**Francesca Martelli 1,\*, Juri Taborri 2, Zaccaria Del Prete 1, Eduardo Palermo <sup>1</sup> and Stefano Rossi <sup>2</sup>**


Received: 30 September 2019; Accepted: 11 November 2019; Published: 13 November 2019

**Abstract:** A deep analysis of ankle mechanical properties is a fundamental step in the design of an exoskeleton, especially if it is to be suitable for both adults and children. This study aims at assessing age-related differences of ankle properties using pediAnklebot. To achieve this aim, we enrolled 16 young adults and 10 children in an experimental protocol that consisted of the evaluation of ankle mechanical impedance and kinematic performance. Ankle impedance was measured by imposing stochastic torque perturbations in dorsi-plantarflexion and inversion-eversion directions. Kinematic performance was assessed by asking participants to perform a goal-directed task. Magnitude and anisotropy of impedance were computed using a multiple-input multiple-output system. Kinematic performance was quantified by computing indices of accuracy, smoothness, and timing. Adults showed greater magnitude of ankle impedance in both directions and for all frequencies, while the anisotropy was higher in children. By analyzing kinematics, children performed movements with lower accuracy and higher smoothness, while no differences were found for the duration of the movement. In addition, adults showed a greater ability to stop the movement when hitting the target. These findings can be useful to a proper development of robotic devices, as well as for implementation of specific training programs.

**Keywords:** ankle impedance; kinematic performance; pediAnklebot; robotics; measurements

#### **1. Introduction**

In recent years, advanced technologies have allowed robot-mediated therapy to become a prominent solution for rehabilitation, as an alternative and/or a supporting solution to traditional rehabilitative programs [1,2]. Robotic devices permit intensive, controlled, and tailored rehabilitation, as well as reducing the therapist's burden [3]. Since the ability of locomotion is fundamental to avoid the worsening of the quality of life [4], one of the main challenges in the robotic field is the design and development of robots for ankle rehabilitation [5]; in fact it is well-known that the ankle joint plays essential roles during walking, such as shock absorption, propulsion, lower limb coordination, adaptation to different environments, and maintenance of stability [6]. From this perspective, an appropriate design of robotic devices for the ankle joint is required for: (i) rehabilitating people affected by neuromuscular diseases [7,8]; (ii) restoring athletes after injuries [9]; and, (iii) augmenting human strength and endurance in industrial and military applications [10,11]. Through the aim of ankle robotic device development, a full insight into kinematic performance and dynamic characterization of ankle appears to be mandatory in order to design robots that operate in accordance with human behavior, leading to a stable and effective physical human–robot interaction [12].

As regards the kinematic performance, several experimental protocols have been developed for quantifying kinematic indices using a robotic device. Among the protocols developed for kinematic performance evaluation, goal-directed movements through the use of serious games represent the most commonly adopted approach in clinical settings [13–16]. Generally, goal-directed tasks are used to understand how the central nervous system optimizes kinematic parameters, such as movement accuracy, smoothness, and speed, when a dynamic task is required [14]. These factors can be also considered the most relevant to be pursued for a proper design of a robotic ankle [12]. However, few studies have been conducted to evaluate ankle kinematic performance during goal-directed tasks. Michmizos and Krebs evaluated the relationship between the speed and the accuracy in both dorsi-plantar (DP) and inversion-eversion (IE) movements performed by adults, assessing the possibility to describe this relation with Fitt's law [17]. The same authors, in [18], compared several models of speed profile in ankle pointing movements, finding that the best fitting models were those already used for upper limbs during pointing movements.

By moving to the dynamic characterization, ankle impedance represents one of the main properties to monitor during rehabilitation programs, as it is one of the most important mechanical components involved in lower body stability during locomotion [19], providing fundamental information for designing robotic devices physically interacting with human lower extremities [20]. In addition, it has been already demonstrated that neurological diseases lead to a significant deterioration of ankle impedance, with respect to healthy subjects [21]. The application of dynamic perturbations to the examined anatomical joint and successive analysis of torque vs. angle graphs is currently the most widespread methodology for the measurement of the dynamic joint mechanical impedance [20,22–24]. Different studies have been proposed in the literature for the objective measurement of ankle impedance in adult subjects. More specifically, Lee et al. validated a stochastic methodology for the quantification of ankle impedance, considering dumping, stiffness, and further dynamic aspects [25,26]. The innovative aspect proposed by the authors is related to the feasibility of impedance evaluation in multiple directions, overcoming the limits of the previously proposed approaches [27–29]. Following a similar approach, Dallali et al. evaluated ankle impedance in the external-internal direction by analyzing the lower limb muscle activation and by applying an artificial neural network that achieved accuracy of 85% in the impedance estimation [22]. Conversely, in the literature, a limited number of studies have focused on ankle impedance evaluation in children. Alhusaini et al. [30] assessed ankle impedance by analyzing the responses to imposed movements in dorsi-plantarflexion in children with cerebral palsy (CP), while Martelli and colleagues [23] applied the methodology proposed by Lee [25] to quantify the effects of botulinum toxin on dynamic ankle impedance.

