the SEE.     

Ability of the planar spring-mass model to predict mechanical parameters in running humans

The planar spring-mass model is a simple mathematical model of bouncing gaits, such as running, trotting and hopping. Although this model has been widely used in the study of locomotion, its accuracy in predicting locomotor mechanics has not been systematically quantified. We determined the percent error of the model in predicting 10 locomotor parameters in running humans by comparing the model predictions to experimental data from humans running in normal gravity and simulated reduced gravity. We tested the hypotheses that the model would overestimate horizontal impulse and the change in mechanical energy of the centre of mass (COM) during stance. The model provided good predictions of stance time, vertical impulse, contact length, duty factor, relative stride length and relative peak force. All predictions of these parameters were within 20% of measured values and at least 90% of predictions of each parameter were within 10% of measured values (median absolute errors: <7%). This suggests that the model incorporates all features of running humans that have a significant influence upon these six parameters. As simulated gravity level decreased, the magnitude of the errors in predicting each of these parameters either decreased or stayed constant, indicating that this is a good model of running in simulated reduced gravity. As hypothesised, horizontal impulse and change in mechanical energy of the COM during stance were overestimated (median absolute errors: 43.6% and 26.2%, respectively). Aerial time and peak vertical COM displacement during stance were also systematically overestimated (median absolute errors: 17.7% and 22.9%, respectively). Care should be taken to ensure that the model is used only to investigate parameters which it can predict accurately. It would be useful to extend this analysis to other species and gaits.     

Testing an emerging paradigm in migration ecology shows surprising differences in efficiency between flight modes

To maximize fitness, flying animals should maximize flight speed while minimizing energetic expenditure. Soaring speeds of large-bodied birds are determined by flight routes and tradeoffs between minimizing time and energetic costs. Large raptors migrating in eastern North America predominantly glide between thermals that provide lift or soar along slopes or ridgelines using orographic lift (slope soaring). It is usually assumed that slope soaring is faster than thermal gliding because forward progress is constant compared to interrupted progress when birds pause to regain altitude in thermals. We tested this slope-soaring hypothesis using high-frequency GPS-GSM telemetry devices to track golden eagles during northbound migration. In contrast to expectations, flight speed was slower when slope soaring and eagles also were diverted from their migratory path, incurring possible energetic costs and reducing speed of progress towards a migratory endpoint. When gliding between thermals, eagles stayed on track and fast gliding speeds compensated for lack of progress during thermal soaring. When thermals were not available, eagles minimized migration time, not energy, by choosing energetically expensive slope soaring instead of waiting for thermals to develop. Sites suited to slope soaring include ridges preferred for wind-energy generation, thus avian risk of collision with wind turbines is associated with evolutionary trade-offs required to maximize fitness of time-minimizing migratory raptors.     

Leg kinematics and muscle activity during treadmill running in the cockroach, Blaberus discoidalis: II. Fast running

We have combined kinematic and electromyogram (EMG) analysis of running Blaberus discoidalis to examine how middle and hind leg kinematics vary with running speed and how the fast depressor coxa (Df) and fast extensor tibia (FETi) motor neurons affect kinematic parameters. In the range 2.5–10 Hz, B. discoidalis increases step frequency by altering the joint velocity and by reducing the time required for the transition from flexion to extension. For both Df and FETi the timing of recruitment coincides with the maximal frequency seen for the respective slow motor neurons. Df is first recruited at the beginning of coxa-femur (CF) extension. FETi is recruited in the latter half of femur-tibia (FT) extension during stance. Single muscle potentials produced by these fast motor neurons do not have pronounced effects on joint angular velocity during running. The transition from CF flexion to extension was abbreviated in those cycles with a Df potential occurring during the transition. One effect of Df activity during running may be to phase shift the beginning of joint extension so that the transition is sharpened. FETi is associated with greater FT extension at higher running speeds and may be necessary to overcome high joint torques at extended FT joint angles. Accepted: 24 May 1997     

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.     

A biomechanical model for size, speed and anatomical variations of the energetic costs of running mammals

Here we propose a model of energetic costs and the muscle-tendon unit function on running mammals. The main goal is to set a simple theoretical framework which gives an understanding of the biomechanical principles behind the size, speed and anatomical variations of the energetic costs of running mammals. The model is a point-like mass withstood by a two-segment leg with an extensor muscle serially attached to a tendon. We considered withstanding body weight during the stance phase as the main role of the muscle-tendon unit during fast locomotion. The ground reaction force dependence on speed and the time of stance phase as well as other biomechanical characteristics were taken from previous empirical studies of running. At the same time, the morphological variations with body mass were taken from empirically well-established allometric equations for mammals. The metabolic cost was estimated from an empirical equation relating metabolic power with muscular force and speed in shortening and stretching. Our model predicts the pattern of mass specific metabolic rate variations with both speed and body mass. It also gives an explanation of the experimentally reported linear inverse relationship between the rate of energy used for running and the time of application of force by the foot to the ground during each stride. It also suggests an explanation of the unusual energy saving adaptations of large macropodids. It provides some predictions on the relationship, between energy costs and muscle-tendon unit characteristics, testable on further experiments.     

