Hemodynamic Adaptations Induced by Short-Term Run Interval Training in College Students

Perceived lack of time is one of the most often cited barriers to exercise participation. High intensity interval training has become a popular training modality that incorporates intervals of maximal and low-intensity exercise with a time commitment usually shorter than 30 min. The purpose of this study was to examine the effects of short-term run interval training (RIT) on body composition (BC) and cardiorespiratory responses in undergraduate college students. Nineteen males (21.5 ± 1.6 years) were randomly assigned to a non-exercise control (CON, n = 10) or RIT (n = 9). Baseline measurements of systolic and diastolic blood pressure, resting heart rate (HRrest), double product (DP) and BC were obtained from both groups. VO2max and running speed associated with VO2peak (sVO2peak) were then measured. RIT consisted of three running treadmill sessions per week over 4 weeks (intervals at 100% sVO2peak, recovery periods at 40% sVO2peak). There were no differences in post-training BC or VO2max between groups (p > 0.05). HRrest (p = 0.006) and DP (p ≤ 0.001) were lower in the RIT group compared to CON at completion of the study. RIT lowered HRrest and DP in the absence of appreciable BC and VO2max changes. Thereby, RIT could be an alternative model of training to diminish health-related risk factors in undergraduate college students.


Introduction
A robust negative relationship exists between weekly amounts of physical activity and cardiometabolic disease morbidity and mortality [1]. The American College of Sports Medicine (ACSM) recommends performing 150 min/week of moderate-intensity or ≥75 min/week of vigorous-intensity physical activity. Although achieving physical activity recommendations is a proven strategy to improve health, undergraduate students, like much of the adult population in developed countries do not meet these recommendations [2][3][4], and report lack of time as the main barrier to achieving these recommendations [5]. In addition, undergraduate students have unhealthy eating habits [6,7]

. Body Composition Analysis
Height was measured using a stadiometer (Biospace Corporation, Seoul, South Korea) to th rest ±1 mm. Body composition measures (i.e., body weight [kg], body fat mass [%], muscle mas ], leg lean mass [kg], and body mass index [BMI = kg/m 2 ]) were recorded by BIA [28]. For quality trol, participants were instructed to refrain from eating and drinking for at least 2 h and to void ir bladders 60 min prior to performing BIA.

. Graded Exercise Test (GXT)
Participants performed a GXT until volitional fatigue to determine fitness level (VO2max) [29] e GXT followed a standard protocol reported before [30,31] with minor modifications. Briefly, th t started with a warm-up run at 5.0 km/h with 1% incline. Then, the treadmill speed was increased 1 km/h every 2 min until participant's exhaustion. During the GXT, members of the research team ouraged participants to give their maximal effort. Breath-by-breath samples of expired CO2 wer lected during the test. The VO2 value recorded at the last stage of the GXT was considered th 2peak (Supplementary Figure S1). The running speed associated with the VO2peak (sVO2peak) was used esign the RIT program. Immediately following the last stage of the GXT, a 4 min cool-down run s performed at 5.0 km/h with 1% incline. The GXT was considered maximal if the participant ched three of the following criteria: (a) respiratory exchange ratio (RER) > 1.10, (b) HRmax within beats of the age-predicted HRmax (220-age), and (c) a VO2 plateau despite an increase in workload inside bars indicate the interval ratio (high-low intensity) and bold numbers represent the 2 min "all out" sprints performed during each session. Cardiovascular measures, bioelectrical impendence (BIA) and graded exercise tests (GXT) were performed 24 h before and after the RIT protocol.

Body Composition Analysis
Height was measured using a stadiometer (Biospace Corporation, Seoul, South Korea) to the nearest ±1 mm. Body composition measures (i.e., body weight [kg], body fat mass [%], muscle mass [kg], leg lean mass [kg], and body mass index [BMI = kg/m 2 ]) were recorded by BIA [28]. For quality control, participants were instructed to refrain from eating and drinking for at least 2 h and to void their bladders 60 min prior to performing BIA.

Graded Exercise Test (GXT)
Participants performed a GXT until volitional fatigue to determine fitness level (VO 2max ) [29]. The GXT followed a standard protocol reported before [30,31] with minor modifications. Briefly, the test started with a warm-up run at 5.0 km/h with 1% incline. Then, the treadmill speed was increased by 1 km/h every 2 min until participant's exhaustion. During the GXT, members of the research team encouraged participants to give their maximal effort. Breath-by-breath samples of expired CO 2 were collected during the test. The VO 2 value recorded at the last stage of the GXT was considered the VO 2peak (Supplementary Figure S1). The running speed associated with the VO 2peak (sVO 2peak ) was used to design the RIT program. Immediately following the last stage of the GXT, a 4 min cool-down run was performed at 5.0 km/h with 1% incline. The GXT was considered maximal if the participants reached three of the following criteria: (a) respiratory exchange ratio (RER) > 1.10, (b) HRmax within 10 beats of the age-predicted HRmax (220-age), and (c) a VO 2 plateau despite an increase in workload or running speed [32,33], and/or (d) when the participant requested to stop the test because of volitional exhaustion [34]. The exhaustion time, HRmax and VO 2max were recorded at the end of the GXT.

