Although endurance-training adaptations to moderate- or high-intensity continuous exercise (CEx) have been well documented for older adults (4,6,11,16,24,26), this is not the case for intermittent exercise (IEx). Intermittent exercise is characterized by periods of exercise separated by periods of rest (22). At the same absolute intensity, IEx results in lower heart rate (HR), oxygen uptake (V̇O2), ventilation (V̇E), and blood lactate responses compared with CEx (1,22). This lower “physiological distortion” suggests that IEx may be a more suitable exercise mode for older adults, particularly if a similar degree of adaptation to endurance training can be demonstrated.
In young adults, endurance-training studies comparing CEx and IEx modes of exercise have produced conflicting results. Whereas some have reported greater endurance-training adaptations with CEx (12,23), others have found greater training adaptations with IEx (9). Moreover, other studies have reported the adaptations to endurance training were not significantly different between CEx and IEx groups (5,7,19). These ambiguous findings may be because previous studies have failed to compare CEx and IEx at the same intensity and have ignored the possible contributing effect of the total work completed upon any resultant endurance-training adaptation. In an apparent attempt to equalize the physiological responses between these two modes of exercise, previous studies have often prescribed IEx exercise at a higher intensity than CEx (5,7,12,19,23). As a result, not only did the intensity of exercise differ, but also the total work completed between CEx and IEx groups was not the same.
Typically, different exercise prescriptions have been compared by taking measurements pre- and post-endurance training (2,3,24,25). Whereas such a research design allows the amplitude of the adaptation to be compared between different exercise prescriptions, no information is obtained with regard to the rate of adaptation. Research designs that include a series of time course measurements allow both the amplitude and rate of adaptation to endurance training to be determined. Information about the rate and amplitude of adaptation to CEx and IEx may in turn provide further insight into how the aged system responds to a prescribed exercise stimulus. Such information will be of value to those with responsibility for prescribing exercise for the older adult.
We hypothesize that CEx and IEx prescriptions, whereby older adults complete the same total amount of work, will result in a similar amplitude and rate of adaptation. Hence, the purpose of the present study was to compare the rate and amplitude of adaptation to 10 wk of either CEx or IEx in a group of older adults, when the training intensity and the total amount of work completed are the same.
Twenty-five subjects, who met the following criteria, participated in the study: nonsmokers for at least 10 yr, resting blood pressure less than 150/90 mm Hg, normal resting 12-lead ECG and spirometry results, and normal physical examination with no evidence of clinically significant exercise-induced myocardial ischemia. Written consent was obtained after a clear explanation of the experimental procedures. The study was approved by The University of Sydney Human Ethics Committee. The physical characteristics of the subjects that participated in the study are presented in Table 1.
Peak exercise test.
Before commencing the study, each subject completed two incremental exercise tests to exhaustion on an electronically braked cycle ergometer (Siemens Elema, 380B, Erlangen, Germany) on separate days. The saddle height was standardized for each subject. Heart rate (HR) and rhythm was monitored by a physician throughout the exercise test using bipolar leads placed in the CM5 position. Oxygen uptake (V̇O2) during exercise was measured using standard open-circuit spirometry techniques. The subject breathed through a mouthpiece connected to a low resistance 2-way valve (Hans Rudolph 2700, Hans Rudolph, inc., Kansas City, MO). Expired gas was passed into a mixing chamber via a pneumotach (47304A Flow Transducer, Hewlett Packard, Waltham, MA). The pneumotach was calibrated before and immediately after each exercise test using a calibrated 3-L syringe (Hans Rudolph Series 5530). Dried expired gases were continuously sampled throughout the exercise test for the determination of % O2 (S-3A/1, Ametek, Paoli, PA) and % CO2 (Ametek, CD-3A). The analyzers were calibrated before and immediately after each exercise test by using precision reference gases (BOC gases, Sydney, Australia). Voltage outputs from the cycle ergometer, ECG, pneumotach, and O2 and CO2 analyzers were relayed to a computer via an 12-bit analog to digital board (Data Translation 2814, Data Translation, Inc., Marlboro, MA) sampling at 100 Hz. Raw and calculated data were stored and displayed using custom written software (Exerstress 1.1, Sydney, Australia).
