Incidence and prevalence

The National Spinal Cord Injury Statistical Center (NSCISC) estimates 291,000 people are living with an SCI in the United States with 17,730 new cases each year, or 54 per million ( ). Other studies have found similar incidence rates, ranging from 52 to 56.4 per million ( ); however, due to varying methodologies, other reports include prevalence estimates of 1.5 million to 2.6 million ( ). For decades, the typical person acquiring SCI was 29; however, that has increased to 43, likely due to an increase in the etiology of falls among an aging population. People with newly acquired SCI are typically male (78%) and white (59.5%) with leading causes of injury vehicle crashes (39.3%) and falls (31.8%). The majority of injuries are incomplete, either tetraplegia (47.6%) or paraplegia (19.9%) followed by complete paraplegia (19.6%) and complete tetraplegia (12.3%).

Healthcare and economic burden

Healthcare burden is largely dependent upon the degree of severity of the SCI with hospitalizations in the first year of injury, which includes the bulk of the direct cost estimated at 1.7 billion in the United States ( ). For example, an individual with tetraplegia and vent-dependent has an average healthcare expenditure over 1.2 million dollars in the first year after injury with almost $200,000 in subsequent years, whereas someone with paraplegia cost estimates include $550,381 for the first year and $72,909 in subsequent years ( ). The first year after following injury is the costliest and the most critical to the life of the person with SCI, as life expectancy increases significantly after living past the first year of injury. For individuals injured at 20 years old, estimated life loss is 7 years among individuals with paraplegia and 48 years of life loss among individuals with higher injury levels and vent dependency. In addition to direct cost and life loss, indirect cost, such as loss of earnings and productivity, is estimated at $76,237 per year.

Depending on the level of injury and personal response, many individuals do not return to full-time work or even part-time work. Data from NSCISC show 66% of individuals working at time of injury with only 17% working 1 year out and 23% working 10 years after injury. Lastly, length of stay for rehabilitation after sustaining an SCI has been reduced from 98 to 34 days over the last 50 years ( ). Post-acute rehabilitation offers people with SCI the benefit of individualized, intensive training before returning home. However, shorter length of stay and lack of intensive programming for the home has been linked to higher incidence of rehospitalization due to secondary health conditions, as well as increased discharge to institutional settings rather than the home, all of which drive healthcare cost ( ; ; ).

Associated risk factors

People with SCI have reduced physical functioning due to loss of motor functioning and loss of muscle mass, which result in deleterious cardiometabolic effects. People with SCI have high rates of secondary health conditions, which are defined by the Institute of Medicine as preventable conditions secondary to the associated condition. These secondary conditions (e.g., pain, pressure ulcer) lead to a downward spiral in health and function for people with SCI and in the aggregate, can impose substantial limitations in rates of participation including employment, social engagement, and performing activities of daily living ( ). The NCISC reported in 2018 that within the first year of sustaining an SCI, 36.5% of patients are re-hospitalized due to infections associated with the urinary, respiratory, or integumentary system, with an average length of hospitalization of 19 days ( ). Additionally, reported that within the first year of sustaining an SCI, 57% used emergency room care. Following rehabilitation, health trajectories in people with SCI are impacted by the onset and course of these secondary conditions and their “additive” effect on changes in health and function ( ). Some of the highest rates of physiological secondary conditions in people with SCI include excess weight gain (i.e., obesity) ( ; ; ; ), extremely low fitness levels ( ; ; ), and pain (neuropathic and musculoskeletal) ( ; ; Fig. 1 ).

Downward spiral of deconditioning among people with SCI.

Health disparities, barriers, and SCI

People with SCI have higher rates of health disparities, which includes lower levels of physical activity. Data show that around half of people with SCI obtain little to no aerobic physical activity, which has resulted in people with SCI at three times more likely to have chronic conditions, such as heart disease, obesity, and stroke than the general population ( ; ). This is largely due to the reduced physical functioning and sedentary lifestyles acquired by many individuals with SCI due to the issues of deconditioning and secondary conditions discussed in the above section ( ; ). This is also related to the low rates of employment previously mentioned, as well as other lower quality of life indicators among people with SCI. There is an extreme challenge faced by people with SCI to lead an active and healthy lifestyle because of muscle loss, reduced motor functioning, and health, environmental, and logistical barriers ( ; ). Deconditioning is only one aspect of a plethora of exercise barriers that span each level of the social ecological model for people with SCI. Many systematic reviews have highlighted the issue of barriers to exercise among people with SCI ( ). At the intrapersonal level, people with SCI have low levels of self-efficacy, fear of injuring themselves, and experience an increase in musculoskeletal pain, as well as the addition of neuropathic pain, due to the SCI. On the interpersonal level, there is a lack of social support for individuals with SCI resulting in cases of learned helplessness, which is detrimental to gaining independence. Next, there is a lack of awareness and knowledge about supporting exercise behavior among people with SCI at the organizational level and lack of accessible fitness centers, exercise programs, and equipment at the community level. Lastly, the largest barrier to exercise has been transportation with many dependent upon public transportation.

