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in children with CP

This section is oriented to give an analysis of human gait linked to the preliminary conceptual design of the new robotic platform for gait rehabilitation: CPWalker. The ambition with the new device is to provide novel robot-based therapies to enhance walking function in children with CP.

2.2.1 Biomechanics of human walking

To specify the structure of the new device and prior to start with the the mechanical design of the rehabilitation equipment, it was necessary to understand the biomechanics of human walking as an essential part for developing such robotic device. The analysis of normal gait will served to define the DOFs in which CPWalker robotic platform should provide assistance.

The process of human walking starts with an impulse in the CNS and concludes with the emergence of reaction forces on the ground. A gait cycle is the period of time between a first foot ground contact and the next ground contact by the same foot. Each gait cycle is commonly classified in two main phases: stance phase and swing phase, both represented in Figure 2.3. Focusing the attention on right leg (blue shadow leg on Figure 2.3), the stance phase comprehends the period in which right foot is in contact with the ground (around 60% of the whole cycle), being the swing phase the time in which the right foot is elevated preparing the next heel strike.

Foot prepositioning is the first subtask in the gait cycle (Figure 2.3) [91]. It means preparing the knee extension and hip flexion to support a good weight acceptance. Once the contralateral foot is lifted of the ground, all the weight is supported by a single limb, so it is necessary to maintain the stability during this stance phase. The shift from stance to swing is preceded by the push o↵ subtask, where the ipsilateral foot takes o↵ the ground. In this moment is very important to keep enough distance from the floor executing a proper step height (foot clearance in Figure 2.3). The swing phase ends ensuring that the step length is adequate to start again a new gait cycle.

The sagittal plane is the dominant plane of motion during human walking. The move- ments referred to this plane are flexion (the limb approaches the body) and extension

Table 2.2: Estimated power for human flexion-extension movements during normal walking

Joint Flexion Power [W/kg] Extension Power [W/kg]

Hip 0.93 -0.57

Knee 0.73 -1.12

Ankle 3.21 -0.42

(the limb is away from the body) for all the joints of the lower limbs. Table 2.2shows the estimated power of each joint for human walking in sagittal plane [92].

2.2.2 Conceptual design of CPWalker

The conceptual design of CPWalker aims to solve the limitations found on the current commercial rehabilitation robots. Concretely, CPWalker will integrate in only one plat- form all the advantages collected from commercial rehabilitation equipments for CP, introducing improvements to address the challenges proposed for these robotic devices (see Table 2.3).

To justify the di↵erent fields of Table 2.3, there are some studies [42, 61, 87, 88] that demonstrate promising results of robotic strategies for gait rehabilitation related to improvements on patients’ kinematics, speed and ambulation endurance. On the other hand, the provision of PBWS in robot-based therapies is beneficial in case of children

Table 2.3: Main aspects and principal advantages of current robotic devices for gait rehabilitation in CP

Actuated Central Peripheral Active Task and Guided PBWS Over-ground Nervous Nervous Postural Specific

Movement Training System System Control Training

Innowalk * * Innowalk-Pro * * NF-Walker * * * Lokomat * * * GT-1 RehaStim * * * Walkbot * * * Autoambulator * * * Multi-robot * * CPWalker * * * * * * *

with a severe level of disability (GMFCS III, IV and V) [49,50,93,94]. Nevertheless, these new therapies should not replace over-ground treatments [95]. The possibility of free over-ground displacement in real rehabilitation environments and the inclusion of CNS and PNS into the treatment, encourage more challenging exercises with an important motivating condition and, as a consequence, a proper physical rehabilitation. At the same time, to maintain a correct posture during walking is a very relevant aspect in case of children with CP [96, 97]. The inclusion of di↵erent sensors to improve postural control during robot-based therapy is expected to lead to better treatment results. Finally, the option of implementing strategies focused on specific and selected subtasks of walking has been demonstrated to be a crucial factor in facilitating functional improvements [84,98].

Figure 2.4shows an overview of the preliminary concept of the CPWalker robotic plat- form. This concept looks for improving the users’ physical and cognitive skills by involv- ing the requirements of Table2.3. The device is composed by two main parts: a walker to provide balance and support to the patients during over-ground walking training, and an exoskeleton to guide the joints of their lower limbs allowing flexion and extension movements in sagittal plane. Several sensors were distributed throughout the platform constituting the MHRI for the interaction between the patient and the robot. These sensors were the foundation for the inclusion of CNS and the implementation of task specific training through AAN strategies. The concept of CPWalker (Figure 2.4) intro- duced a new change on the rehabilitation treatments, which was focused on four main pillars: first, the possibility of free over-ground movement with PBWS (not restricted to treadmill training) in rehabilitation environment, which could be an important motivat- ing condition for this population; second, the use of AAN strategies in specific subtasks of walking might optimize the treatment by increasing the active patient’s participation; third, the inclusion of di↵erent sensors to carry out novel strategies as the improvement of postural control of head and trunk during robot-based therapy, was expected to pro- vide progressions of the child’s gait patterns; and finally, the integration of CNS was expected to boost the e↵ects of the therapy.

The new trainer promotes the progression of patients with CP into the rehabilitation therapy, increasing the level of intensity and frequency of the exercises as well as enhanc- ing the motivation and tailoring the therapy to each user. CPWalker is the first trainer with PBWS and active driven gait in over-ground environments, a platform with an interface that corrects the child’s posture while participating in robot-based therapies, and a device which includes PNS and CNS into the treatment through the incorporation of di↵erent technologies [99]. Overall, CPWalker provides the child with a structure that rehabilitates their gait to physiological patterns.

Figure 2.4: Overview of CPWalker concept.

Following sections describe in depth the mechanical design of CPWalker, each active system, their justifications, re-designs and the control architecture of the project.