The authors would like to thank the ANRT and the LADAPT.

The protocol described below follows the guideline of the human research ethics committee of the Universit&#233; Paris-Saclay. Participants approved and signed a consent form.
1. Participants
<ol>
	<li>Include at least 15 healthy young adult participants in the experiment (aged 20 to 40 years old). 
	NOTE: This recommended number of subjects corresponds to what is classically considered in the literature on GI.</li>
	<li>Exclude participants with walking aids, visual, h...

The authors have no competing interests.

The goal of this paper was to provide scholars, clinicians, and higher education students information on the method (the &#34;global&#34; method) used in our laboratory to investigate the biomechanical organization of gait initiation (GI). Critical steps of the protocol, limitations of the method, and alternative methods and applications are discussed below.
A critical step in the protocol is the detection of the timing events of GI (i.e., APA onset, swing heel-off and toe-off, and rear foot-o...

Gait initiation (GI), the transient phase between orthograde posture and steady-state locomotion, is a functional task and an experimental paradigm that is classically used in the literature to investigate postural control during a complex motor task requiring simultaneous whole-body propulsion and stability1. Patients with neurological conditions, such as Parkinson&#39;s disease2, stroke3, progressive supranuclear palsy4, and &#34;higher level gait disorders&#34;5, are known to have difficulty initiating gait, which exposes them to an increased risk of falling. It is therefore important for both basic and clinical sciences to develop concepts and methods to gain insight into the postural control mechanisms in play during gait initiation, to gain scientific knowledge and a better understanding of the pathophysiology of gait and balance disorders and be able to remediate them through adequate interventions.
The concept of biomechanical organization of gait initiation is described below, and the classical method designed to investigate this organization is detailed in the protocol section. GI can be subdivided into three successive phases: the &#34;anticipatory postural adjustments&#34; (APA) phase corresponding to the dynamic phenomena occurring in the whole body before swing heel-off, the &#34;unloading&#34; phase (between swing heel-off and toe-off), and the &#34;swing&#34; phase that ends at the time of swing foot contacting the support surface. This classical subdivision of the GI process originates from the pioneering studies of Belenkii et al.6 and others7,8, focusing on the coordination between posture and movement during voluntary arm raising to horizontal in the erect posture. In this paradigm, the body segments that are directly involved in the arm raising correspond to the &#34;focal&#34; chain, while the body segments that are interposed between the proximal part of the focal chain and the support surface correspond to the &#34;postural&#34; chain9. These authors reported that raising the arm was systematically preceded by dynamic and electromyographical phenomena in the postural chain, which they called &#34;anticipatory postural adjustments&#34;. For GI, swing heel-off (or swing toe-off, depending on the authors) is considered as the onset of gait movement10. Consequently, the dynamic phenomena occurring before this instant correspond to APA, and the swing limb is considered to be a component of the focal chain11. This statement is in agreement with the classical conception of movement biomechanical organization, according to which any motor act must involve a focal and a postural component12,13.
From a biomechanical point of view, APA associated with GI manifests as a backward and mediolateral (swing leg side-oriented) displacement of the center of pressure, which acts to propel the center of gravity in the opposite direction - forward and toward the stance leg side. The larger the anticipatory backward center of pressure displacement, the higher the motor performance in terms of the forward center of gravity velocity at foot contact10,14. In addition, by propelling the center of gravity toward the stance leg side, APA contribute to maintain mediolateral stability during the swing phase of GI1,15,16,17. The current literature stresses that alteration in this anticipatory control of stability is a major source of falls in the elderly1. Stability during GI has been quantified in the literature with an adaptation of the &#34;margin of stability&#34;18, a quantity that takes into account both the velocity and the position of the center of gravity within the base of support. In addition to the development of APA, the fall of the center of gravity during the swing phase of GI under the effect of gravity has been reported to be actively braked by the triceps surae of the stance leg. This active braking facilitates stability maintenance after foot contact, allowing a smooth foot landing on the support surface4.
The goal of this paper is to provide scholars, clinicians, and higher education students information on the material and method developed in our laboratory to investigate the postural organization of GI via a biomechanical approach. This &#34;global&#34; method (which can also be assimilated to a &#34;kinetic&#34; method for the reasons detailed below) was initiated by Breni&#232;re and collaborators10,19. It is based on the direct principle of mechanics to calculate both the acceleration of the center of gravity, as well as the instantaneous positions of the center of pressure. Each of these points is a global expression specific to the movement.
One is the instantaneous expression of the movements of all body segments related to the purpose of the movement (the center of gravity; e.g., the progression velocity of the body during GI); the other (the center of pressure) is the expression of the support conditions necessary to reach this objective. The instantaneous positions of these two points reflect the posturo-dynamic conditions to be satisfied for gait initiation. The force platform is the appropriate instrument for this model because it allows the direct measurement of the external forces and moments acting at the supporting surface during movement. It also allows the performance of natural movements and requires no special preparation.
Many factors are known to influence the postural organization of GI, including biomechanical, (neuro)physiological, psychological, environmental, and cognitive factors1,20. This paper focuses on the influence of two factors - velocity of GI and temporal pressure - and provides typical values obtained in healthy young adults.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Force platform(s)</td>
			<td>AMTI</td>
			<td></td>
			<td>One large [120 cm x 60 cm] or two small [60 cm x 40 cm] force platform(s)</td>
		</tr>
		<tr>
			<td>Python or Matlab</td>
			<td>Python or MathWorks</td>
			<td></td>
			<td>Programming language for the computation of experimental variables</td>
		</tr>
		<tr>
			<td>Qualisys track manage</td>
			<td>Qualisys</td>
			<td></td>
			<td>Software for the synchronization of the force platform(s), the recording and the on-line visualization of raw biomechanical traces (3D forces and moments)</td>
		</tr>
		<tr>
			<td>Visual3D</td>
			<td>C-Motion Inc</td>
			<td></td>
			<td>Software for the processing of raw biomechanical traces (low-pass filtering)</td>
		</tr>
	</tbody>
</table>

