Health and exercise performance have traditionally been considered on a continuum in which the athlete, viewed as the epitome of human physiological performance, is at one end and individuals with disability, traditionally medicalized as a condition to be treated, at the other (1). The para-athlete and the Paralympic Games, encompassing the wider Paralympic Movement, have challenged such dogma. While athletes with disabilities were not unheard of (2), it was the Stoke Mandeville Games (established in 1948) and Paralympic Games (retroactively established in 1960) that have most rapidly advanced para-sport and the Movement (3).

The International Paralympic Committee (IPC; established 1989), recognized as the global governing body of the Movement (4), operates under the vision of enabling para-athletes to achieve sporting excellence. In doing so, the IPC has a classification code that governs the process by which athletes are categorized into a number of groups on the basis of common properties (5). The system aims to determine who is eligible to compete at the Paralympic Games, while ensuring that it is not the degree of impairment but sporting excellence that ultimately determines which athlete or team is victorious (6). Presently, there are 10 eligible impairment types: impaired muscle power, impaired passive range of movement, limb deficiency, ataxia (uncoordinated movement), athetosis (involuntary movements), hypertonia (increased muscle tension), short stature, leg length difference, vision impairment, and intellectual impairment (7).

Though research studies dating back to the 1970s and 1980s have documented exercise responses and the unique physiology of “active” individuals with an impairment, it is only in the last 30 years that research into the “para-athlete” has started to emerge as a field of its own. For example, a search of PubMed databases indicates that in 1990 there were 57 articles on “disability sport” and that by 2020 this number had increased more than twenty-fold. Despite the expansion in research and many excellent para-sport research groups globally there still exists a relative paucity of para-sport research and evidence-based practice—particularly in the physiology domain. There is currently no complete overview of how the different classifiable impairment types impact the physiological response to exercise. Therefore, practitioners within the para-sport field may have trouble incorporating data from isolated case studies and/or studies that focus on a single disability type to develop a comprehensive understanding of how various impairments impact the physiological response to exercise. This lack of cohesive understanding can prevent new practitioners from providing appropriate and timely support of para-athletes and may result in researchers having to spend additional time sourcing background material, rather than designing studies to advance the field.

The present review was written as part of a collaborative effort between Own the Podium and members of the Own the Podium Paralympic Professional Development Working Group that is made up of practitioners, athletes, and academics in Canada. The purpose of this review was to provide a comprehensive overview of the physiological considerations that should be taken into account when supporting para-athletes in applied sports performance roles. Nevertheless, the content of this review is structured in such a way that it will benefit researchers and clinicians who want to gain an overview of para-sport physiology as well those wanting to develop interventions to improve the health and performance of para-athletes. Within this review we discuss the neural control of the physiological response to exercise and the current state of research aimed at enhancing exercise performance in athletes with classifiable impairments under the major sub-groups of limb deficiency, cerebral palsy, SCI, and other classifiable neurological impairments. Whilst athletes with visual and intellectual impairments have practical and biomechanical considerations, they are not expected to exhibit an altered physiological response to exercise or energy requirements and hence are omitted from this review for brevity. Finally, we discuss research gaps and areas of interest for future research and innovation.

Given that many of the abovementioned impairments impact neurological function, it follows that the degree of function is highly variable across impairment groups. This can result in some para-athletes exhibiting an unaffected cardiorespiratory, metabolic, and thermoregulatory response to exercise (e.g., visual impairment), and others exhibiting a severely attenuated cardiorespiratory, metabolic, and thermoregulatory response to exercise compared to able-bodied athletes (e.g., cervical SCI). As such, we believe that para-sport researchers and practitioners should familiarize themselves with fundamental neuroanatomy so as to develop appropriate individualized interventions and understand unique practical considerations of the para-athlete. Figure 1A provides a general guide to neuroanatomy of the autonomic pathways regulating the thermoregulatory and cardiovascular response to exercise. Figure 1B gives an overview of the autonomic and somatic pathways that regulate the ventilatory and skeletal muscle response to exercise. Below we discuss how various impairments may impact the physiological response to exercise in relation to these pathways. It should be noted, however, that there may be a large degree of variability between athletes with the same impairment depending on where the specific lesion (e.g., SCI or cerebral palsy) or injury (e.g., limb deficiency) is located.