Each subject commenced cycling at 20 W. After a 3-min warm-up period, the power was increased by 5 W every 20 s until exhaustion or clinically significant findings prevented further exercise. Throughout the exercise test, metabolic and HR data were averaged over each 20-s period. Metabolic and HR data obtained during the final minute of the exercise test were averaged and reported as the subject’s peak oxygen uptake (V̇O2peak), ventilation (V̇Epeak), power, and HR (HRpeak) provided the subject met two of the following criteria: (i) attainment of a plateau in V̇O2 with increasing power, (ii) HR within at least 10 beats·min−1 of age-predicted maximum, and (iii) respiratory exchange ratio in excess of 1.1. Similar criteria for the attainment of peak or maximal V̇O2 in older adults have been used by other researchers (11,16,26). If the subject did not meet the criteria, then the incremental exercise test was repeated on a separate day.
Cardiac output during peak and submaximal exercise.
Cardiac output (Q̇) was measured using the modified acetylene (C2H2) rebreathing technique (27). To measure Q̇, the subject rebreathed a gas mixture of 0.9% C2H2, 9.9% He, 40% O2 balance N2 from a closed-circuit anesthetic bag. The concentrations of He, C2H2, and CO2 were sampled continuously at the mouth using a mass spectrometer (Airspec 3000 MGA, Airspec, Biggin Hill, U.K.). Before each test, the mass spectrometer was calibrated using a chemically analyzed alpha standard gas (Linde Gases, Sydney, Australia). Output from the mass spectrometer was normalized to allow for the effects of water vapor. Cardiac output was determined from the exponential decay of C2H2 relative to He over six to eight sequential end-tidal breaths by using custom written software (18). The day-to-day intraclass correlation coefficient for repeated estimates of Q̇ during submaximal exercise in men (64 ± 1 yr) was 0.92.
To minimize any learning effects, each subject was thoroughly familiarized with the rebreathing technique before the peak and submaximal exercise tests. Peak cardiac output was measured after the determination of V̇O2peak. The subject initially cycled at 20 W for 2 min while breathing through a three-way valve (Hans Rudolph). The power on the cycle ergometer was progressively increased over a 6- to 8-min period, until the V̇O2 and HR were within 95–100% of the previously determined peak values, at which point Q̇peak was measured. Peak cardiac output was measured on two separate days before commencing the study with the highest Q̇ recorded taken as the subjects pretraining Q̇peak.
Each subject completed a submaximal exercise test at the same absolute intensity before and after the 10-wk endurance-training or control period. During the submaximal exercise test, Q̇ was measured on at least two occasions, with the first measurement occurring 6 min after the subject had commenced cycling at the submaximal exercise intensity. Subsequent measurements of Q̇ were separated by 3 min to allow for the washout of C2H2. Oxygen uptake and HR were measured in the minute immediately before the determination of Q̇. Submaximal exercise Q̇, V̇O2, and HR were taken from the average of the two measurement periods. Stroke volume was calculated as Q̇/HR (expressed as mL·beat−1) and arteriovenous O2 difference [(a-v̄)O2 difference] as V̇O2/Q̇ (expressed as mL O2·100 mL blood−1).
The subjects in the CEx and IEx groups trained three times per week for 10 wk. Subjects in the CEx and IEx groups trained at an absolute power calculated to elicit 70–75% of their pretraining V̇O2peak during continuous exercise. The present study used a similar exercise prescription as previous studies that modeled the time course of the peak exercise responses to endurance training (10,13). As a result, subjects in the CEx and IEx groups trained at the same absolute exercise intensity for the entire 10-wk period and completed the same total amount of work during each training session. Subjects in the CEx group exercised for 30 min each training session at the required intensity. Subjects in the IEx group exercised for 60 s then passively rested for 60 s with repeats of 60-s exercise, 60-s rest for 60 min. Hence, the total exercise time (i.e., 30 min) for the IEx group and the CEx group was the same. To compare the physiological response between the CEx and IEx groups, V̇O2 and HR were recorded at the midpoint of the first training session for each subject. For the IEx group, V̇O2 and HR reported in the results were recorded during the 60-s exercise period.