Exercise benefits

Exercise research for SCI is a relatively new area of research inquiry that emerged around the 1980s. Because of this, there is limited high-level evidence that confirms precise physiological benefits of exercise among this population. Unfortunately, the relatively small prevalence of SCI has made it difficult for researchers to confirm the benefits of exercise in SCI within large high-quality trials, such as randomized controlled trials. This lack of trials has resulted in few systematic reviews for SCI compared with other disability groups (e.g., stroke and multiple sclerosis) and, ultimately, no meta-analyses. Therefore, while the benefits for SCI are likely similar to those observed in the general population, there is a lack of research evidence supporting interventions that can lead to actual benefits in SCI. This makes it difficult for clinicians to prescribe exercise and improve participation. Nevertheless, we hereby present the latest high-quality evidence of benefits in SCI as of the time of this publication.

In 2017, a high-quality systematic review of exercise studies for people SCI reported that exercise could improve fitness cardiometabolic health ( ; ). The review included a total of 211 studies where exercise interventions were delivered. The review included non-randomized studies (non-randomized trials, case studies, etc.) in addition to randomized studies, which means that we should interpret not only their findings but also the confidence they had in their findings. There were insufficient high-quality studies that included people with acute SCI (≤ 12 months post-injury). Thus, the findings were presented only for people with chronic SC (189/211 studies). The key findings are presented below:

Benefits summary: ( ; )

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  Low to moderate confidence evidence demonstrates that exercising 3 to 5 times per week at a moderate to vigorous intensity can improve cardiorespiratory fitness, muscular strength and metabolic health (body composition and cardiovascular risk factors).

  
  
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  Different doses of exercise result in different health benefits.

  
  
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  Doses of lower training durations and frequencies can improve fitness (i.e., cardiorespiratory fitness and muscular strength/power).

  
  
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  Whereas doses with higher training durations and frequencies appear to be necessary to result in cardiometabolic health improvements.

These findings had important implications and informed the development of the latest SCI exercise guidelines.

Of note, the published studies primarily base their evidence off of short-term clinical studies. In other words, the studies are conducted in highly controlled laboratory or clinical settings, as opposed to real-world changes in the lifestyle of the person with SCI after the study. A scoping review examined whether exercise benefits could be sustained after completing the formal intervention of an intervention for adults with disabilities ( ). The report identified that, from exercise trials published from 2006 to 2016, only one study for SCI found a sustained benefit toward a health outcome that lasted throughout a follow-up period (a period after completing a formal intervention component). Consequently, there is a need to identify programs that not only improve health and function in the short term for SCI, but also demonstrate long-term benefits, which will ultimately require strategies that promote long-term participation.

Exercise guidelines

Given the infancy of the field, exercise guidelines or recommendations designed specifically for SCI are quite new and still evolving. The latest and most widely acknowledged exercise guidelines were created in 2017 by a large international team of highly respected disability-exercise scientists, led by Dr. Kathleen Martin Ginis the University of British Columbia, Canada and Victoria Goosey-Tolfrey at Loughborough University, United Kingdom ( ; ). The guidelines were split into two different categories, fitness benefits and cardiometabolic health, as stated below:

Guidelines summary: ( ; )

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  Aerobic exercise should be performed 2 to 3 sessions per week at a moderate to vigorous intensity for 20 to 40 min (at minimum, 20 min).

  
  
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  Strength exercise should be performed 2 to 3 times per week and each session should include 3 sets of 10 repetitions for large muscle groups (weight should be set at 50% to 80% of the person’s maximum weight; at minimum 3 sets should be performed in a week).

