Is human gait initiation program affected by a reduction of the postural basis?

The aim of this study was to access the adaptability of the gait initiation program by imposing before and during gait a posture that partially prevents the backward shift of the center of foot pressure. Six healthy subjects performed normal gait in the control situation (CS) and gait in the absence of heel ground contact in the test situation (TS) on a force platform at three different speed conditions. It is shown that an increase in the duration of the anticipation phase in TS is necessary to create conditions for progression which allow the subjects to reach a gait velocity similar to the one obtained in CS at the end of the anticipatory movements and also at the end of the first step. Modifications of the gait initiation program occur in order to fulfil the performance in terms of gait velocity.

Changes to the gait initiation programme following a running exercise in human subjects.

The question addressed by the present study was whether the programme for gait initiation is fully invariant or can be modified by a prior long duration running exercise. Eight subjects performed a series of gait initiation movements on a force platform in three different speed conditions before and after running on a treadmill (3.3 m/s, 0% slope). Following the running exercise, the first step length and velocity were greater and the double stance phase duration was shorter, while the relationships between the temporal parameters of gait initiation and the velocity of the centre of gravity at the end of the first step were not significantly different. During the anticipation phase, the maximal variation of the ankle joints moment remained significantly correlated with the integrated EMG activity of the Tibialis Anterior after the run. However the displacement of the centre of foot pressure, displayed some post-exercise changes, indicating a modification to the postural part of the gait initiation programme. The greater velocities reached after the running exercise could be explained by a recalibration of gravitational force control, while the invariance of time necessary to reach these velocities showed the rigidity of the initiation gait.

Heel-off perturbation during gait initiation: biomechanical analysis using triaxial accelerometry and a force plate.

This study analyzes the movements of the hips, shoulders and of the body center of gravity before and at heel-off, when step execution begins to initiate gait from an upright posture. The heel-off movement was considered as a dynamic perturbation induced by the stepping movement. The experimental paradigm used for studying this perturbation was the single-step movement, in which the initial posture and voluntary movements are identical to those of gait initiation. Data were collected from accelerometer recordings of the triaxial accelerations at the joints of the upper part of the body, and by calculating the triaxial accelerations of the center of gravity using force plate measurements. The resultant vectors were used to establish and compare the magnitude and direction of the accelerations at different joints, and from them, the roles of the pelvis and the scapular girdles with respect to the objectives of the gait movement.

Estimating the centre of gravity of the body on the basis of the centre of pressure in standing posture.

This study proposes to estimate the horizontal positions of the body's centre of gravity (CoG) in a standing posture, on the basis of the horizontal positions of the centre of pressure (CoP). The latter were measured with a force plate, and using a low-pass filter defined by a mathematical relationship of the relative magnitude of the CoG with respect to the magnitude of the CoP, as a function of the frequency oscillations (Brenière, 1996, Journal of Motor Behaviour 28, 291-298). This relationship was computed from the angular momentum equation applied to the whole body with respect to the CoG using the inverse dynamics approach and force plate recordings, and considering the CoP and CoG oscillations as simple periodic functions. Five subjects were asked to perform voluntary oscillations along medio-lateral and antero-posterior axes, keeping their bodies straight, and without moving their feet. The CoG accelerations measured by the force plate were compared with the CoG accelerations derived from the estimated CoG positions. The average root-mean-square difference between these accelerations was very small, confirming the accuracy of this method. This simplified way to calculate the CoG positions, rarely measured in standing, allows a comparative assessment of motion performance. This method could also be applied to other kinds of movement such as walking.

When and how does steady state gait movement induced from upright posture begin?

The aim of this research was to study when and how the stationary process of gait begins when walking starts from upright posture. The subject initially stood up on a large force plate, then walked. Three conditions of speed (slow, normal, fast) were examined. Five subjects participated in the experiment. A total of 105 trials were performed. The results show that, at the end of the first step, the progression velocity of the center of gravity is not significantly different from the mean progression velocity of gait during the second step of gait and that the time necessary to reach steady state gait from initial posture phase is constant. Furthermore, the frequency of the first step, when compared to published values of the steady state gait frequency, is not significantly different from these frequencies. It can be concluded that the aim of the gait initiation process is to place the subject in steady-state gait within the first step, in an invariant time which is dependent only on the body segment parameters of each subject.

A biomechanical study of balance recovery during the fall forward.

