Scholarly article on topic 'Effects of Non-Wingate-based High-intensity Interval Training on Cardiorespiratory Fitness and Aerobic-based Exercise Capacity in Sedentary Subjects: A Preliminary Study'

Effects of Non-Wingate-based High-intensity Interval Training on Cardiorespiratory Fitness and Aerobic-based Exercise Capacity in Sedentary Subjects: A Preliminary Study Academic research paper on "Medical engineering"

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{"aerobic fitness" / "intermittent exercise" / "interval training" / "proportional-assist ventilation"}

Abstract of research paper on Medical engineering, author of scientific article — Tom K. Tong, Pak Kwong Chung, Raymond W. Leung, Jinlei Nie, Hua Lin, et al.

This study examined two hypotheses: (1) a non-Wingate-based high-intensity interval training protocol of 20×30-second cycle exercise at 120% of peak aerobic power interspersed with 60-second recovery per session, 3 sessions per week for 6 weeks, can enhance cardiorespiratory fitness and aerobic-based exercise capacity; and (2) proportional-assist ventilation (PAV) can augment interval training intensity, and, in turn, enhance the adaptations to the training in sedentary and mild obese individuals. Sixteen subjects were paired up and assigned into an interval training (IT) group or IT plus PAV (IT + PAV) group. During the 6-week interval training program, the increase in training intensity was not different between the IT and IT + PAV groups (p > 0.05). Nevertheless, significant improvements were found in the peak work rate and peak O2 during the post-training incremental cycling test in both groups. Moreover, the limit of tolerance during the post-training constant-load cycling test (70% of pre-training peak aerobic power) was enhanced, while blood lactate accumulation, heart rate, ratings of breathing discomfort, and perceived exertion at the iso-time point of the pre-training test at exhaustion were reduced (p < 0.05). The interaction effect on the change in each variable between the two groups was not significant (p > 0.05). In conclusion, the 6-week non-Wingate-based high-intensity interval training protocol was preliminarily found to enhance cardiorespiratory fitness and aerobic-based exercise capacity in sedentary and mild obese subjects. The provision of PAV during the interval training did not augment training intensity and subsequent aerobic adaptations.

Academic research paper on topic "Effects of Non-Wingate-based High-intensity Interval Training on Cardiorespiratory Fitness and Aerobic-based Exercise Capacity in Sedentary Subjects: A Preliminary Study"

Original Article

Effects of Non-Wingate-based High-intensity Interval Training on Cardiorespiratory Fitness and Aerobic-based Exercise Capacity in Sedentary Subjects:

A Preliminary Study

Tom K. Tong1, Pak Kwong Chung1, Raymond W. Leung2, Jinlei Nie3, Hua Lin4, Jun Zheng4

1Department of Physical Education, Hong Kong Baptist University, Hong Kong, CHINA 2Department of Physical Education and Exercise Science, Brooklyn College of the City University of

New York, Brooklyn, NY, USA 3School of Physical Education and Sports, Macao Polytechnic Institute, Macao, CHINA 4Physical Education Department, Liaoning Normal University, Dalian, CHINA

This study examined two hypotheses: (1) a non-Wingate-based high-intensity interval training protocol of 20x30-second cycle exercise at 120% of peak aerobic power interspersed with 60-second recovery per session, 3 sessions per week for 6 weeks, can enhance cardiorespiratory fitness and aerobic-based exercise capacity; and (2) proportional-assist ventilation (PAV) can augment interval training intensity, and, in turn, enhance the adaptations to the training in sedentary and mild obese individuals. Sixteen subjects were paired up and assigned into an interval training (IT) group or IT plus PAV (IT + PAV) group. During the 6-week interval training program, the increase in training intensity was not different between the IT and IT + PAV groups (p > 0.05). Nevertheless, significant improvements were found in the peak work rate and peak Vo2 during the post-training incremental cycling test in both groups. Moreover, the limit of tolerance during the post-training constant-load cycling test (70% of pre-training peak aerobic power) was enhanced, while blood lactate accumulation, heart rate, ratings of breathing discomfort, and perceived exertion at the iso-time point of the pre-training test at exhaustion were reduced (p < 0.05). The interaction effect on the change in each variable between the two groups was not significant (p > 0.05). In conclusion, the 6-week non-Wingate-based high-intensity interval training protocol was preliminarily found to enhance cardiorespiratory fitness and aerobic-based exercise capacity in sedentary and mild obese subjects. The provision of PAV during the interval training did not augment training intensity and subsequent aerobic adaptations. [J Exerc Sci Fit • Vol 9 • No 2 • 75-81 • 2011]