Run Interval Training
The RIT program consisted of 12 exercise sessions, with a progressively increasing training volume. Weekly training sessions were performed on Monday, Wednesday, and Friday for four weeks. All training sessions were performed from 0900 to 1300 h in the student's personal leisure time between classes. The initial three sessions started with a 2 min run warm-up at 40% sVO 2peak . Then, a high-intensity interval was performed for 2 min at 100% sVO 2peak , for a total of three high-intensity and low-intensity bouts. The mean intensity was calculated using the equation reported by Billat et al., (2001): (100 + 40)/2 = 70% sVO 2peak . The high:low interval ratio for these sessions was 2:2 = 1 [38], and the total training time was 12 min. The next four sessions consisted of four cycles of RIT. The high:low interval ratio was 2:1 = 2, for a total duration of 12 min. Finally, for the last five sessions, the number of cycles increased to five (Supplementary Figure S1), the high:low interval ratio was 2:1 = 2, for a total duration of 15 min. This protocol can be considered as moderate-volume (MV-HIIT), and short duration intervention (ST-HIIT) [17]. To confirm the effect of the workload changes during the intervention, the total distance (km) covered in each exercise session was recorded for volume and intensity. Finally, the HR was monitored continuously throughout all exercise sessions.

Statistical Analysis
Statistical analysis was performed using IBM SPSS version 20.0 (IBM SPSS-Statistics, Armonk, NY, USA). Data are reported as means ± standard deviation (SD). Distribution of the data was assessed with the Shapiro-Wilk's test. Independent samples t-test compared mean baseline anthropometry and body composition between CON and RIT groups. One-way ANOVA was computed to examine mean differences in distance run and HRmax responses in the training sessions S1 (start of program), S6 (middle of the program), and S12 (end of program). Two-way ANOVA with Tukey post-hoc testing was computed to evaluate VO 2max , hemodynamic, and body composition variables over time (pre vs. post) and groups (CON vs. RIT). Pearson correlation was used to analyze the association among the change (∆%) in the HRrest and SBP. Effect sizes were computed as Cohen's d, and were interpreted as small (0.2-0.5), moderate (0.5-0.8) and large (>0.8). The 95% confidence intervals (95% CI) around the point estimates are reported. Statistical significance was set a priori at p ≤ 0.05.

Results
Descriptive and inferential statistics of the participants in the CON and RIT groups are presented in Table 1. Following randomization, baseline age, height, weight, BMI, fat mass (%), muscle mass (kg) and lean leg mass (kg) were similar between CON and RIT groups (p > 0.05 for all). Distance run during RIT was different between the program stages (p = 0.002, d = 1.12, 95%CI = 1.11, 1.12). Post-hoc analysis showed that the distance run during S1 (2.8 ± 0.4 km) and S6 (3.0 ± 0.2 km) was similar (p = 0.091, 95%CI = 0.0, 0.4). The distance run during S1 was shorter than S12 (3.1 ± 0.2 km, p = 0.005, 95%CI = 0.2, 0.6), and the distance run during S12 was longer than S6 (p = 0.004, 95%CI = 0.1, 0.2, Figure 2A). These data indicate the positive effects of the workload changes applied during the treatment. Mean HR response to high-intensity bouts was similar between S1, S6 and S12 (p = 0.543) ( Figure 2B). The mean HR recorded during S1, S6 and S12 were 90% of the HRmax reached during the GXT. CON = control group; RIT = run interval training group; BMI = body mass index; HR rest = resting heart rate; SBP = systolic blood pressure; DBP = diastolic blood pressure; DP = double product; VO 2max = maximal oxygen uptake. * p = 0.031 indicates significant differences between post vs. pre in the CON group; # p = 0.001 indicates significant differences between post vs. pre in the RIT group; § p = 0.006 indicates significant differences between CON vs. RIT in the post-test measurement; ¥ p = 0.012 indicates significant differences between post vs. pre in the CON group; ¶ p = 0.005 indicates significant differences between post vs. pre in the RIT group; ¤ p ≤ 0.001 indicates significant differences between CON vs. RIT groups in the post-test measurement. CON = control group; RIT = run interval training group; BMI = body mass index; HRrest = resting heart rate; SBP = systolic blood pressure; DBP = diastolic blood pressure; DP = double product; VO2max = maximal oxygen uptake. * p = 0.031 indicates significant differences between post vs. pre in the CON group; # p = 0.001 indicates significant differences between post vs. pre in the RIT group; § p = 0.006 indicates significant differences between CON vs. RIT in the post-test measurement; ¥ p = 0.012 indicates significant differences between post vs. pre in the CON group; ¶ p = 0.005 indicates significant differences between post vs. pre in the RIT group; ¤ p ≤ 0.001 indicates significant differences between CON vs. RIT groups in the post-test measurement.

