Active Voice: Is It Possible to Simultaneously Improve Endurance and Strength?
By Laura Karavirta, Ph.D.

Viewpoints presented in SMB commentaries reflect opinions of the authors and do not necessarily reflect positions or policies of ACSM.

Laura Karavirta, Ph.D., received her doctoral degree from the Department of Biology of Physical Activity at the University of Jyväskylä in Finland early this year. For her thesis, she investigated cardiorespiratory, neuromuscular and cardiac autonomic adaptations to endurance and strength training in aging adults, with special reference to individual training adaptations. This commentary presents her views associated with the research article she and her colleagues published in the March 2011 Medicine & Science in Sports & Exercise® (MSSE).

The most recent ACSM physical activity guidelines released in 2007 recommend three to five endurance training sessions and two strength training sessions each week. The rationale behind this recommendation is that both cardiorespiratory fitness and muscular strength are independently related to health and longevity. Given the training-mode-specific nature of adaptations and the rapid effects of detraining, the aim should be to simultaneously train for both endurance and strength. However, the complex interplay of several control mechanisms at the muscular and cellular level, which enable the human body to adapt to training, seem to be on a collision course when endurance and strength training are performed in tandem.

The incompatibility of simultaneous endurance and strength training has been extensively studied. Training design variables – such as training volume, frequency and intensity – have been suggested to partly explain why incompatibility is sometimes, but not always, observed. In addition, the relative timing of endurance and strength training seems to favor training on alternate days, allowing a longer recovery period between sessions. To amplify the complexity of training prescription, the adaptations to combined endurance and strength training seem to be individually determined. Our recent findings, reported in MSSE, showed that approximately half of the subjects performing combined endurance and strength training were able to simultaneously improve both cardiorespiratory fitness and muscular strength during a 21-week period. In other words, a large number of subjects were able to notably improve either cardiorespiratory fitness or muscular strength, but not both, despite the carefully standardized and fully supervised training program.

Large individual differences in training adaptations are not a new finding, but they have caught only minor research attention considering the large number of studies investigating the effects of physical training. Even when training programs are highly controlled and supervised, training responses may vary from a negative change to a doubling of initial values. Age, gender and initial fitness level seem to be only minor determinants of training response. In fact, data from the HERITAGE Family Study with 720 subjects showed that the maximal oxygen uptake at baseline accounted for only one percent of the variation in the training response. Thus, the information about the effects of a certain type of training prescription, on average, gives us only a rough estimate for an individual’s response.

That there are individual adaptations to endurance and strength training does not mean their combination is not beneficial for everyone. Rather, it indicates that training prescription should be carefully designed and target-oriented. Further, the lack of large training-induced improvements in physical fitness does not rule out other beneficial effects of exercise, such as improved body composition, functional capacity or quality of life. Individual variations in training responses highlight the importance of exercise testing and physical examination in identifying individual needs and monitoring progress. For example, strength training may be more important for older adults with considerable loss of skeletal muscle mass while endurance training may be more beneficial for individuals with increased risk of cardiovascular disease. Thus, health-and-fitness professionals should be aware of the need for individualization and periodization in training prescription, since adaptations seem to be complex, including an individual optimum for the combination of design variables as well as proportions of endurance and strength training.