Each subject from the CEx and IEx groups completed the same total amount of work during each training session. The total work completed during each training session was determined from the instantaneous power output from the cycle ergometer downloaded to the computer. By using custom written software (18), the signal from the cycle ergometer was collected, integrated, and summed to give the total work completed.
Each subject completed two peak exercise tests before being allocated to the CEx (N = 10), IEx (N = 10), or CON (N = 5) groups. The highest V̇O2 achieved in the two peak exercise tests was taken as the subject’s V̇O2peak. The three groups were initially matched so that there were no significant differences in age, height, body mass, and peak V̇O2, power, V̇E, Q̇, or HR. To determine the time course model and the rate of change of adaptation, peak exercise tests were repeated every 2 wk for subjects in both the exercise and control groups. The posttraining V̇O2peak and Q̇peak were measured within 3 d of completing the final training session. All other measurements including anthropometry, spirometry, and the submaximal exercise test were completed within 1 wk of the completion of the 10-wk training or control period.
Mean and standard error of the mean (SE) were calculated for all dependent variables. The differences in the training V̇O2, HR and total amount of work completed between CEx and IEx groups were assessed using independent t-tests assuming equal variance. Amplitude adaptations to endurance training were assessed using a two-way analysis of variance with repeated measures. Post hoc group differences were assessed using Tukey’s confidence intervals. A polynomial fit was used to determine order of the time course models for endurance-training adaptations (28). To determine the order of the time course data, linear (x), quadratic (x2), cubic (x3), quartic (x4), and quintic (x5) components were sequentially fitted to the time course data. Significant components were then incorporated into the appropriate time course model and residual plot analysis performed on the fitted data. The normalcy of the residuals was assessed by analysis of the scatter plot by using the methods outlined by Rice (20). Statistical significance was accepted at P < 0.05.
Each subject in the CEx and IEx groups completed 29 endurance-training sessions over the 10-wk period. The absolute training intensity (CEx 112 ± 5 W; IEx 112 ± 5 W) and the total amount of work completed (CEx, 199 ± 9 kJ; IEx 195 ± 9 kJ, P = 0.67) for each training session were not significantly different between the exercise groups. The IEx group trained at a significantly (P < 0.01) lower HR (CEx, 139 ± 4 beats·min−1; IEx, 109 ± 2 beats·min−1) and V̇O2 (CEx, 1.65 ± 0.06; IEx 1.23 ± 0.06 L·min−1) than the CEx group.
Peak exercise responses.
Peak exercise responses, measured before and after endurance training, are presented in Table 2. Both the CEx and IEx groups demonstrated a significant increase (P < 0.01) in V̇O2peak (L·min−1 and mL·kg−1·min−1) after 10 wk of endurance training. The amplitude increase in V̇O2peak with endurance training did not differ significantly between exercise groups (CEx +3.7 ± 0.5 mL·kg−1·min−1, IEx +3.7 ± 0.4 mL·kg−1·min−1, P = 0.97).
Both exercise groups had a significant increase (P < 0.01) in peak power and V̇Epeak after 10 wk of endurance training. The amplitude increase in the peak power (CEx +25.5 ± 3.2 W, IEx +26.1 ± 4.1 W P = 0.91) and V̇Epeak (CEx +23.9 ± 2.9 L·min−1, IEx +18.1 ± 4.5 L·min−1, P = 0.29) was not significantly different between exercise groups. Both the CEx and IEx groups had significant (P < 0.01) increases in HRpeak after 10 wk of endurance training. Peak HR increased by 7 ± 2 beats·min−1 for the CEx group and 8 ± 1 beats·min−1 for the IEx group. The increase in HRpeak was not significantly different between exercise groups. There were no significant changes in peak V̇O2 (L·min−1 and mL·kg−1·min−1), power, V̇E, or HR for the CON group.