The study deals with the biomechanics of balance recovery of human subjects in a falling forward situation. Eight subjects took part in the experiment. The subject, held in the initial leaning forward position, was released without his knowledge. The instruction was to recover the induced disequilibrium by walking. The biomechanical analysis shows two phases in the balance recovery. The first phase--preparation phase--is characterized by three events at fixed timing whatever the initial inclination. (i) Dynamic reaction time, showing no significant inter-individual variation (mean value = 90.8 ms). (ii) Braking of the forward fall, between 184 ms and 237.2 ms, depending on the subject. (iii) Beginning of the swing phase--i.e. toe-off instant--between 235.9 ms and 328.3 ms, depending on the subject. The second phase--gait execution phase--is characterized by the duration of the swing phase, the duration of the stance phase, the stride length and execution speed. The durations diminish whereas the stride length and the execution speed increase with respect to the initial inclination. For the same execution speed, the stride length is shorter than in normal walking. It has been concluded that balance recovery following an induced fall forward begins with an invariable preparation process which is followed by an adaptable recovery one.

Why we walk the way we do.

By using inverse dynamics and forceplate recordings, this study established the principle of oscillating systems and the influence of gravity and body parameters on the programming of the gait parameters, step frequency and length. Calculation of the ratio of the amplitude of the center of mass (CM) and the center of foot pressure (CP) oscillations yielded an equation and established a biomechanical constant, the natural body frequency (NBF). NBF appears to be an absolute invariant parameter, specific to human standing posture and gait in terrestrial gravity, which influences the relative positions of CM and CP and whose value separates the frequency bands of standing posture from those for gait. This equation was tested by using the experimental paradigm of stepping in place and then used in calculating the magnitude of CM oscillations during gait. The biomechanical analysis of the experimental observations allows one to establish the relationships between body parameters and gravity and the central programming of locomotor parameters.

Postural requirements and progression velocity in young walkers.

This article describes developmental changes in gait velocity and relates these changes to gait parameters that index postural stability (step width and lateral acceleration) and two components of velocity (cadence and step length). Five children were observed longitudinally over a 2-year period after onset of independent walking. Their range of speed increased threefold in the first 6 months of independent walking and then remained constant. In contrast, step width decreased approximately twofold. Whereas in adults, cadence and step length contribute approximately equally to speed, when infants first begin to walk independently, increase in velocity is due mostly to increased step length. After 5 months of independent walking, the pattern reverses, and increase in velocity is due primarily to increased cadence. The pattern remains constant over the next 18 months. From a developmental point of view, the data lead us to interpret early walking (the first 5 months) as a process of integration of postural constraints into the dynamic necessities of gait movement. A second phase, beginning after 4 to 5 months of independent walking, is considered to be a tuning phase characterized by a more precise adjustment of the gait parameters.

Analysis of the transition from upright stance to steady state locomotion in children with under 200 days of autonomous walking.

The aim of this paper was to study, from a developmental perspective, the transient phase of gait during the period between the standing posture and the achievement of steady state gait, using temporal and biochemical parameters. Eight children who had been walking autonomously for 90 to 200 days were observed. A total of 64 sequences of steps were analyzed. A sequence of steps began with the child standing still and was executed on a large force plate. From the determination of the instantaneous velocity of center of gravity results establish that, unlike adults, progression velocity in children end of the first step, but after two to four steps. The gait initiation process does not depend on the steady state velocity, but results from an initial fall. The duration of the movement up to the end of the first step is independent of the progression velocity but depends only upon the body mass and moment of inertia of the children.

Control of gait initiation.

The initiation of gait from a standing posture by 6 subjects, who took controlled-length steps, was analyzed. Using an inverted pendulum model, we found that the duration of gait initiation was independent of gait velocity. This finding suggests that subjects' biomechanical constants are the determining factors for initiating movement. Both the instantaneous velocity of the center of gravity at the end of the first step (resulting in the propulsive forces measured on the ground) and the steady-state velocity (resulting in the step length and frequency) varied with step length, whereas step frequency did not. But step frequency and progression velocity were linked, for step frequency always increased in parallel with increased progression velocity. We interpret the correlation between velocity and frequency variations to be a peripheral expression of the posturodynamic control of the step parameters by the progression forces.

Are dynamic phenomena prior to stepping essential to walking?

The aim of our research was to examine the function of the pre-gait weight shifts in generating the dynamic forces needed to start walking at different speeds. Five subjects participated in the experiment, and a total of 105 gait initiation movements, executed on a large force plate, for three speed conditions (slow, normal, and fast), were examined. Results, which related to durations of the anticipation and of the step execution phases and to biomechanical parameters (progression velocity of the center of gravity, backward shift of the center of foot pressure, and magnitude of propulsive forces at heel-off time), suggested that dynamic phenomena prior to stepping are essential to walking as far as they contribute to the creation of convenient conditions for progression. The configuration of the support basis prior to stepping limits the progression velocity reached at the end of the first step.