Mechanical output of the cat soleus during treadmill locomotion: in vivo vs in situ characteristics

To study the mechanical output of skeletal muscle, four adult cats were trained to run on a treadmill and then implanted under sterile conditions and anesthesia with a force transducer on the soleus tendon and EMG electrodes in the muscle belly. After a two-week recovery period, five consecutive step cycles were filmed at treadmill speeds of 0.8, 1.3 and 2.2 m s-1. Locomotion data in vivo included individual muscle force, length and velocity changes and EMG during each step cycle. Data for an average step cycle at each speed were compared to the force-velocity properties obtained on the same muscle under maximal nerve stimulation and isotonic loading conditions in situ. Results indicate that the force and power generated at a given velocity of shortening during late stance in vivo were greater at the higher speeds of locomotion than the force and power generated at the same shortening velocity in situ. Strain energy stored in the muscle-tendon unit during the yield phase in early stance is felt to be a major contributor to the muscle's enhanced mechanical output during muscle shortening in late stance.     

Stretching Your Energetic Budget: How Tendon Compliance Affects the Metabolic Cost of Running

Muscles attach to bones via tendons that stretch and recoil, affecting muscle force generation and metabolic energy consumption. In this study, we investigated the effect of tendon compliance on the metabolic cost of running using a full-body musculoskeletal model with a detailed model of muscle energetics. We performed muscle-driven simulations of running at 2–5 m/s with tendon force–strain curves that produced between 1 and 10% strain when the muscles were developing maximum isometric force. We computed the average metabolic power consumed by each muscle when running at each speed and with each tendon compliance. Average whole-body metabolic power consumption increased as running speed increased, regardless of tendon compliance, and was lowest at each speed when tendon strain reached 2–3% as muscles were developing maximum isometric force. When running at 2 m/s, the soleus muscle consumed less metabolic power at high tendon compliance because the strain of the tendon allowed the muscle fibers to operate nearly isometrically during stance. In contrast, the medial and lateral gastrocnemii consumed less metabolic power at low tendon compliance because less compliant tendons allowed the muscle fibers to operate closer to their optimal lengths during stance. The software and simulations used in this study are freely available at simtk.org and enable examination of muscle energetics with unprecedented detail.     

Alteration in community-dwelling older adults' level walking following perturbation training

While perturbation training is promising in reducing fall-risk among older adults, its impact on altering their spontaneous gait pattern has not been investigated. The purpose of this study was to determine to what extent older adults' gait pattern would be affected by exposure to repeated slips. Seventy-three community-dwelling older adults (age: 72.6±5.4 years) underwent 24 repeated-slip exposure induced by unannounced unlocking and relocking of low-friction sections of a 7-m pathway upon which they walked. Full body kinematics and kinetics were recorded during the training. The gait parameters and the center of mass (COM) stability against backward balance loss were compared before and after the training. The results revealed that the training reduced fall incidence from 43.8% upon the novel slip to 0 at the end of training. After the training, subjects significantly improved gait stability by forward positioning of their COM relative to the base of support without altering gait speed. This forward COM shift resulted from a shortened step at the end of single stance and forward trunk leaning during double stance. They also adopted flat foot landing with knee flexed at touchdown (with an average change of 6.9 and 4.1 degrees, respectively). The perturbation training did alter community-dwelling older adults' spontaneous gait pattern. These changes enabled them to improve their volitional control of stability and their resistance to unpredictable and unpreventable slip-related postural disturbance.     

The influence of joint rigidity on impact efficiency and ball velocity in football kicking

Executing any skill with efficiency is important for performance. In football kicking, conflicting and non-significant results have existed between reducing ankle plantarflexion during foot-ball contact with impact efficiency, making it unclear as to its importance as a coaching instruction. The aims of this study were to first validate a mechanical kicking machine with a non-rigid ankle, and secondly compare a rigid to a non-rigid ankle during the impact phase of football kicking. Measures of foot-ball contact for ten trials per ankle configuration were calculated from data recorded at 4000 Hz and compared. The non-rigid ankle was characterised by initial dorsiflexion followed by plantarflexion for the remainder of impact, and based on similarities to punt and instep kicking, was considered valid. Impact efficiency (foot-to-ball speed ratio) was greater for the rigid ankle (rigid = 1.16 ± 0.02; non-rigid = 1.10 ± 0.01; p < 0.001). The rigid ankle was characterised by significantly greater effective mass and significantly less energy losses. Increasing rigidity allowed a greater portion of mass from the shank to be used during the collision. As the ankle remained in plantarflexion at impact end, stored elastic energy was not converted to ball velocity and was considered lost. Increasing rigidity is beneficial for increasing impact efficiency, and therefore ball velocity.     