Peak cardiac output and stroke volume.
The changes in peak Q̇ and SV (SVpeak), for the CEx and IEx groups are presented in Figure 1. Peak Q̇ increased significantly (P < 0.05) for both the CEx and IEx groups after endurance training. The increase in Q̇peak was associated with a significant increase (P < 0.05) in SVpeak for both exercise groups. The increase in peak Q̇ and SV was not significantly different between the exercise groups. There was no significant change in the peak (a-v̄)O2 difference for either the CEx or IEx groups. Peak Q̇, SV, and (a-v̄)O2 difference did not change significantly over the 10-wk period for the CON group.
Submaximal exercise test.
There were no significant changes in the submaximal exercise V̇O2, Q̇, and (a-v̄)O2 difference measured before or after the 10-wk endurance-training period for the CEx, IEx, and CON groups. Changes in the submaximal exercise HR and SV for the CEx and IEx groups are presented in Figure 2. The submaximal exercise HR for the CEx and IEx groups decreased significantly (P < 0.01) after 10 wk of endurance training. Accordingly, the submaximal exercise SV increased significantly (P < 0.01) for both exercise groups after training. The decrease in the submaximal exercise HR, and the increase in SV, was not significantly different between the CEx and IEx groups. The submaximal exercise HR and SV did not change significantly for the CON group.
Time course changes.
For both exercise groups, only the linear components of the time course models describing the change in V̇O2peak, peak power, and V̇Epeak were significant (P < 0.01). Higher-order components of the time course model were not significant. As a result, linear models were used to describe the time course change in peak V̇O2, power, and V̇E for the CEx and IEx groups. Time course changes in peak V̇O2, power, and V̇E for the CON group were not significant. Time course changes in V̇O2peak (mL·kg−1·min−1) for the three groups are shown in Figure 3.
Time course changes in V̇O2peak (L·min−1) were described by the linear models y(t) = 2.20 + 0.02(t) and y(t) = 2.34 + 0.02(t) for the CEx group and IEx groups, respectively. The linear models accounted for 91% and 76% the variance for the CEx and IEx groups, respectively. The rate of change in V̇O2peak (L·min−1) over the 10-wk training period was not significantly different between CEx and IEx groups (CEx: 0.02 ± 0.00 L·min−1·wk−1, IEx 0.02 ± 0.00 L·min−1·wk−1, P = 0.34). When V̇O2peak was expressed relative to body mass, the respective linear models for the CEx and IEx groups were y(t) = 27.5 + 0.3(t) and y(t) = 29.0 + 0.3(t), which accounted for 92% and 75% of the variance, respectively (Fig. 3). The rate of change in V̇O2peak (CEx: 0.3 ± 0.0 mL·kg−1·min−1·wk−1, IEx: 0.3 ± 0.0 mL·kg−1·min−1·wk−1, P = 0.32) was not significantly different between the CEx and IEx groups over the 10-wk training period.
The linear models describing the time course changes in peak power were y(t) = 191 + 2.6(t) for the CEx group and y(t) = 194 + 2.6(t) for the IEx group which accounted for 95% and 96% of the variance, respectively. The rate of change in the peak power was not significantly different between the CEx and IEx groups (CEx: 2.6 ± 0.4 W·wk−1, IEx: 2.6 ± 0.4 W·wk−1, P = 0.92) over the 10-wk training period. The linear model describing the time course change in V̇Epeak for the CEx group was y(t) = 89.7 + 2.0(t), which accounted for 93% of the variance. The time course change in V̇Epeak for the IEx group was described by the linear model y(t) = 103 + 1.2(t), which accounted for 78% of the variance. There was no significant difference in the rate of change in V̇Epeak between the two exercise groups (CEx: 2.0 ± 0.2 L·min−1·wk−1, IEx: 1.2 ± 0.5 L·min−1·week−1, P = 0.10).
