While there are a number of prospective studies evaluating powered exoskeletons in SCI patients, to date, not a single well-designed, randomized clinical trial has been published. However, there is evidence for beneficial effects of over-ground exoskeleton therapy on walking function post-intervention from a meta-analysis on non-randomized, uncontrolled studies. Functional electrical stimulation (FES), on the other hand, is a common and established method for the rehabilitation of persons with SCI and has been demonstrated to be beneficial in, e.g., improving muscle force, power output and endurance.

Combining FES and overground robotic therapy within the same therapy session could potentially merge and potentiate the effects of each separate treatment, making it a very powerful and efficient therapy method. Up to date, however, comparative studies evaluating benefits of this combined approach (i.e., powered exoskeleton and FES) to robotic therapy without FES are missing.

Paraplegia is a serious event that leads to a complete or partial loss of motor, sensory and vegetative functions. Regaining of gait, balance and mobility are important priorities for persons with a spinal cord injury (SCI). In the last decade the technological development of exoskeletons allowed persons with SCI getting closer to their desired goal. Wearable robotic exoskeletons are motorized orthoses that facilitate untethered standing and walking over ground. Supporting multiple step repetitions while having full weight bearing on the body, these devices represent a task-specific and -oriented training approach for rehabilitation of gait function after SCI. However, in cases where rehabilitation of gait function is not the aim, the need to target secondary health problems associated with SCI like pain, spasticity, bowel and bladder function can still be a rationale for engaging in exoskeleton training.

Another well-established technique for the treatment of such secondary health problems is functional electrical stimulation (FES). FES is a common and established method for the rehabilitation of persons with spinal cord injury. Several studies have documented positive effects of FES like, e.g., avoiding disuse and denervation atrophy, improving muscle force, power output and endurance, changing muscle fibre type, increasing cross sectional area of muscles, increasing muscle mass, activation of nerve sprouting, motor learning and reducing spasticity. In addition, FES has been shown to improve bladder, bowel and sexual function, cardiovascular fitness (by increasing aerobic capacity), reduce body fat mass and prevent and treat pressure ulcers by increasing muscular blood flow. Moreover, FES treatment has also been shown to have an impact on body function by improving lower limb function as well as trunk stability and function.

The power elicited by the muscle through electrical stimulation can be used for locomotion. To do so, undesired limb motion is often restricted by passive orthoses or pedals in order to efficiently use the muscle contraction from the user to safely provide the power for forward propulsion. The usefulness of such systems, however, is often limited due to the rapid initiation of muscle fatigue. This is one reason (amongst others) why hybrid FES-robotic solutions have been developed, which supplement the power produced by electrical stimulation with motorized assistance. This approach reduces the power that needs to be produced by the muscles, allowing for FES application for longer training sessions before fatigue occurs. By doing so, such hybrid powered exoskeletons offer the physiological health benefits similar to FES cycling, while simultaneously enhancing the user's mobility. The addition of FES to a powered exoskeleton also synergistically reduces the motor torques of the device, reducing battery drain and therefore increasing the maximum range of the exoskeleton.

While it sounds perfectly reasonable, from a technical and physiological perspective, to combine powered exoskeletons and FES to such hybrid bionic systems, there is only anecdotal evidence for their clinical usefulness and efficacy in patients with SCI. Here the investigators propose a randomized controlled trial investigating the effect of the combined application of the EksoNR powered exoskeleton (Ekso Bionics, Richmond, CA, USA) and FES (FES RehaMove2, Hasomed, Magdeburg, Germany) compared to Ekso therapy alone on functional outcomes and secondary health parameters.

Inclusion Criteria:

• chronic, incomplete SCI (> 1 year, AIS B-D)
• traumatic or non-traumatic lesion
• capacity to stand up and perform a 10MWT with or without medical aids
• partially wheelchair dependent
• intact lower motoneuron on the segmental innervation level of M. glutaeus maximus, Mm. ischiocrurales, M. tibialis anterior and M. quadriceps (to guarantee the stimulability with FES)

Exclusion Criteria:

• Exoskeleton device related contraindications: > 100 kg body weight; Body height: < 155 cm or > 190 cm; pelvic width: > 46 cm
• orthopedic limitations (acute fractures of the lower limb)
• contractures
• heterotrophic ossification
• spasticity (modified Ashworth Scale >3)
• skin injuries of the lower limbs in areas where the skin has contact with the exoskeleton
• Unstable circulation (unable to stand for at least 10 minutes)
• acute deep vein thrombosis
• pregnancy (tested in women of childbearing age (15 - 49 years))

Assessors will be blinded to treatment allocation and study participants will be instructed not to talk to assessors about their treatment within the study.