Kinematics and mechanical changes with step frequency at different running speeds.

PURPOSE: Running at a given speed can be achieved by taking large steps at a low frequency or on the contrary by taking small steps at a high frequency. The consequences of a change in step frequency, at a fixed speed, affects the stiffness of the lower limb differently. In this study, we compared the running mechanics and kinematics at different imposed step frequencies (from 2 step s-1 to 3.6 step s-1) to understand the relationship between kinematic and kinetic parameters. METHODS: Eight recreational male runners ran on a treadmill at 5 different speeds and 5 different step frequencies. The lower-limb segment motion and the ground reaction forces were recorded. Mechanical powers, general gait parameters, lower-limb movements and coordination were investigated. RESULTS: At low step frequencies, in order to limit the magnitude of the ground reaction force, the vertical stiffness is reduced and thus runners deviate from an elastic rebound. At high step frequencies, the stiffness is increased and the elastic rebound is optimised in its ability to absorb and restore energy during the contact phase. CONCLUSION: We studied the consequences of a change in step frequency on the bouncing mechanics of running. We showed that the lower limb stiffness and the intersegmental coordination of the lower-limb segments are affected by running step frequency rather than speed. The runner rather adapts their lower limb stiffness to match a step frequency for a given speed than the opposite.

Postural control in the elephant.

As the largest extant legged animals, elephants arguably face the most extreme challenge for stable standing. In this study, we investigated the displacement of the centre of pressure of 12 elephants during quiet standing. We found that the average amplitude of the oscillations in the lateral and fore-aft directions was less than 1.5 cm. Such amplitudes for postural oscillation are comparable with those of dogs and other species, suggesting that some aspects of sensorimotor postural control do not scale with size.

The bouncing mechanism of running against hindering, or with aiding traction forces: a comparison with running on a slope.

PURPOSE: Much like running on a slope, running against/with a horizontal traction force which either hinders/aids the forward motion of the runner creates a shift in the positive and negative muscular work, which in turn modifies the bouncing mechanism of running. The purpose of the study is to (1) investigate the energy changes of the centre of mass and the storage/release of energy throughout the step during running associated with speed and increasing hindering and aiding traction forces; and (2) compare these changes to those observed when running on a slope. METHODS: Ground reaction forces were measured on eight subjects running on an instrumented treadmill against different traction forces at different speeds. RESULTS: As compared to unperturbed running, running against/with a traction force increases/decreases positive external work by ~ 20-70% and decreases/increases negative work by ~ 40-60%, depending on speed and traction force. The external power to maintain forward motion against a traction is contained by increasing the pushing time and step frequency. When running with an aiding force, the external power during the brake is limited by increasing braking time. Furthermore, the aerial time is increased to reduce the power required to reset the limbs each step. CONCLUSION: Our results show that the bouncing mechanism of running against/with a hindering/aiding traction force is equivalent to that of running on a positive/negative slope.

Neuromechanical adjustments when walking with an aiding or hindering horizontal force.

PURPOSE: Walking against a constant horizontal traction force which either hinders or aids the motion of the centre of mass of the body (COM) will create a discrepancy between the positive and negative work being done by the muscles and may thus affect the mechanics and energetics of walking. We aimed at investigating how this imbalance affects the exchange between potential and kinetic energy of the COM and how its dynamics is related to specific spatiotemporal organisation of motor pool activity in the spinal cord. To understand if and how the spinal cord activation may be associated with COM dynamics, we also compared the neuromechanical adjustments brought on by a horizontal force with published data about those brought on by a slope. METHODS: Ten subjects walked on a treadmill at different speeds with different traction forces. We recorded kinetics, kinematics, and electromyographic activity of 16 lower-limb muscles and assessed the spinal locomotor output by mapping them onto the rostrocaudal location of the motoneuron pools. RESULTS: When walking with a hindering force, the major part of the exchange between potential and kinetic energy of the COM occurs during the first part of stance, whereas with an aiding force exchanges increase during the second part of stance. Those changes occur since limb and trunk orientations remain aligned with the average orientation of the ground reaction force vector. Our results also show the sacral motor pools decreased their activity with an aiding force and increased with a hindering one, whereas the lumbar motor pools increased their engagement both with an aiding and a hindering force. CONCLUSION: Our findings suggest that applying a constant horizontal force results in similar modifications of COM dynamics and spinal motor output to those observed when walking on slopes, consistent with common principles of motor pool functioning and biomechanics of locomotion.