The principal finding of the present study is that both the amplitude and rate of adaptation were independent of training mode (i.e., continuous and intermittent exercise) when the training intensity is the same and the total work completed by each exercise group (CEx and IEx) are equal. The present study also found that both the rate and amplitude of increase in V̇O2peak, V̇Epeak, and peak power were not significantly different between continuous and intermittent modes of exercise.
Our finding that the increase in V̇O2peak did not differ significantly between CEx and IEx groups is consistent with some studies of young subjects (5,7,19) but not others (9,12). In the present study, the CEx and IEx groups completed the same total amount of work during each training session. Studies of younger subjects, which equalized the total amount of work completed by the CEx and IEx groups, also found similar increases in V̇O2max for both continuous and intermittent groups (5,7,19). In contrast to our findings, studies of younger subjects that did not equalize the total amount of work completed found the increase in V̇O2max to be significantly different between the exercise groups (9,12). The results of the present study and those of others (5,7,19) suggest that the total amount of work completed may be the primary stimulus for adaptation to endurance training for elderly subjects, regardless of whether the exercise is performed continuously or intermittently.
Unlike other studies (5,7,19), the CEx and IEx groups of the present study trained at the same absolute training intensity (112 ± 5 W) for 10 wk. Studies of younger subjects, which found similar increases in V̇O2max for CEx and IEx groups that completed the same amount of work, prescribed IEx at a higher absolute intensity than CEx. For example, Eddy et al. (7) reported a 15% increase in V̇O2max for CEx and IEx groups after 7 wk of training; however, the CEx and IEx groups trained at an intensity of 70% and 100% V̇O2max, respectively. Poole and Gaesser (19) trained two continuous exercise groups at 50 and 70% V̇O2max, respectively, whereas an intermittent exercise group trained at 105% V̇O2max. All three groups completed the same total amount of work and had similar and significant increases in V̇O2max. Cunningham et al. (5) trained 2 groups of female subjects using continuous and intermittent exercise at 70 and 100% V̇O2max respectively and found similar increases in V̇O2max for both exercise groups.
Typically, endurance-training studies of younger subjects have prescribed IEx at higher intensity than CEx in an attempt to equalize the physiological responses to the different modes of exercise. The present study did not attempt to equalize the physiological responses to continuous and intermittent exercise. Rather, we selected two modes of exercise (CEx vs IEx) and compared the adaptations to endurance training when the absolute training intensity and the total amount of work completed were the same. The IEx group had similar increases in peak V̇O2, V̇E, power, Q̇, and SV as the CEx group. Moreover, the rate of increase in peak V̇O2, power, and V̇E were the same for the CEx and IEx groups. Our results suggest that older subjects would be able to exercise using IEx and still achieve similar increases in peak exercise responses as subjects performing high-intensity CEx for the first 10 wk of training.
Several studies have compared CEx performed at different intensities in older subjects and reported the increase in V̇O2peak to be independent of the training HR and V̇O2. Badenhop et al. (2) found no significant difference in the increase in V̇O2peak for two groups that trained for 9 wk at intensities corresponding to 30–45% of heart rate reserve (HRR) and 60–75% of HRR. Likewise, Belman and Gaesser (3) reported an 8% increase in V̇O2peak for each of two groups that trained for 8 wk at intensities either below (35% of HRR) or above (75% of HRR) the blood lactate threshold. These findings (2,3) and those of the present study support the hypothesis that endurance-training adaptations may be independent of the training HR and V̇O2 during the first 8–10 wk of training in older men.
The primary mechanisms for the increase in peak V̇O2 with endurance training for the CEx and IEx groups was an increase in Q̇peak. Our results agree with those reported in some studies (16,26) but conflict with others (24). Unlike the present study, Seals et al. (24) reported that the primary mechanism for an increase in V̇O2max after endurance training in a group of older men and women was an increase in the maximal (a-v̄)O2 difference rather than an increase in the Q̇max. However, Seals and colleagues (24) possibly failed to detect a change in Q̇max because both male and female subjects were included in their experiment.(It has been reported that for older female subjects, the increase in V̇O2max after endurance training is only the result of an increase in maximal (a-v̄)O2 difference (26). Conversely, studies of older male subjects have reported increases in the maximal Q̇ and (a-v̄)O2 difference after endurance training (16,26).