Figure 2.
Distance run during the first session of training (S1), at the middle of the protocol (S6), and during the last session of RIT (S12) (A). Heart rate recorded during the RIT sessions (B). * p = 0.005 S1 vs. S12; # p = 0.004 S6 vs. S12.

Figure 2.
Distance run during the first session of training (S1), at the middle of the protocol (S6), and during the last session of RIT (S12) (A). Heart rate recorded during the RIT sessions (B). * p = 0.005 S1 vs. S12; # p = 0.004 S6 vs. S12.

Discussion
The aim of the current study was to determine the effects of short-term RIT on cardiorespiratory fitness, hemodynamic responses, and body composition in healthy undergraduate students. The main finding of the study was a significant reduction in DP and HRrest following 4 weeks of RIT. However, the 12 sessions of RIT did not modify fitness levels or body composition.
The total training volume throughout all sessions averaged ≤15 min, which agrees with the training duration suggested for HIIT or SIT [39]. The HR reached during each high-intensity bout was approximately 90% of HRmax. Since the high-intensity bouts were performed at 100% of sVO2peak, our results show that monitoring HR is not a good indicator for controlling the intensity of in interval training, this phenomena was also previously reported in healthy young adults [40][41][42]. In contrast to previous reports, VO2max was not increased following short-term training compared to CON [18,27]. Participants in the current study showed higher baseline VO2max values than those previously reported [18,27], which could partially explain the lack of significant changes in aerobic power. Others have suggested a similar hypothesis [41]. Moreover, although the studies that reported an improvement in VO2max directly had comparable intervention durations (12 sessions over 4 weeks) compared with the current study, the sessions' design were completely different (short-interval vs. moderate interval in the current study) [17,18,27]. These data suggest that interval duration is relevant to increase cardiorespiratory fitness in young adults [24,43]. Finally, although we did not examine peripheral adaptations, other reports show that 18 sessions of interval training improved VO2max in undergraduate students by inducing skeletal muscle adaptations [26,44].
In this study, we report that a short-term RIT protocol reduced HRrest; these data are in agreement with other authors that used short-term interval training in cycling exercises [45]. In the current study, heart rate variability was not measured; thus, we cannot determine whether the lower HRrest resulted from an elevated vagal activity following training. Others have previously demonstrated that the physiological mechanism induced by short-term interval training responsible for reducing HRrest is an intrinsic adaptation of the sinoatrial node rather autonomic activity changes [45]. Therefore, it is possible that similar physiological adaptations might have occurred in our participants to reduce HRrest. Contrary, the CON group increased HRrest, despite this outcome, their values match with previous HRrest data reported in healthy young adults [46][47][48].
The RIT program designed for the present study produced a decrease in SBP compared to CON, yet, this finding did not reach statistical significance. The trend observed agrees with a recent report

