High-Intensity Interval Training in Older Adults: a Scoping Review

High-intensity interval training (HIIT) is an increasingly popular form of aerobic exercise which includes bouts of high-intensity exercise interspersed with periods of rest. The health benefits, risks, and optimal design of HIIT are still unclear. Further, most research on HIIT has been done in young and middle-aged adults, and as such, the tolerability and effects in senior populations are less well-known. The purpose of this scoping review was to characterize HIIT research that has been done in older adults including protocols, feasibility, and safety and to identify gaps in the current knowledge. Five databases were searched with variations of the terms, “high-intensity interval training” and “older adults” for experimental or quasi-experimental studies published in or after 2009. Studies were included if they had a treatment group with a mean age of 65 years or older who did HIIT, exclusively. Of 4644 papers identified, 69 met the inclusion criteria. The average duration of training was 7.9 (7.0) weeks (mean [SD]) and protocols ranged widely. The average sample size was 47.0 (65.2) subjects (mean [SD]). Healthy populations were the most studied group (n = 30), followed by subjects with cardiovascular (n = 12) or cardiac disease (n = 9), metabolic dysfunction (n = 8), and others (n = 10). The most common primary outcomes included changes in cardiorespiratory fitness (such as VO2peak) as well as feasibility and safety of the protocols as measured by the number of participant dropouts, adverse events, and compliance rate. HIIT protocols were diverse but were generally well-tolerated and may confer many health advantages to older adults. Larger studies and more research in clinical populations most representative of older adults are needed to further evaluate the clinical effects of HIIT in these groups.


Introduction
Globally, the number of people aged 65 years or older is expected to more than double in the next 30 years, making it the fastest growing age demographic [1]. It has been estimated that these older adults experience 23% of the global burden of disease and that this number increases to nearly 50% in high-income countries and is about 20% in low-and middle-income countries [2]. Chronic non-communicable diseases make up the majority of this burden [2]. This accounts for a significant and growing financial burden on our health care systems and the UN National Assembly (2012) has acknowledged the urgent need for governments to scale up and transition towards universal, affordable, and quality health-care services [3]. Exercise is known to be an important part of healthy aging and is useful in preventing and managing chronic disease [4]. The Physical Activity Guidelines for Americans [5] recommend that adults over 65 years of age achieve at least 150 min of moderate-or 75 min of vigorous-intensity aerobic physical activity per week, in addition to muscle-and bonestrengthening activities at least 2 days per week. They also highlight that in this population, exercise can be done to improve both health outcomes and functional abilities and that they should include multicomponent training that incorporates balance and flexibility training [5]. As such, identifying modes of exercise which achieve these goals and are tolerable and feasible in older adults is an important step towards improving the health of these populations.
High-intensity interval training (HIIT) is an increasingly popular form of aerobic exercise which includes bouts of high-intensity exercise, typically lasting seconds to minutes, interspersed with periods of rest [6]. HIIT has been proposed to be equal or advantageous to continuous endurance training both in terms of physiologic results [7,8] and in enjoyability [9]. However, the health benefits, risks, and optimal design of HIIT are still unclear. Further, most of the research on the effects and benefits of HIIT has been done in younger and middleaged adults, and as such, the tolerability and effects in older populations are less well-known.
The purpose of this scoping review is to identify and characterize existing research on the effects of HIIT in older adults to assist in knowledge translation and recommend further areas of study. Specifically, it aims to describe which study populations are included, the training protocol designs, whether this training is feasible and/or tolerable for older adults, the main outcomes being addressed, and to identify gaps in the current knowledge.