THE EFFECTS OF THREE DIFFERENT REAR KNEE ANGLES ON KINEMATICS IN THE SPRINT START

The purpose of this study was to investigate the rear knee angle range in the set position that allows sprinters to reach greater propulsion on the rear block during the sprint start. Eleven university-track team sprinters performed the sprint start using three rear knee angle conditions: 90°, 115° and 135°. A motion capture system consisting of 8 digital cameras (250 Hz) was used to record kinematic parameters at the starting block phase and the acceleration phase. The following variables were considered: horizontal velocity of the centre of mass (COM), COM height, block time, pushing time on the rear block, percentage of pushing time on the rear block, force impulse, push-off angle and length of the first two strides. The main results show that first, horizontal block velocity is significantly greater at 90° vs 115° and 135° rear knee angle (p<0.05 and p<0.001 respectively) at block clearance and the first two strides; second, during the pushing phase, the percentage of pushing time of the rear leg is significantly greater at 90° vs 135° rear knee angle (p<0.01). No significant difference was found for block time among the conditions. These results indicate that block velocity is the main kinematic parameter affected by rear knee angle during the starting block phase and acceleration phase. Furthermore, the 90° rear knee angle allows for a better push-off of the rear leg than larger angles at the set position. The findings of this study provide some direction and useful practical advice in defining an efficient rear leg biomechanical configuration at the set position.     

Preferred step frequency during downhill running may be determined by muscle activity

It is well established that metabolic cost is minimized at an individual’s running preferred step frequency (PSF). It has been proposed that the metabolic minimum at PSF is due to a tradeoff between mechanical factors, however, this ignores muscle activity, the primary consumer of energy. Thus, we hypothesized that during downhill running, total muscle activity would be greater with deviations from PSF. Specifically, we predicted that slow step frequencies would have greater stance activity while fast step frequencies would have greater swing activity. We collected metabolic cost and leg muscle activity data while 10 healthy young adults ran at 3.0 m/s for 5 min at level and downhill at PSF and ±15% PSF. In support of our hypothesis, there was a significant main effect for step frequency for both metabolic cost and total muscle activity. In addition, there was greater muscle activity in the stance phase during the slower step frequency while muscle activity was greater in the swing phase during the fast step frequency. This suggests that PSF is partially determined by the tradeoff between the greater cost of muscle activity in the swing phase and lower cost in the stance phase with faster step frequency.     

Age-related mechanical work expenditure during normal walking: The Baltimore Longitudinal Study of Aging

The aim of this cross-sectional study was to delineate age-associated kinematic and kinetic gait patterns of normal walking, and to test the hypothesis that older adults exhibit gait patterns that reduce generative mechanical work expenditures (MWEs). We studied 52 adult Baltimore Longitudinal Study of Aging participants (means age 72±9, from 60 to 92 years) who could walk 4 m unaided. Three-dimensional kinematic and kinetic parameters assessed during rotation-defined gait periods were used to estimate MWEs for the rotation of lower extremities about the medial–lateral (ML) and anterior–posterior (AP) axes of proximal joints, which represent MWEs in the AP and ML sides, respectively. Relationships between gait parameters and age were examined using regression analysis with adjustments for walking speed, sex, height, and weight. Older age was associated with slower self-selected walking speed (p<0.001), shorter stride length (p<0.001), and greater propensity of landing flat-footed (p=0.003). With older age, hip generative MWE for thigh rotation was lower about the AP axis (hip abduction and adduction) during stance (p=0.010) and higher about the ML axis (hip extension and flexion) during late stance (p<0.001). Knee absorptive MWE for shank rotation about the AP axis (knee abduction and adduction) during early stance was also lower with older age (p<0.003). These age-related gait patterns may represent a compensatory effort to maintain balance and may also reflect mobility limitations.     

Predicted threshold against backward balance loss following a slip in gait

The purpose of this study was to use a 7-link, moment-actuated human model to predict, at liftoff of the trailing foot in gait, the threshold of the center of mass (COM) velocity relative to the base of support (BOS) required to prevent backward balance loss during single stance recovery from a slip. Five dynamic optimization problems were solved to find the minimum COM velocities that would allow the simulation to terminate with the COM above the BOS when the COM started 0.25, 0.5, 0.75, 1.0, and 1.25 foot lengths behind the heel of the stance foot (i.e., behind the BOS). The initial joint angles of the model were based on averaged data from experimental trials. Foot-ground contact was modeled using 16 visco-elastic springs distributed under the stance foot. Slipping was modeled by setting the sliding coefficient of friction of these springs to 0.02. The forward velocity of the COM necessary to avoid a backward balance loss is nearly two times larger under slip conditions under non-slip conditions. The predicted threshold for backward balance loss following a slip agreed well with experimental data collected from 99 young adults in response to 927 slips during walking. In all trials in which a subject's COM had a velocity below the predicted threshold, the subject's recovery foot landed posterior to the slipping foot as predicted. Finally, combining experimental data with optimization, we verified that the 7-link model could more accurately predict gait stability than a 2-link model.