Keywords: aerobic fitness, intermittent exercise, interval training, proportional-assist ventilation

Introduction

Poor dietary habits have been known for years to be a key risk factor for chronic diseases; the role of physical

Corresponding Author Pak Kwong Chung, Department of Physical Education, DLB621A, L6, David C. Lam Building, Shaw Campus, Hong Kong Baptist University, Renfrew Road, Kowloon Tong, Hong Kong, CHINA. E-mail: pkchung@hkbu.edu.hk

ELSEVIER

activity is now also firmly established as an increasingly important determinant of health (Roberts & Barnard 2005). Though researchers and policymakers continue to stress the importance of leading healthy lifestyles, much is still to be understood concerning the minimum dose of physical activity necessary to improve health status (Blair et al. 2004). Burgomaster et al. (2008) reported that a low-volume interval training protocol requiring participants to repeat the 30-second Wingate test four to six times per training session, three sessions per week for 6 weeks resulted in a number of

metabolic adaptations usually associated with traditional aerobic training. However, the Wingate-based training protocol requires an individual to perform at an extremely high level of motivation. It is doubtful if sedentary persons could perform the high-intensity interval training safely and practically. On the other hand, for reducing the work rate of the repeated 30-second exercise bouts to 120% of peak aerobic power (interspersed with 60-second recovery), the intensity of the intermittent exercise has been demonstrated to be approximate to the maximum lactate steady state in untrained subjects—a borderline intensity with relevance for the prescription of aerobic training (Turner et al. 2006; Beneke et al. 2003). The relatively lower work rate performed intermittently, as compared with that of the Wingate-based training protocol, may elicit fewer feelings of difficulty and be more applicable in sedentary people. Whether or not a non-Wingate-based high-intensity interval training protocol with an initial work rate set at 120% of peak aerobic power, with the total work and recovery duration close to that of the previous Wingate-based protocol for repeating six Wingate tests, can enhance cardiorespiratory fitness and aerobic-based exercise capacity in sedentary persons is not known.

Proportional-assist ventilation (PAV) was originally designed for clinical use of noninvasive ventilation (Dolmage & Goldstein 1997). The ventilation assist is accomplished by an amplification of the inspiratory airway pressure in proportion to the patient's instantaneous inspiratory effort for generating inhaled airflow and volume. As a result, resistive and elastic loads of ventilation are lessened. PAV has been shown to be effective in unloading inspiration in patients with chronic obstructive pulmonary disease, and in improving submaximal exercise endurance by easing the sensation of breathlessness (Bianchi et al. 1998). Similar beneficial effects of PAV have also been reported in athletes during exercise (Kleinsasser et al. 2004; Harms et al. 2000). It was previously reported that inferior exercise tolerance in obese persons might be partly attributable to the early onset of the sensation of breathlessness (Tong et al. 2006). It is thought that the provision of PAV during non-Wingate-based interval training might allow mild obese subjects to sustain the training load at a high level.

The purposes of this study were to examine the following two hypotheses: (1) a non-Wingate-based high-intensity interval training protocol of 20 x 30-second cycle exercise at 120% of peak aerobic power interspersed with 60-second recovery per session, 3 sessions per week for 6 weeks, can enhance cardiorespiratory

fitness and aerobic-based exercise capacity; and (2) PAV can augment interval training intensity, and, in turn, enhance the adaptations to the interval training in sedentary and mild obese individuals.

Methods Participants

Twelve women and four men, who were mild obese but otherwise healthy and not engaged in regular physical work, volunteered to participate in the present study. Each participant was paired up with another of identical sex and similar obesity and aerobic fitness. Each pair was assigned randomly into either of the two groups: interval training (IT) and IT plus PAV (IT + PAV). The sample size for each group was computed based on the findings reported in the study of Burgomaster et al. (2005) and that of our pilot study. A power of 0.9 was selected to detect the expected increase in endurance capacity with a = 0.05. We used the formula N =a2(-z0 + zt)2/(^0~M-i)2 (Toothaker & Miller 1996), where is the pre-training endurance exercise capacity with a (± standard deviation), is the expected post-training endurance capacity, z0 is the critical value for effect size under the null distribution, and z1 is the critical value that is associated with the alternative distribution, N < 8.