Methodological Framework
According to Arksey and O'Malley [10], the purpose of a scoping review is to examine the extent, range, and nature of research activity, to summarize research findings, or to identify gaps in the existing literature. To achieve this, they established a 5-step framework which was used in the undertaking of this review. The steps are detailed below and include the following: 1. Identifying the research question, 2. Identifying relevant studies, 3. Study selection, 4. Charting the data, 5. Collating, summarizing, and reporting the results [10].
The research question was as follows: What is known in the literature about HIIT in older adults, including which protocols are used, outcomes measured, its feasibility and safety in this population, and what are the gaps in the current knowledge?
A full description of the study protocol including search strategy and detailed reasons for article exclusion are available in the supplemental materials. In summary, five databases were searched (Scopus, Medline (Ovid), Embase (Ovid), CINAHL, and SportDiscus) for articles published up to February 2021. Search terms included combinations and variations of the following: "high-intensity interval training," "interval aerobic training," "HIIT," "older adult," and "senior." A description of the complete search strategy is included in the supplemental materials. These searches identified 4644 potential studies. Of these, 2019 references were removed as duplicates. The non-duplicate titles and abstracts were read by authors CM and AP to determine if the studies were relevant to the research question. Initial exclusion criteria were as follows: (1) published before 2009 (as it was not feasible for the authors to screen all of the potential studies and the authors wanted to include the most recent and relevant studies), (2) review papers or not peer-reviewed, (3) did not include high-intensity exercise protocols, (4) did not use human subjects, or (5) the mean age of the study subjects was less than 50 years old.
In keeping with the iterative nature of scoping review methods, inclusion criteria were then developed in collaboration with author RP. The inclusion criteria were as follows: (1) the mean age of all participants was at least 65 years of age or older, or one mean cohort age was at least 65 years and was not statistically different from the other groups, (2) the study was an experimental or semiexperimental trial, (3) the study was an original source (for example, letters to the editor, correspondences, and editorials were not included) published as full-text in English, and (4) the exercise protocol used was exclusively high-intensity interval aerobic training, and was not combined with another intervention, such as resistance training (RT). Many exercise training modalities which have some similarities to HIIT were excluded from this review. These included RT or high-intensity resistance training, which primarily aims to overload the musculoskeletal system by causing the muscles to contract against an external force [11], circuit training and body-weight interval training (which includes RT), moderate-intensity interval training (MIT), and moderate-intensity continuous training (MCT). Additionally, high-intensity functional exercise is a form of functional weight-bearing exercise training designed for the elderly populations dependent on activities of daily living. This type of training more closely resembles RT than HIIT, and as such, it was also excluded from this review [12]. Exercise intensity is often measured using heart rate (HR), heart rate reserve (HRR), or oxygen uptake (VO 2 ). "High-intensity" was defined and categorized as vigorous effort (70-89% of peak HR; 60-84% of HRR; 60-79% of peak VO 2 ) or "very hard" effort (≥ 90% of peak HR; ≥ 85% of HRR; ≥ 80% of peak VO 2 ) [13,14]. The anaerobic threshold (VT 2 ) was included as highintensity, but the aerobic threshold (VT 1 ) was not [15]. The percentage of peak power output (%PPO) is occasionally used to measure and report exercise intensity. There are reports that %PPO does not correlate with the same percentage of HR max in different exercise modalities/patient populations [16]. To illustrate this point, Hood et al. [17] used a target intensity of 60% PPO which correlated with approximately 80% HRR and increased over time to 95% HRR. Where a range of target intensities was described in the study protocols, the average intensity was used to determine eligibility. For example, if the study had participants exercise at 60-70% HR peak , it was excluded as the average intensity was presumed to be 65% HR peak . Sprint interval training (SIT) is an interval exercise involving maximal or supramaximal intensity activity for short periods of time (typically seconds) [7]. Though different from HIIT, SIT treatment groups were included in this review as they may offer further insight into the tolerability, safety, and acceptance of similar interval protocols. The remaining articles were read in full by authors CM and AP to assess for eligibility. Any discrepancy was discussed by these authors until consensus was achieved.
The eligible studies were read and grouped by clinical populations. Data from these studies were extracted and charted by CM, including the population(s) studied, the study design, and the main outcomes measured. Details of the HIIT protocol intervention were also charted and included exercise frequency, intensity and duration of interval, intensity and duration of the rest period, and modality (such as treadmill, cycling, etc.). If it was noted in the publication, information on whether the HIIT was feasible and/or tolerated by study participants was also extracted. This was done by measuring outcomes such as attendance, adherence, drop-outs/withdrawals, "enjoyability" or acceptance of protocol, and adverse events. These data were validated by research assistant, EM. In charting the study design, the HIIT interventions were summarized to allow for ease of comparison between studies. Controls or other treatment groups compared to HIIT in the literature were noted and included RT and MCT. MCT was defined as an intensity of 55-69% HR max or 40-59% VO 2max and is representative of typical endurance training [13]. RT was defined as exercise primarily aiming to overload the musculoskeletal system by causing the muscles to contract against an external force [11]. Data were summarized and reported as per the emerging themes.