Differential activation of lumbar and sacral motor pools during walking at different speeds and slopes.

Organization of spinal motor output has become of interest for investigating differential activation of lumbar and sacral motor pools during locomotor tasks. Motor pools are associated with functional grouping of motoneurons of the lower limb muscles. Here we examined how the spatiotemporal organization of lumbar and sacral motor pool activity during walking is orchestrated with slope of terrain and speed of progression. Ten subjects walked on an instrumented treadmill at different slopes and imposed speeds. Kinetics, kinematics, and electromyography of 16 lower limb muscles were recorded. The spinal locomotor output was assessed by decomposing the coordinated muscle activation profiles into a small set of common factors and by mapping them onto the rostrocaudal location of the motoneuron pools. Our results show that lumbar and sacral motor pool activity depend on slope and speed. Compared with level walking, sacral motor pools decrease their activity at negative slopes and increase at positive slopes, whereas lumbar motor pools increase their engagement when both positive and negative slope increase. These findings are consistent with a differential involvement of the lumbar and the sacral motor pools in relation to changes in positive and negative center of body mass mechanical power production due to slope and speed.NEW & NOTEWORTHY In this study, the spatiotemporal maps of motoneuron activity in the spinal cord were assessed during walking at different slopes and speeds. We found differential involvement of lumbar and sacral motor pools in relation to changes in positive and negative center of body mass power production due to slope and speed. The results are consistent with recent findings about the specialization of neuronal networks located at different segments of the spinal cord for performing specific locomotor tasks.

Effect of walking speed on the intersegmental coordination of lower-limb segments in elderly adults.

BACKGROUND: Ageing brings profound changes in walking gait. For example, older adults reduce the modification of pelvic and trunk kinematics with walking speed. However, the modification of the coordination between lower-limb segments with age has never been investigated across various controlled speeds. RESEARCH QUESTION: Is the effect of speed on the intersegmental coordination different between elderly and young adults? METHODS: Nineteen senior and eight young adults walked on a treadmill at speeds ranging from 0.56 to 1.94 m s-1. The motion of the lower-limb segments in the sagittal plane was recorded by cinematography. When the angles of the thigh, shank and foot during a stride are plotted one versus the other, they describe loops constraint on a plane. The coordination between lower-limb segments was thus evaluated by performing a principal component analysis between the thigh, shank and foot elevation angles. The effect of speed and age on the intersegmental coordination was examined using a two-level linear mixed model ANOVA. RESULTS: In both age groups the orientation of the plane changes with speed, due to a more in-phase shank and foot motion. However, the effect of speed on the covariation plane is lessened with age. SIGNIFICANCE: Our results demonstrate that there is an age-related specific adjustment of the intersegmental coordination to speed. In particular, older adults restrict their repertoire of angular segment motion. These differences in coordination are mainly related to different foot-shank coordination.

Running on a slope: A collision-based analysis to assess the optimal slope.

When running, energy is lost during stance to redirect the center of mass of the body (COM) from downwards to upwards. The present study uses a collision-based approach to analyze how these energy losses change with slope and speed. Therefore, we evaluate separately the average collision angle, i.e. the angle of deviation from perpendicular relationship between the force and velocity vectors, during the absorptive and generative part of stance. Our results show that on the level, the collision angle of the absorptive phase is smaller than the collision angle of the generative phase, suggesting that the collision is generative to overcome energy losses by soft tissues. When running uphill, the collision becomes more and more generative as slope increases because the average upward vertical velocity of the COM becomes greater than on the level. When running downhill at a constant speed, the collision angle decreases during the generative phase and increases during the absorptive phase because the average downward vertical velocity of the COM becomes greater. As a result, the difference between the collision angles of the generative and absorptive phases observed on the level disappears on a shallow negative slope of ∼-6°, where the collision becomes 'pseudo-elastic' and collisional energy losses are minimized. At this 'optimal' slope, the metabolic energy consumption is minimal. On steeper negative slopes, the collision angle during the absorptive phase becomes greater than during the generative phase and the collision is absorptive. At all slopes, the collision becomes more generative when speed increases.

