Recruitment
The recruitment rate reached in this preliminary study (28.6%) was acceptable although it remains relatively low considering the number of potential participants (N = 49) who initially expressed their interest in participating into the proposed study and completed the different steps of the recruitment process. Moreover, considering that the study was conducted in a rehabilitation center hosting an ultra-specialized SCI rehabilitation program servicing the western part of the Province of Quebec, the fact that numerous strategies were implemented to overcome potential barriers (e.g., a dedicated research professional in charge of the recruitment, multiple recruitment strategies implemented, telephone pre-screening interview to minimize the number of visits, free parking, free training sessions), and that all participants were allocated to the locomotor training program with the robotic exoskeleton, a higher recruitment rate was anticipated (i.e., ≥50%). Nonetheless, this recruitment rate is 1.7 times greater than the one reported in other feasibility studies investigating locomotor training programs with a robotic exoskeleton in individuals with complete or incomplete SCI in England (17%) [15] and Germany (12%) [18]. Only one recent multi-center study investigating a single training session with a self-stabilizing robotic exoskeleton in individuals with SCI has reached a recruitment rate near 50% (i.e., 20 participants recruited among 46 screened for eligibility) [17]. Nonetheless, the recruitment rate of the present study compares relatively well to the rates reached in other feasibility studies investigating various task-specific gait-training programs offered to relatively homogeneous samples of individuals with neurological impairments (e.g., stroke = 6.7% [20], Parkinson disease = 11% [21]). In the present study, the most important reason (16 out of 35 potential participants = 46%) for not qualifying for the training program was due to musculoskeletal impairments with the leading cause being a reduced passive range of motion at the ankle, knee, or hip. For the same reason, other preliminary studies have excluded up to 77.8% of potential participants (7 out of 9 potential participants excluded) [16]. The second most important reason was linked to time constraints (7 out of 35 potential participants = 20%). Contrary to other preliminary studies [16, 22], transportation did not emerge as a major barrier to the recruitment process nor the dropout rate in the present study since only two potential participants based their decision on this criterion (2 out of 35 potential participants = 5.7%). Taking these reasons together, developing a home-based pre-training program with indirect supervision of a therapist that would target gains in passive range of motion at the lower extremity and progressive standing time prior to initiating the locomotor training program may be warranted.
Different strategies may need consideration to optimize recruitment rate and facilitate attendance in future clinical trials [e.g., offering training sessions during the evening and weekend; offering training sessions away from the main rehabilitation center affiliated with the project (e.g., other rehabilitation centres, community-based physical activity centers, living labs in shopping malls); adjusting the training schedule to best match participants’ availability with a minimum of two training sessions per week; proposing temporary housing alternatives for potential participants living further away who demonstrate an interest in participating]. Last, it is important to highlight that the recruitment rate may have been lower if potential participants had had a chance to be allocated to an alternative experimental group undergoing a different training program or a control group with no training program.
Attendance
The attendance rate reached in the present preliminary study (97.9%) was excellent. The high attendance with respect to the scheduled training sessions confirms the commitment of the participants who engaged into the locomotor training program. This is further supported by the fact that only one participant dropped out of the program following an adverse event (i.e. calcaneus fracture). Hence, a completion rate of 92.9% (n = 13 participants/14 participants) was reached and is greater than the 50% documented in another feasibility study proposing a comparable program [15]. The importance of the familiarization sessions needs to be highlighted since 5 out of the 19 participants (26.3%) decided not to engage into the locomotor training program at that time. Although not formally documented, these familiarization sessions allowed the research team, to some extent, to further screen potential participants who were hesitant to engage into the locomotor training program and potential participants to take an informed decision about their commitment based on a lived experience. Additionally, it supports the relevance of adopting a flexible approach when scheduling the training sessions to accommodate all stakeholders, especially the participants. Since the training sessions involved no or very limited socialization with individuals with similar sensorimotor impairments and functional disabilities (i.e., individualized approach), aside from the interaction with one or two therapists, the commitment of participants to complete the training session and, to some extent, their acceptance of the new technology, especially with regard to its perceived usefulness and ease-of-use, is also established.
Learnability and performance
Individuals with a complete motor SCI demonstrated a capability to quickly learn to ambulate overground with a robotic exoskeleton. Overall, the standing and walking time (including the number of steps/session) progressed at a faster rate during the first half than during the second half of the locomotor training program. Overall, the participants stood and walked at least 30 and 20 min, respectively, at the first training session which is recommended in clinical practice to anticipate beneficial effects among long-term manual wheelchair users with a spinal cord injury [23, 24]. The level of therapist assistance also rapidly decreased over the course of the locomotor training program with most participants requiring no more than minimal assistance after the 8th and 9th training sessions (halfway into the program) and only contact guard or stand-by assistance by the end of the program when walking. Additionally, most participants walked with forearm crutches, with or without the use of the self-controller that allows the user to drive few basic functions of the robotic exoskeleton (e.g., initiation of the first step, continuous walking in ‘prostep’ mode, and stops), by the end of the program. Overall, the learnability trajectory, illustrated for the first time in the present study using measures systematically collected at each training session, compares to some extent with the ones reported using only pre- and post-intervention measures in previous study using a similar or different robotic exoskeletons [3, 16, 18, 22].
