Prison populations are growing in many Western countries (Pérez-Moreno et al., 2007). The overriding goal of prisons is resocialization and sport is considered to play a significant role in resocialization. Sport activities have a high priority in prisons and represent the most popular leisure activity of inmates (Kuhn, 2019). Participation in sports allows inmates to overcome the monotony and boredom of daily prison routines (Müller and Mutz, 2019). Nevertheless, studies examining the impact of sport-based interventions within prison populations were described as limited (Meek and Lewis, 2014) and embryonic (Gallant et al., 2015).

A few studies have examined outcomes of sport-based interventions in prisons. For example, Gettman et al. (1976) studied running programs of different frequencies and reported significant improvements in maximum oxygen uptake, resting and recovery heart rates, maximum treadmill stress test, oxygen pulse, and pulmonary ventilation. The inmates in Pérez-Moreno et al. (2007), who were HIV/HVC co-infected and enrolled in a methadone maintenance program, could enhance their cardiorespiratory fitness in a gradual cycle ergometer test and their dynamic strength. Male inmates with chronic illness in an Australian prison (Cashin et al., 2008) have achieved significant improvements in resting heart rate and aerobic endurance (6-min walk test). Battaglia et al. (2013) showed that a cardiovascular and resistance training (CRT) is more effective than a high-intensity strength training (HIST): performance in a 10 × 5 shuttle run test, dynamic strength endurance, and estimated maximum oxygen uptake improved as a result of both types of training, while the body-mass-index (BMI) decreased only through CRT.

What all four studies have in common is that they were time-consuming, both in terms of the length of a session (minimum 1h) and the overall intervention (minimum 12weeks). It is possible that the large amount of time required explains the low adherence to the training programs: 57% in Battaglia et al. (2013) and 71% in Pérez-Moreno et al. (2007). Also, the mentioned studies (except for Gettman et al., 1976) use a variety of materials in training. The intensive reliance on training equipment is challenging because exercise equipment is usually difficult to access in prisons (Ghanbarzadeh and Mohamadi, 2012). Consequently, given the time and equipment constraints of a prison, an effective training method is required that has an attractive time-use ratio and does not require any material by using bodyweight exercises.

High-intensity interval training (HIIT) represents such a possible method in the prison context. This method became popular due to its attractive time-use ratio and is characterized by unimodal movements like running (Dransmann, 2020). By now, relatively novel variations of the traditional HIIT have been developed that maintain HIIT characteristics in terms of training parameters, but change the focus from purely endurance training to more functional and strength-oriented training protocols (Sperlich et al., 2018). Due to the different responses evoked by endurance and strength, the functional HIIT leads to synergistic improvements in both cardiorespiratory and metabolic parameters (Sperlich et al., 2017). While cardiorespiratory fitness is associated with improved health and less premature death (Bouchard et al., 2015), strength exercise is related to elevated muscle mass and improved body composition (Poehlman et al., 2000). These beneficial health outcomes are particularly relevant for inmates, as imprisoned people usually have a poor health status compared with the general population (Mannocci et al., 2015).

Functional HIIT programs are not as widely explored as the traditional HIIT protocols. The circuit-like functional high-intensity training of Sperlich et al. (2017) increased the peak oxygen uptake, lowered body mass, decreased BMI, reduced fat mass, enhanced fat-free mass, and improved functional strength of overweight women. The overweight women trained 9weeks with three sessions per week of an average duration of around 30min. A mobile based Circuit_HIIT performed once or twice per day by untrained individuals just improved the functional strength, whereas body mass, fat mass, fat-free mass, and peak oxygen uptake did not change (Sperlich et al., 2018). The authors explained this finding by the short duration of the sessions (6min each) and the shortness of the intervention (4weeks).

A functional HIIT should not be confused with a high-intensity functional training (HIFT). HIFT workouts are based on completing a certain number of repetitions in the fastest time possible or on completing as many repetitions as possible within a given time frame for a series of exercises. Thus, HIFT protocols are based on the exclusion of defined rest intervals – in contrast to more traditional HIIT protocols (Feito et al., 2018).

A functional HIIT has not yet been investigated in the context of a prison. Accordingly, this study examined the effects of a functional HIIT program on anthropometric and motor performance parameters among a group of inmates. The group of inmates was composed of male, German offenders who were serving their prison sentence in a German penitentiary. Following the presented findings of scientific research, our hypothesis was that the applied functional HIIT program improved body composition (body mass, BMI, fat mass, and fat-free mass) and motor performance (muscular endurance and aerobic endurance) of participants.