The physical characteristics of the two groups, shown in Table 1, were not different (p > 0.05). All participants were asymptomatic for respiratory disease and had no orthopedic or cardiovascular contraindications to exercise. Following explanation of the purpose of the study and the potential benefits and risks involved in the interval training and exercise tests, the participants gave written informed consent for participating in this study. The local Ethics Committee for the Use of Human and Animal Subjects in Research provided ethical approval of the study.

Procedures

Both the IT and IT + PAV groups participated in an identical 6-week interval training program. A pre-set PAV was provided to the IT + PAV group, but not to the IT group, during the training. Identical incremental and constant-load exercise tests were performed by the two groups pre- and post-training. The aerobic adaptations to the interval training in subjects w. ere examined by investigating the changes in peak VO2 and work rate achieved during an incremental cycling test, and in the limit of tolerance and blood lactate accumulation

Table 1. Physical characteristics of the study participants*

IT + PAV ( n = 8) IT (n = 8)

Age (yr) 22.5 ± 3.2 22.9 ± 2.4

Height (cm) 163.5 ± 7.2 161.2 ± 9.3

Weight (kg) 66.8 ± 15.7 62.0 ± 13.8

%BF 29.9 ± 3.0 31.3 ± 5.7

Rairway (cmH2O • L— • s_1) 1.40 ± 0.45 1.52 ± 0.47

FVC (L) 3.78 ± 0.91 3.44 ± 0.85

FEV1 (L) 3.20 ± 0.64 2.96 ± 0.61

FEV1/FVC (%) 85.6 ± 6.0 86.6 ± 4.6

RV (L) 1.47 ± 0.40 1.44 ± 0.52

FRC (L) 3.57 ± 1.74 2.60 ± 0.96

IRV (L) 1.67 ± 0.80 1.61 ± 0.53

IC (L) 2.32 ± 1.07 2.30 ± 0.59

ERV (L) 1.48 ± 0.52 1.14 ± 0.51

TLC (L) 5.27 ± 1.17 4.90 ± 1.28

*All variables were not different between the two groups, p > 0.05. %BF = percent body fat; Rairway = airway resistance; FVC = forced vital capacity; FEV1 = forced expiratory volume in 1 second; RV = residual volume; FRC = functional residual capacity; IRV = inspiratory reserve volume; IC = inspiratory capacity; ERV = expiratory reserve volume; TLC = total lung capacity.

in performing a constant-load cycling test post-training in the two groups. To examine the potential advantage of applying PAV during the interval training, the increase in training intensity within the 6-week training period and the resultant aerobic adaptations were compared between the two groups. In this study, all exercise tests were conducted on the same cycle ergometer (Monark 834E; Monark Exercise AB, Varberg, Sweden) in an air-conditioned laboratory. The seat and handle of the cycle ergometer were modified to facilitate the participants to sustain exercise on it. During the exercise tests, the participants were verbally encouraged to make a maximum effort. All exercise tests were separated by a minimum of 3 days. Participants were requested to refrain from vigorous physical activity 24 hours prior to any exercise test and to stick to their normal eating habits during the training and testing periods.

Preliminary testing and familiarization Before the experimental trials, the body height and weight of the participants were measured. Percent body fat (%BF) was assessed with a leg-to-leg bioimpedance body fat analyzer (Tanita TBF-410; Tanita Corporation, Tokyo, Japan). Maximum static inspiratory mouth pressure (PImax), forced spirometry, resting airway resistance and subdivision lung volumes were measured using body plethysmography (V6200 Autobox; Sensormedics, Yorba Linda, CA, US). The measurements of %BF and PImax were repeated after the 6-week intervention.

Two exercise tests identical to the experimental trials were undertaken to familiarize the participants with the sensation of exercising to exhaustion and with the testing equipment and procedures.