The cardiac (8), vascular (9), ventilatory (10), metabolic (11), and thermoregulatory (12, 13) response to exercise in able-bodied individuals has been reviewed extensively elsewhere and are only referred to here for comparison. Briefly, at the onset of exercise, the sympathetic nervous system (SNS) (Figure 1A) is activated to increase cardiac output and blood pressure to meet the oxygen demands of exercising musculature. Simultaneously, activity of the parasympathetic nervous system (PNS) is gradually withdrawn (14), though present to maximal exercise (14), and increased drive to respiratory muscles (Figure 1B) increases tidal volume to facilitate oxygen supply to, and carbon dioxide removal from, the arterial blood. There also exists a complex interplay between peripheral reflexes (i.e., the baroreflex, chemoreflex and exercise pressor reflex) that provide afferent feedback to the brainstem to “fine-tune” cardiorespiratory, metabolic, and thermoregulatory function (15).

Energy availability (EA) represents the amount of energy left over for optimal physiological function after exercise energy expenditure (EEE) is subtracted from energy intake (EI), and corrected for fat free mass (FFM) (54). It is well established that chronic low EA (LEA) results in myriad negative health, psychological and performance outcomes and is the underpinning etiology of Relative Energy Deficiency in Sport (RED-S) (55). However, laboratory and clinical quantification of EA is impressively difficult, and challenged by methodological considerations that introduce risk for significant under- or overestimation of EI (56) and/or EEE (57); although to our knowledge this has primarily been only examined in able bodied athletes. Indeed, we are not aware of studies that have utilized the “gold-standard” approach of double-labeled water to assess total daily energy expenditure (TDEE). Within TDEE, there is significant work to be done to better understand the EEE demands in para-athletes, of which most data is predominantly in wheelchair athletes. However, even our understanding of EEE in wheelchair athletes is limited. For example, our current compendium of energy costs of physical activities for individuals who use manual wheelchairs is now 10 years old and, despite identifying 266 studies, only 11 studies met the inclusion criteria (58). Only four of these studies included para-athletes, of which 91 were male and 6 female. Given the progress of wheelchair technology advances utilized by modern para-athletes, which would potentially impact gross efficiency outcomes of EEE, much more data needs to be developed to accurately estimate EEE. Therefore, the accurate appreciation of EEE in most classifications of para-athletes remains to be elucidated.

Since the assessment of EA is challenging in evaluating LEA, more chronic indicators of LEA tend to be used for RED-S diagnosis in able-bodied athletes (59), and common outcomes include: clinically low hormones involved in the hypothalamic-pituitary-gonadal-adrenal (HPGA)-axis; amenorrhea; clinically low bone mineral density (BMD); restricted eating leading to disordered eating or eating disorders; increased risk of injuries; poor training adaptations and performance outcomes. However a validated RED-S diagnosis tool in able and para-athletes remains to be developed.

Within para-sport, several recent reviews have highlighted that depending on the impairment many para-athletes have significant challenges with optimizing EI, coupled with potentially altered aspects of EEE, EA, and FFM and thus may be especially at risk for LEA and RED-S (60). Despite over 70 papers published since the 1980's on either RED-S or the Female Athlete Triad in able-bodied athletes (55, 61), we are only aware of three recent publications that have assessed the risk of LEA and/or other chronic indicators of RED-S in para-athletes (62, 63). Depending which assessment tool was used, there were indications of ~73% of elite female athletes with SCI having LEA (62), which was significant more than male athletes with SCI. In another study, 78% of female athletes with SCI were deemed at “risk” for LEA using the LEA in Females Questionnaire (LEAF-Q), although in this study EA assessments were not compromised (63). Taken together and similar to the able-bodied literature, there are considerable discrepancies in RED-S assessment across various tools/parameters, and much more research is required. Among a cohort of male and female track and field para-athletes with cerebral palsy, vision impairment, or limb deficiency >82% had reduced EA and approximately one-third had LEA (64).

The actual prevalence of RED-S may be profound, as a recent survey study in 260 elite para-athletes demonstrated that ~32% had elevated disordered eating questionnaire scores, ~45% of premenopausal females had oligomenorrhea/amenorrhea, ~55% had reported low BMD, but < ~10% had awareness of the RED-S (65). Indeed, athletes with SCI may be especially at risk. For example, wheelchair athletes often have significant lower body muscle atrophy resulting in lower whole-body FFM, which may artificially elevate EA calculations compared to able-bodied athletes. Additionally, ~50% of male athletes with SCI have low testosterone (66), due either to altered HPGA-axis outcomes related to SNS dysfunction and/or aspects of LEA. It is important to note that as of yet, no clinical and laboratory normative LEA data has been published in para-athletes linked to adverse clinical outcomes. Therefore, we have to encourage much more research to develop RED-S specific data and assessment tools in these unique para-athlete populations.

Elite able-bodied athletes and their support staff constantly strive to gain a competitive advantage within the rules of their sport to enhance performance. Two of the more common physiological performance enhancing strategies used by able-bodied athletes may include altitude training (67) and heat acclimation training (68). Although there are extensive research and review articles on these, and other, interventions for the able-bodied athlete, there is often little to no research on how these interventions may affect elite para-athletes—while some may be beneficial, some may be detrimental to performance and/or health.