postural organization of gait initiation for biomechanical analysis using force platform recordings

Gait initiation (GI), the transient phase between orthograde posture and steady-state locomotion, is a functional task and an experimental paradigm that is classically used in the literature to obtain insight into the basic postural mechanisms underlying body motion and balance control. Investigating GI has also contributed to a better understanding of the physiopathology of postural disorders in elderly and neurological participants (e.g., patients with Parkinson&#39;s disease). As such, it is recognized to have important clinical implications, especially in terms of fall prevention.
This paper aims to provide scholars, clinicians, and higher education students information on the material and method developed to investigate GI postural organization via a biomechanical approach. The method is based on force platform recordings and the direct principle of mechanics to compute the kinematics of the center of gravity and center of pressure. The interaction between these two virtual points is a key element in this method since it determines the conditions of stability and whole-body progression. The protocol involves the participant initially standing immobile in an upright posture and starting to walk until the end of an at least 5 m track.
It is recommended to vary the GI velocity (slow, spontaneous, fast) and the level of temporal pressure - gait may be initiated as soon as possible after the deliverance of a departure signal (high level of temporal pressure) or when the participant feels ready (low level of temporal pressure). Biomechanical parameters obtained with this method (e.g., duration and amplitude of anticipatory postural adjustments, step length/width, performance, and stability) are defined, and their computation method is detailed. In addition, typical values obtained in healthy young adults are provided. Finally, critical steps, limitations, and significance of the method with respect to the alternative method (motion capture system) are discussed.

Description of representative biomechanical time plots obtained from the force platform during gait initiation 
Whatever the level of temporal pressure or the instruction on GI velocity, swing heel-off is systematically preceded by APA. These APA can be characterized by a backward and swing leg side shift of the center of pressure (Figure 2). This anticipatory center of pressure shift promotes the acceleration of the center of gravity in the opposite direction (i.e., fo...

