Sprint Interval Running and Continuous Running Produce Training Specific Adaptations, Despite a Similar Improvement of Aerobic Endurance Capacity: A Randomized Trial of Healthy Adults

purpose of the present study was to investigate training-speci c adaptations to eight weeks of moderate intensity continuous training (CT) and sprint interval training (SIT). Young healthy subjects (n=25; 9 males and 16 females) performed either continuous training (30–60 min, 70–80% peak heart rate) or sprint interval training (5–10 near maximal 30 s sprints, 3 min recovery) three times per week for eight weeks. Maximal oxygen consumption, 20 m shuttle run test and 5 60 m sprint test were performed before and after the intervention. Furthermore, heart rate, oxygen pulse, respiratory exchange ratio, lactate and running economy were assessed at ve submaximal intensities, before and after the training interventions. Maximal oxygen uptake increased after CT (before: 47.9 1.5; after: 49.7 1.5 mL kg 1 min 1 ,p<0.05) and SIT (before: 50.5 1.6; after: 53.3 1.5 mL kg 1 min 1 ,p<0.01), with no statistically signi cant di erences between groups. Both groups increased 20 m shuttle run performance and 60 m sprint performance, but SIT performed better than CT at the 4th and 5th 60 m sprint after the intervention (p<0.05). At submaximal intensities, CT, but not SIT, reduced heart rate (p<0.05), whereas lactate decreased in both groups. In conclusion, both groups demonstrated similar improvements of several performance measures including VO2max, but sprint performance was better after SIT, and CT caused training-speci c adaptations at submaximal intensities. Keywords: maximal oxygen consumption; heart

and 5th 60 m sprint after the intervention (p<0.05). At submaximal intensities, CT, but not SIT, reduced heart rate (p<0.05), whereas lactate decreased in both groups. In conclusion, both groups demonstrated similar improvements of several performance measures including VO2max, but sprint performance was better after SIT, and CT caused training-speci c adaptations at submaximal intensities. Keywords: maximal oxygen consumption; heart rate; oxygen pulse; shuttle run; repeated sprint ability 1. Introduction Manipulation of duration and intensity of exercise bouts change the demands of metabolic pathways within muscle cells, as well as oxygen delivery to exercising muscles [1]. The training adaptations that occur after repeated bouts of exercise are to some degree speci c to that particular exercise [1,2], but both high intensity interval training and continuous training bouts increase VO2max and oxidative capacity in skeletal muscles [1–3]. Within this context, it is of interest to clarify the speci c adaptations of di erent training protocols to optimize endurance training, health and performance. There has recently been a lot of interest in a type of high intensity interval training known as sprint interval training (SIT). SIT is (often) performed as 30 s of “all out” sprints with 2.5–4.5 min of rest between sprints [4–6]. Several cycling studies have reported that this type of training Int. J. Environ. Res. Public Health2020,17, 3865; doi:10.3390 /ijerph17113865 /journal/ijerph

Int. J. Environ. Res. Public Health2020,17, 3865 2 of 12 improves maximal oxygen consumption (VO2max), endurance performance and the oxidative capacity of skeletal muscle [3–12]. Previous studies have also demonstrated that the magnitude of improvement in endurance performance and VO2maxafter SIT is comparable to continuous cycling at moderate intensity [3,4]. Furthermore, research also suggest that SIT is an e cient approach to improve several important health parameters in addition to VO2max, such as insulin sensitivity, blood pressure, cardiovascular function, and body composition [13]. Because most previous studies on SIT adaptations have used a cycling protocol, there is limited knowledge about sprint interval running [14]. This is unfortunate, as running is a basic and popular type of exercise. More importantly, there are several fundamental di erences between cycling and running exercise. Power output during sprint exercise is substantially higher in cycling than in running [15]. There are also several physiological di erences, such as higher heart rate (HR), higher fat oxidation and higher muscle mass activation in running than in cycling [16,17]. Thus, results from sprint interval cycling may not be directly applicable to sprint interval running [18]. Only a few previous studies have investigated the e ects of sprint interval running. In most of these studies, SIT is added to the training program of trained endurance athletes [19–21]. However, one previous study has compared the e ect of sprint interval and traditional endurance running in healthy untrained subjects [22]. Macpherson et al. [22] reported similar improvements of VO2max and endurance performance after SIT and continuous running at moderate intensity. Interestingly, VO2maximproved in the SIT group without a ecting cardiac output, whereas continuous running increased cardiac output, as expected. The study by Macpherson et al. [22] revealed that sprint interval running and continuous running produced similar improvements of aerobic performance, but still caused training-speci c physiological adaptations. Because there is limited data available on this topic, it is of great interest to investigate training-speci c adaptations of sprint interval running and continuous running. The purpose of this study was therefore to compare performance and health related adaptations of continuous training (CT)

