The primary aim of our current analysis was to examine predictors of no change in maximal aerobic fitness (VO2max) across different doses of cardiorespiratory endurance training in a sample of sedentary, overweight/obese, moderately hypertensive, post-menopausal women. Despite a high retention rate and uniform high compliance within each treatment group, there was a large amount of individual variability in response to exercise. The overall predictors of VO2max non-response to cardiorespiratory training were baseline VO2max, age, and volume of training, with those groups exercising for longer durations (all participants exercised at the same intensity) having a lower prevalence of non-response to training. Variables that were not significant included the participant’s level of exertion (i.e., RER), ethnicity, BMI, body composition, and smoking status. Within the treatment groups, findings were similar. The most important finding of this study, especially pertaining to future exercise program development, is that as women increase the volume of exercise, the percent who do not improve VO2max significantly decreases. Additionally, on the individual level there was a decrease in prevalence of non-response with increasing training volume.
The large range of individual variability in response to training (−33.2% to 76.0% change) is similar to other trials (9
). Approximately 32% of the participants in the entire sample were non-responders to the exercise training but that varying between treatment groups. While few studies have examined the response to exercise in this manner, those studies that have do not show such a large proportion of non-responsive individuals (5
). This phenomenon may be due to the uniqueness of the study design and training protocol. All DREW participants exercised at a heart rate established intensity corresponding to 50% of baseline VO2max
. Participants in other trials exercised between 70–85% VO2max
). The higher intensity of these other trials may partially explain the discrepancy in the proportion of non-responders between our trial and previous studies. Nonetheless, while the exercise prescription in our current study was lower than in other studies, it was of a significant intensity to increase VO2max
in most participants. The increase in VO2max
was especially strong in the group that exercised 50% above the current recommendation (i.e., 192 minutes/week (8
In contrast to our findings, the HERITAGE Family Study (7
) did not find initial VO2max
to be a significant predictor of heterogeneity of response. A trend towards the significance of initial VO2max
was reported in another trial of older adults (11
) and 30 to 40 year old men (16
) however, the HERITAGE Family Study had the largest sample size of the trials examining non-response to training. Volume of exercise was a significant predictor in the DREW trial meaning that those exercising at a level of 8 kcal/kg per week were 55% more likely to increase their VO2max
than participants exercising at 4 kcal/kg per week. Furthermore, those in the 12 kcal/kg per week were 87% more likely to increase their VO2max
than the 4 kcal/kg per week group. These findings are in contrast to an 8 week aerobic exercise trial in men where no difference was found between moderate and high volume treatment groups (10
). Specifically, Hautala et al. (n=39), the men in the moderate group engaged in 180 minutes of exercise at 70–80% heart rate max per week. This level of exercise corresponds well with the highest dose group in our current cohort who exercised approximately 192 minutes per week or exercise at 50% VO2max
). Kohrt et al. (11
) also examined this issue by separating participants into quartiles based on percent improvement in VO2max
. When their exercise volume and intensity were examined no differences were found; however, these participants all received the same exercise treatment (11
). Additionally, existing studies show mixed findings for the influence of age on fitness non-response with one trial showing significance of age (10
), one showing a trend (11
), and another showing no influence (7
). Further, the age range of our current cohort was relatively narrow. Thus, studies examining a greater range of ages may offer better insights regarding the influence of age and exercise training response.
The primary limitation of this trial is the homogenous nature of the participants. Therefore, the generalizability of our findings to other populations of women or to men is not possible. However, this was an efficacy trial and the limited variability of the sample allowed for the examination of the effectiveness of the dose-response exercise intervention. Another possible limitation could be the sensitivity of the VO2max
testing to detect a change between groups and over time, however, there were no significant differences between test 1 and test 2 at baseline or post-test and the differences between test 1 and 2 were significantly smaller than the difference between baseline and post test (data not shown). Furthermore, the intraclass correlation was high at both time points (8
). It is also important to note that due to the ramping protocol, participants in the 8 and 12 kcal/kg per week groups spent less time, five and four months rather than six, at their maximum training volume. It is not expected that this difference would yield any meaningful differences in these findings. A strength of the DREW study is that it includes a large sample of sedentary, overweight, post-menopausal women. Additionally, the adherence to the tightly controlled and supervised exercise training was extremely high and the attrition rate in all groups was low, maintaining adequate sample sizes within each group for these analyses. Another strength of this study was the dose-response exercise recommendation that allowed for the examination of non-response to exercise training in three different treatment groups.
In conclusion, initial levels of VO2max, volume of training, and age were significant predictors of VO2max non-response following a training program in this sample of sedentary, overweight, post-menopausal women. Those women that were younger, less fit initially or exercised more during the trial had greater odds of improving their VO2max with training. The most important finding of this study, especially pertaining to future exercise program development, is that as women increase the volume of exercise, the more likely they were to improve their VO2max. Practically speaking, older, postmenopausal women continuing or beginning an exercise regimen interested in increasing aerobic fitness should consider increasing the total volume of exercise to increase the likelihood of reaching their goals. In this relatively homogeneous sample of women, race does not appear to have an influence on which participants improve with training and which do not. Future research should explore training programs of longer durations, diverse populations and different intensities in order to better examine why some profiles of women appear to not respond to cardiorespiratory endurance training.