Results
The search yielded 4644 references. Duplicates were removed and inclusion and exclusion criteria were applied.
Age was a common reason for exclusion. As such, if the mean age was not provided in the abstract, this information was found in the full text as part of the screening process. In the papers assessed for eligibility, resistance or circuit training were often combined with HIIT. However, as many papers focused exclusively on HIIT, these combined programs were excluded from the final subset. This left 69 studies to be included in the review (Fig. 1).
The studies included were classified by clinical population. The largest grouping was of non-clinical populations (n = 30), followed by cardiovascular diseases (n = 12), cardiac disease (n = 9), metabolic disease (n = 8), and other (n = 10). Study design, sample size, population(s) included, and baseline characteristics of included studies, grouped by clinical cohort, are reported in Table 1. These studies had sample sizes ranging from 10 to 473 (mean [SD] = 47.0 [65.2]) and included a total of 3243 individuals. The mean ages of the study participants ranged from 61.4 to 80.8 years (mean [SD] = 67.9 [3.4] years). Forty-eight studies were randomized controlled or crossover designs; 21 were quasi-experimental. Tables 2, 3, 4, 5, and 6 show the HIIT protocols, outcomes, and feasibility/tolerability (where available) of studies grouped by clinical population. Across all clinical populations, the interval training interventions ranged from a single session (n = 14) to 6 months (n = 4) of training (mean [SD] = 7.9 [7.0] weeks). In non-acute training interventions, session frequency ranged from 2 to 5 training sessions per week. The training interventions included SIT (n = 12), and intervals defined by Buchheit and Laursen [86] as short (< 1 min) (n = 3), and long (≥ 1 min) (n = 57) in duration. The most common modality for achieving HIIT was to use a cycle ergometer (n = 46) followed by treadmills/walking, waterbased aerobic training, all-extremity non-weight-bearing ergometers, and recumbent steppers. Most training interventions measured the intensity using HR or VO 2 achieved (as percentage of HR peak , HR max , VO 2peak , or VO 2max ). However, some used percentage of PPO or work (W) (W peak , W max ) as metrics. Where it was reported, most authors agreed that the HIIT intervention was generally well-tolerated by study participants. The protocols used, outcomes, and feasibility findings were further examined by clinical groups. Trends seen are discussed below.

Non-clinical Populations
Studies including non-clinical populations made up the largest subgroup in this scoping review (n = 30) and included people who were sedentary or active at baseline and were typically free of significant disease or had wellcontrolled medical conditions (Table 2) [18-46, 87, 88]. Studies in this category were more likely to use SIT as the high-intensity exercise intervention (n = 7) and the most common exercise modality was cycling (n = 22). In HIIT studies, the interval durations were most commonly 1 or 4 min. Notably, a large number of these studies examined the effects of HIIT after only a single session (n = 11). Where HIIT was compared to MCT after a single training session in this group, HIIT generally caused more significant attenuation of flowmediated dilation (FMD) than MCT [20,46]. Nederveen    et al. [37] found that HIIT induced greater satellite cell response to exercise than MCT while both had similar completion rates. Stockwell et al. [40] reported that across a single session, HIIT was generally preferred over MCT by participants. Six studies examined HIIT vs. MCT over longer training periods [21,23,24,[28][29][30]. Here, HIIT was found to be both tolerable and feasible and had a greater impact on VO 2peak [21,30], ejection fraction, and insulin resistance compared to MCT [28]. Kim and colleagues found that arterial stiffness improved only in MCT and not after HIIT [29]. In regards to memory and cognition, high-interference memory was found to be improved after HIIT but not after MCT [30]. Brown et al. (2021) found there to be no direct impact of exercise (HIIT or MCT) on cognition [21].