Discussion
The aim of the current study was to determine the effects of short-term RIT on cardiorespiratory fitness, hemodynamic responses, and body composition in healthy undergraduate students. The main finding of the study was a significant reduction in DP and HRrest following 4 weeks of RIT. However, the 12 sessions of RIT did not modify fitness levels or body composition.
The total training volume throughout all sessions averaged ≤15 min, which agrees with the training duration suggested for HIIT or SIT [39]. The HR reached during each high-intensity bout was approximately 90% of HRmax. Since the high-intensity bouts were performed at 100% of sVO 2peak , our results show that monitoring HR is not a good indicator for controlling the intensity of in interval training, this phenomena was also previously reported in healthy young adults [40][41][42]. In contrast to previous reports, VO 2max was not increased following short-term training compared to CON [18,27]. Participants in the current study showed higher baseline VO 2max values than those previously reported [18,27], which could partially explain the lack of significant changes in aerobic power. Others have suggested a similar hypothesis [41]. Moreover, although the studies that reported an improvement in VO 2max directly had comparable intervention durations (12 sessions over 4 weeks) compared with the current study, the sessions' design were completely different (short-interval vs. moderate interval in the current study) [17,18,27]. These data suggest that interval duration is relevant to increase cardiorespiratory fitness in young adults [24,43]. Finally, although we did not examine peripheral adaptations, other reports show that 18 sessions of interval training improved VO 2max in undergraduate students by inducing skeletal muscle adaptations [26,44].
In this study, we report that a short-term RIT protocol reduced HRrest; these data are in agreement with other authors that used short-term interval training in cycling exercises [45]. In the current study, heart rate variability was not measured; thus, we cannot determine whether the lower HRrest resulted from an elevated vagal activity following training. Others have previously demonstrated that the physiological mechanism induced by short-term interval training responsible for reducing HRrest is an intrinsic adaptation of the sinoatrial node rather autonomic activity changes [45]. Therefore, it is possible that similar physiological adaptations might have occurred in our participants to reduce HRrest. Contrary, the CON group increased HRrest, despite this outcome, their values match with previous HRrest data reported in healthy young adults [46][47][48].
The RIT program designed for the present study produced a decrease in SBP compared to CON, yet, this finding did not reach statistical significance. The trend observed agrees with a recent report that showed the efficacy of short-term interval training to reduce blood pressure [45]; additionally, a recent meta-analysis identified a significant benefit of interval training to improve daytime resting blood pressure [12]. It has been reported that peripheral vascular adaptations are the main interval training-related mechanisms responsible for reducing resting blood pressure [49,50]. Cross-sectional studies have reported that abnormal blood pressure values (e.g., pre-hypertension, hypertension) are common in young adult populations [51,52]. In undergraduate students, high blood pressure is a consequence of higher sympathetic tone [53], and our data are in agreement with this physiological mechanism, where a lower HRrest was associated with lower SBP (Figure 3). Therefore, RIT could be an effective strategy to regulate resting blood pressure in a predisposed population of students characterized by having high-levels of sedentary time, lack of time for physical activity/exercise, and high levels of school burnout [53,54]. In addition, resting DP changed differentially in the RIT and CON groups. DB was lower at the end of the study in the RIT and higher in the CON (within-group analysis), and DP was lower in the RIT compared to the CON following intervention (between-group analysis). These data are in concordance with a lower HRrest [55] ( Table 1). The DP is a non-invasive method to estimate MVO 2 [56]. Others have reported lower resting DP after long-term resistance training in hypertensive women [57]; however, the effects of short-term interval training on resting and exercise DP have not been reported. The usefulness of DP at rest has been previously suggested in the context of MVO 2 [58,59] in that a higher DP at rest was more strongly associated with cardiovascular disease (CVD) mortality, and non-CVD mortality than other cardiovascular biomarkers (e.g., SBP, DBP, HRrest) [53]. A low MVO 2 is an indicator of improvement in left ventricular relaxation, and changes in myocardial substrate metabolism [60,61]. Although we did not evaluate MVO 2 directly, our data suggest that 12 RIT sessions may have induced metabolic changes in the heart that lead to lower estimates of cardiac muscle VO 2 .
In the present study, body composition did not change with 12 sessions of RIT. Our results are in agreement with others who employed a similar short-term running interval training protocols [18]. In contrast, studies using longer interval training programs (≥6 weeks) have reported positive changes in body composition [19,26,62]. These findings suggest that the length of the training program is a relevant variable to induce positive changes in body composition. It is worth noting that in the current study, the calorie intake and composition were not controlled, which may have dampened the training effects on body composition [63,64].
A potential limitation of the current study was that we only included males. Additional shortand long-term studies examining both men and women in parallel cohorts will be important in future studies to further examine hemodynamic and cardiorespiratory responses to exercise. In addition, participants in the current study had high baseline fitness levels which along with the short duration of the study could have limited the magnitude of improvement in both fitness and body composition. In addition, we did not monitor changes in physical activity levels throughout the study; however, participants were instructed to maintain unchanged their physical activity levels throughout the duration of the study. Perhaps accelerometry might be used in future studies to accurately confirm habitual physical activity levels. Finally, this study did not determine ventilatory thresholds (VT), a variable used to assess functional capacity in individuals [65]. Scientific evidence indicates significant changes in VT after long and mid-term interval training [27,66,67]; therefore, we do not discard the possibility that RIT could modify VT in the participants.

Conclusions
Twelve RIT sessions performed over 4 weeks, and executed during short breaks of daily academic activities significantly decreased HRrest and an estimate of MVO 2 in male undergraduate students with high cardiorespiratory fitness levels. In contrast, the exercise intervention did not modify VO 2max and body composition. Based on previous studies using longer exercise duration, it seems plausible that a threshold training length is needed to observe positive changes in cardiorespiratory fitness and body composition. However, reduced heart rate and DP indicate that metabolic changes occur rapidly even in the absence of reductions in body weight or adiposity (at least in males). Further studies are needed to examine the time course of effects of RIT on cardiorespiratory fitness and body composition in male and female undergraduate students.