Robot-Assisted Rehabilitation of Ankle Plantar Flexors Spasticity: A 3-Month Study with Proprioceptive Neuromuscular Facilitation

Abstract

In this paper, we aim to investigate the effect of proprioceptive neuromuscular facilitation (PNF)-based rehabilitation for ankle plantar flexors spasticity by using a Robotic Ankle-foot Rehabilitation System (RARS). A modified robot-assisted system was proposed, and seven poststroke patients with hemiplegic spastic ankles participated in a 3-month robotic PNF training. Their impaired sides were used as the experimental group, while their unimpaired sides as the control group. A robotic intervention for the experimental group started from a 2-min passive stretching to warming-up or relaxing the soleus and gastrocnemius muscles and also ended with the same one. Then a PNF training session including 30 trials was activated between them. The rehabilitation trainings were carried out three times a week as an addition to their regular rehabilitation exercise. Passive range of motion, resistance torque, and stiffness were measured in both ankles before and after the interventions. The changes in Achilles tendon length, walking speed, and lower limb function were also evaluated by the same physician or physiotherapist for each participant. Biomechanical measurements before interventions showed significant difference between the experimental group and the control group due to ankle spasticity. For the control group, there was no significant difference in the 3 months with no robotic intervention. But for the experimental group, passive dorsiflexion range of motion increased (p < 0.01), resistance torque under different dorsiflexion angle levels (0°, 10°, and 20°) decreased (p < 0.05, p < 0.001, and p < 0.001, respectively), and quasi-static stiffness under different dorsiflexion angle levels (0°, 10°, and 20°) also decreased (p < 0.01, p < 0.001, and p < 0.001, respectively). Achilles's tendon length shortened (p < 0.01), while its thickness showed no significant change (p > 0.05). The robotic rehabilitation also improved the muscle strength (p < 0.01) and muscle control performance (p < 0.001). In addition, improvements were observed in clinical and functional measurements, such as Timed Up-and-Go (p < 0.05), normal walking speed (p > 0.05), and fast walking speed (p < 0.05). These results indicated that the PNF-based robotic intervention could significantly alleviate lower limb spasticity and improve the motor function in chronic stroke participant. The robotic system could potentially be used as an effective tool in poststroke rehabilitation training.

Keywords: ankle spasticity; plantar flexors; proprioceptive neuromuscular facilitation; robot-assisted rehabilitation; stroke.

Figure 1

Overview of the proposed PKU-RARS. It consists of an adjustable seat, a sliding platform, a leg support, a robotic footplate, an actuator, and a control cabinet, two handheld emergency switches, and a human–machine interface (HMI). The blue arrows represent the passive adjustable degrees of freedom, while the red arrow represents the active movement of the ankle joint.

Figure 2

The overall structure of the proposed PKU-RARS. It is functionally divided into two parts: the graphical user interface and the hardware.

Figure 3

The control framework of the proposed PKU-RARS. It is a hierarchical architecture comprising a low-level control layer, which implements a basic torque, velocity, and position closed-loop control, and a high-level control layer, which outputs control signal to low-level controller and performs the execution of the whole specific rehabilitation protocol.

Figure 4

The customized graphical user interface developed in the Labview environment.

Figure 5

The protocol of a PNF training session. It mainly involves three steps, namely, preparation, initialization, and stretching. (a) Measurement of ankle height D0 and rear foot length D1; (b) attach foot to the footplate and strap them; (c and f) neutral position (θ_a = 0); muscle relaxation; (d) maximum dosiflexion position; muscle relaxation; measure of EMG baseline (default: 10 s); (e) maximum dosiflexion position; muscle contraction; measure of EMG MVC (default: 10 s); (g and i) neutral position (θ_a = 0); rest break (default: 10 s); muscle relaxation; (h) maximum dosiflexion position; muscle contraction; tracking (default: 15 s); *A stretching trial: g (rest) → h (tracking) → i (rest) (default: 30 trials).

Figure 6

Passive properties of ankle joint. The left figure: hysteresis loop. Joint angle and resistance torque are measured in the passive movement (three times). Each hysteresis loop is divided into 2 limbs, the ascending limb for dorsiflexion direction movement and the descending limb for plantar flexion direction movement. The right figure: the averaged torque-angle curve (the shade denotes the SD). It is used to calculate resistance torque (RT) and the quasi-static stiffness (QS) under three specified joint angle (0°, 10°, and 20°). QS is calculated as the slope of the regression curve to fit 11 data points [5 points before and after (red point)] around a specific joint angle.

Figure 7

Ultrasonic measurement of AT. (A) The ankle is held at the natural position (90° between the leg and the foot) using a fixed apparatus. (B) The length of the AT is defined from the muscle–tendon junction (MTJ) (Point A) to its insertion into the calcaneus notch (Point C). The thickness of the AT is defined the distance between two points (D and E).

Figure 8

Resistance torque under different joint angle level. Exp- and Con- represent experimental group and control group, respectively. -Pre and -Post represent before and after the 3-month interventions (* stands for a significance level, and ^# represents no significant difference).

Figure 9

Quasi-static stiffness under different joint angle level. Exp- and Con- represent experimental group and control group, respectively. -Pre and -Post represent before and after the 3-month interventions (* stands for a significance level, and ^# represents no significant difference).

Figure 10

Active properties of ankle joint. (A) Joint of maximum voluntary contraction (TMVC); (B) train score during PNF stretching (TS). Exp- and Con- represent experimental group and control group, respectively. -Pre and -Post represent before and after the 3-month interventions (* stands for a significance level, and ^# represents no significant difference).

Figure 11

Achilles’s tendon properties. (A) The length of Achilles’s tendon; (B) the thickness of Achilles’s tendon. Exp- and Con- represent experimental group and control group, respectively. -Pre and -Post represent before and after the 3-month interventions (* stands for a significance level, and ^# represents no significant difference).

Figure 12

Clinical and functional evaluation. (A) Time Up-and-go (TUG); (B) normal walking speed (NWS); (C) fast walking speed (FWS). Pre and Post represent before and after the 3-month interventions (* stands for a significance level, and ^# represents no significant difference).