Currently, the design of robotic devices for the rehabilitation of children is an increasingly appealing and challenging field [16,31–34]. However, properly scaling robotic devices designed for adults to match the characteristics of children, remains an existing challenge, and is generally recognized as an important goal to achieve in the robotic field [35]. Considering this aspect, the quantification of age-related differences in terms of kinematic and dynamic performance clearly represents the starting point. However, no studies, to the best of the authors' knowledge, have been conducted for investigating the age-related differences in terms of ankle impedance between adults and children, as well as regarding kinematic performance, such as accuracy and smoothness, during goal-directed movements. Thus, this study aims at providing full insight into the ankle properties' maturation by comparing kinematic and dynamic performance indices in both DP and IE directions related to healthy young adults and healthy children using a robotic device. The outcomes of the study could offer important guidelines for the correct design and development of robotic devices and rehabilitation protocols addressed for adults and children, as well as serving as a starting point for solving the issue related to the scalability of robotic devices.

## **2. Materials and Methods**

#### *2.1. Subjects*

Sixteen healthy adults aged from 22 to 30 years old and ten healthy children aged from 5 to 9 years old were enrolled in the study. The inclusion criteria were: (i) absence of neurological and visual deficits, (ii) physiological range of motion (ROM) for ankle, (iii) adequate anthropometric measures in order to freely move the ankle in the robot workspace, and (iv) right footedness. The dominant leg was established by asking them to kick a ball [36].

Written informed consent was obtained from all subjects. The protocol was compliant with the ethical standards outlined in the Declaration of Helsinki.

#### *2.2. Experimental Setup and Procedure*

All measurements were conducted by means of the pediAnklebot [31]. Subjects were seated in front of a monitor with knee flexed at 45◦. They wore a knee brace fixed to the limb by means of Velcro straps, and a shoe of proper size, firmly tightened to the foot with shoelaces to prevent foot slippage. The main body of the robot was attached to the knee brace and the end-effectors were connected to the bracket attached to the bottom of the shoe. The calf of the subjects leaned against an aluminum support, covered with foam rubber and linked to the chair. The robot was laterally attached to the chair to ensure collected data were free from the weight of the robot and to improve repeatability.

The robot was equipped with two linear encoders and two load cells to acquire displacements and forces of each end-effector at 200 Hz. From the acquired data, rotations and moments of the ankle were obtained as reported in [29,37]. The robot was mainly designed for children, but can be used by adults in the sitting configuration, by changing the dimensions of shoe and knee brace, and using a new ad-hoc developed linkage between the brace and chair. Different anthropometric features across enrolled subjects do not affect moment arm, which only depend on robot design. In particular, torque was computed from the sensors as follows [29]:

$$\mathbf{x}\_{DP} = \begin{pmatrix} F\_{right} + \, ^\circ F\_{left} \end{pmatrix} \mathbf{x}\_{length} \tag{1}$$

$$\mathbf{x}\_{IE} = \begin{pmatrix} F\_{right} - F\_{left} \end{pmatrix} \mathbf{x}\_{widlh} \tag{2}$$

where τ*DP* and τ*IE* are the dorsi-plantar and inversion-eversion net torques at the ankle joint; *Fright* and *Fle f t* are the forces measured by the right and left force sensors; and *xlength* and *xwidth* are the distances between the line of action of the actuator force and the point of attachment between the ankle and robot in the sagittal and the frontal planes, respectively. The experimental setup is shown in Figure 1.

**Figure 1.** Experimental setup: an adult (**a**) and a child (**b**) wearing the robot.

The experimental protocol consisted of two different phases: the first aimed at the evaluation of the ankle mechanical impedance, while the second aimed at the evaluation of ankle kinematic performance in a goal-directed task. Before starting the experimental procedure, the initial and reference positions were set by positioning the foot at 90◦ relative to the shank, for both the experimental phases. In addition, each participant performed a familiarization session that lasted until participants felt familiar with the equipment and the tasks. All subjects performed the entire protocol with the dominant limb.
