Sixty-one male and female young adults were recruited from the students and staff of Old Dominion University for this study. All subjects were at a low risk for cardiovascular disease according to ACSM's Guidelines for Exercise Testing and Prescription, that is, they were between 18 and 44 yr, they had no more than one risk factor for coronary heart disease, they had no signs or symptoms of cardiovascular disease, and they did not have known cardiovascular, pulmonary, or metabolic disease. Exclusionary criteria included anyone classified higher than low risk, anyone taking medications that influence HR (such as beta-blockers), anyone who was pregnant, and anyone with recent, significant bicycle training, that is, a competitive cyclist or one who had engaged in at least 3 h of cycling per week over the past 3 months.
The study was reviewed and approved by the university's institutional review board. Upon the date of initial testing, subjects were informed of the nature, the risks, and the potential benefits of the study orally and in writing, and then subjects provided written, informed consent.
Subjects were instructed to refrain from caffeine, heavy meals, or heavy exercise 3 h before testing. Upon arrival to the laboratory, investigators obtained informed consent from each subject. Height, mass, and three-site skinfolds were measured, and body mass index (BMI) and percent body fat were calculated. Skinfolds were measured by a single, experienced investigator.
Subjects were fitted with a chest strap monitor (Polar, Kempe, Finland) for HR measurement and an automated brachial BP device (HEM-422CR; Omron, Vernon Hills, IL) and lay supine quietly for 15 min. HR and BP were measured at 14 and 15 min and averaged to report resting values.
After the rest period, each subject completed a maximal incremental exercise test on a cycle ergometer (Cycle 828 E; Monark, Varberg, Sweden). Calibration of the cycle ergometer was completed before initiation of the study and at 1-month intervals. The seat height was adjusted to allow a slight bend in the knee with the leg at full extension and the foot held parallel to the floor. The subject continued to wear the HR monitor and was fitted with a mouthpiece and head gear for the collection of expired gases. For the exercise testing protocol, the subject pedaled at a cadence of 60 rpm at an initial resistance of 0.75 kg for males and 0.5 kg for females to produce workloads of 45 and 30 W, respectively. Resistance was increased every 3 min by 0.75 kg for males and 0.5 kg for females. HR was recorded during the last 10 s of each minute of exercise. Testing was terminated when the subject was no longer able to continue or could not maintain a cadence of 60 rpm despite encouragement.
A metabolic cart (Vmax 29c; SensorMedics, Yorba Linda, CA) was used during pre- and posttesting sessions to measure V˙O2max and RER. The flow sensor was calibrated against a 3.0-L syringe, and CO2 and O2 sensors were calibrated against known gases before each test. V˙O2max was calculated as the average of the three highest, continuous 20-s intervals (typically, but not necessarily, the last three 20-s intervals of the test). Criteria for attainment of V˙O2max were an RER ≥1.10 or a plateau in V˙O2 (an increase in V˙O2 from the penultimate stage to the last completed stage that was less than one half the expected increase).
To reduce potential sources of variability, the pretests and the posttests were administered at the same time of day for each subject. The testing procedures of the posttest matched those of the pretest. Posttesting occurred the week immediately after the completion of the 6-wk experimental period.
Subjects were matched according to sex and V˙O2max and were randomly assigned to one of four groups: 1) moderate intensity (50% V˙O2R), 2) vigorous intensity (75% V˙O2R), 3) near-maximal intensity (intervals at 95% V˙O2R), and 4) nonexercising control. Age was not used in the assignment process, because the age range of the subjects was relatively narrow. Table 1 presents the training protocol. The training protocol varied in duration and frequency to ensure that each intensity group performed the same volume of exercise, defined in aerobic training units (ATU) based on V˙O2R, that is, volume = intensity (%V˙O2R) × duration (minutes per session) × frequency (sessions per week). During exercise, intensity was controlled by establishing target HR at the equivalent percentages of HR reserve (HRR) based on the resting and the maximum HR values measured during testing.