Kinematic patterns while walking on a slope at different speeds.

During walking, the elevation angles of the thigh, shank, and foot (i.e., the angle between the segment and the vertical) covary along a characteristic loop constrained on a plane. Here, we investigate how the shape of the loop and the orientation of the plane, which reflect the intersegmental coordination, change with the slope of the terrain and the speed of progression. Ten subjects walked on an inclined treadmill at different slopes (between -9° and +9°) and speeds (from 0.56 to 2.22 m/s). A principal component analysis was performed on the covariance matrix of the thigh, shank, and foot elevation angles. At each slope and speed, the variance accounted for by the two principal components was >99%, indicating that the planar covariation is maintained. The two principal components can be associated to the limb orientation (PC1*) and the limb length (PC2*). At low walking speeds, changes in the intersegmental coordination across slopes are characterized mainly by a change in the orientation of the covariation plane and in PC2* and to a lesser extent, by a change in PC1*. As speed increases, changes in the intersegmental coordination across slopes are more related to a change in PC1 *, with limited changes in the orientation of the plane and in PC 2*. Our results show that the kinematic patterns highly depend on both slope and speed. NEW & NOTEWORTHY In this paper, changes in the lower-limb intersegmental coordination during walking with slope and speed are linked to changes in the trajectory of the body center of mass. Modifications in the kinematic pattern with slope depend on speed: at slow speeds, the net vertical displacement of the body during each step is related to changes in limb length and orientation. When speed increases, the vertical displacement is mostly related to a change in limb orientation.

The mechanics of head-supported load carriage by Nepalese porters.

In the Everest valley of Nepal, because of the rugged mountain terrain, roads are nothing more than dirt paths and all material must be conveyed on foot. The Nepalese porters routinely carry head-supported loads, which often exceed their body mass, over long distances up and down the steep mountain footpaths. In Africa, women transport their loads economically thanks to an energy-saving gait adaptation. We hypothesized that the Nepalese porters may have developed a corresponding mechanism. To investigate this proposition, we measured the mechanical work done during level walking in Nepalese porters while carrying different loads at several speeds. Our results show that the Nepalese porters do not use an equivalent mechanism as the African women to reduce work. In contrast, the Nepalese porters develop an equal amount of total mechanical work as Western control subjects while carrying loads of 0 to 120% of their body mass at all speeds measured (0.5-1.7 m s-1), making even more impressive their ability to carry loads without any apparent mechanically determined tricks. Nevertheless, our results show that the Nepalese porters have a higher efficiency, at least at slow speeds and high loads.

Human motor control of landing from a drop in simulated microgravity.

Landing on the ground on one's feet implies that the energy gained during the fall be dissipated. The aim of this study is to assess human motor control of landing in different conditions of fall initiation, simulated gravity, and sensory neural input. Six participants performed drop landings using a trapdoor system and landings from self-initiated counter-movement jumps in microgravity conditions simulated in a weightlessness environment by different pull-down forces of 1-, 0.6-, 0.4-, and 0.2 g External forces applied to the body, orientation of the lower limb segments, and muscular activity of 6 lower limb muscles were recorded synchronously. Our results show that 1) subjects are able to land and stabilize in all experimental conditions; 2) prelanding muscular activity is always present, emphasizing the capacity of the central nervous system to approximate the instant of touchdown; 3) the kinetics and muscular activity are adjusted to the amount of energy gained during the fall; 4) the control of landing seems less finely controlled in drop landings as suggested by higher impact forces and loading rates, plus lower mechanical work done during landing for a given amount of energy to be dissipated. In conclusion, humans seem able to adapt the control of landing according to the amount of energy to be dissipated in an environment where sensory information is altered, even under conditions of non-self-initiated falls.

The rebound of the body during uphill and downhill running at different speeds.

