Severe mental illnesses (SMI), including schizophrenia spectrum disorder, bipolar disorder and major depressive disorder (MDD) are leading causes of years lived with disability, affecting approximately 3–5% of the population (1–3). Along with the impact of the mental health symptoms and reduced daily functioning, SMIs are associated with physical health comorbidity and an associated premature mortality amounting to 13–20 years (4, 5). Cardiometabolic diseases, including the metabolic syndrome, diabetes mellitus, obesity, and coronary heart disease are two-fold higher in people with SMI (6, 7). In addition, people with SMI typically experience cognitive impairment, socio-occupational difficulties and reduced quality of life (QoL) which may worsen over time (8–11).
The newly published WHO guidelines report that frequent physical activity and structured exercise have a beneficial impact on cardiometabolic and cardiovascular disease risk, and ameliorate symptoms and QoL for those living with these complications (12). Further, regular exercise can improve mental health, cognition and sleep quality, in both the general population and those with severe mental health conditions (13–16), and may aid smoking cessation in the long term (17, 18). Acute exercise bouts can also have immediate effects on appetite and cigarette cravings (19–21), cognitive functioning (22), and positive wellbeing in healthy and clinical populations (22–24). This said, people with SMI engage in significantly less moderate and vigorous intensity exercise than the general population and are less likely to meet physical activity guidelines of 150 minutes of exercise per week (25). Thus, low levels of physical activity are recognised as contributing towards physical, mental and cognitive ill health in this population group (13), along with higher levels of tobacco consumption and antipsychotic induced weight gain. Physical activity is a modifiable lifestyle risk factor; thus, a focus of current research is to increase levels of physical activity in people with SMI and to promote uptake of structured exercise interventions (26). For example, the recent European Psychiatric Association (EPA) guidance draws on 20 past systematic reviews in SMI population and suggests physical activity should be used as an adjunctive treatment in SMI to improve mental health symptoms, physical health, cognition and quality of life (13).
High intensity interval training (HIIT) is a type of exercise characterised by alternating short bursts (typically 30 seconds to 4 minutes) of high intensity exercise interspersed by similar length periods of light exercise or rest, repeated for typically 10-25minutes. This popular fitness approach yields positive effects on physical and mental health in the general population and may be more beneficial in terms of improvements in cardiometabolic health when compared to traditional moderate intensity continuous training (MICT) approaches (27). Preliminary work has sought to establish the effectiveness of HIIT in people with schizophrenia spectrum disorders (28–34), major depressive disorders (35–38), and adults with self-reported psychiatric problems (39) in interventions of duration 12 days to 6 months. This work has been summarised in two recent meta-analyses, whereby significant improvements in depressive symptoms and cardiorespiratory fitness were reported post HIIT intervention, and there was a suggestion that HIIT may be more beneficial in terms of improvements in depressive symptoms when compared to MICT (40, 41). Further, HIIT may improve cognitive measures including verbal learning and overall neurocognition (34). This said, there is inconsistency concerning the effects of HIIT on psychopathology, social and global functioning, and anthropometric measures, which is hampered by small sample sizes and a paucity of clinical trials. Total number of participants with an SMI allocated to HIIT ranges from 8 (39) to 43 (28, 34) across trials, and meta-analysis data summarised findings from just 366 participants with SMI, including comparison groups. To add, previous work in people with SMI has reported a low rate of adverse events (AE) including no acute injuries or cardiovascular events, completion rates of approximately 71% and a mean attendance at sessions of 74%, suggesting good feasibility and acceptability, although these measures have only been reported in roughly half of HIIT trials in people with SMI to date (40, 41).
To add, two trials have sought to establish the acute effects of HIIT in those with a mental illness (42, 43). In patients with schizophrenia and depression, there was an improvement in positive affect and wellbeing, and a reduction in psychological distress and state anxiety from pre-training to 15 minutes post HIIT (42). In adolescents hospitalised with MDD, suicidal ideation, stress and anxiety disorders, there was a suggestion that acute bouts of HIIT may improve inhibitory control for up to 30 minutes post exercise (43).
Despite the initial evidential support for HIIT in people with SMI, little work has been undertaken with patients receiving treatment on psychiatric wards, in particular in those with schizophrenia spectrum disorders. To date, four papers have assessed the effect of HIIT in inpatients with MDD (35–38), one has assessed HIIT in inpatients with schizophrenia (31), and another has looked at the acute effects of a single bout of HIIT in adolescents receiving inpatient mental health treatment (43), highlighting a need for further research in this population and setting. Moreover, a recent qualitative analysis explored perspectives on implementing HIIT interventions in inpatient mental health settings (44). Across seven focus groups, in inpatients with SMI, carer and staff groups, HIIT was seen positively, with beliefs that it would help inpatients feel more relaxed, build their fitness, and provide a break from the monotony of ward environments. This said, concerns were noted related to patient motivation, safety, especially for those with chronic physical health comorbidities, and practical logistical factors, including having access to the right sports clothing and staff availability to supervise (44).
The primary aim of the study, therefore, is to determine whether HIIT is acceptable and feasible amongst inpatients with a broad range of severe mental illnesses, and the secondary aim is to investigate if the HIIT intervention improves mental health symptoms, including psychiatric symptoms, depression, anxiety, stress, sleep and mental wellbeing; cognition; and physical activity measures including increases in weekly physical activity, motivation to engage in exercise and anthropometric measures. The tertiary aim is to determine whether a single bout of HIIT leads to acute changes in psychological states and appetite and cigarette cravings.