Ankle rehabilitation robot training for stroke patients with foot drop: Optimizing intensity and frequency

1Introduction

Stroke is the third leading cause of morbidity and mortality worldwide, and approximately 80% of stroke survivors suffer from varying degrees of limb movement disorders after a stroke occur (Brewer et al., 2013). Foot drop is a common lower extremity sequela of stroke and at least 20% to 30% of post-stroke patients have foot drop (Peishun et al., 2021). Patients with stroke are unable to dorsiflex or have inadequate dorsiflexion due to triceps spasm and ankle dorsiflexion muscle weakness, resulting in a “circling gait” when walking (Fernandez-Gonzalez et al., 2016; Nikamp et al., 2018). The compensatory gait pattern affects balance and walking efficiency, and increases the risk of falls, thus restricting their participation in many activities of daily life and in the community (de Haart et al., 2004; Mao et al., 2022). Therefore, it is crucial to discover effective treatments for foot drop that facilitate the recovery of lower limb functions in individuals with hemiplegia, thereby enhancing their quality of life and facilitating reintegration into family and society.

In traditional ankle exercises, physiotherapists manually manipulate the affected ankle to perform internal/external rotation, dorsiflexion/plant-arflexion, and inversion/eversion movements during ankle rehabilitation (Park et al., 2019). The reliance on manual training methods heavily depends on the hands-on skills and experience of the therapists. This approach presents challenges in providing targeted, repeated, and intense rehabilitation training, as well as inhibiting real-time monitoring of the effects of rehabilitation treatment (Hussain et al., 2021). Ankle rehabilitation robots represent a novel intelligent intervention for enhancing post-stroke ankle dysfunction. This advanced approach offers personalized, high-intensity, multi-modal rehabilitation training, which is efficient and time-saving. Extensive clinical studies have been conducted to investigate its effectiveness in neurological rehabilitation (Yoo et al., 2018; Alvarez-Perez et al., 2020). Wearable robotic orthoses/exoskeletons and end-effector-based parallel robotic mechanisms are two major research directions in the field of ankle rehabilitation assistance (Jamwal et al., 2015; Hussain et al., 2016). Several clinical trials have consistently shown the potential of ankle rehabilitation robots in improving foot drop (Renfrew et al., 2019; Zhai et al., 2021). However, to date, there is a lack of research exploring the optimal training protocols for ankle robot-assisted training, making it challenging for patients to receive precise treatment during their recovery.

This study aims to compare four different training methods with varying frequencies and intensities, utilizing a parallel structured robotic system. The primary objective is to investigate the optimal approach for quantitative ankle motion training, thereby providing guidelines for robotic rehabilitation training in the clinical setting.

2Methods

2.1Participants

This was a single-centre, single-blind, prospective, randomized controlled trial with a two-by-two factorial design (Figs. 1 and 2). A total of 36 post-stroke foot drop subjects who were admitted to the Inpatient Rehabilitation Center of Shenzhen Second People’s Hospital between December 2020 and October 2021 were recruited. All subjects were diagnosed with stroke, as confirmed by computed tomography or magnetic resonance imaging. The main inclusion criteria for subjects were as follows: (1) first episode of unilateral stroke with hemiparesis, (2) the duration of disease ranged from 2 weeks to 12 months, (3) age between 18 to 75 years, (4) adequate cognitive function to follow simple verbal instructions, and (5) presence of voluntary motor ability on the healthy side. The exclusion criteria were (1) limb dysfunction due to other causes than stroke, (2) unable to cooperate with training or poor compliance with treatment, (3) fractures, infections or active bleeding in the affected ankle joint, and (4) a fixed contracture at the affected ankle. All subjects provided written informed consent.

Fig. 1

2×2 factorial trial design.

Fig. 2

Flow diagram of the study.

2.2Ankle intelligent rehabilitation robot

The Ankle Intelligent Rehabilitation Robot (Guangdong Minkai Medical Robotics Co., Ltd., Zhuhai, China) is a parallel robot configuration-based ankle robot for rehabilitation in the seated position (Liu et al., 2018). The device consists of a robot body, a chair, a control cabinet and a display system. The robot body has two symmetrical laminated platforms which can provide assistance for patients to exercise all three degrees of freedom (DOFs) including internal/external rotation, dorsiflexion/plantarflexion, and inversion/eversion movements to the ankle-foot complex (Fig. 3). Patients sit in the chair and their feet are placed on the platforms which allow them to use their able ankle joint to teach the affected ankle joint to perform ankle exercises under the guidance of a physical therapist. The included virtual reality software provides interactive game training with audio-visual feedback.

Fig. 3

a Ankle intelligent rehabilitation robot, b Robot body.

3Results

A total of 45 stroke patients were screened for eligibility and 36 were included in our study. All subjects completed the study and no serious adverse event or important harm was reported (Fig. 2).

4Discussion

This study aims to explore the optimal training scheme with Ankle Intelligent Rehabilitation Robot to guide its clinical application. Previous investigations of ankle robots in subjects with post-stroke foot drop primarily focused on robot development and validating their clinical efficacy. To our knowledge, this is the first study to compare the impact of trainings with different frequencies and intensities using an ankle robot in individuals with post-stroke foot drop. When training at the same level of intensity, patients who received 2 sessions/day training had better improvement in dorsiflexion PROM of the affected ankle than those who received 1 session/day. In addition, patients who received 5Nm and 2 sessions/day of robotic training showed more significant improvement in both AROM and FMA-LE compared to the other 3 groups.