and continuous running produced similar improvements of aerobic performance, but still caused training-speci c physiological adaptations. Because there is limited data available on this topic, it is of great interest to investigate training-speci c adaptations of sprint interval running and continuous running. The purpose of this study was therefore to compare performance and health related adaptations of continuous training (CT) and SIT, performed as running, on VO2max, 20 m shuttle run performance, repeated sprint ability (RSA) and the physiological response to submaximal exercise. We hypothesized that both types of training would improve VO2maxand 20 m shuttle run similarly, and that training-speci c adaptations would occur at submaximal exercise in favor of CT and during RSA in favor of SIT. 2. Materials and Methods 2.1. Participants Participants were recruited through the o cial webpage of the Norwegian School of Sport Sciences, and printed and electronic yers posted in various places in the local area of northern Oslo and in social media, respectively. Forty-eight subjects volunteered and were screened for participation. The inclusion criteria for participation were: (1) non-smokers; (2) body mass index (BMI)<30 kg m 2 ; (3) no cardiovascular or metabolic disease; (4) no systematic endurance training during the last two years ( 2 sessions per week). Twenty-nine subjects met these criteria and were invited to participate. Subjects were matched based on gender and VO2max, and then randomly assigned by coin toss to either CT or SIT. Four subjects dropped out during the training intervention; One dropped out during week 1 due to receiving a job o er (CT, male 21 years), one, during week 2, after realizing that participation in the intervention was not compatible with his life situation (SIT, male, 21 years), one during week 5, due to unspeci ed reasons (CT, male, 22 years), and one during week 8, due to moving to a di erent region (SIT, female, 22 years). Thus, 25 subjects (9 males and 16 females) completed the training intervention.

unspeci ed reasons (CT, male, 22 years), and one during week 8, due to moving to a di erent region (SIT, female, 22 years). Thus, 25 subjects (9 males and 16 females) completed the training intervention.

Int. J. Environ. Res. Public Health2020,17, 3865 3 of 12 2.2. Training Protocol Both groups completed eight weeks of training. Each week consisted of three training sessions, separated by at least one resting day. Training sessions were organized and supervised by quali ed instructors. Subjects were occasionally allowed to perform sessions at home if participation in organized sessions was problematic. The training intensity was controlled during all sessions by heart rate monitors (Polar Sport Tester RS800CX, Polar Electro, OY, Kempele, Finland). An adherence of >85% (19 of 24 training sessions, including sessions performed at home) was required. Subjects were instructed to maintain their normal diet and lifestyle throughout the intervention. The CT group was instructed to maintain an intensity corresponding to 70–80% HRpeakat all training sessions. Organized training sessions were performed on slightly undulating terrain. During the rst week, the CT group performed 30 min of running. The time then increased by ve minutes per week, up to a total of 60 min. The SIT group consisted of 30 s sprints at near maximal e ort, with three minutes of rest between each sprint. The training intensity of SIT sessions was evaluated subjectively during sessions, while the HR data was used to verify that the individual participant did not have a session or interval that deviated from their usual level of e ort. During the rst week, the SIT group performed ve sprints per session. The number of sprints then increased gradually, until a total of 10 sprints per session in weeks 7 and 8. When the number of sprints reached seven, subjects were given six minutes of rest midway through the training session. All sprints were performed on slightly uphill terrain. Prior to all training sessions, the CT group performed a ten-minute warm-up at an intensity corresponding to 60–75% of HRpeak. The SIT group performed a ten-minute warm-up at an intensity corresponding to 60–85% of HRpeak, followed by three incremental strides of about 80 m. After each training session, all subjects performed ve minutes of walking or running at intensities below 70% of HRpeak. The training volume in CT

