Date Published: April 18, 2016
Publisher: Public Library of Science
Author(s): Lora A. Cavuoto, Rammohan V. Maikala, Yoshihiro Fukumoto.
The objective of this study was to compare gas exchange, cardiac and cerebral hemodynamic responses between 10 non-obese and 10 obese men during submaximal work. With the increasing prevalence of obesity, there is a need to understand the impact of obesity on work-induced responses. Participants completed a step-wise incremental cycling until they reached 60% of their age-predicted maximum heart rate. Gas exchange, cardiac and pre-frontal cortex hemodynamic responses were simultaneously measured during rest, work, and recovery. The non-obese group reached ~43% of their predicted maximal aerobic capacity as compared to ~34% in the obese group, with the non-obese working at a relatively higher workload and for more duration than the obese. The obese had elevated baseline heart rate and reduced whole-body oxygen uptake per body weight at baseline and task termination. Other cardiac and cerebral responses, although increased from baseline, were similar between groups during submaximal effort. In the obese, during recovery oxygen uptake and heart-rate recovery were slowest; cardiac output and rate pressure product were greatest, and left ventricle ejection time was shortest. However, both groups exhibited similar cerebral hemodynamics during recovery. These finding imply that, irrespective of their low aerobic fitness, obesity does not impair myocardial performance and cerebrovascular function during graded submaximal work, however, recovery from a short duration of work was influenced by their fitness level. Since a majority of activities of daily living are performed at individual’s submaximal level, understanding influence of obesity on submaximal work is critical.
With an increasing trend toward sedentary living and the associated prevalence of obesity (defined by standard body mass index (BMI) > 30 kg/m2) , examining the role of these modifiable personal factors on cardiovascular fitness and responses to submaximal work is critical. However, there has been mixed evidence on the role of obesity on cardiorespiratory fitness and exercise capacity. Typical exercise modes and occupational tasks are most often performed at submaximal levels, therefore it is essential to examine BMI-related differences during submaximal exercise testing. Lafortuna et al.  observed a 23% increase in energy consumption for obese compared to non-obese women during submaximal cycling. While levels of deconditioning are still unclear in general, obesity-related compromises in ventilatory mechanics may require adaptations to training modalities for effective and safe exercise or work . Based on treadmill exercise testing, Goran et al.  demonstrated a reduced submaximal aerobic capacity in both overweight and obese participants as evidenced by a higher submaximal heart rate, respiratory exchange ratio, and percent maximal oxygen uptake, and shorter time to exhaustion, suggesting that the limiting factor in aerobic-type activities for the obese is not the cardiorespiratory system per se, but rather a limitation in their submaximal aerobic capacity. Nevertheless, the role of obesity as a function of graded submaximal work and exercise tolerance has not been extensively explored.
At task termination, the groups reached an average of 60.9 (3.3)% age-predicted maximum heart rate. However, the duration of the task in mean (SD) differed by group, with the non-obese terminating after 432 (67) seconds and the obese stopping after 330 (101) seconds (p = 0.016). All participants completed at least four minutes of cycling before reaching the stopping criteria. When gas exchange responses were normalized to task duration, the obese group exhibited higher VO2 compared to the non-obese group until 60% of task duration, at which point it dropped lower (see Fig 2B). The group level responses were significant at 20% and 100% of the task duration. VO2 per kg of body mass was equivalent between groups from 0% to 40% of task duration and then the non-obese group had more than 50% higher uptake (see Fig 2C).
This study examined the physiological responses between men of healthy weight and obese during a submaximal graded cycling task until participants attained their 60% of age-predicted maximum heart rate. The non-obese group cycled at a higher workload and for a longer duration than the obese group. The obese group had elevated baseline HR and reduced VO2 per body mass at baseline and task termination. Recovery of VO2 and HRR was slower in the obese group, while rate pressure product during recovery was higher in the obese group. However, both groups exhibited similar cerebral hemodynamics during recovery. These findings imply that irrespective of their low physiologic fitness, obesity does not impair myocardial performance and cerebrovascular function during sub-maximal exercise.
While maximal aerobic capacity is often considered the key indicator of cardiovascular fitness, submaximal exercise tests are safer to perform and remain a valuable indicator of health risk and for evaluation of task demands. The obese group had reduced fitness as evidenced by the elevated baseline HR and reduced VO2 per body mass at baseline and task termination. Based on the RERs observed at task termination, we speculate a greater shift in carbohydrates as the prominent metabolic substrate over fat in the non-obese group. Cardiac hemodynamics and cerebral responses, although increased from baseline during submaximal effort, were similar between groups, suggesting that during submaximal work, irrespective of their low fitness, obesity does not impair myocardial performance and cerebrovascular function. Reduced fitness of the obese during task performance was further corroborated by the elevated HR and VO2 responses after the two-minute recovery. Both groups exhibited similar cerebral hemodynamics during recovery, implying restoration of cerebrovascular function. Understanding these group-level differences in fitness can allow for improved evaluation of abnormal responses to submaximal work, which is critical as individuals who are obese tend to be at higher risk of cardiovascular disease.