In summary, walkers exercised less frequently than stretchers and as pregnancy advanced, walkers exercised significantly less frequently than stretchers (an interaction effect). Furthermore, stretchers experienced more favorable blood pressure changes over the course of pregnancy than walkers did; they were more likely to stay within recommended weight gain; and they experienced less change in resting HR in late pregnancy. These findings suggest that the type of exercise and advances of pregnancy influenced adherence to exercise and, thus, resulted in different levels of effects on risks of preeclampsia. The differences observed in physiologic measures of risk of preeclampsia may have been due to the low adherence in the walking exercise group, physiologic exercise effects specific to the stretching exercises, or a combination of both.
The study results highlight the importance of the relationship between adherence and physiologic dose effects of exercise interventions, and suggest that the type of exercise may mediate this relationship. The stretching exercise was performed more frequently than the walking exercise until the end of pregnancy. Because of higher adherence, the participants in the stretching group may have benefited more than the walking group did. These results do not deny the effects of walking exercise on risk of preeclampsia. Rather, the findings suggest that previously sedentary pregnant women may not adhere to walking exercise, and thus may fail to benefit from it.
Different types of exercise require different arrangements to execute. McAuley et al., defined exercise adherence as “execution of life-style changes with an exercise” (1994, p. 498). That is, life-style needs to be changed to perform exercises, but different exercises require different changes in life-style. Participants may have experienced less difficulty in executing life-style changes with stretching than with walking exercise. The difficulty may stem from the requirements each exercise type imposes. For example, walking may require walking shoes, taking a shower and management of hair, selection of favorite music or programs during exercise, arrangement of childcare for older children, and other accommodations.
Individual’s perception about required modifications for execution of exercises may influence adherence. Hallam and Petosa (2004)
linked exercise adherence among adults to social cognitive variables, including self-efficacy (one’s perceived confidence to overcome barriers to exercise), outcome-expectancy values (values set on the expectation that exercising will lead to less weight gain, less fatigue, or better pregnancy outcomes), and self-regulation (the skills used to implement exercise intentions and to overcome personal and situational barriers to regular exercise). Self-regulation may be less demanding with stretching than walking. In the current study, because no group differences were observed in terms of beliefs about exercise benefit or any of the other potential confounders at baseline, the difference in adherence may be attributed to the difference in the type of exercise.
Better adherence to exercise interventions may have another benefit unrelated to the exercise itself. In the current study, two participants in the stretching group gained weight within the amount recommended by the IOM (1990)
; all participants in the walking group gained more than the recommended amount. Because stretching does not expend energy as walking exercise does, it was not the exercise itself that helped participants manage weight gain. Further research is needed to determine whether adherence to exercise helps pregnant women manage other lifestyle behaviors such as eating.
An important component in the discussion of adherence is the intensity and duration of exercise. Although a limited number of studies of exercise adherence in sedentary pregnant women were found, findings for non-pregnant sedentary women indicate that they may adhere to exercise better if it is of moderate, rather than high intensity (Cox et al., 2003
). The intensity of the stretching exercise was lower than that of the walking exercise, because stretching had neither aerobic nor muscle strengthening components. On the other hand, many walkers in the current study adjusted the intensity of their exercise by walking at HRs lower than their target HRs and also for shorter durations. Even with these self-adjustments, walkers still exercised less frequently than stretchers. Thus, proposing walking exercise at lower intensity or a shorter duration (e.g., two 15-minutes sessions a day) may not result in sufficient adherence to bring protective physiologic effects for sedentary pregnant women.
The duration of prescribed exercise also influences adherence. For example, Schachter, Busch, Peloso, and Sheppard (2003)
, who compared adherence to two short daily versus one long daily exercise in sedentary women with fibromyalgia, found that participants adhered better to the one long exercise. Although the long exercise required more physical effort to complete, it was considered better than repeating an exercise twice a day. The current study made the duration of both exercises equal; however, different types of exercise may require different durations to achieve high adherence.
Protective physiological effects from daily stretching exercise also were observed in the current study. These are important findings, because pregnancy-specific diseases such as preeclampsia (Ros et al., 1998
) bring risks for cardiovascular disease and diabetes for women in later life (Wolf et al., 2004
). Studies on the effects of stretching exercises are limited. However, researchers who have examined the effects of Yoga and Tai Chi, which both contain stretching (Lu & Kuo, 2003
; Motivala, Sollers, Thayer, & Irwin, 2006
; Narendran, Nagarathna, Gunasheela, & Nagendra, 2005
), reported some improvement in HR variability and attributed this to autonomic nervous system regulation. The current findings provide indications that the autonomic nervous system may be involved in the effects of stretching exercises when regularly performed. Another possible mechanism may be oxidative stress (Yeo & Davidge, 2001
). Oxidative stress has been associated with the development of preeclampsia (Rogers et al., 2006
), and findings from a few small studies have suggested a possible decrease in oxidative stress with yoga (Bhattacharya, Paney, & Verma, 2002
). Further examination of the physiologic autonomic and oxidative stress models of the effects of stretching on the risk of preeclampsia, followed by testing in a large clinical trial, is needed.
A limitation of this study was that the majority of the participants were White and they were relatively affluent and well educated; the results therefore can not be generalized to other groups of pregnant women, especially those at risk for pregnancy-related complications by virtue of race/ethnicity and social class. Also, inability to separate out walking exercise from total daily physical activity was a design limitation. Finally, data on duration and intensity of exercise were not collected for stretchers.
Despite these limitations, the results of the study suggest the value of stretching exercises for sedentary women during pregnancy. Regular stretching in the latter half of pregnancy may be more effective than walking for previously sedentary women either because of higher adherence with this form of exercise or because of its physiologic effects.