performed a ten-minute warm-up at an intensity corresponding to 60–75% of HRpeak. The SIT group performed a ten-minute warm-up at an intensity corresponding to 60–85% of HRpeak, followed by three incremental strides of about 80 m. After each training session, all subjects performed ve minutes of walking or running at intensities below 70% of HRpeak. The training volume in CT and SIT was not matched. 2.3. Measures Incremental treadmill test to exhaustion. The test was performed on a motorized treadmill (Woodway pps55 sport, Woodway Gmbh, Weil an Rhein, Germany). Oxygen consumption (VO2) was measured through a 2-way mouthpiece (Hans Rudolph Instr., Shawnee, KS, USA) and a sling, which was connected to an O2and CO2analyzer (Oxycon Champion, Jaeger Instruments, Hoechberg, Germany). Samples of O2and CO2were collected continuously from a mixing chamber, with average values obtained over 30-s intervals. The gas analyzer was calibrated before each test with ventilated indoor air and standardized gas concentrations, to span the concentration range observed during exercise. The expired volume was measured with a turbine (Triple V volume transducer, Leipzig, Germany), and volume calibration was performed regularly with a 3-L syringe. The incremental test to exhaustion followed current recommendations for test duration [23], and was performed according to the standard protocol of the Norwegian Olympic Sports Centre (see e.g., [24]). Prior to the pre-test, subjects performed two familiarization tests to reduce the learning e ect, following the recommendations of Edgett et al. [25]. Identical procedures were conducted for familiarization, pre- and post-test. All subjects performed a 15-min warm-up of gradually increasing intensity. The last ve minutes of the warm-up were performed with an inclination of 5.3%, as was the incremental test. The starting speed was chosen in order to exhaust the subjects after ~5 min. Running speed was initially increased by 1 km h 1 every minute. At the end of the test, running speed was either maintained or increased by 0.5 km h 1 , to allow at least one minute running at the nal speed. VO2maxwas determined as the average of the highest values achieved over two subsequent 30-s measurements. Verbal encouragement was given

speed was initially increased by 1 km h 1 every minute. At the end of the test, running speed was either maintained or increased by 0.5 km h 1 , to allow at least one minute running at the nal speed. VO2maxwas determined as the average of the highest values achieved over two subsequent 30-s measurements. Verbal encouragement was given throughout the test. Two minutes after completion, a capillary blood sample was obtained and 20 l of blood was injected into a lactate analyzer (1500 SPORT, YSI Inc., Yellow Springs Instr., Yellow spring, OH, USA), with the help of a standard injector. The lactate analyzer was calibrated before each test with a 5.0 mM lactate standard. The main criterion for evaluating whether VO2maxwas achieved was a plateau in

Int. J. Environ. Res. Public Health2020,17, 3865 4 of 12 oxygen consumption. A levelling-o of the VO2curve was used in conjunction with a lactate value 6 mmol l 1 and respiratory exchange ratio (RER)>1.10 as secondary criteria. HR was monitored throughout the test (Polar Sport Tester RS800CX, Polar Electro, OY, Kempele, Finland) and the highest value achieved was de ned as HRpeak. Submaximal treadmill test. The submaximal treadmill test was conducted with the same equipment as described above and consisted of four stages of ve minutes on a motorized treadmill. The running speed at each stage was individualized based on each subject's VO2maxand a general relationship between running speed and VO2. This relationship was estimated based on data from a pilot study. The purpose of the procedure was to establish four individualized stages of gradually increasing running velocities at approximate intensities of 50%, 60%, 70% and 80% of VO2max. The same velocity (absolute intensity) was used for both the pre- and post-test. Measurements of VO2and RER were made between the third and fourth minute. After the fourth minute, the mouthpiece was removed and HR was monitored until the end of the stage. Between each stage, the subjects were given one minute rest for measurement of lactate, as described above. The post-test was conducted at the same running velocities as the pre-test. Running economy (RE; mL kg 1 km 1 ) was de ned as VO2divided by body mass and running speed. O2pulse (mL beat 1 ) was calculated by dividing VO2(mL min 1 ) by HR (beat min 1 ). Training adaptations at the same relative intensity were evaluated by examining the running speed that elicited the VO2value closest to 70% of the individual subject's VO2max. This intensity was chosen because it produced the least variation in VO2values. Repeated sprint test. After completing the submaximal treadmill test, all subjects performed a 5 60 m repeated sprint test in an indoor sports hall. The test was considered appropriate to induce the performance decrement associated with repeated sprint exercise [26]. All subjects performed a test-speci c warm-up prior to the sprint test

