# Switched Control of Motor Assistance and Functional Electrical Stimulation for Bieps Curls +
## 1 Introduction  
Robot-assisted therapy has been shown to be beneficial for rehabilitation and neurological relearning for people with movement impairments. Specifically, in [1], robot-assisted therapy was used to aid in stroke recovery. More recently in [2], robot-assisted rehabilitation resulted in significant gains in motor impairment and functional recovery of the upper limb of acute stroke patients. The reviews in [3; 4; 5] compare various therapeutic robots that have been developed for the upper extremity. While exercises involving robotic assistance are advantageous for rehabilitation, intensive active involvement by the person (rather than only passive motion) is beneficial for motor recovery [6; 7; 8]. Balancing human and robot contributions often requires patient-specific adaptations of the controller [9]. Many methods have been used to facilitate maximal human contribution, such as challenge-based robots [10], assist-as-needed controllers [11; 12], an adaptive algorithm based on lead/lag performance [13], and an algorithm for altering allowable error and decaying disturbance rejection [14]. Functional electrical stimulation (FES) is another rehabilitation tool that can be used alongside robot-assisted therapy. Used to elicit involuntary muscle contractions, FES is particularly useful for people with neurological disorders. For instance, FES has been found to increase strength in the biceps brachii [15] and motor function on the post-stroke paretic arm [16]; however, for people that have some sensory feedback intact, there is often discomfort at high levels of stimulation. Thus, stimulation oftenmust be limited to levels below that necessary to perform certain exercises with muscle force alone. To compensate, it may be necessary to incorporate motor/robotic assistance. In [17], a combination of robotic assistance and FES resulted in arm mobility improvements in severely impaired stroke survivors. In this paper, we design an upper arm robot-assisted FES rehabilitation device and protocol where the robot only assists when the necessary stimulation input reaches a saturation limit (e.g., due to a predefined comfort level by the participant). This paper focuses on biceps curls since they are a common upper body exercise with a simple, one-degree-of-motion movement. Moreover, repetitive movements, such as biceps curls, are known to be beneficial for people with neurological conditions by improving coordination [3,18]. In [19], FES is used to induce biceps curls and motor assistance is provided intermittently whenever the stimulation reaches a pre-set comfort threshold; however, tracking performance may be further improved by extending bouts of motor assistance, during which stimulation is decreased (a byproduct of motor assistance decreasing the tracking errors), but the muscle is still activated and contracting. In this paper, and in [19,20], FES of the biceps brachii is used as the primary actuator for tracking a desired elbow angle trajectory that represents a biceps curl. Multiple electrodes (i.e., stimulation channels) are placed across the muscle, and the motor contributes when the stimulation threshold is reached; however, here, as in the preliminary works in [20], from which this paper builds upon, the motor will continue to contribute until the stimulation decays to a selected lower stimulation value, which prevents the FES control input from oscillating at the stimulation threshold and causing motor chatter. This novel approach eliminates the potential for motor chatter without the need to activate the motor throughout the entire biceps curl exercise, which may be too much assistance for the patient. Additionally, a strategy is developed to decrease the lower stimulation threshold throughout the biceps curl, such that the motor is active for longer as the person fatigues and/or performance decreases, yielding individualized adjustments.