The 11 inmates completed 91.92±7.60% of the 18 training sessions. They achieved an average overall intensity of 77.95±5.58% of their individual maximum heart rate.

The key findings concerning the effects of the 6-week functional HIIT were as follows. Body mass and BMI significantly decreased. Fat mass, fat-free mass, and sprint times did not change significantly. A significantly longer distance was covered in the standing long jump. The number of repetitions in push-ups, sit-ups, and lateral side-to-side jumps increased significantly. Also, the maximum running speed in the 30–15_IFT and the maximum oxygen uptake increased significantly.

On average, the body mass decreased by 1.1% and the BMI by 1.2%. In both cases, this decrease corresponds to about half the improvement of Sperlich et al. (2017), where the 11 overweight women trained 3weeks longer with the same frequency (three sessions/week) and a similar session duration (30min). Because not all participants in the present study were overweight and the inmates continued to be exposed to everyday prison life including unhealthy behaviors like physical inactivity, drugs, alcohol, smoking, and violence (Fazel and Baillargeon, 2011), this finding represents a good result. In the context of a prison, maintaining body mass and BMI can already be considered a beneficial outcome. For example, existing research has indicated that maintaining the status quo can be quite challenging. The control group in Battaglia et al. (2013) had a BMI increase of 1.4% after 9months (n=18), the control group of Pérez-Moreno et al. (2007) witnessed an increase in body mass of 0.3% after 4months (n=10).

The decrease in body weight and BMI cannot be explained through the development of the absolute fat mass and the relative fat-free mass. These changes may have occurred to a significant decrease of the body water from 51.04 (±6.52) to 50.09 (±6.46) L, which represents a large effect (F=26.959, p<0.001, η²=0.729).

The sprint times have become 0.3% slower on average. This lack of improvement was likely given that the focus of the training was on muscular and aerobic endurance. To improve speed, different components, such as technique, coordination, and maximum muscular strength should have been featured more prominently in the program. In Wong et al. (2010), the 30m sprint times of professional soccer players improved significantly after 8weeks of running HIIT and muscular strength training (both twice a week). The improvements – opposite to our study – can be explained by the shorter and faster running intervals (16 sprints of 15s at 120% of maximum aerobic speed), and especially the high loads and low number of repetitions in strength training (four sets of six repetitions maximum).

By far the greatest improvements can be seen for both muscular endurance exercises. The number of push-ups increased by 27.1% and the number of sit-ups by 31.2%, with both improvements being statistically significant. Such an improvement was expected for two reasons: both exercises were performed in almost every session and the focus of the training program was on muscular endurance. The improvements were similar to studies in which training was performed more frequently or over a longer time period. For instance, in Sperlich et al. (2018), the group that trained twice daily for 4weeks improved the number of sit-ups in 1min by 29.6%. In Battaglia et al. (2013), inmates in both training groups enhanced their push-up performance after 9months with two sessions per week: the CRT-group by 24%, the HIST-group by 28.1%.

The improvements in muscular endurance cannot be explained by changes in body composition. The gains in muscle protein mass may take several months (Zinner et al., 2017). Consequently, the improvement in muscular endurance likely results from neural adaptation and improvements in intramuscular coordination.

The number of lateral side-to-side jumps increased by 14.8%, the distance in long jump by 7.7%. Even though coordinative abilities under time pressure and high-speed strength were not explicitly trained, the improvements can be explained with the same type of activity (Bös and Schlenker, 2016). The present program included many jumping movements. Side jumps (without spatial restriction) were trained in seven sessions, with other exercises like burpees and jumping jacks also carrying jumping elements in them. Moreover, if possible, elements such as squats and lunges were instructed to be jumped or performed explosively.

The maximum running speed increased by 16.1%. Converting this value into VO_2max, the average improvement was equivalent to 14.3%. This change was higher than improvements in comparable research: for example, Sperlich et al. (2017) reported only a 10.1% improvement in maximum oxygen uptake, although providing an additional 3weeks of training. The main reason for this difference might be the increased proportion of running within the present training program: In 16 of 18 sessions, inmates had to run. The improvement in VO_2max can be explained by better central adaptation, including augmented plasma and blood volumes (Graham et al., 2016) with elevated stroke volume (Goodman et al., 2005).

