Sounds like a trick question, right? In general, any mode of exercise will reap health benefits (Ehrman et al., 2013). In addition, if one is not currently doing any type of exercise, incorporating aerobic, resistance, or flexibility exercises will result in some improvement in overall health. However, when it comes to health promotion and disease prevention, I believe aerobic exercise is the most important. In my opinion, resistance training and flexibility exercises help make up a well-rounded/balanced routine when combined with aerobic exercise. Although, I would say the substantial list of benefits from aerobic exercise alone makes this my choice for the most important mode of exercise for health promotion and disease prevention.

Aerobic exercise is categorized as any activity that uses large muscle groups and can be sustained over a period of time (relative to level of fitness) and includes activities such as walking, swimming, or dancing (Taylor, 2014). The current recommendation from the American Heart Association (2021) is to get at least 150 minutes of moderate-intensity aerobic exercise or 75 minutes of vigorous aerobic exercise each week. According to the Mayo Clinic (2020), those who participate in regular aerobic exercise live longer and have a lower risk of a variety of medical conditions and diseases than those who do not participate in regular aerobic exercise. Furthermore, evidence has shown that a more physically fit individual may be better suited to survive a myocardial infarction if one were to occur (Blair et al., 1989). Therefore, while strengthening the heart and pulmonary functions is certainly possible to some extent through resistance training and flexibility exercise (and I think it is important to incorporate some of each type of exercise into a program!), I believe the best and most noticeable changes for health promotion and disease prevention happen when aerobic activity is included to a large extent in an exercise regimen.   

*Always make sure to consult your physician when adding or changing anything to your current exercise routines!

References

American Heart Association. (2021). American Heart Association Recommendations for Physical Activity in Adults and Kids. Retrieved from: https://www.heart.org/en/healthy-living/fitness/fitness-basics/aha-recs-for-physical-activity-in-adults 

Blair, S. N., Kohl, H. W., 3rd, Paffenbarger, R. S., Jr, Clark, D. G., Cooper, K. H., & Gibbons, L. W. (1989). Physical fitness and all-cause mortality. A prospective study of healthy men and women. JAMA, 262(17), 2395–2401. https://doi.org/10.1001/jama.262.17.2395 

Ehrman, Gordon, Visich, & Keteyian. (2013). Clinical Exercise Physiology. Human Kinetics. 

Mayo Clinic. (2020). Aerobic exercise: Top 10 reasons to get physical. Retrieved from: https://www.mayoclinic.org/healthy-lifestyle/fitness/in-depth/aerobic-exercise/art-20045541#:~:text=Studies%20show%20that%20people%20who,heart%20disease%20and%20certain%20cancers 

Taylor, D. (2014). Physical activity is medicine for older adults. Postgraduate Medical Journal;90:26-32.