Simulation of gait and gait initiation associated with body oscillating behavior in the gravity environment on the moon, mars and Phobos.

A double-inverted pendulum model of body oscillations in the frontal plane during stepping [Brenière and Ribreau (1998) Biol Cybern 79: 337-345] proposed an equivalent model for studying the body oscillating behavior induced by step frequency in the form of: (1) a kinetic body parameter, the natural body frequency (NBF), which contains gravity and which is invariable for humans, (2) a parametric function of frequency, whose parameter is the NBF, which explicates the amplitude ratio of center of mass to center of foot pressure oscillation, and (3) a function of frequency which simulates the equivalent torque necessary for the control of the head-arms-trunk segment oscillations. Here, this equivalent model is used to simulate the duration of gait initiation, i.e., the duration necessary to initiate and execute the first step of gait in subgravity, as well as to calculate the step frequencies that would impose the same minimum and maximum amplitudes of the oscillating responses of the body center of mass, whatever the gravity value. In particular, this simulation is tested under the subgravity conditions of the Moon, Mars, and Phobos, where gravity is 1/6, 3/8, and 1/1600 times that on the Earth, respectively. More generally, the simulation allows us to establish and discuss the conditions for gait adaptability that result from the biomechanical constraints particular to each gravity system.

How locomotor parameters adapt to gravity and body structure changes during gait development in children.

The principles of direct dynamics and oscillating systems have been used to study the development of gait parameters in children, with respect to their kinetic consequences on the oscillations of body center of mass (CM). In particular the equations established (a) a natural body frequency (NBF), a body parameter specific to oscillating movements which is invariant for adults and decreases with age for children, and (b) the amplitude ratio of CM to center-of-foot pressure (CP) oscillations as a parametric function of the step frequency, whose parameter is the NBF. This function was used to analyze the development of gait locomotors with respect to their kinetic effects on balance in the frontal plane. Five children were examined longitudinally during their first 5 years of independent walking (IW), and two cross-sectional groups between 5 and 7 years of IW were also considered. The results showed a shift toward the low end of step frequency bands as the NBF decreased along with invariances in the amplitudes of CM oscillation in both the frontal and sagittal planes, regardless of age and gait velocity. The biomechanical meaning of the NBF, of its decrease and of postural invariances associated with the decrease of the frequency, are discussed as well as how the programming of locomotor parameters adapts to changes in body structure during gait development.

Adaptation of the gait initiation process for stepping on to a new level using a single step.

During the gait initiation in level walking, the anticipatory postural adjustments (APA) which precede heel off consist of a forward fall of the whole body and their duration depends on the intended gait velocity related to the step length. The present study examines the adaptation of the gait initiation process for stepping on to a new level. Five subjects performed a single step at natural speed in five experimental conditions. The first condition (C1) was a level walking task whereas the other (stair) conditions required stepping on to a new level (from 8 to 32 cm). The horizontal step length was the same under all conditions. Results showed that the center of mass (CM) forward velocity at the end of the APA, and also until foot contact of the leading limb, decreased from C1 to the stair conditions whereas the peak of forward velocity was similar under all conditions. Moreover, the CM forward displacement up to foot contact was smaller in the stair conditions than in C1. These results suggest the use of a sequential mode of control for the organization of the CM forward dynamics during the stair conditions. This adaptation of the gait initiation process for stepping up is examined mainly from the result that the majority of body lift, which occurred only from the beginning of the double-stance phase, involved a larger CM forward translation than in level walking. As the horizontal step length was the same in all conditions, it can be suggested that the CNS had to reduce the CM forward displacement up to foot contact in the stair conditions, in order to take into account the subsequent greater forward translation.

The role of anticipatory postural adjustments and gravity in gait initiation.

This study analyzes the anticipatory postural adjustments which precede heel-off by considering the participation of the gravitational and muscular actions about the ankle joints during the gait initiation process. The resultant moment about the ankle joints and the gravitational moment were calculated using a biomechanical model in five normal subjects for three different speed conditions. The results show that the variations of these two moments are correlated to the velocity at the end of the first step. Nevertheless, a significant variation of the ankle joints moment occurs at the beginning of the anticipatory phase, whereas the gravity effect is still insignificant. These findings show how the successive controls of the muscular actions acting during the anticipatory movement and of the gravity action acting principally during the step execution allow the subject to reach the velocity which has been initially and centrally decided, by the end of the first step.