When prescribing these guidelines to people with SCI, there are a few important concepts to note. The duration component for aerobic exercise necessary to anticipate health benefits is, at minimum, 20 min. If improving health is the primary goal, people with SCI should aim to exceed the SCI guidelines, particularly since guidelines for SCI are lower than those for the general adult population. The US Department of Health and Human Services recommends the general adult population and adults with disabilities should aim to achieve at least 150 to 300 min of moderate-intensity aerobic exercise per week; or 75 to 150 min of vigorous intensity aerobic exercise ( ). The collective evidence suggests that the SCI exercise guidelines should be considered as a minimum recommendation or a starting point for exercise promotion. The long-term goal should be to progress toward higher volumes (frequency of exercise sessions per week and session durations) of exercise overtime. As a final note, there will certainly be many people with SCI who will have difficulty achieving or may never achieve the SCI guidelines (barriers discussed in the last subsection). Although achieving the guidelines is an important target for exercise prescription, know that some exercise is always better than none: gains can be made from even low levels of exercise participation, particularly in people who are physically deconditioned or new to exercise.

Exercise participation

Although there is an encouraging amount of literature demonstrating benefits, ultimately, published studies have not demonstrated much success with identifying programs that can increase exercise participation. Specifically, there have been no evidence-based programs successful at maintaining exercise participation over a long term ( ) and, most importantly, there is substantial difficulty with getting people with SCI enrolled into an exercise intervention. A review study ( ) found the mean sample size of randomized controlled trials published between the years 2006 to 2016 was 25 people; 14 people per study group when they were randomized to a treatment group and control group. This means exercise interventions may have poor generalizability (i.e., findings may not be able to be projected to the population as a whole), and we must be careful when interpreting study findings. This limitation is currently the largest factor that impedes the growth of exercise research in SCI.

To address this issue, we would urge researchers to use theory-based strategies to enroll people with SCI into exercise interventions or programs. Behavioral change strategies that are framed upon theories (e.g., social cognitive theory) have been found to be successful for promoting physical activity throughout an intervention for people with SCI, albeit for short-to-mid terms ( ). Conceivably, theory-based strategies should also frame how we enroll people with SCI into exercise interventions. Our team conducted a qualitative investigation that generated a theory for enhancing the likelihood of success with enrollment ( ). The theory posits that wheelchair users with SCI ( n = 33) conceive three core concerns with enrolling into an exercise intervention: (1) they first assess their capability to participate in the program due to scheduling, transportation, and secondary health conditions; (2) they mentally balance anticipated benefits versus the difficulty of starting the program; and (3) they assess the desirability of the program characteristics based upon their needs and preferences. Successful recruitment will depend on a recruiters ability to address these three concerns while presenting time-efficient, accessible, and desirable programs ( ).

High-intensity interval training in able-bodied individuals

One mode of training that may overcome some of these barriers and provide similar cardiometabolic health benefits is low-volume high-intensity interval training (HIIT). HIIT can be performed many different ways, but usually consists of brief intervals of higher intensity exercise performed between (~ 80%–200% of VO ₂ max) separated by rest intervals of lower intensity exercise or complete rest ( ; ). Furthermore, interval training can be broken down to HIIT, which consists of repeated bouts of exercise performed at workloads between the ventilatory threshold and VO ₂ max, or sprint interval training (SIT), which is performed at workloads greater than VO ₂ max ( ; ). At this point, there are insufficient data to support the idea that individuals with SCI are able to perform SIT, which requires maximal or supramaximal efforts for the duration of the interval. Thus, for simplicity, we will refer to both of these terms as HIIT when referring to the effects on individuals with SCI throughout this chapter. A growing body of evidence in non-disabled individuals, from our group ( ) and others ( ; ; ; ) has demonstrated the potential for low-volume high-intensity interval training (HIIT) to provide comparable or superior improvements in cardiometabolic health outcomes compared to continuous moderate-intensity training (MIT) requiring 60% to 80% greater time commitment. For example, we recently found similar improvements in % fat, blood lipids, and insulin sensitivity in obese males following 6 weeks of training despite HIIT only requiring 20% of the total time commitment as MIT. Importantly, this mode of exercise has been shown to be well tolerated in many different clinical populations, including type 2 diabetes (T2D) ( ; ), overweight/obesity ( ; ; ), cardiovascular disease ( ), and metabolic syndrome ( ). Thus, HIIT appears to be a mode of exercise that is well-tolerated, requires minimal time commitment, and provides many of the same cardiometabolic health benefits as MIT in non-disabled individuals.