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Postural Organization of Gait Initiation for Biomechanical Analysis Using Force Platform Recordings

This paper describes the material and method developed to investigate the postural organization of gait initiation. The method is based on force platform recordings and on the direct principle of mechanics to compute center of gravity and center of pressure kinematics.

Gait initiation (GI), the transient phase between orthograde posture and steady-state locomotion, is a functional task and an experimental paradigm that ...

This method make it possible to obtain anticipatory postural adjustment, or APA. APA reflects the functioning of the central level system. It therefore make it possible to insert the question of what the APA, the MOS, and the BI look like.The main advantage of this technique is precision. Moreover, it is a technique that has the possibility of being adapted, and less tiring for patient who have motor fixation, for example. The implication of this technique can be extended to all working pathology.For Parkinson disease, it is especially interesting because it could help to find the mechanical marker for providing an earlier diagnosis or detecting subclinical balance and gait disorder. This method is applicable in certain sport, such as running start or sport with gesture with a still position, like shooting basketball or baseball. I think a person who have never practiced this method can also perform well.You just have to be rigorous. To begin, ask the participants to stand barefoot and immobile on a force platform in their natural upright posture. With the arms hanging loosely against their sides and their gaze directed to a target at eye level at least five meters away.Determine the participant's preferential starting leg by pushing lightly against their back while in the initial posture with their eyes closed to provoke a step forward. Explain to the participants that the tasks they are to perform is to initiate gait from the standing posture with the preferred leg. To continue walking to the end of the track and then return quietly to the initial standing posture.Explain that gait is to be initiated following two successive signals, a preparatory signal and a departure signal. Then explain the instructions on velocity and temporal pressure. Trigger data acquisition from the force platform and deliver the first or preparatory signal to the participants.Instruct them to stand immobile and avoid anticipating gait initiation at this first signal. Ensure that the participants are visually immobile. Check the immobility online with the time plots of the antero-posterior or medial lateral center of pressure displacement.Then deliver the second or departure signal. Vary the conditions of temporal pressure imposed on gait initiation, and then vary the conditions of gait initiation velocity. Instruct the participants to perform a series of 10 successive trials in each experimental condition and impose a rest of at least two minutes between successive conditions to avoid the effects of fatigue.In each condition, allow the participants to perform two familiarization trials before the recordings. Finally, stop the data acquisition once the participant has left the force platform. The biomechanical time plots obtained from the force platform during gait initiation and selected spacial temporal variables are shown here.Gait was initiated quickly in a reaction time condition. The acceleration of the center of gravity and the velocity of the center of gravity along the antero-posterior, medial lateral and vertical directions are depicted here. The displacement of the center of pressure along the antero-posterior and medial lateral directions is also depicted in this figure.The onset of anticipatory postural adjustments, or APA, along the media lateral and antero-posterior directions, time of swing heel off, time of swing toe off, time of swing foot contact and time of rear foot off are shown here. The figure also depicts the representative values of different temporal variables like time windows for APA, unloading phase and swing phase of gait initiation. The representative values of different spatial variables like antero-posterior velocity of the center of gravity at foot off and foot contact.The maximal anticipatory center of pressure displacement, step length, step width, peak downward center of gravity velocity and vertical center of gravity velocity at swing foot contact time are shown here. The most important part of the foot occur is to ensure that the participant are visibly immobile in order to have a good baseline. Beside it is also important that the participant perform two formalization to have before the recording.This will allow us to have a really better trial during the passages. So kinematic method can be used to add data during stationary working. I think might interest some researchers and think how they can use this technique in the field, from engineering to clinical replication.

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2.3K visualizações.  Centre de Soins de Suite et de Réadaptation. Este método permite obter o ajuste postural antecipatório, ou APA. A APA reflete o funcionamento do sistema de nível central. Portanto, torna possível inserir a questão de como é o APA, o MOS e o BI.A principal vantagem desta técnica é a precisão. Além disso, é uma técnica que tem a possibilidade de ser adaptada, e menos cansativa para pacientes que possuem fixação motora, por exemplo. A implicação desta técnica pode ser estendida a todas as patologias de trabalho....