Experimental trials

Incremental cycling test: After sitting at rest on the Monark cycle ergometer for 2 minutes, subjects started pedaling at 50 ± 2rev • min-1 with no load applied for 2 minutes. The work rate was then increased by 15 W every minute while the pedal frequency was maintained throughout the test until volitional exhaustion (Marques-Magallanes et al. 1997). Respiratory and metabolic responses during the test were recorded online breath-by-breath with the SensorMedics Cardiopulmonary Exercise Testing Instrument (Vmax229d; SensorMedics). The peak work rate (Wpeak) was defined as the highest work rate that could be maintained for 1 minute. The highest averaged Vo2 for each 10 seconds was the peak VO2 (VO2peak).

Constant-load cycling test: Subjects performed the test on the same cycle ergometer at a constant work rate of 70% Wpeak with the pedal frequency maintained at 50 ± 2rev • min-1. The limit of tolerance (Endtime) was defined as the time to volitional exhaustion. During the exercise test, heart rate (HR) response was recorded with a heart rate monitor (Polar Electro Oy, Kempele, Finland) while perceived intensity of breathless sensation (RPB) and physical exertion (RPE) were assessed with the aid of Borg category scales (0-10) and (6-20), respectively, every 2 minutes. Blood samples were collected immediately before and after the exercise to assess the accumulation of blood lactate (LA). In the identical test after the 6-week interval training program, an additional blood sample was drawn at the time when the pre-training test ended. For each LA measurement, a sample of fingertip blood of 25 ^L was drawn for analysis using the YSI 1500 Sport Analyzer (YSI Inc., Yellow Springs, OH, USA). The blood sampling and LA analysis were performed by following the manufacturer instructions.

6-week interval training program

Both groups performed interval training three times a week for 6 weeks. The protocol of the interval training consisted of 20 x 30-second exercise bouts on the same cycle ergometer interspersed with 60-second recovery. The initial loading of the 30-second exercise bouts was 120% Wpeak, while it was reduced to 20 W during the recovery intervals. The exercise load was increased

voluntarily by at least 6 W each time when the participant was able to complete the 30-minute training session. For participants who could not follow the protocol with an increased loading, an extension of the recovery time to 90 seconds for each resting interval was allowed, while the exercise bout repetitions were reduced in order to keep the training time to 30 minutes for each session. The increase in training load during the 6-week intervention was the difference between the initial loading and the highest loading that could be sustained with the pre-set recovery time.

During the training, PAV was provided to the IT + PAV group by using a BiPAP Vision Ventilatory Support System (Respironics Inc., Pittsburgh, PA, USA) in PAV mode. The adjustment of PAV entails assistance to unload elastic and resistive burdens and has been described previously (Wysocki et al. 2004). Briefly, positive airway pressure generated from the ventilatory support system was applied to the participant. The inspired gas passed through a humidifying chamber and was directly delivered to the participant through a bore tubing that was connected to the inspiratory port of the non-rebreathing valve to which the participant was attached. The adjustment of the magnitude of PAV was started from volume assist with flow assist set at 1 cmH2O • L-1 • s-1. The level of volume assist was increased in steps of 2 cmH2O • L-1 until the ventilator inspiratory time suddenly extended beyond the value of previous breaths with a visible activation of expiratory muscles before inspiration off. After that, the volume assist was turned to 0 cmH2O • L-1 and started to increase the flow assist in steps of 2 cmH2O • L-1 • s-1 until the detection of rapid boosts of inspiratory airway pressure immediately after starting inspiration. Eighty percent of these pre-set

values of volume and flow assists were used in order to neutralize most, but not all, of the elastic and resistive work during exercise.

During the exercise training, mild adjustment on the magnitude of the pre-set PAV was performed voluntarily by the participant to minimize the discomfort mediated by the excessive positive airway pressure. The mean volume and flow assist in the IT+PAV group were 3.79 ± 0.60cmH2O • L-1 and 2.00 ± 0.19cmH2O • L-1 • s-1, respectively. The provision of the pre-set PAV to participants was mandatory during the 30-second exercise bouts starting from the sixth bout; the utilization of PAV during the recovery intervals was optional.