The learning process also may have been facilitated by optional distinct auditory feedbacks automatically generated when the participant respectively reached the lateral and forward body weight shift targets required prior to initiating steps, especially early on during the learning stage [25]. Moreover, although not formally assessed in the present study, some participants periodically filmed their performance, especially at the beginning of the study, to facilitate their learning and complement the therapist’s subjective feedback (i.e., visual feedback-induced performance improvement) [26]. Hence, in addition to the adjustability of some exoskeleton parameters (e.g., reducing body shift amplitudes to initiate steps, reducing step height, increasing step length), numerous clinical strategies (e.g., reducing level of human assistance, changing walking aid) are also possible to adjust the level of challenges during overground walking with the robotic exoskeleton as the participant’s level of proficiency improves. Maintaining a level of challenge during learning is also known to positively impact a participant’s level of motivation and attendance, both of which are crucial in the context of any clinical trial in which participants are assigned to receive an intervention [27].
As for the performance, the walking speed was found to increase significantly between the start and end of the training program. In fact, the mean walking speed reached in the present study (i.e., 0.25 ± 0.05 m/s) is similar to the weighted mean gait speed of 0.25 ± 0.14 m/s reported in a recent meta-analysis investigating a heterogeneous group of individuals with a complete SCI who completed, with different models of overground robotic exoskeletons, various training protocols encompassing a wide range of training sessions [2]. Nonetheless, reaching faster walking speed after 18 sessions may still be possible with additional training since able-bodied adults, who have completed basic training with the robotic exoskeleton, reach on average a self-selected comfortable walking speed of 0.38 ± 0.09 m/s when they were asked to avoid all voluntary muscular contraction of their lower extremities (i.e., passive walking) [28].
Safety
Among all participants, one serious adverse event occurred during the study. One participant sustained bilateral type I non-displaced fracture of the calcaneus after completing the two familiarization and the first training sessions. Although uncertainties exist about the specific cause of the fractures, both the fragility fracture risk of the calcaneus and the elevated vertical ground reaction force, known to reach about 36% ± 15% of the bodyweight at heel strike when walking with an overground robotic exoskeleton [29], are potential explanatory factors. This participant was withdrawn from the study and referred to the medical team until the fractures were healed. These fractures occurred even though the screening process was thoroughly completed by an experienced research physiotherapist and the minimal standing time tolerance (i.e., ≥30 min), recommended by the manufacturer of the exoskeleton, was verified. Unfortunately, another preliminary study also reported a comparable fracture of the talus during a locomotor training program with another overground robotic exoskeleton [15] whereas a review recently suggested an overall incidence rate of bone fracture of 3.4% [3]. The fact that some studies have predominantly included individuals with recent SCI (≤ 1 year), a time period during which bone mineral density declines at the L/Es and distal vertebrae (i.e., infralesional osteoporosis) may not have yet stabilized at levels significantly below those of age and gender-matched able-bodied individuals [30], may explain why this risk may have been underestimated [16]. Further investigation will be needed to explore all potential causes to implement additional screening elements for severe osteoporosis into the process (e.g., fracture risk stratification algorithms for adults with SCI [31], threshold for bone mineral density or architecture at the ankle and foot) and to develop solutions addressing the complex challenges linked to physical activities performed in standing position in individuals with SCI in the future. Other minor adverse events, predominantly linked to exacerbation of pre-existing (N = 4) or the development of new (N = 1) musculoskeletal-related non-debilitating pain at the upper extremity, were also documented (n = 5/14; 35.7%) over the course of the locomotor training program. Yet, all these participants opted to continue the training sessions while exploring personalized solutions to alleviate or even eliminate pain over the course of the program (e.g., increased number and duration of rest periods during sessions; cushioning at the handle of the walking aid; recommendation of stretching exercises post-training; use of nonsteroidal anti-inflammatory drugs) with the certified trainer(s). Unexpectedly, no exoskeleton-related skin or soft tissue issue was observed in the present study although it affected up to 50% of participants in previous studies and typically leads to interruptions of the intervention or withdrawal of participants from the studies [15]. All the above-identified risks remain impossible to eliminate, warrant thoughtful consideration, and should be carefully explained within the informed consent form along with the strategies implemented to minimize or overcome them. Finally, two events linked to the robotic exoskeleton itself (device malfunction) occurred over the course of the locomotor training program. The first event was a battery failure that required its replacement while the second event was a mechanical problem with a hip joint bearing assembly malfunction due to a damaged bolt holding the proximal and distal joint components together. In both cases, the problems were solved within a 48-h period with the prompt assistance of the company’s customer service department and had minimal impact on the conduct of the study.
Limits of the study
Limitations in the present study were the small sample size of relatively homogeneous participants recruited at a single site as well as the absence of a control group. Uncertainties about the best research design and outcome measures to adopt in future clinical trials continue. Because the study only included long-term manual wheelchair users with a chronic SCI living in the community, the generalizability of the results beyond this reference population, such as in ambulatory individuals with an incomplete SCI, requires caution. Prudence is also suggested when inferring about participants’ acceptance and satisfaction, particularly when addressing attendance and learnability, as this dimension was not reported. Hence, the findings of the present study should be considered preliminary, but it is anticipated that they will stimulate interest in conducting future larger-scale level I or II clinical trials investigating the efficacy or effectiveness of locomotor training programs with an overground robotic exoskeleton in long-term manual wheelchair users.