HIGH-INTENSITY INTERVAL TRAINING IN PEOPLE WITH SPINAL CORD INJURY: A SYSTEMATIC REVIEW

Objective: Analyze the effects of high-intensity interval training (HIIT) on cardiometabolic parameters, and cardiorespiratory fitness to compile the most used HIIT training types in adults with spinal cord injury (SCI). Methods: This is a systematic review of searches performed in the electronic databases PubMed / Medline, Science Direct, and Google Scholar. Studies included I) needed to apply HIIT training II) adults with SCI to analyze III) cardiometabolic aspects and cardiorespiratory fitness. Two independent reviewers selected the articles for inclusion, extracted their data, and assessed their methodological quality. Results: 654 studies were found. Thus, 12 studies, 11 pre-and postintervention, and one control group (CG) with 106 participants were analyzed. Pre-and post-HIITT intervention results revealed significant improvement in cardiorespiratory fitness and cardiometabolic aspects (VO 2 peak, LDH, HDL, insulin resistance). In addition, GC results revealed significant improvement in cardiorespiratory fitness observed in the intervention group (HIIT) compared to the moderate-low intensity (GC) group. Seven studies used the arm ergometer as the primary exercise modality. Two studies described functional electrical stimulation (FES) performed with the arm ergometer plus electrical stimulation in the lower limbs. None reported heart rate dynamics during the study period. Conclusion: High-intensity interval training improves physical fitness and cardiometabolic health in adults with SCI. Evidence level II; Systematic Review of level II studies.


INTRODUCTION
Spinal cord injury (SCI) is a damage along the spinal cord's length, causing motor and sensory deficits. 18][9] In contrast, this sedentary profile and the associated cardiometabolic risk could be prevented by physical activity. 102][13] Furthermore, high-intensity physical activity through sport has also been related to cardiovascular risk reduction, 14,15 improved diastolic function 16 , and better body composition. 17In addition to sports practice, other models of high-intensity interval exercise (HIIT) performed in different contexts have received attention.
Thus, HIIT is defined as a training modality in which the practitioner alternates high-intensity efforts (i.e., above the anaerobic threshold) with rest periods that can be active or passive. 18Specifically, classification models are derived from manipulating intervening variables, which are organized, among other proposals, into long intervals, short intervals, repeated sprint training (RST), and sprint interval training (SIT). 18These models stand out by allowing higher intensity levels to be maintained for longer,19 which seems stung for adaptive cardiorespiratory changes. 20][23][24][25][26][27][28] Therefore, considering the potential of HIIT to promote positive adaptations in individuals with SCI, the present study aims to systematize the knowledge already available about the effects of HIIT on cardiometabolic and cardiorespiratory parameters in people with SCI.

METHODS Preliminary Information
This systematic review is based on PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analysis) terms. 29The study had the following PICO strategy as its question: People with spinal cord injury (participants); High-intensity interval training (HIIT) (intervention); Descriptive data on HIIT (comparison); Improvement of cardiometabolic and fitness aspects (results).

Search Strategy
Searches were performed in PubMed / Medline, Science Direct, and Google Scholar electronic databases in January 2022.They were used: PubMed / Medline: high-intensity interval training AND spinal cord injury; Science Direct: "High-intensity interval training" AND "Spinal cord injury" and Google Scholar: High-intensity interval training AND spinal cord injury AND cardiorespiratory AND metabolic AND improve AND elicit AND ergometer AND adaptations AND paraplegic.The articles were selected by reading the title and abstract, and duplicates were excluded.After selection, the articles were read fully, and the inclusion criteria were applied.

Criteria for election
This systematic review included articles that i) applied the HIIT method; ii) in adult individuals with SCI and; iii) performed analyses of cardiometabolic and fitness aspects, showing the results by mean and standard deviation.Pilot studies, systematic and scoping review articles, book chapters, conference abstracts, and those not meeting the inclusion criteria were excluded.

Data Extraction
Data extraction was performed based on an extraction table developed by the authors.The data collected were: the purpose of the study, sample, groups, training methods, research duration, weekly training frequency, session duration in minutes, and main results.

Quality Assessment
The methodological quality of the studies presented in Table 1 was assessed with the Apprisal for Cross-Sectional Studies (AXIS) tool. 30wo authors (CM, KS) performed the quality assessment separately, and disagreements were resolved by consensus in the presence of a third reviewer (AC).In the AXIS tool, for each correct answer, a score of one was assigned to each of the twenty questions.These tools aim to evaluate the methodological rigor of these studies.

Study Selection
The initial search found 654 articles (PubMed = 19; Science direct = 39; Google Scholar = 596).After excluding duplicates and applying the inclusion and exclusion criteria, 11 articles remained for this systematic review.(Figure 1)

Description of the study
Table 2 describes the sample characteristics of the selected studies.A total of 106 individuals underwent the HIIT method interventions.Of those, 14 (13.2%) were female, and 92 (86.7%) were male.One of the studies did not describe the gender of the participants. 31Only one study had a control group. 23egarding the training duration, the studies showed differences in the total intervention time with training protocols lasting from one to twelve weeks.The study that applied the training protocol for only one week and also obtained improvement in aerobic capacity. 21Improvements in cardiometabolic parameters (e.g., LDH, HDL, insulin resistance) were observed in two studies. 27,31None of the reviewed studies reported the heart rate dynamics during the study period.The number of training sessions also varied from 1 to 3 times a week.
26][27]32 Eight studies used the arm ergometer as the primary exercise modality.In addition, two studies have described exercises performed with the arm ergometer plus FES. 28,31Regarding the applied stimulus time, the minimum observed was 30 seconds 33 and the maximum was 5 minutes. 26Two of the twelve studies included in this review studied cardiometabolic and cardiorespiratory parameters. 27,31Two reviewers determined the quality assessment of the studies.