Statistical analysis

Two-way ANOVA was performed to assess the differences in the variables measured during the exercise tests and the interval training program between pre-and post-training and across IT and IT + PAV groups. Post-hoc analyses using Newman-Keuls were performed when the main effects of ANOVA were significant. All tests for statistical significance were standardized at an alpha level of p < 0.05, and all results are expressed as mean ± standard deviation.

Results

Table 2 shows the results of the variables measured pre- and post-training in the IT and IT + PAV groups. Pre-training %BF and PImax were not different between the IT and IT + PAV groups (p > 0.05). No significant difference was found in pre-training Wpeak and VO2peak measured during the incremental cycling test between

Table 2. Adaptations to the 6-week non-Wingate-based high-intensity interval training

IT + PAV (n = 8) IT (n = 8)

Pre Post Pre Post

Wpeak (W) 162.0 ± : 50.9 186.8 d t53.5* 144.0 d 134.7 171.0 d d 39.1*

VO2peak (mL-kg^-min-1) 35.8 ± : 5.3 37.7d t5.0* 36.2 d 15.8 38.6d t5.0*

Endtime (min) 30.3 ± : 8.5 47.7d 1116.7* 28.0 d 17.3 49.8 d 115.3*

LA (mmol-L-1) 5.02 ± : 1.82 3.74 d 11.40* 5.55 d 12.88 3.29d 11.95*

HR (b-min-1) 159.8 ± = 16.1 148.5 d 114.4* 172.4d 110.0 152.9 d 111.7*

RPB 8.88 ± = 1.25 6.88 d 11.73* 9.25 d 11.16 7.13d 11.89*

RPE 19.6 ± : 0.5 17.6 d t0.92* 19.6 d 10.7 17.0 d 11.3*

%BF 29.9 ± :3.0 29.8 d t2.9 31.3d 15.7 29.1 d 15.6*

PImax (cmH2O) 110.5 ± : 26.2 110.3d t25.9 106.9 d 128.9 107.9d t24.6

•Significantly different from corresponding Pre values, p < 0.05. Wpeak and VO2peak are the peak work rate and VO2, respectively, recorded in the incremental cycling test; Endtime, LA, HR, RPB and RPE are the limit of tolerance, blood lactate accumulation, heart rate, rating of perceived intensity of breathless sensation and rating of perceived exertion, respectively, recorded during the constant-load cycling test. %BF = percent body fat; PImax = maximum static inspiratory mouth pressure.

the two groups (p>0.05). For Endtime and LA accumulation during the pre-training constant-load cycling test, there was no significant difference between the two groups (p > 0.05). No significant difference was found between the two groups in HR, RPB and RPE measured at exhaustion during the cycling test (p > 0.05).

For the interval training, the initial training load was similar between the IT (173.5 ± 41.5 W) and IT + PAV (193.5 ± 61.4 W) groups. During the 6-week intervention, the training load increased significantly in the two groups (p < 0.05), with a maximum training load of 207.0 ± 42.1 W and 232.3 ± 67.0W in the IT and IT + PAV groups, respectively. The increase in the training load between the two groups was not significantly different (p > 0.05). Following the interval training program, PImax did not change significantly (p> 0.05) in the two groups, whereas a minor but significant %BF reduction (p< 0.05) was observed in the IT group but not in the IT + PAV group. Significant increases in Wpeak and VO2peak (p < 0.05) during the incremental cycling test were found in both the IT and IT + PAV groups (Table 2). Moreover, Endtime increased during the post-training constant-load cycling test in the two groups, while LA, HR, RPE and RPB at the iso-time point of the pre-training test at exhaustion decreased (Table 2). However, none of the changes in the variables of cardiorespiratory fitness and aerobic-based exercise capacity subsequent to the 6-week interval training was different between the IT and IT + PAV groups (p > 0.05).