An intensity of just under 80 percent of the maximum heart rate can be considered relatively high for a mix of strength and endurance training. For example, the corresponding value was 72.6% in Sperlich et al. (2018). Hence, the level of purely endurance based HIITs (90% of maximum heart rate) was not reached in both our study and in Sperlich et al. (2018). These values illustrate a recurring finding: whereas a HIFT protocol resulted in heart rates of around 90% of the age-predicted maximum throughout the whole workout, the average heart rates for a multimodal/functional HIIT protocol were around 76%, with only the peaks for each interval approaching 90% of the maximum heart rate (Butcher et al., 2015). The analysis of the heart rate data suggests that the average heart rates were lowest in sessions 1.2 and 2.1. These were the sessions including a static exercise (isometric squat) and a lying exercise (rowing). In the only session, where these two exercises were included again (3.2), the increased running portion compensated the lower heart rates resulting from static and lying exercises.

The high training participation of about 92% shows how well the training was received in this setting. The adherence was much higher than in comparative prison studies. It was also higher than participation rates in other functional HIIT programs such as 89% in Sperlich et al. (2017) and 88.2% in Sperlich et al. (2018). This high degree of adherence may be attributed to the intervention being relatively short (6weeks) and representing a welcome distraction from inmates’ daily routine. It was also important that the training sessions did not take up too much time, i.e., only about 40min. The inmates in this prison, which is similar to other open prisons in Germany, have a maximum of 5h of free time per day. This period begins with the end of work at around 5 pm and ends with the closing of the cells at 10 pm. Hence, the attractive time-use ratio of HIIT aligns well with the conditions of an open German prison. The only material that was used was gymnastic mats, a music system, and sports watches. Collectively, the present training program only requires few resources in terms of time and equipment and can, therefore, be carried out in any other detention center.

The main limitations originate from the setting, i.e., the prison context. The most difficult circumstances were the small sample size and the lack of a control group. Both restrictions resulted from the small number of inmates in this institution. Twenty of the total 40 inmates agreed to participate. This participation rate corresponds to 50%. Of the remaining 20 inmates, no volunteers could be found who agreed to participate in the pre- and post-tests. They could have been obliged by the management of the prison, but due to the lack of motivation, the data for motor performance requiring maximum effort would not have been reliable. The present study shares the limitations of other fitness interventions in prison also relying on a small sample size. The 13 participants in the present study exceeded the two participants of Amtmann and Kukay (2016) by a margin.

Although we instructed our participants not to alter their eating habits, we cannot exclude the possibility that some inmates nevertheless altered their nutritional intake. Because participation in the training already involved a significant restriction of free time, we did not want to overburden the participants with keeping a food diary. However, after consultation with the kitchen management, the menu did not differ regarding protein or carbohydrate intake from the weeks before the pre-test.

Additionally, the non-exercise activity level plays a role in energy consumption. Since the prisoners were only allowed to leave the prison for work, such activity only occurred in the context of work. Because all participants were employed in a craft profession (mainly gardening and kitchen), the non-exercise physical activity was comparable for all inmates.

Drug use can also impact body composition (Hartgens et al., 1996) and motor performance (Singh et al., 1984). Accordingly, not taking drugs was a prerequisite for inclusion in the study. Notably, the use of drugs is generally forbidden for prisoners. In the event of a positive drug test – which were carried out frequently and without previous notice – the inmate would be transferred to a locked facility. Consequently, the inmates would not admit to taking drugs and have an incentive to not take any.

Testing of motor skills was limited to field tests. Even though only tests with a high reliability and validity were selected (Buchheit, 2008; Bös and Schlenker, 2016), laboratory tests would have a higher accuracy. The high improvements suggest that learning effects through test repetitions occurred. Likewise, Bös and Schlenker (2016) showed that the test results in the DMT improved significantly with an average by 6.3% in the retest (without training).

Another limitation is the lack of further cardiovascular data for the training or the 30–15_IFT, because the Polar FT1 just exported the average and peak heart rate. In future studies, recording heart rate variability could provide additional information about physical stress or psychological strain, especially in everyday prison life. Another avenue for future research is the conduction of a purely running based HIIT or a HIFT. To date, such interventions have not yet been conducted and evaluated in a prison context. Moreover, female inmates should be considered in future sport interventions, as they have received little or no attention to date.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The studies involving human participants were reviewed and approved by Ethics Committee of Bielefeld University. The patients/participants provided their written informed consent to participate in this study.

MD, MK, BG, and PW contributed to the conception and design of the study and wrote sections of the manuscript. MD organized the database and performed the statistical analysis. MD and MK planned and led the intervention as well as wrote the first draft of the manuscript. PW copyedited the draft for content and language, and organized the revision process. All authors contributed to manuscript revision, read, and approved the submitted version.

We acknowledge the financial support of the German Research Foundation (DFG) and the Open Access Publication Fund of Bielefeld University for the article processing charge.

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.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.