When running on the level, muscles perform as much positive as negative external work. On a slope, the external positive and negative work performed are not equal. The present study analysed how the ratio between positive and negative work modifies the bouncing mechanism of running. Our goals are to: (1) identify the changes in motion of the centre of mass of the body associated with the slope of the terrain and the speed of progression, (2) study the effect of these changes on the storage and release of elastic energy during contact and (3) propose a model that predicts the change in the bouncing mechanism with slope and speed. Therefore, the ground reaction forces were measured on 10 subjects running on an instrumented treadmill at different slopes (from -9 to +9 deg) and different speeds (between 2.2 and 5.6 m s(-1)). The movements of the centre of mass of the body and its external mechanical energy were then evaluated. Our results suggest that the increase in the muscular power is contained (1) on a positive slope, by decreasing the step period and the downward movements of the body, and by increasing the duration of the push, and (2) on a negative slope, by increasing the step period and the duration of the brake, and by decreasing the upward movement of the body. Finally, the spring-mass model of running was adapted to take into account the energy added or dissipated each step on a slope.

Motor control of landing from a countermovement jump in simulated microgravity.

Landing from a jump implies proper positioning of the lower limb segments and the generation of an adequate muscular force to cope with the imminent collision with the ground. This study assesses how a hypogravitational environment affects the control of landing after a countermovement jump (CMJ). Eight participants performed submaximal CMJs on Earth (1-g condition) and in a weightlessness environment with simulated gravity conditions generated by a pull-down force (1-, 0.6-, 0.4-, and 0.2-g0 conditions). External forces applied to the body, movements of the lower limb segments, and muscular activity of six lower limb muscles were recorded. 1) All subjects were able to jump and stabilize their landing in all experimental conditions, except one subject in 0.2-g0 condition. 2) The mechanical behavior of lower limb muscles switches during landing from a stiff spring to a compliant spring associated with a damper. This is true whatever the environment, on Earth as well as in environments where sensory inputs are altered. 3) The motor control of landing in simulated 1 g0 reveals an increased "safety margin" strategy, illustrated by increased stiffness and damping coefficient compared with landing on Earth. 4) The motor command is adjusted to the task constraints: muscular activity of lower limb extensors and flexors, stiffness and damping coefficient decrease according to the decreased gravity level. Our results show that even if in daily living gravity can be perceived as a constant factor, subjects can cope with altered sensory signals, taking advantage of the remaining information (visual and/or decreased proprioceptive inputs).

The mechanics of jumping over an obstacle during running: a comparison between athletes trained to hurdling and recreational runners.

PURPOSE: This study compares the mechanism of running in trained athletes (TA) experienced in hurdling and in recreational runners (RR), as they approach and jump over an obstacle. METHODS: The movements of the centre of mass of the body (COM), the external muscular work (W ext) and the leg-spring stiffness (k leg) were evaluated in athletes approaching an obstacle at 18 km h(-1), from the ground reaction forces (measured by force-platforms) and the orientation of the lower-limb segments (measured by camera). These results were compared to those obtained in RR. RESULTS: Two steps before the obstacle, k leg is reduced by 10-20 %; so, the COM is lowered and accelerated forward. During the step preceding the obstacle, k leg is increased by 40-60 %; so the COM is raised and accelerated upwards, whereas its forward velocity is reduced. This change in the running pattern is similar to the one observed in RR while leaping an obstacle. However, in TA, the change in stiffness is less pronounced. As a result, the orientation of the velocity vector at the beginning of the aerial phase over the obstacle is more horizontal than in RR, which involves a 10-20 % greater horizontal velocity and a 40-60 % smaller vertical excursion of the COM when crossing the obstacle; subsequently, W ext during contact before the obstacle is 10-20 % less. CONCLUSION: Athletes use the same mechanisms as non-specialists to cross an obstacle. However, athletes adapt the mechanism of jumping to reduce the loss in the velocity of progression when crossing an obstacle.

Leg stiffness and joint stiffness while running to and jumping over an obstacle.