Cardiovascular Disease
12 studies examined the impacts of HIIT in people with known cardiovascular disease including coronary artery disease, hypertension, stroke, abdominal aortic aneurysms, and peripheral arterial disease ( Table 3) [47][48][49][50][51][52][53][54][55][56][57][58]. The most common training intervention in these populations was a single HIIT session (n = 7). Four of the study protocols used short intervals lasting 1 min in duration, alternating with rest periods also lasting 1 min. Six studies in this group measured interval intensity by power or work output. The most common exercise modality used was cycling (n = 8).
Where studies examined outcomes of acute HIIT sessions compared to acute MCT sessions, some conflicting results were found. Bailey et al. [47] found that HIIT causes a decrease in flow-mediated dilation (FMD) while MCT causes this to increase. Currie et al. found that both HIIT and MCT cause an increase in FMD [48] and similar findings were reported by the same authors in longer training interventions [49]. HIIT was found to induce greater hypotensive effects compared to MCT by both dos Santos et al. [50] and Sosner et al. [56]. One study reported that participants in this category demonstrated a preference for HIIT over MCT [50]. Across population cohorts, there were mixed results as to the effects of HIIT compared to continuous training on blood pressure. Blood pressure was shown to decrease from baseline to 1 h after interval training, up to 10 mmHg in systolic blood pressure, though it was variable whether or not this was statistically greater than a MCT group [50,56,59]. Reichert et al. had participants with hypertension complete 28 weeks of HIIT or HCT [55]. Here, it was found that systolic and diastolic blood pressures were decreased in both training groups but that the diastolic decrease was greater in the continuous training group [55].

Cardiac Disease
Of the 9 studies which employed HIIT in patients with heart failure [59][60][61][62][63][64][65][66][67], the majority (n = 6) used the "4 × 4" HIIT protocol (Table 4). This intervention includes 4 bouts of long, 4-min intervals at high intensity, interspersed with 3-min rest periods. Every study in this population involved a non-acute training session of 4 to 12 weeks in duration, with 12 weeks being the most common (n = 7). Cycle ergometers and treadmills were the most common means of exercising in this group.
In studies where HIIT was compared to MCT, HIIT was found to result in a larger reduction in blood pressure [59,62] and had a greater or similar improvement in VO 2peak [59][60][61][62]67]. HIIT was also found to have similar effects compared to MCT on left ventricular end-diastolic function and left ventricular end-diastolic diameter [60,62] and on metabolic improvements [62].