High-intensity interval training in individuals with spinal cord injury

Exercise guidelines for non-disabled individuals have been developed based on modes of exercise that require weight bearing activities such as running and walking or lower-body exercise such as cycling. This is uniquely different than exercise programs in individuals with SCI, in which upper body exercise training (arm crank cycling, wheelchair movement, and upper body resistance training) is the primary mode used and consists of upper body muscle mass. When SCI occurs, there is a decline in aerobic capacity, onset of skeletal muscle atrophy, increases in body fat, and impaired cardiometabolic health. Thus, this rapid deconditioning greatly enhances risk of cardiovascular and metabolic diseases then able bodied populations ( ). Furthermore, while there have been few long-term studies assessing the benefits of exercise on individuals with SCI a recent study showed improvements in VO ₂ max and muscular strength compared to no-exercise controls. However, there were no changes in body composition, blood lipids, or blood glucose and insulin following 16 weeks of combined aerobic and resistance training exercise performed twice a week ( ), suggesting the potential need for greater volume or intensity to modify cardiometabolic risk factors. Thus, HIIT is an attractive form of training to address these potential limitations following lower volume or moderate-intensity exercise. To date, there have been few well-powered studies assessing the potential benefits for improving cardiometabolic health in individuals with SCI. Overall, studies ( ; ; ; ) not including a MIT exercise comparison group showed increases in VO ₂ peak and peak power output, and have also shown improved stroke volume ( ), and decrease % body fat ( ). Whereas studies that have compared HIIT to MIT training groups have shown similar improvements in VO ₂ peak ( ; ), insulin sensitivity ( ; ), and peak power output ( ; ). Thus, it appears that HIIT can improve some components of cardiometabolic health, similar to that seen following MIT exercise; however, it remains difficult to compare results between studies due to a lack of standardized exercise guidelines when implementing HIIT.

While there is much work needed to draw conclusions of the efficacy of HIIT for improving health in individuals with SCI, several recent studies have shown greater exercise enjoyment ( ) following HIIT compared to MIT (15, 20). Furthermore, HIIT studies to date have reported minimal ( ; ; ) or no adverse responses ( ; ) to HIIT. Therefore, HIIT appears to be a feasible and tolerable form of training in individuals with SCI. Future well powered and long-term studies are needed to determine if HIIT is a more attractive form of training that can address the major barriers (lack of time, enjoyment, and access to facilities) to exercise participation and exercise adherence in individuals with SCI, and lead to long-term improvements in cardiometabolic health.

Telehealth strategies to promote exercise adherence to high-intensity interval training

Participation in exercise remains low in individuals with SCI (50% sedentary) ( ). Adherence to exercise treatments in disabled individuals is often reported to be less than 50% ( ; ; ). In addition to low adherence rates, the majority of SCI exercise training studies to date have had small sample sizes (mean = 14) and were often underpowered to determine primary treatment effects ( ). Thus, it is critical to identify novel strategies and tools that can increase exercise participation and improve rates of adherence in SCI. Recent advances in technology, such as smartphones and internet streaming, have been used to deliver health care services to individuals at home. Thus, with emerging technology it is possible for the implementation of home-based telehealth exercise, which may enable us to overcome many of the reported barriers, such as transportation, accessibility to exercise facilities, and access to knowledgeable exercise personnel ( ), in individuals with SCI.

To date, there have been a few pilot studies testing the efficacy of various telehealth exercise interventions (telephone, video, and web-based approaches) in individuals with SCI ( ; ; ; ). Telephone-based exercise counseling has been shown to increase exercise participation in adults with SCI from 35% at baseline to 52% after 6 months ( ). A recent pilot study by our group assessed an aerobic exercise training program delivered remotely using an upper body ergometer, tablet, physiological monitor, and custom application that delivered video feed to a remote trainer and monitored and recorded exercise data in real time ( ). This approach resulted in completion of all 24 sessions during the 8 weeks of training and 100% adherence. Another advantage is that technology-based programs might also increase the likelihood that outcomes are maintained after an exercise trial is completed. In a recent review, we observed that technology-driven interventions sustained 56% of all outcomes measured at follow-up, which was higher than all other exercise modalities and the mean of all studies (32%) ( ). Collectively, these studies demonstrate that home-based telehealth exercise training can be implemented safely and allows individuals and investigators to overcome many of the common barriers to exercise participation and research in SCI. Given the potent health improvements observed following low-volume HIIT and the ability to implement a home-based telehealth exercise approach, HIIT delivered via telehealth may be an optimal exercise training program that improves cardiometabolic health, requires little time, and reduces environmental barriers that typically prevent exercise in SCI. Current research by our group will assess the potential for home-based telehealth HIIT exercise for improving cardiometabolic health in individuals with SCI. We anticipate greater adherence to exercise and significant health improvements as it has been shown that one-on-one training and attention from the trainers are key factors that will make HIIT enjoyable, as it has been shown that working with personal trainers significantly increase adherence to exercise ( ).