Discussion

Despite the well-established benefits of regular physical activity on health and wellbeing, more than 50% of adults in Hong Kong, a cosmopolitan city in China, failed to meet the guidelines of physical activity participation recommended by the American College of Sport Medicine (Department of Health, Hong Kong SAR 2008). This was also true in the US, where only 49.1 % of adults met the physical activity recommendation (Haskell et al. 2007). "Lack of time" seems to be a universal reason for not regularly participating in physical activity (Gibala & McGee 2008). In fact, the minimum training volume necessary to enhance aerobic fitness and associated work capacity in sedentary persons is still unclear. Recently, high-intensity interval training with minimal time commitments was recommended in an attempt to overcome the barrier of "lack of time" to exercise participation and to eventually increase the physical activity level and health status in general populations

(Wisl0ff et al. 2009; Gibala & McGee 2008). Moreover, low-volume interval training, usually performed with an all-out effort, could induce metabolic adaptations and enhance endurance exercise capacity associated with traditional high-volume endurance training within a short period (Gibala & McGee 2008). Furthermore, exercise performed intermittently tended to enhance exercise adherence and reduce attrition compared with single-bout continuous exercise; interval training may be more beneficial from a practical approach to weight control or weight loss (Jacobsen et al. 2003; Jakicic et al. 1995). However, the recommended intense interval training protocol, which is Wingate-based (Burgomaster et al. 2008), was often considered unsafe, impractical and intolerable for general populations, especially sedentary and obese ones. Macfarlane et al. (2006) reported that 5 x 6-minute light-to-moderate physical activities per day, 4-5 days per week for 8 weeks, enhanced the VO2max of sedentary subjects. The training protocol, to a certain extent, was high-volume and endurance in nature.

In the present study, a 6-week interval training protocol consisting of 20 x 30-second exercise bouts with initial intensity set at 120% of peak aerobic power was prescribed to the participants. The training volume was close to the previously reported Wingate-based protocol while the training load was lower (Burgomaster et al. 2008). We preliminarily found that the non-Wingate-based interval training protocol could enhance cardiores-piratory fitness and aerobic-based exercise capacity in sedentary and mild obese subjects. Moreover, minor %BF reduction was observed in the IT group after the interval training. However, as we had not controlled for diet during the training period, we could not come to a conclusion that the current training protocol reduced %BF. After the 6-week intervention, a small but significant improvement of ~7% in VO2peak was noted in both the IT and IT + PAV groups. This finding was in contrast to the result of unchanged Vo2peak subsequent to the 2-week Wingate-based interval training program (Gibala & McGee 2008), and may possibly be due to greater total work performed by our participants. Nevertheless, the magnitude of improvement in Vo2peak was much less than the ~18% increase in Wpeak, which suggests that the improved exercise capacity was largely attributable to peripheral adaptations. Such a result supports the previous notion that intense interval training is an effective means of enhancing central and peripheral adaptations in O2 transport and utilization, whereas continuous exercise training is mainly associated with increased O2 extraction (Daussin et al. 2008). Nonetheless, both trainings

permit significant functional improvements in sedentary individuals.

The increased muscular work capacity in the two groups after the 6-week intervention was also revealed by the ~69% improvement in Endtime during the constant-load cycling test. This result was in agreement with the marked improvement in the endurance capacity of submaximal exercise resulting from a 2-week Wingate-based high-intensity interval training reported previously (Burgomaster et al. 2005). The increase in Endtime in the present study was concomitant with the reductions in post-training LA, HR and RPE from the corresponding values of the pre-training trial at exhaustion. Such changes were consistent with the decreases in net muscle glycogenolysis and lactate accumulation during matched-work exercise subsequent to a 2-week Wingate-based high-intensity interval training reported previously (Burgomaster et al. 2006). Further oxidative phenotype upregulated by the 2-week intense interval training including increases in the maximal activity of mitochondrial enzymes such as pyruvate dehydroge-nase, citrate synthase and cytochrome c oxidase have also been reported (Burgomaster et al. 2006, 2005). Talanian et al. (2007) further noted that the markers of fat metabolism during submaximal exercise, including muscle mitochondrial P-hydroxyacyl-CoA dehydroge-nase and muscle plasma membrane fatty acid-binding protein content, and whole-body fat oxidation, increased when the 2-week interval training was performed at a lower intensity (90% VO2peak) and for a longer exercise duration (10 x 4-minute bouts). In the present study, we did not examine changes in the metabolic markers of locomotor muscles of the participants in the posttraining constant-load exercise. The underlying mechanism for the Endtime enhancement is not clear. In light of the previous and current findings, we can only hypothesize that the cardiorespiratory fitness and oxidative capacity of the locomotor muscles of the participants were enhanced subsequent to the 6-week non-Wingate-based interval training program, contributing to the increased Endtime. Moreover, despite the fact that our findings provide reasonable information regarding the non-Wingate-based high-intensity interval training protocol for improving cardiorespiratory fitness and aerobic-based exercise capacity in sedentary individuals, further interpretation of the current findings is limited by lack of comparison with a control group. Investigations of the underlying mechanism for the aerobic adaptations and the temporal dimension of the adaptations secondary to the non-Wingate-based interval training protocol in sedentary individuals by inclusion of a control

group for comparison are recommended for future research.