Metabolic Disease
Studies considered to be metabolic diseases included populations with type 2 diabetes mellitus (T2DM) [71-  There was a significant increase in bone mineral density of the lumbar spine, total femur, and whole body of HIIT compared to the control group. Functional ability was also improved in HIIT compared to control as measured by the timed up-and-go test (improved by − 11 ± 4%) and Chair Stand (improved by 17 ± 3%).   (12); −9(9)mmHg, P= 0.002) groups. ere was no significant change in either the RIPC (138 (15) vs. 134 (14); −4(5), P= 0.17) or control (130 (10) vs. 128(10); −1(6), P= 0.96) groups Systolic blood pressure significantly decreased in the HIIT (− 9 ± 9 mmHg) and isometric handgrip training groups (− 9 ± 9 mmHg). There was no significant change in the control or remote ischemic preconditioning groups. Frequency: 4×/week for 8 weeks Intervals: 4 intervals at 90% HR peak for 4 min Rest: 3 min at 70% HR peak Modality: Non-weight-bearing all-extremity ergometer Primary outcome-see feasibility and tolerability. Secondary outcomes-VO 2peak improved by 11% and ejection fraction improved by 4% in HIIT, no change was seen in MCT or control. Insulin resistance decreased by 26% in the HIIT group only. Diastolic function, body composition, lipid, and glucose did not change.
Dropouts: Of 51 participants randomized, 16% did not complete the study (control: 2 unable to contact; MCT: 2 lack of motivation, 1 family conflict, 1 schedule conflict; HIIT: 1 family conflict, 1 schedule conflict) Compliance: rate by participants was 88% AEs: None in HIIT HIIT was deemed to be feasible Kim et al. (2017) [29] Frequency: 4×/week for 8 weeks Intervals: 4 intervals at 90% HR peak for 4 min Rest: 3 min active recovery at 70% HR peak Modality: All-extremity non-weight-bearing ergometer Arterial stiffness improved after MCT only (decrease in carotid to femoral pulse wave velocity and increase in common carotid artery compliance). No change was seen in HIIT.
Dropouts: 9 of 49 subjects did not complete intervention (HIIT: family issues, schedule conflict; MCT: lack of motivation, inability to contact for follow-up. 2 subjects were excluded from analysis (1 in MCT due to unrelated illness, 1 in control group due to noncompliance). Compliance: Similar between MCT and HIIT (both 90%). AEs: None reported Exercise training was described as "welltolerated." Kovacevic et al. (2020) [30] Frequency: 3×/week for 12 weeks Intervals: 4 intervals at 90-95% HR peak for 4 min Rest: 3 min active recovery at 50-70% HR peak Modality: Treadmill HIIT and MCT interventions induced similar cardiorespiratory fitness adaptations from preto post-test, such that exercise training led to the greatest increases in predicted VO 2peak . High-interference memory significantly improved following HIIT but not MCT or control. There was no main effect of group on brainderived neurotrophic factor Dropouts: 13 participants withdrew during training (HIIT = 3, MCT = 5, stretching = 5) Compliance: All participants completed at least half of training protocol. 3 SIT protocols: 1) 6 intervals of 5 s at "all-out" intensity (90-s rest), 2) 3 intervals of 30 s at "all-out" intensity (4min rest), or 3) 3 intervals of 60 s at submaximal intensity (4-min rest) Frequency: Single session Modality: Cycle ergometer Decrease in torque ratio represented lowfrequency fatigue and was more evident in the 30 s and 60 s protocols. In young women, low volume (6 × 5 s) exercise induces physiological stress and is effective. In older women, longer intervals (3 × 60 s) are more stressful than shorter but still tolerable.
Acceptance of protocol: 6 × 5 s cycling was the most preferred method in both age groups. Perceived enjoyment was similar in both groups. Linares et al. (2020) [32] Frequency: Single session Intervals: 10 intervals at 90% PPO for 1 min Rest: 1 min at 10% PPO Modality: Cycle ergometer There were similar peaks in oxygen consumption (in women alone) and in HR (both women and men) when comparing HIIT and maximal exercise. There was a greater cardiopulmonary response to HIIT compared with MCT. When PPO was used for exercise prescription there was considerable individual variability in work intensity seen.

McSween
Frequency: Single session In lower baseline learning performers, the MCT Not given  [68,69], and obesity [70,74] (Table  5). Similar to protocols used in the cardiac disease grouping, these interventions tended to have variable durations of training, ranging from 2 to 16 weeks. The most common duration of training was for 8 weeks (n = 3). A treadmill-based exercise was the most common modality in these populations (n = 4).
In studies comparing HIIT to MCT, the results can be divided by length of intervention. In studies of 8 weeks duration, little difference was seen between HIIT and MCT in terms of change in body composition, metabolic profile, cardiovascular risk, and aerobic capacity [70,71]. In studies lasting 12 and 16 weeks, HIIT was seen to have a larger decrease compared to MCT in total abdominal and visceral fat mass [73] and in BMI and metabolic profile [75]. HIIT was found to be similarly enjoyed and tolerated compared to MCT [70,71]. Maillard et al. [73] observed that over a 16-week intervention, both HIIT and MCT groups had a similar decrease in whole-body fat mass of (mean ± SD) − 2.5 ± 1.3% in the HIIT group, and − 3.2 ± 1.2% in the MCT group. Notably, after this longer intervention, there was only one reported dropout from the MCT group for personal reasons.