Although intensity was controlled via HR and total volume of exercise was based on ATU, the external work that was performed in each exercise session was recorded to determine whether energy expenditure was similar across the three training groups. Cadence on the bike ergometer was maintained at 60 rpm, and resistance (R, in kilograms) was recorded every 5 min, not including the warm-up and cool-down. The average R across all training sessions in a given week was determined for each subject, and external work (in joules) was calculated as
where t is the total duration (not including warm-up and cool-down) in minutes of the exercise sessions in the given week.
Subjects were informed that they must complete at least 90% of all training sessions to fulfill the requirements of the study. Training during the 6-wk experimental period was performed on the same model of cycle ergometer as used in testing. Each training session was supervised to ensure that the target HR was maintained and to ensure that cadence was maintained at 60 rpm. A visual display of cadence was available for the subject and the investigator to monitor. For week 1, each group performed moderate-intensity cycling at 50% HRR for 30 min on three nonconsecutive days. Each cycling session contained 5-min warm-up and cool-down periods not included in the 30 min. HR and resistance were recorded every 5 min during each exercise session. The resistance against which subjects pedaled was adjusted if needed at the start of the next 5-min period to keep the subjects' HR close to the target value.
During week 2, the moderate-intensity group increased the exercise duration to 45 min and frequency to 4 d of exercise while maintaining 50% HRR. The vigorous-intensity group increased the duration to 40 min and the intensity to 75% HRR, while maintaining a frequency of 3 d. The near-maximal-intensity group exercised with the same prescription as the vigorous-intensity group during week 2.
For the remaining 4 wk of exercise, subjects were exercising at their final levels of duration, frequency, and intensity. The moderate-intensity group exercised at 50% HRR for 60 min, 4 d·wk-1. The vigorous-intensity group exercised at 75% HRR for 40 min, 4 d·wk-1. The near-maximal-intensity group exercised at 75% HRR for 5 min followed by five intervals of 5 min at 95% HRR (the work phase) and 5 min at 50% HRR (the recovery phase), 3 d·wk-1. All three groups performed a 5-min warm-up and a 5-min cool-down with each exercise session throughout the 6 wk. From the HR values recorded every 5 min during each training session, an average HR for each subject was calculated (separately for the work and recovery phases of the intervals for the near-maximal-intensity group) and expressed in %HRR units. For the final 4 wk of training, the mean %HRR for all sessions completed by all subjects within each group was determined.
Subjects were asked to not vary their usual physical activity patterns during the study and were asked to maintain a log of all physical activity performed outside of the supervised training. Volume of physical activity was estimated using the compendium of physical activity. Each activity was assigned an approximate intensity in METs from the compendium; 1 MET was subtracted from the value in the compendium to express the net rather than gross intensity. The MET value was multiplied by the duration of the activity to obtain MET-hours of activity, and these values were summed for each subject for any given week.
Six participants withdrew from the study due to scheduling conflicts, and only the remaining 55 were included in the analysis. Descriptive statistics are presented as mean and SD. Effects of training on the principal dependent variables (V˙O2max, resting HR, resting systolic BP, and resting diastolic BP) were analyzed using two-way ANOVA, one factor being time (with two levels: before and after) and the other being treatment (with four levels corresponding to the four training groups); repeated measures were used on one factor (time). For significant F-ratios, a post hoc Turkey's test was used to determine which group means differed from each other. To evaluate percent changes in variables, a one-way ANOVA with post hoc Tukey's test was used. One-way ANOVA was also used to determine whether the work performed on the bike ergometers during any given week was different between the three training groups, and whether the physical activity performed outside of the study during any given week was different between the four subject groups. For all tests, statistical significance was set at an alpha level of 0.05.
Med Sci Sports Exerc. 2008;40(7):1336-1343. © 2008 American College of Sports Medicine
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