A number of previous studies confirmed that passive stretching was effective in expanding the ankle range of motion by reducing the triceps surae muscle spasticity (Zhao et al., 2011; Freitas et al., 2015; Lecharte et al., 2020). Our study further explored the efficacy of passive stretching, utilizing robotic equipment that provided “flexibility training”. Our findings revealed that a higher frequency of flexibility training proved more effective in enhancing ankle dorsiflexion PROM. This can be explained as large repetitions of passive training may be more effective in relieving muscle tension and stiffness, enhancing muscle elasticity and flexibility, and improving neuromuscular coordination, which was more helpful in reducing muscle spasticity and increasing PROM. A study of Thibaut et al. (2018) had similar results in traumatic brain injury patients with disorders of consciousness. They identified a negative correlation between the frequency of stretching and the severity of spasticity. They also found that subjects who stretched more than 4 sessions per week experienced better relief muscle spasticity compared to those who stretched less than 4 sessions. However, the work that has been done on the effect of stretching frequency on muscle spasticity after stroke is scarce. Our findings may provide some insights for the development of training programs to improve spasticity in patients with stroke.

The intensity of resistance, on the other hand, was found not significantly meaningful for improving PROM in this study. This was probably because the low/high intensity in this study was applied in ankle dorsiflexion training, which was active resistance training. The intensity of the resistance mainly focuses on muscle strength and muscle mass gains and is not closely related to the reduction of muscle spasticity (Lasevicius et al., 2018).

Our results also showed that both the frequency and intensity of robotic training had a significant effect on improving ankle dorsiflexion AROM, with a superimposed effect observed between the two factors. Specifically, 2 sessions/day with high intensity training exhibited greater effectiveness. One plausible explanation was that the higher frequency of robotic training strengthened the dorsiflexors, which in turn improved the ankle AROM. A meta-analysis conducted by Grgic et al. (2018) found that increasing the frequency of resistance training could significantly enhance muscle strength gains and this was observed when comparing resistance training programs with different frequencies (1, 2, 3, and ≥4 days per week) in healthy adults. In addition, a study utilizing electromyography (EMG) also confirmed a greater muscular activation in high- versus low-intensity, suggesting that the full spectrum of motor units may not be fully stimulated when training at a lower intensity (Schoenfeld et al., 2014). Furthermore, from a neurological perspective, engaging in high-intensity repetitive training could enhance neuroplasticity and facilitate motor relearning in stroke patients (Langhorne et al., 2009; French et al., 2016). Leech et al. (2017) also discovered a positive correlation between exercise intensity and serum concentration of brain-derived neurotrophic factor (BDNF), which served as a peripheral blood indicator for nerve remodeling. The elevated levels of BDNF suggested that higher intensity rehabilitation treatments could facilitate more effective nerve remodeling. Thus, this superimposed effect of frequency and intensity may have a greater ameliorative effect on ankle dorsiflexion AROM.

The 2 sessions/day with high intensity robotic training was also found effective in improving FMA-LE scores. Ankle dorsiflexion plays a key role in the motor function of lower limbs and affects activities of daily life such as walking. Training with higher frequency and intensity in rehabilitation could lead to better improvements in PROM and AROM of ankle dorsiflexion which allowed for more effective gait training. This contributed to the improvement of lower limb motor function. Similar findings can be found in Forrester et al.’s (2013) study. They applied a seated ankle robot training device for patients with stroke which could successfully improve ankle-foot movement angles and increase walking speeds, thus enhancing lower limb motor function.

There is a growing trend towards incorporating robots into traditional rehabilitation practice. Ankle rehabilitation robots can play an important role due to their distinctive features, including repetitive and safe sensory motor stimulation, precise and effective motor training patterns, and the provision of objective and accurate motor feedback (Zhang et al., 2013; Payedimarri et al., 2022). The robotic device utilized in this study was developed based on the mirror neuron theory. In addition to offering passive activities, a distinctive combined training mode was developed, involving both the healthy and affected side. In this mode, the healthy side drives the affected side to perform repeated active ankle dorsiflexion movements (Liu et al., 2018). This approach enables patients to engage in coordinated and controlled movements of both ankles, facilitating biomechanical balance in the muscle groups surrounding both feet. Furthermore, the incorporation of virtual reality training, utilizing computer games, offers visual and auditory feedback, effectively enhancing patient motivation during the training sessions (Kayabinar et al., 2021). To ensure optimal therapeutic outcomes, it is essential to establish clear protocols guiding its clinical application. Our findings can provide evidence for policymakers when formulating such a protocol. The results are not only applicable to the device but can also be relevant and valuable for the implementation of similar ankle robotic devices in clinical settings.

This study has several limitations. First, the small sample size of participants might limit the results. Second, we only used PROM, AROM and FMA-LE to evaluate the ankle function and lack of measurements for relevant muscle (e.g., tibialis anterior and triceps surae) kinematics and kinetics. The assessment of balance function and gait was also lacking. Third, we have only compared two levels of frequency and intensity of robot training, while other levels have not been investigated. Additionally, the training duration was 20 minutes for all four protocols, and the effect of different training durations was not studied. In future research, we will conduct a multi-centre study containing a larger sample size for further verification. More assessment metrics will be added for a multi-faceted study such as the use of EMG to collect information regarding muscle activation around the ankle joint and the evaluation of balance and gait to assess overall motor function. In addition, the effect of different training durations and levels of frequency and intensity will be compared to explore a more effective robot training programme.

5Conclusion

Utilizing an Ankle Intelligent Rehabilitation Robot, this study explored the optimal solution for quantitative exercise training of stroke patients with foot drop for the needs of clinical rehabilitation. In light of this study, we suggest that the frequency of 2 sessions/day ankle robot training could be used to expand PROM and the frequency of 2 sessions/day and the intensity of 5Nm to train could be used to improve AROM and lower limb motor function. In addition, the findings of this study can serve as valuable references for the clinical application of other ankle joint robotic devices.