Robotic-Assisted Leg Stretching Techniques Facilitated by a Powered Exercise Machine and Functional Electrical Stimulation

Flexibility training involving stretching aims to increase the range of motion (ROM) about a joint and mitigate factors that limit mobility. Stretching has been used for improving human performance and preventing injuries. Further, stretching can be used as spasticity treatment in people with spinal cord injury to provide relief from muscle spasms and improve passive ROM. Therapists apply manual stretching to participants; however, providing personalized and repeatable stretching forces is caregiver intensive. This paper describes the methods to apply three stretching techniques and determines the feasibility of implementing their protocols using a powered device and open-loop Functional Electrical Stimulation (FES) patterns. This paper develops a robust kinematic closed-loop controller to emulate the static, activeisolated (AIS), and proprioceptive neuromuscular facilitation (PNF) stretching techniques targeting the hamstring complex in which the human lies in a supine position. The robust electric motor controller rotates the leg from its initial position through a desired ROM using a rigid pivot arm actuated by a Bowden cable. When the leg reaches the target end ROM, FES inputs are applied for AIS and PNF stretching. Experiments were safely conducted in two able-bodied individuals to demonstrate the device's feasibility of implementing each stretching technique on the hamstring complex. A Lyapunov stability analysis ensures exponential tracking of the motor controller.