because it produced the least variation in VO2values. Repeated sprint test. After completing the submaximal treadmill test, all subjects performed a 5 60 m repeated sprint test in an indoor sports hall. The test was considered appropriate to induce the performance decrement associated with repeated sprint exercise [26]. All subjects performed a test-speci c warm-up prior to the sprint test consisting of 3 60 m incremental runs. The sprints were performed with a 1 m ying start and each sprint was separated by 30 s of rest. Time was measured by photoelectric detectors (Brower Speed Trap II Timing system, Brower Timing system, Salt Lake USA). Verbal encouragement was given throughout the test. 20 m shuttle run test. The 20 m shuttle run test procedure was the same as previously described [27]. In short, subjects ran repeatedly between two lines, 20 m apart. The test started at a running speed of 8.5 km h 1 , which then increased by 0.5 km h 1 per minute. The test was terminated when subjects failed to reach the 20 m line before the signal on two successive occasions. To stimulate competition, the subjects ran in groups. 2.4. Procedures All tests were performed before and after the training interventions. The submaximal treadmill test and the repeated sprint test were performed on the same day, and only separated by the time to relocate from the laboratory to the sports hall. All other tests were separated by at least one resting day. Subjects were familiarized with testing procedures to minimize any potential learning e ect. The data for this study were collected in relation to a larger study [28]. The study was approved by the Regional Ethics Committee of Oslo, Norway (ref. number 2010/1567-1) and was performed according to the Declaration of Helsinki. All subjects were informed about the purpose of the study and associated risks before they gave their written informed consent to participate. A few subjects did not obtain valid results for all tests due to sickness, injury and unspeci ed withdrawal from the study. These subjects were excluded from both pre and post

performed according to the Declaration of Helsinki. All subjects were informed about the purpose of the study and associated risks before they gave their written informed consent to participate. A few subjects did not obtain valid results for all tests due to sickness, injury and unspeci ed withdrawal from the study. These subjects were excluded from both pre and post analysis for these particular tests. The number of participants for each test is stated in the captions of tables and gures. 2.5. Analysis Data are presented as group means SEM. All statistical analyses were performed in SPSS version 18 (SPSS inc., Chicago, IL, USA). The assumption of normality was evaluated by a Shapiro–Wilk test. Student's pairedt-test was used to investigate within-group di erences, and a Student's

Int. J. Environ. Res. Public Health2020,17, 3865 5 of 12 unpairedt-test was used to investigate between-group di erences. A repeated measures ANOVA with a Greenhouse–Geisser correction was used to evaluate a potential increase in VO2maxas a function of the number of tests performed before the intervention. In cases where data was not normally distributed, a Wilcoxon signed-rank test was used to verify within-group di erences, and a Mann–Whitney test was used to verify between-group di erences. Statistical signi cance was accepted at thep<0.05 level. 3. Results The number of females in each group was eight, while the number of males was four in CT and ve in SIT. The mean age, height, weight and BMI was 25 1 years, 175 2 cm, 72.6 3.8 kg and 23.6 0.9 kg m 2 in CT at the start of the intervention. In SIT, the mean age, height, weight and BMI was 25 1 years, 173 3 cm, 71.2 4.1 kg and 24.0 0.8 kg m 2 . There was no statistical di erence between groups and these characteristics did not change during the intervention. Heart rate registrations at all training sessions con rmed that the subjects performed the training as recommended, including the sessions performed at home (19% of sessions). Three participants experienced minor injuries during the training intervention, including one injury unrelated to the intervention. All three were in the SIT group, and all managed to complete>85% of training sessions. Maximal oxygen consumption and 20 m shuttle run performance. Maximal oxygen uptake was measured three times prior to the intervention, and VO2maxincreased from test to test. The repeated measures ANOVA revealed that VO2maxincreased from 48.2 1.1 at the rst familiarization test to 49.3 1.3 in the second, and eventually to 49.9 1.3 mL kg 1 min 1 at the third test when combining both groups (F(1.434, 28.683)=10.320,p<0.01). VO2maxwas improved in both CT (p<0.05) and SIT (p<0.01) after training (Table). The improvement of VO 2maxcorresponded to a 3.8% increase in CT and 5.5% in SIT. The increase in VO2maxvaried between subjects and ve subjects did not increase VO2max(Figure). In accordance with the

This study compares the effects of sprint interval and continuous running on aerobic capacity and training adaptations.