Other Clinical Populations
Other studies examined HIIT in populations with chronic obstructive pulmonary disease (COPD) [83,84], Parkinson's disease [78,85], Alzheimer's disease [79], cancer [76,77,82], knee osteoarthritis [80], and systemic sclerosis [81] (Table 6). Cycling was the most common exercise modality in these groups (n = 8). Interventions with COPD populations lasted between 8 and 12 weeks. In this population, HIIT was found to have a similar impact on cardiac autonomic function, aerobic fitness, tolerability, and compliance compared to MCT [83,84]. In   [79] showed that 76% of the HIIT group attended more than 80% of the sessions and 78% of the participants exercised at over 70% of their HR max .
There were 35 adverse events and 7 serious adverse events reported during this study. The adverse events suspected to be related to the intervention included musculoskeletal problems, dizziness, and one episode of atrial fibrillation. In this study, there was no change from baseline in cognitive scores, quality of life, or activities of daily living in the HIIT group. Across all studies in this diverse range of clinical populations, HIIT was generally found to have a high exercise adherence [76,[79][80][81][82]85]. Where it was compared to MCT, attendance ranged from 70.1-94% of sessions and was similar between treatment groups [83,85].

Discussion
This scoping review characterized existing literature on HIIT in older adults including exercise protocols administered, main outcomes, and feasibility and safety. The purpose of this review was to assist in knowledge translation for clinicians, as well as to recommend areas of further research. In brief, 69 studies involving 3243  In HIIT only, significant increase in VO 2peak was seen. Some improvements in anxiety and depression scores (SF-36 and HADS-D) were seen in HIIT group at 12 weeks. At 2-year follow-up, the HIIT group had maintained scores, or scores trended towards baseline values. This was significantly improved over controls who had no change at 12 weeks and had deteriorated scores at 2-year follow-up. At 2-year follow-up, the control group reported significantly more time spent sitting during the day compared to the HIIT group. Nonsignificantly, the HIIT group also had more physical activity per week. No significant differences between groups regarding hospitalization and implantable cardioverter defibrillator shocks at 2-year follow-up. In both HF and healthy populations, HIIT increased aerobic capacity and improved ability to override sympathetic vasoconstriction (arterial infusion of tyramine) during exercise. The peak vasodilatory Not given individuals belonging to both clinical and non-clinical populations were included in this review. The main findings and recommendations of this scoping review are discussed below.

HIIT Protocols Used for Older Adults
The studies in this review used a range of HIIT protocols which varied in frequency and duration (single sessions to 6-month interventions), length of interval (seconds to minutes), intensity of interval (70% HR peak to supramaximal intensity), and modality. In spite of the lack of consensus on which protocols are optimal for maximal benefits, some protocols were seen more commonly with certain groups. The "4 × 4" protocol (n = 15), which is 4 bouts of 4-min intervals interspersed with 3-min rest-usually 3×/week for at least 4 weeks-was most popular for use in subjects with heart failure [59,60,[62][63][64]66], but was also used in non-clinical populations [28-30, 33, 45, 46, 87], peripheral arterial disease [52], T2DM [71], and in colorectal cancer survivors [77]. It has been reported previously that longer work intervals and higher weekly energy expenditure may be more effective in increasing adaptations such as VO 2max and cardiac function in populations with heart failure [89]. Protocols of alternating 1-min high intensity and 1-min rest for 10-12 intervals ("10 × 1" protocol) were the next most common in this age group (n = 9). In all but one study, these 10 × 1 protocols were used in single training sessions. As such, it is difficult to measure the effects as well as tolerability and feasibility of longer interventions in this age group. SIT was also commonly seen (n = 10).
With the exception of one study in participants with limited cutaneous systemic sclerosis [81], all studies using SIT were in healthy populations. Cycle ergometers were the most commonly used means of facilitating HIIT in this age group, followed by treadmills. These methods are likely to be among the most accessible to individuals who want to incorporate HIIT into their exercise routines, either at a gym or by biking or walking/running outdoors. Less commonly used methods of achieving aerobic exercise included aquatic training, non-weight-bearing all-extremity ergometers, and recumbent steppers. Although these modalities may seem less accessible to the average older adult, these alternative modalities are important to continue to include in future research as they may allow for greater participation among older adults with mobility limitations.

