Closed-loop controlled functional electrical stimulation (FES) applied to the lower limb muscles can be used as a neuroprosthesis for standing balance in neurologically impaired individuals. The objective of this study was to propose a methodology for designing a proportional-integral-derivative (PID) controller for FES applied to the ankle muscles toward maintaining standing balance for several minutes and in the presence of perturbations. First, a model of the physiological control strategy for standing balance was developed. Second, the parameters of a PID controller that mimicked the physiological balance control strategy were determined to stabilize the human body when modeled as an inverted pendulum. Third, this PID controller was implemented using a custom-made Inverted Pendulum Standing Apparatus that eliminated the effect of visual and vestibular sensory information on voluntary balance control. Using this setup, the individual-specific FES controllers were tested in able-bodied individuals and compared with disrupted voluntary control conditions in four experimental paradigms: (i) quiet-standing; (ii) sudden change of targeted pendulum angle (step response); (iii) balance perturbations that simulate arm movements; and (iv) sudden change of targeted angle of a pendulum with individual-specific body-weight (step response). In paradigms (i) to (iii), a standard 39.5-kg pendulum was used, and 12 subjects were involved. In paradigm (iv) 9 subjects were involved. Across the different experimental paradigms and subjects, the FES-controlled and disrupted voluntarily-controlled pendulum angle showed root mean square errors of <1.2 and 2.3 deg, respectively. The root mean square error (all paradigms), rise time, settle time, and overshoot [paradigms (ii) and (iv)] in FES-controlled balance were significantly smaller or tended to be smaller than those observed with voluntarily-controlled balance, implying improved steady-state and transient responses of FES-controlled balance. At the same time, the FES-controlled balance required similar torque levels (no significant difference) as voluntarily-controlled balance. The implemented PID parameters were to some extent consistent among subjects for standard weight conditions and did not require prolonged individual-specific tuning. The proposed methodology can be used to design FES controllers for closed-loop controlled neuroprostheses for standing balance. Further investigation of the clinical implementation of this approach for neurologically impaired individuals is needed.

Functional electrical stimulation (FES), applied to paralyzed or paretic muscles, can facilitate arm-free standing in patients with neurological disorders. At the same time, it has been shown that the able-bodied central nervous system regulates standing balance using a control strategy similar to proportional-integral-derivative (PID) control. The objective of this study was to investigate the capability of a PID controller for regulating an FES system applied to the ankle flexors for maintaining standing balance. We compared the performance of this controller to voluntary balance control of able-bodied individuals with compromised visual, vestibular and proprioceptive senses. According to our experimental results, the proposed PID control strategy outperformed the disrupted voluntary control of the ankle flexors while it did not require larger control efforts, in terms of the ankle torque. Therefore, this control strategy could be considered in for use in neurological patients, including spinal cord injured individuals, after further validations in the future.

Modeling the muscle response to functional electrical stimulation (FES) is an essential step in the design of closed-loop controlled neuroprostheses. This study was aimed at identifying the dynamic response of ankle plantar-flexors to FES during quiet standing. Thirteen healthy subjects stood in a standing frame that locked the knee and hip joints. The ankle plantar-flexors were stimulated bilaterally through surface electrodes and the generated ankle torque was measured. The pulse amplitude was sinusoidally modulated at five different frequencies. The pulse amplitude and the measured ankle torque fitted by a sine function were considered as input and output, respectively. First-order and critically-damped second-order linear models were fitted to the experimental data. Both models fitted similarly well to the experimental data. The coefficient of variation of the time constant among subjects was smaller in the case of the second-order model compared to the first-order model (18.1% vs. 79.9%, p < 0.001). We concluded that the critically-damped second-order model more consistently described the relationship between isometric ankle torque and surface FES pulse amplitude, which was applied to the ankle plantar-flexors during quiet standing.

Considerable demand exists for a device to facilitate hands-free, stable stance in individuals with neurological disorders such as spinal cord injury (SCI) and stroke. In this regard, applying functional electrical stimulation (FES) to muscles of the lower limbs in closed loop has shown promise. In particular, it has been suggested that a PID control strategy could offer functional benefits to stability by mimicking the neurological control strategy employed in able-bodied stance. In this proof of concept study, we tested this assertion by examining the potential of a PID control strategy with gravity compensation to effectively maintain balance during quiet stance by regulating FES-induced contractions of the ankle plantarflexors and dorsiflexors in able-bodied individuals. A novel Inverted Pendulum Standing Apparatus (IPSA) was employed to simulate quiet stance whilst minimizing the voluntary control of able-bodied subjects. Quiet and perturbed standing trials were performed in 3 able-bodied subjects. Performance metrics including those pertaining to stability during quiet stance (root mean square difference), perturbation rejection capabilities (settling time, peak deviation), and ability to transition from an offset initial position (settling time), were examined. For all 3 subjects and for all of the metrics examined, our results showed that the proposed closed-loop controlled FES system improved performance in comparison to voluntary control. These results indicate that the PID plus gravity control strategy used in this study offers meaningful benefits over voluntary control in terms of standing stability. Thus, the controller could potentially be applied to the problem of improving or restoring standing ability in some neurologic patient populations.