DISCUSSION
This systematic review aimed to analyze the effect of HIIT on cardiometabolic parameters indicators and cardiorespiratory fitness in people with SCI.The main finding indicates that HIIT is considered a "time efficient" strategy, compared to continuous training, for improving cardiometabolic health and fitness markers.Special populations may benefit from this training.
Seven of the eleven studies that used arm exercises with HIIT protocol report improved cardiorespiratory fitness, indicating that this exercise effectively promotes positive changes in this parameter.][38] HIIT requires less training time than continuous training 39 , and in people, without disabilities, it presents itself as an alternative to continuous training to promote improvements in cardiometabolic parameters. 40The effects observed in research using short single-session HIIT protocols showed increased VO 2 max. 21,33These results agree with those of HIIT use in non--disabled people. 41One study compared energy expenditure between HIIT and moderate-intensity continuous training, concluding that similar caloric expenditure occurs between the two training models (115.9 ± 21.8 vs. 116.6 ± 35.0 kcal). 25However, another study in obese adults identified that more duration time was required in moderate-intensity continuous training compared to HIIT (39.8 ± 4.6 vs. 32.2± 6.2 min). 42fficial exercise guidelines for cardiorespiratory fitness benefits for adults with SCI suggest moderate to vigorous intensity at least two to three times per week, further indicating that for cardiometabolic health benefits, adults with SCI should perform at least 30 minutes of moderate to vigorous aerobic intensity three times per week. 2,43The protocols observed in this review, except for one study that did not report the timing of the HIIT protocol, 28 follow the timing recommendations proposed in the guideline for people with SCI.(Table 1).-Pre = 7.9 -Post = 9.9The studies in this review did not show how heart rate behaved during HIIT.To better understand the isolated impact of prescribed exercise interventions on cardiovascular health, future studies may control for heart rate response throughout the protocol, providing a better understanding of the overall impact of high-intensity exercise interventions in people with SCI.5][46] Knowledge of heart rate responses at different running intensities is essential for correct exercise prescription. 47t is evident in this review that most studies presented a sample composed of people with low spinal cord injury (paraplegia), which seems to influence cardiorespiratory outcomes. 48,49Metabolic parameters seem to be influenced by the time of injury of the individual.No improvement in cardiometabolic parameters was observed in people with chronic SCI after six weeks of HIIT. 31On the other hand, it proved sufficient to alter metabolic markers in people with acute SCI undergoing eight weeks of HIIT. 27The difference observed between the studies seems to be influenced by the time of injury of the sample.
Previous studies in non-disabled people have observed the effectiveness of HIIT over continuous training in improving cardiometabolic risk factors. 40,50owever, the spinal cord injury population results are discrete and should be evaluated carefully. 27,31Most studies to evaluate the effectiveness of HIIT in improving cardiometabolic parameters have been conducted with a small sample size, 27,31 which justifies the need for new studies.The data presented are summary evidence of the effect of HIIT on cardiometabolic risk factors such as HDL-c, LDL-c, triglycerides, glucose, and total cholesterol.
The glucose response during the application of HIIT and continuous training in people with SCI was reduced. 27,51The insulin results must be analyzed cautiously since the small sample size (n = 3) and the difference in age and sex may sensitize responses to exercise. 27Two of the twelve studies in this review, which measured insulin concentration before and HIIT and MICT HIIT: Participants were asked to propel their wheelchair at a high and low intensity during 30 and 60 second intervals, respectively, and repeat this sequence 20 times over 30 minutes.During the 30-second high-intensity interval, participants needed to achieve an RPE between 6 and 8 (very difficult).Each high-intensity interval was followed by a low-intensity interval of 60 seconds at an RPE between 1 (very light) and 2 (light).
MICT: Participants were then asked to propel their wheelchairs for 30 minutes at a constant speed, maintaining an RPE between 4 (somewhat difficult) and 5 (difficult).after the training intervention, reported very discrete results after HIIT for the upper body, suggesting that this training form needs further exploration to demonstrate its effectiveness.This review highlights the use of HIIT as an effective strategy for inducing positive cardiometabolic responses in people with SCI.In addition, we believe the results observed in this research should serve as a data source to clarify and safely and efficiently conduct this training method in people with SCI.Finally, this review highlights the magnitude of using HIIT as a "time efficient" strategy that effectively induces cardiometabolic responses in people with SCI.

Figure 1 .
Figure 1.Prisma 2020 flow diagram for new systematic reviews which included searches of databases and registers only.*Consider,if feasible to do so, reporting the number of records identified from each database or register searched (rather than the total number across all databases/registers). **If automation tools were used, indicate how many records were excluded by a human and how many were excluded by automation tools.From: Page MJ, McKenzie JE, Bossuyt PM, Boutron I, Hoffmann TC, Mulrow CD, et al.The PRISMA 2020 statement: an updated guideline for reporting systematic reviews.BMJ 2021;372:n71.doi: 10.1136/bmj.n71.
Caption: Y = Yes, N = No, and NI = Not Informed.

Table 2 .
Characteristics of the selected studies.