HIIT and Aerobic Fitness
The most common outcome measured in the included studies was a change in VO 2peak as a measure of aerobic fitness. Across clinical and non-clinical populations, HIIT was shown to increase VO 2peak more than the control [21, 27, 28, 30, 34, 45, 52, 60, 62-64, 67, 71, 81, 85]. Conversely, one study found no change between HIIT and control [57]. Where change in VO 2peak or PPO was compared between age cohorts, HIIT was shown to increase aerobic fitness similarly across both younger and older age groups [41,43,87]. When directly compared between two groups in one study, HIIT was also shown to increase aerobic capacity between a healthy cohort and a cohort with heart failure [65]. The evidence becomes conflicting when comparing change in VO 2peak between HIIT and traditional endurance training. Most   [30,49,60,62,67,70,71,81,84,85]. However, some studies found HIIT to be superior [21,28,59,61,82]. These studies were diverse in clinical populations as well as in HIIT protocols, and as such, it remains unclear which factors are most likely to correlate with maximum success of the intervention. These results are suggestive that HIIT can be an effective means of improving aerobic fitness in older adults, and that it may confer a small advantage over traditional endurance training. This is consistent with the existing literature on non-elderly adults which showed that HIIT may have a small benefit compared to MCT on improving VO 2peak , but that this improvement is likely to be increased by longer intervals and greater work-to-rest ratios, and in older or less-fit subjects [90,91].

HIIT and Vascular Outcomes
The most common outcomes of vascular function included in the literature were blood pressure measurements (systolic, diastolic, 24-h ambulatory) and FMD, and these yielded mixed results. A decrease in blood pressure was larger in HIIT when compared to RT [22,38] and when compared to MCT [38]. In one study, there was no change seen in ambulatory blood pressure in either MCT or HIIT group [62]. In studies measuring FMD, results were variable. In non-acute studies, no change was seen in FMD between MCT and HIIT [49,59,67]. In acute studies, changes in FMD varied depending on the cohort's fitness [20], sex [46], and were conflicting whether HIIT attenuated, increased, or had no impact on FMD compared to MCT [47,48]. One study examined change in arterial stiffness, and this was only seen to improve after MCT [29].

HIIT and Cardiac Function
Measures of cardiac function were examined in populations with heart failure and in one study of non-clinical populations. HIIT was found to increase left ventricular end-diastolic diameter compared to control either similar to MCT [60] or to be superior to MCT [28,59]. Conversely, HIIT was not seen to impact cardiac function in three studies [62,66,67].

HIIT and Metabolic Factors
Metabolic factors were most often measured in populations with obesity, T2DM, and pre-diabetes, as well as in non-clinical populations. In these studies, glycemic control was seen to improve compared to controls and was superior compared to MCT [28,72] or to be similar to MCT [62]. Most studies using body fat or body composition as an outcome measure, including a study in participants with knee osteoarthritis, did not find that interval or continuous training resulted in significant changes [70,72,80]. Maillard et al. however, observed that HIIT resulted in a larger decrease in total abdominal and visceral fat mass, but not whole-body fat compared to MCT [73]. Conversely, Hwang et al. noted that MCT had a larger decrease in body fat than HIIT [71]. Regarding lipid markers and cholesterol changes, Boukabous et al. found that HIIT and MCT had resulted in similar improvements [70] whereas no change was seen by Hwang et al. [71].
In non-elderly adults, two systematic reviews and metaanalyses by Keating et al. [92] and Wewege et al. [93] compared the effect of HIIT or SIT to MCT on changes in body adiposity. Both studies found interval and continuous training to result in similarly decreased total body fat over a range of intervention durations. This is similar to the findings of this scoping review which identified that the HIIT interventions ≤ 8 weeks in duration had a similar effect on body composition and adiposity compared to MCT. The findings of this scoping review, however, suggest that longer interventions in older adults may reveal higher efficacy of HIIT compared to MCT.