In recent years there have been a number of studies that highlight the impacts of respiratory muscle training (69, 70), abdominal binding (71), and heat acclimation (72, 73) in para-athletes (see 23). However, there remains a large gap in the research and applied knowledge for how these protocols may translate to benefit para-athletes in all impairment groups. It is beyond the scope of this review to fully review these interventions. Nevertheless, some key interventions and practical considerations to working with para-athletes are highlighted in Tables 2 and 3, respectively.

The Tokyo 2020 Summer Paralympic Games featured 4,403 athletes from 162 participating countries who competed in a total of 539 events were contested across 22 sports (83). Despite growing interest and participation in Paralympic sport, along with the influx of published articles on disability sport over the last quarter century, there remains a lack of evidence-based physiological interventions to improve performance in the para-athlete, especially those without SCI. This is in part due to the barriers associated with conducting research on small heterogenous cohorts of para-athletes. For example, Stephenson et al. (73) conducted a heat acclimatization intervention in seven elite para-triathletes across five different impairment groups. While conducting such a research study is commendable from a logistical perspective, the heterogeneity of the cohort makes it difficult to form conclusions as to how the intervention may be applied to para-athletes with various impairments. To overcome these limitations, practitioners often rely on knowledge transfer from experience, colleagues, and anecdotal evidence through case study approaches to inform their practice. Current practice is often adapted based on able-bodied research, yet there is little validated data on how many of the interventions, protocols, training methods and assessment tools used by practitioners directly translate to support athletes in all impairment groups or across athletes with varying severities of impairment (i.e., SCI level, hemiplegia vs. diplegia, level of amputation). Furthermore, there is a distinct lack of published data on the physiological demands of para-sports from which practitioners can base physiological interventions and training programs. This includes simple, relatively non-invasive data such as game duration/intensity profiles integrated with heart rate, oxygen consumption, and/or lactate profiles.

Thermoregulation is an area that has been relatively well researched in para-athletes. However, the majority of research focuses on athletes with a SCI due to the known higher risk of thermal strain in this population (22). Therefore, we can only hypothesize as to whether other impairment groups would experience increased thermal strain relative to able-bodied athletes, or would benefit from cooling strategies and/or heat acclimation training. The current literature on heat acclimation is limited in elite para-athletes (72, 73). This not only highlights the difficulty in recruiting large homogeneous samples of para-athletes but emphasizes the need to determine impairment specific physiological responses, adaptations, and performance outcomes to training in the heat for practitioners to individualize training preparation for Paralympic athletes.

Competing in the heat is not the only environmental condition that impacts para-athletes. We believe that further research should also examine the impact air quality, cold environments, and altitude on performance. In particular, altitude training is a common intervention adopted by elite able-bodied endurance athletes (84), but we are unaware of any altitude training interventions in para-athletes whom commonly compete at moderate altitudes. Additionally, the areas of travel and immune function are only starting to emerge in conversation and the scientific literature (85) and warrant further research.

It is understood that elite para-athletes have a high prevalence of injury and illness (86), and is therefore imperative for the integrated support staff to first support the para-athletes health and well-being for them to train and perform at the highest level. There is a plethora of data indicating the negative health, psychological and performance impacts that LEA has on able-bodied athletes (87), however, future research is critical to understanding the prevalence and consequences of LEA and metabolic considerations across all impairment groups. Currently, a challenge for practitioners is the accuracy and validity of assessment tools available for monitoring EA, as all practical methods were developed with no consideration of athletes with an impairment. Even though many para-athletes may be at high risk for developing RED-S, we have no normative data or valid assessment tools to accurately monitor resting metabolic rate, EE, and RED-S in athletes with an impairment.

The present review has outlined the neurophysiology of the most common impairment groups and the practical considerations when supporting para-athletes along with performance enhancing interventions. We acknowledge that a highly individualized approach to supporting para-athletes is needed due to the variability not only between but within impairment groups and emphasize the need to further enhance our approach to providing practitioners with evidence-based research. We suggest that this may be achieved by various knowledge translation strategies, including (i) modules to educate current and future practitioners in the para-sport field, (ii) encouraging para-sport practitioners to publish and/or present on unique field observations, and/or (iii) the sharing of data between sport systems including cross collaborative projects between research groups from different nations as well as sports with para-athletes of similar impairment groups.

All authors provided substantial contributions to the conception and design of the work and drafting the work or revising it critically. Final approval of the version submitted/published and consent for publication has been agreed by all authors.

CG and EG were supported in the writing of this manuscript by funding from Own the Podium.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The reviewer EB declared a past co-authorship with one of the authors TS to the handling editor.

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