In the present study, PAV was provided to the IT + PAV group during the 6-week interval training to examine if it could augment the interval training intensity, and, in turn, enhance the training adaptations. The provision of PAV has been shown previously to reduce respiratory muscle work by 35-42% (Wysocki et al. 2004). In comparison of the IT + PAV and IT (control) groups, we found that the work rate of the training bouts increased ~20% from the initial values in both groups over the period with no significant change in inspiratory muscle function. Further, there was no significant augmentation in aerobic adaptations in the IT + PAV group compared to the IT group. Such findings were in contrast to those reported in previous studies. Hawkins et al. (2002) found that PAV augmented the training intensity and the increase in post-training work capacity in patients with chronic obstructive pulmonary disease. The training adaptation was further evidenced by a significant correlation between reduction in blood lactate at equivalent workload and increase in training intensity. Wysocki et al. (2004) reported that PAV lessened the work of breathing markedly in healthy subjects during exercise with external thoracic restriction that was imposed for mimicking patients with increased elastic work of breathing. The contrasting findings in the present study may be due to different physical conditions in the participants—chronic obstructive pulmonary disease versus sedentary, and to different exercise modes—continuous versus intermittent (Coquart et al. 2008). Moreover, factors limiting voluntary increase in training intensity in sedentary individuals during the training program are multidimensional. Although the participants in the IT + PAV group expressed "comfort" brought about by PAV during the intermittent exercise, the willingness of sedentary individuals to increase their effort to match the required level of perceived exertion during physical training may not be comparable to that of trained athletes previously observed (Tong et al. 2010).

In conclusion, the 6-week non-Wingate-based high-intensity interval training with an initial work rate set at 120% of peak aerobic power was preliminarily found to enhance the cardiorespiratory fitness and aerobic-based exercise capacity of sedentary and mild obese individuals. The underlying mechanism for the aerobic adaptations and the temporal dimension of the adaptations await further investigation. In addition, there was no evidence found in these participants that the provision of PAV during interval training can augment training intensity and subsequent aerobic adaptations.

References

Beneke R, Hutler M, Von Duvillard SP, Sellens M, Leithauser RM (2003). Effect of test interruptions on blood lactate during constant workload testing. Med Sci Sports Exerc 35:1626-30. Bianchi L, Foglio K, Pagani M, Vitacca M, Rossi A, Ambrosino N (1998). Effects of proportional assist ventilation on exercise tolerance in COPD patients with chronic hypercapnia. Eur Respir J 11:422-7.

Blair SN, LaMonte MJ, Nichaman MZ (2004). The evolution of physical activity recommendations: how much is enough? Am J Clin Nutr 79:913S-20S.

Burgomaster KA, Heigenhauser GJ, Gibala MJ (2006). Effect of short-term sprint interval training on human skeletal muscle carbohydrate metabolism during exercise and time-trial performance. J Appl Physiol 100: 2041-7.

Burgomaster KA, Howarth KR, Phillips SM, Rakobowchuk M, Macdonald MJ, McGee SL, Gibala MJ (2008). Similar metabolic adaptations during exercise after low volume sprint interval and traditional endurance training in humans. J Physiol 586:151-60.

Burgomaster KA, Hughes SC, Heigenhauser GJ, Bradwell SN, Gibala MJ (2005). Six sessions of sprint interval training increases muscle oxida-tive potential and cycle endurance capacity in humans. J Appl Physiol 98:1985-90.

Coquart JB, Lemaire C, Dubart AE, Luttembacher DP, Douillard C, Garcin M (2008). Intermittent versus continuous exercise: effects of perceptually lower exercise in obese women. Med Sci Sports Exerc 40: 1546-53.