HIIT and Neurocognitive Decline
Neurocognitive decline is prevalent among adults of advanced age and is an important contributor to reduced health and quality of life [94]. Three studies included in   [83] Acute bout followed by 12-week training intervention Responses to acute bout: All but one subject were able to achieve the target exercise Compliance: Mean attendance not significantly different between groups, means were 70.1-this review used HIIT in populations with known cognitive decline: two in Parkinson's disease [78,85], and one in mild Alzheimer's disease [79]. Two more examined cognitive outcomes in non-clinical populations [21,33]. Of these studies which measured cognitive outcomes in both HIIT and MCT, in some, HIIT was not found to be associated with any improvement on cognitive scores [21,33,79]. Conversely, in one study, it was also found to improve immediate auditory memory similar to MCT and improve both attention and sustained attention greater than MCT [78]. In Northey et al.'s study on breast cancer survivors, cognitive performance was also measured, and HIIT was seen to have a moderate to large positive but statistically insignificant effect compared to MCT and control [82].

HIIT and Osteoarthritis
Only one study examined HIIT in osteoarthritis, and it showed that HIIT had higher adherence than the MCT group (94% compared to 88%) and that both groups had improvements in health-related quality of life scores. Though neither intervention resulted in a change in body composition, HIIT was shown to improve physical function similar to or greater than MCT [80]. Though this is only one small study, these results are supportive of HIIT use in this population and further research should be pursued.

Feasibility and Tolerability of HIIT
One of the aims of this review was to examine whether HIIT protocols are feasible and/or tolerable in the older adult population. The HIIT protocols employed in the included studies were generally well tolerated, had few adverse events which were comparable to those of MCT interventions, and had good attendance which was also comparable to attendance for MCT interventions. One exception to this was the 6-month study by Uc et al. [85], where three participants in the HIIT group dropped out due to exercise-related knee pain. After initially randomizing participants to HIIT or MCT, later cohorts in this study were allocated to MCT only. In several studies, HIIT was deemed to be as or more enjoyable by participants than MCT [40,50,81].

Recommendations and Future Directions
Of the studies included in this review, only 57% examined the impact of HIIT in clinical populations and many prevalent chronic diseases among the elderly were not represented at all. This percentage is low when considering that in some countries, more than 85% of people over the age of 65 are reported to live with at least one chronic Table 6 HIIT studies in other clinical populations (Continued) medical condition [95,96] and a significant proportion of those may have multiple medical comorbidities [95,97]. Cardiovascular diseases, including chronic ischemic heart disease, CHF, and arrhythmia are the leading causes of death in older adults followed by cancer [94]. These were present in the literature, although given their significant mortality, they should be further studied. Additionally, the most common chronic diseases in adults over 85 years of age are hypertension, osteoarthritis, T2DM, and osteoporosis [94]. Notably, osteoarthritis was represented in only one small study and clinical groups with osteoporosis were absent from the available literature altogether. Other prevalent conditions among the elderly include frailty and depression [94]. More studies on HIIT in these populations will be important additions to the existing knowledge of its impact and tolerability.
There is still no consensus on which HIIT protocol is most effective in older adults. Though some HIIT protocols appeared in the literature more often, namely the "4 × 4" and the "10 × 1" protocols, there was still much variation in frequency and duration of training, clinical population studied, and outcomes measured. As such, it was very difficult to directly compare which of these common training methods are most likely to induce training results. Further research should compare these and other HIIT and SIT protocols for clinical outcomes as well as feasibility and tolerability.
The current Physical Activity Guidelines for Americans were established for adults over 65 years of age with emphasis on preserving or improving functional independence and quality of life [5]. These goals are not adequately represented by the measured outcomes of the included studies and as such, further research is needed.

Conclusion
In summary, as the global population continues to age, early research on the impact of aerobic HIIT in older adults suggests that this training method is generally well-tolerated, feasible, and may confer many health advantages to this population. The majority of studies included in this review were in non-clinical populations and compared HIIT to MCT or to a control group. These studies are still few in number, small in sample sizes, and do not yet represent the scope of chronic diseases which are nearly ubiquitous in this age group. As such, more research is needed on the effects, feasibility, and tolerability of HIIT in these clinical populations in order to support our aging population with best health practices and exercise recommendations.