During running, muscles of the lower limb act like a linear spring bouncing on the ground. When approaching an obstacle, the overall stiffness of this leg-spring system (k(leg)) is modified during the two steps preceding the jump to enhance the movement of the center of mass of the body while leaping the obstacle. The aim of the present study is to understand how k(leg) is modified during the running steps preceding the jump. Since k(leg) depends on the joint torsional stiffness and on the leg geometry, we analyzed the changes in these two parameters in eight subjects approaching and leaping a 0.65 m-high barrier at 15 km h(-1). Ground reaction force (F) was measured during 5-6 steps preceding the obstacle using force platform and the lower limb movements were recorded by camera. From these data, the net muscular moment (M(j)), the angular displacement (θ(j)) and the lever arm of F were evaluated at the hip, knee and ankle. At the level of the hip, the M(j)-θ(j) relation shows that muscles are not acting like torsional springs. At the level of the knee and ankle, the M(j)-θ(j) relation shows that muscles are acting like torsional springs: as compared to steady-state running, the torsional stiffness k(j) decreases from ~1/3 two contacts before the obstacle, and increases from ~2/3 during the last contact. These modifications in k(j) reflect in changes in the magnitude of F but also to changes in the leg geometry, i.e. in the lever arms of F.

The mechanics of running while approaching and jumping over an obstacle.

When leaping an obstacle, the runner increases the vertical velocity of his/her centre of mass (COM) at takeoff to augment the amplitude and duration of the aerial phase over it. This study analyses the modification of the bouncing mechanism of running when approaching a barrier. The forces exerted by the feet on the ground are measured by a 13-m-long force platform during the four to nine running steps preceding the jump over a 0.45- to 0.85-m-high barrier, at an approaching speed between 9 and 21 km h(-1). The movements of the COM are evaluated by time-integration of the forces and the stiffness of the bouncing system by computer simulation. The running mechanism is modified during the two steps preceding the barrier. During the contact period, two steps before the barrier, the leg-spring stiffness decreases; consequently, the COM is lowered and accelerated forward. Then during the contact period preceding the obstacle, the leg-spring stiffness increases and the COM is raised and accelerated upwards, whereas its forward velocity is reduced. During this phase, the leg-spring acts like a pole, which stores elastic energy and changes the direction of the velocity vector to release this energy in a vertical direction. At high speeds, this storage-release mechanism of elastic energy is sufficient to provide the energy necessary to leap the obstacle. On the contrary, at low speeds, the amount of elastic energy stored and released in the leg-spring is not sufficient to jump over the obstacle and additional positive muscular work must be done.

Biomechanics of locomotion in Asian elephants.

Elephants are the biggest living terrestrial animal, weighing up to five tons and measuring up to three metres at the withers. These exceptional dimensions provide certain advantages (e.g. the mass-specific energetic cost of locomotion is decreased) but also disadvantages (e.g. forces are proportional to body volume while supportive tissue strength depends on their cross-sectional area, which makes elephants relatively more fragile than smaller animals). In order to understand better how body size affects gait mechanics the movement of the centre of mass (COM) of 34 Asian elephants (Elephas maximus) was studied over their entire speed range of 0.4-5.0 m s(-1) with force platforms. The mass-specific mechanical work required to maintain the movements of the COM per unit distance is approximately 0.2 J kg(-1) m(-1) (about 1/3 of the average of other animals ranging in size from a 35 g kangaroo rat to a 70 kg human). At low speeds this work is reduced by a pendulum-like exchange between the kinetic and potential energies of the COM, with a maximum energy exchange of approximately 60% at 1.4 m s(-1). At high speeds, elephants use a bouncing mechanism with little exchange between kinetic and potential energies of the COM, although without an aerial phase. Elephants increase speed while reducing the vertical oscillation of the COM from about 3 cm to 1 cm.

Effect of an increase in gravity on the power output and the rebound of the body in human running.

The effect of an increase in gravity on the mechanics of running has been studied by using a force platform fixed to the floor of an aircraft undergoing flight profiles, resulting in a simulated gravity of 1.3 g. The power spent to maintain the motion of the centre of mass of the body is approximately 1.3 times greater than on Earth, due to a similar increase of both the power spent against gravity and to sustain the forward speed changes. This indicates that the average vertical displacement per unit distance and the average direction of the push are unchanged. The increase in power is mainly due to an increase in step frequency rather than to an increase in the work done at each step. The increase in step frequency in turn is mainly due to a decreased duration of the effective aerial phase (when the vertical force is less than body weight), rather than an increase in the stiffness of the bouncing system. The maximal speed where step frequency can match the resonant frequency of the bouncing system is increased by approximately 5 km h(-1) at 1.3 g. These results suggest a similar running mechanics at higher gravity, maintained at the expense of greater energy expenditure.