[How does a child modulate his progression speed during the first 18 months of independent gait?]. / Comment l'enfant module-t-il la progression de sa vitesse durant les premiers dix-huit mois de sa marche autonome?

Our results clearly suggest that during the 18 months following onset of independent walking, toddlers undergo a two-stage development process. Firstly, up to the sixth month of independent walking, there is a striking evolution of all the gait parameters analysed here. The correlation between velocity and length of steps is highly significant. Those data lead to interpret early walking as a process of "integration" of postural constraints to the dynamic necessity of gait movements. In the second stage, velocity becomes highly correlated with foot's frequency. It appears to be more of a turning phase of all the gait components.

Development of postural control of gravity forces in children during the first 5 years of walking.

The aim of this work was to propose developmental indexes relative to the control of balance and gravity forces, using force-plate data, for children in their first 5 years of independent walking. The first part of this paper is devoted to the definition of an index to quantify postural capacity during walking. Based on the assumption that the vertical acceleration of center of mass (CM) reflects the capacity of muscular forces between the head-arms-trunk and the stance leg segments to control the external forces, the value of the CM vertical acceleration at foot contact is proposed as a developmental index of the postural capacity of the child to control gravitational forces. This index was analyzed longitudinally in five children, over the course of eight experimental sessions. The children were examined during their first 5 years of independent walking (for a total of 457 step sequences). The covariation between the CM vertical acceleration at foot contact and the gait velocity was considered as a second index characterizing the development of coordination between the postural and dynamic requirements of body progression. From these indexes it was established that the postural capacity needed just to control balance with the leg muscles was not attained before 4-5 years of independent walking, i.e., at about 5-6 years of age.

A double-inverted pendulum model for studying the adaptability of postural control to frequency during human stepping in place.

In order to analyze the influence of gravity and body characteristics on the control of center of mass (CM) oscillations in stepping in place, equations of motion in oscillating systems were developed using a double-inverted pendulum model which accounts for both the head-arms-trunk (HAT) segment and the two-legged system. The principal goal of this work is to propose an equivalent model which makes use of the usual anthropometric data for the human body, in order to study the ability of postural control to adapt to the step frequency in this particular paradigm of human gait. This model allows the computation of CM-to-CP amplitude ratios, when the center of foot pressure (CP) oscillates, as a parametric function of the stepping in place frequency, whose parameters are gravity and major body characteristics. Motion analysis from a force plate was used to test the model by comparing experimental and simulated values of variations of the CM-to-CP amplitude ratio in the frontal plane versus the frequency. With data from the literature, the model is used to calculate the intersegmental torque which stabilizes the HAT when the Leg segment is subjected to a harmonic torque with an imposed frequency.

[Why do children walk when falling down while adults fall down in walking?]. / Pourquoi les enfants marchent en tombant alors que les adultes tombent en marchant?

Some posturo-dynamical features of early gait have been studied in 5 children a few weeks after onset of independent walking and are analysed in comparison with adult gait. During the single support phases, the vertical acceleration of center of gravity of the child is negative and remains positive only during the double support phases. For the adult, the vertical acceleration of the center of gravity is positive before the time of heel contact. During the single support phases of the independent gait, the child does not have the same postural capacity to maintain his balance with respect to the gravitational forces and control of the vertical position of his center of gravity is different of the adult.

The build-up of anticipatory behaviour. An analysis of the development of gait initiation in children.

This study analyses the anticipatory postural adjustments during the gait initiation process in children aged 2.5, 4, 6 and 8 years. In adults, anticipation during gait initiation includes a shift in the centre of foot pressure (CP) both backwards and towards the stepping foot. Backward displacement and the duration of the anticipation phase covary with the gait progression velocity reached by the subject at the end of the first step. In the present study, the children walked on a force plate that allowed us to calculate the acceleration of the centre of mass and the displacements of the CP. The results showed three main characteristics of the development of anticipatory behaviour: (1) The occurrence of anticipatory displacements of the CP increased progressively with age. Systematic backward anticipation was found for all children except one of the youngest, whereas the lateral displacement was systematically observed later, in the 6-year group; (2) the amplitude of the spatial parameters showed a significant increase with age; (3) contrary to the adult, the amplitude of the backward shift did not covary with the forthcoming velocity in the youngest groups. This covariation became significant at 6 years and remained significant at 8 years. The results showed that even if anticipatory behaviour was present in 2.5-year-old children it is only later that the child is able of more accurate tuning of feedforward control, probably due to better control of the overall postural adjustments.