# Control Strategies for Gait Tele-Rehabilitation System Based on Parallel Robotics
## 4 Discussion  
To the best of the authors' knowledge, this paper presents a first attempt to implement control strategies such as SMM and TMM in an SGP-based lower limb robot for tele-rehabilitation. Nevertheless, while the proof-of-concept experiments helped us to evaluate the overall usability of the system, some of its potential limitations became evident. Within the SMM trial, the user needed to maintain his balance, while simultaneously changing his trunk inclination to modulate gait speed. Despite the supplementary action required in this case, user acceptance was immediate, and no specific concerns were raised. The provided control interface was sufficiently intuitive for the user to control gait speed, as illustrated in Figure 5.  
Nonetheless, it may be also noted that other movement intention sources could be used to modulate gait speed, such as myoelectric activity of lower limb muscles, upper limb oscillatory movement, and others. While each of those measures feature specific drawbacks, the strategy employed in this work (i.e., trunk inclination) also present disadvantages. For instance, momentary loss of balance could be wrongly interpreted as  
Figure 5: Experimental data from an SMM trial. Top panel shows in blue the trunk inclination in degrees acquired by a wireless inertial sensor during a 70-s experiment. The red line depicts the simultaneous gait speed in km/h, calculated using Equation (2). Bottom panel depicts the stride execution, i.e., the relative phase of movement of the mobile platform when executing one stride. intention to accelerate or decelerate. While additional signal processing may assist in extracting the volitional movement, we believe further clinical study is required to validate this hypothesis. Furthermore, sensor location in this configuration may be less practical if a harness for body weight support is used. In this case, an alternative is to place the sensor on the neck or another body part. Regarding the TMM trial, the participant that took the role of the patient mentioned that being unaware of future robot movements produced a degree of anxiety. Since we consider it is an undesirable effect in rehabilitation, strategies must be designed to prevent it. One alternative involves a common practice in physiotherapy: verbally explaining each movement before execution. One of the potential limiting features when using TMM is that there may be the need to scale therapist movements for a patient of different body dimensions. Nevertheless, particularly considering that the system operates at a limited speed, one finding from these preliminary tests is that the therapist adapts their movement based on the observed patient movement, i.e., avoiding the requirement of additional technical solutions. Indeed, this particular adaptation is commonly observed in human-robot interaction [21; 22]. The lack of feedback to the therapist of interaction forces involved may be one additional limitation of the proposed framework when using TMM. Indeed, different modalities exist to provide this real-time information to the therapist [23]. While we recognize this issue may limit the range of interventions that may be applied, we also understand visual (and kinematic) feedback alone are sufficient to enable different therapeutic interventions using such a mechatronic system. The use of position control on both control strategies, instead of techniques based on force control, features additional implications. For instance, strategies based on force control would enable a higher degree of adaptability depending on the interaction forces between the user and the platform. Nevertheless, when comparing to exoskeleton-based systems, an end-effector system using position control would be safer to exoskeletons using a similar approach, particularly in cases of spasticity, for instance. Certainly, testing of the tele-rehabilitation feature requires a higher number of experiments from different locations. Intercontinental connection implies that only one country is within the peak hours.