Both exercise groups (CEx and IEx) had an increase in the peak HR after endurance training. Increases in peak HR after endurance training for men aged 60–70 yr have been reported previously by other investigators (16,17). Markides et al. (16) reported a 12 beat·min−1 increase in peak HR after 12 wk of endurance training. Meredith et al. (17) also reported that the peak HR increased from 154 ± 10 beats·min−1 to 163 ± 13 beats·min−1 after 12 wk of endurance training in a group of 10 subjects aged 65 ± 1 yr.
Like the present study, both Markides et al. (16) and Meredith et al. (17) measured peak V̇O2 on the cycle ergometer. Although it is recognized that a true maximal response is difficult to achieve on a cycle ergometer, particularly in elderly subjects (14), the present study undertook several steps to ensure that the pretraining V̇O2peak was representative of a peak effort. First, each subject completed at least two incremental exercise tests to exhaustion, on separate days, before being allocated to either the exercise or control group. The highest V̇O2, power, V̇E, and HR achieved in these two exercise tests were taken as the subject’s pretraining peak exercise response. Second, the criteria used in the present study to determine the attainment of V̇O2peak or V̇O2max were consistent with other investigations of elderly subjects (11,16,26). We believe that the pretraining peak exercise test was representative of a peak effort and the HR achieved during the pretest was the highest that could be obtained on a cycle ergometer at that point.
Both exercise groups had a similar increase in SV and a decrease in HR during submaximal exercise. These changes in the submaximal exercise response for the CEx and IEx groups are consistent with those reported by others (2,3,15,16,24,26). The decrease in submaximal exercise HR with endurance training in the elderly has been associated with an increase in the diastolic filling time and end diastolic volume (8,21). Such changes in the end diastolic volume may have contributed to an increase in the SV via the Frank-Starling mechanism (21).
Our finding that the increase in V̇O2peak (L·min−1, mL·kg−1·min−1), power, and V̇Epeak in older men, can best be described by a linear time course model, conflict with the exponential time course model, y(t) = c1(1 − e−c2t), proposed by Hickson et al. (13) and Govindasamy et al. (10). Like the present study, both Hickson et al. (13) and Govindasamy et al. (10) employed an experimental design whereby the absolute training intensity remained unchanged during the period that the time course model was fitted. The analyses employed in the present study could provide no evidence to support the exponential time course model proposed by other investigators (10,13). When increasing order polynomials were fitted to the change in V̇O2peak, peak power, and V̇Epeak, only the linear components were significant. If the time course model describing the increase in V̇O2peak, power, and V̇Epeak, was exponential, then higher order polynomials would have been significant.
Although the duration of the present study is longer than others studies that have modeled the time course change in peak exercise responses (10,13), the 10-wk duration of the training program does represent an experimental limitation. Conceptually, a linear increase is unlikely to continue indefinitely and a longer duration study (20 wk) may have indicated that the rate of change in the in V̇O2peak, peak power, and V̇Epeak would be asymptotic. It is possible that if the training duration had been extended then the CEx and IEx groups may have had different amplitude and rates of training adaptations.
In summary, the present study found the amplitude and rate of adaptation to 10 wk of CEx and IEx were independent of the mode of exercise (CEx and IEx), when the absolute training intensity and the total amount of work completed were similar. For both exercise groups, the principal mechanism for the increase in V̇O2peak was an increase in the Q̇peak. The results of the present study suggest that intermittent exercise should be considered as an alternative exercise mode for older individuals.
Dr. N. Morris was supported by the National Health and Medical Research Council of Australia, grant no. 961088.
Address for correspondence: Norman R. Morris, Ph.D., School of Physiotherapy and Exercise Science, Griffith University, Gold Coast, PMB 50 Gold Coast Mail Centre, Queensland 9276, Australia; E-mail: N.Morris@mailbox.gu.edu.au.
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