Daussin FN, Zoll J, Dufour SP, Ponsot E, Lonsdorfer-Wolf E, Doutreleau S, Mettauer B, Piquard F, Geny B, Richard R (2008). Effect of interval versus continuous training on cardiorespiratory and mitochondrial functions: relationship to aerobic performance improvements in sedentary subjects. Am J Physiol Regul Integr Comp Physiol 295: R264-72.

Department of Health, Hong Kong SAR (2008). Behavioural Risk Factor Survey, April 2007.

Dolmage TE, Goldstein RS (1997). Proportional assist ventilation and exercise tolerance in subjects with COPD. Chest 111:948-54.

Gibala MJ, McGee SL (2008). Metabolic adaptations to short-term high-intensity interval training: a little pain for a lot of gain? Exerc Sport Sci Rev 36:58-63.

Harms CA, Wetter TJ, St Croix C, Pegelow DF, Dempsey JA (2000). Effects of respiratory muscle work on exercise performance. J Appl Physiol 89:131-8.

Haskell WL, Lee IM, Pate RR, Powell KE, Blair SN, Franklin BA, Macera CA, Heath GW, Thompson PD, Bauman A (2007). Physical activity and public health: updated recommendation for adults from the American

College of Sports Medicine and the American Heart Association. Circulation 116:1081-93.

Hawkins P, Johnson LC, Nikoletou D, Hamnegard CH, Sherwood R, Polkey MI, Moxham J (2002). Proportional assist ventilation as an aid to exercise training in severe chronic obstructive pulmonary disease. Thorax 57:853-9.

Jacobsen DJ, Donnelly JE, Snyder-Heelan K, Livingston K (2003). Adherence and attrition with intermittent and continuous exercise in overweight women. Int J Sports Med 24:459-64.

Jakicic JM, Wing RR, Butler BA, Robertson RJ (1995). Prescribing exercise in multiple short bouts versus one continuous bout: effects on adherence, cardiorespiratory fitness, and weight loss in overweight women. Int J Obes 19:893-901.

Kleinsasser A, Von Goedecke A, Hoermann C, Maier S, Schaefer A, Keller C, Loeckinger A (2004). Proportional assist ventilation reduces the work of breathing during exercise at moderate altitude. High Alt Med Biol 5:420-8.

Macfarlane DJ, Taylor LH, Cuddihy TF (2006). Very short intermittent vs continuous bouts of activity in sedentary adults. Prev Med 43:332-6.

Marques-Magallanes JA, Koyal SN, Cooper CB, Kleerup EC, Tashkin DP (1997). Impact of habitual cocaine smoking on the physiologic response to maximum exercise. Chest 112:1008-16.

Roberts CK, Barnard RJ (2005). Effects of exercise and diet on chronic disease. J Appl Physiol 98:3-30.

Talanian JL, Galloway SD, Heigenhauser GJ, Bonen A, Spriet LL (2007). Two weeks of high-intensity aerobic interval training increases the capacity for fat oxidation during exercise in women. J Appl Physiol 102:1439-47.

Tong TK, Fu FH, Eston R, Chung PK, Quach B, Lu K (2010). Chronic and acute inspiratory muscle loading augment the effect of a 6-week interval program on tolerance of high-intensity intermittent bouts of running. J Strength CondRes 24:3041-8.

Tong TK, Lu K, Quach B (2006). Simulated obesity-related restrictive ven-tilatory load impairs moderate exercise sustainability in nonobese men. J Exerc Sci Fit 4:42-51.

Toothaker LE, Miller L (1996). Introductory Statistics for the Behavioral Sciences, 2nd ed. Brooks-Cole Publishing Co., Belmont, CA.

Turner AP, Cathcart AJ, Parker ME, Butterworth C, Wilson J, Ward SA (2006). Oxygen uptake and muscle desaturation kinetics during intermittent cycling. Med Sci Sports Exerc 38:492-503.

Wisloff U, Ellingsen 0, Kemi OJ (2009). High-intensity interval training to maximize cardiac benefits of exercise training? Exerc Sport Sci Rev 37:139-46.

Wysocki M, Meshaka P, Richard JC, Similowski T (2004). Proportionalassist ventilation compared with pressure-support ventilation during exercise in volunteers with external thoracic restriction. Crit Care Med 32:409-14.