Review
Individual responses to aerobic exercise: The role of the autonomic nervous system

https://doi.org/10.1016/j.neubiorev.2008.04.009Get rights and content

Abstract

It is well established that regular aerobic exercise training reduces all-cause mortality and improves a number of health outcomes. However, a marked heterogeneity in the training-induced changes, e.g. in terms of aerobic fitness, has been observed in healthy human subjects, even with highly standardized training programs. Mean improvements in aerobic fitness, expressed as maximal oxygen consumption, have been about 10–15% of the baseline values, but the training-induced changes have ranged from almost none to a 40% increase. The exact nature of the mechanisms responsible for this heterogeneity in response to regular aerobic exercise is not well known. In this review, we consider evidence of the association between the autonomic nervous system (ANS), aerobic fitness and aerobic training-induced changes in fitness. Results of recent studies support the hypothesis that assessment of ANS functioning includes important information concerning acute and chronic physiological processes before, during and after aerobic exercise training stimulus. Moreover, we show that daily assessment of ANS activity could serve as an indicator of appropriate physiological condition for aerobic training.

Introduction

Marked changes in heart rate (HR) occur during aerobic exercise and after training interventions. These changes in HR are primarily due to alterations in autonomic nervous system (ANS) activity. HR accelerates during acute exercise due to the reduced cardiac vagal modulation of HR and increased sympathetic activity. After chronic aerobic training, autonomic balance is altered toward parasympathetic predominance due to increased vagal modulation of HR and probably also due to decreased sympathetic activity. These changes in autonomic activity can be studied non-invasively using a HR variability technique (Akselrod et al., 1981, Akselrod et al., 1985) or by invasively measuring sympathetic activity from the peroneal nerve with a microneurography technique (Delius et al., 1972, Hagbarth and Vallbo, 1968). In this review we focus on the association between aerobic exercise training and autonomic regulation measured with the HR variability and microneurography techniques.

Regular physical activity and good aerobic fitness are widely accepted as factors that improve a number of health outcomes and reduce all-cause mortality (Blair et al., 1989, Ekelund et al., 1988, Haskell et al., 2007, Kesaniemi et al., 2001, Myers et al., 2002, Thompson et al., 2003). Previous studies have shown that aerobic fitness is related to cardiovascular autonomic regulation, providing evidence that aerobic training improves ANS functioning (Davy et al., 1996, De Meersman, 1993, Goldsmith et al., 1992, Tulppo et al., 1998). Aerobic training has been suggested to protect the heart against harmful cardiac events by increasing cardiac vagal modulation of HR and also by decreasing sympathetic outflow (Billman, 2002).

Interestingly, considerable heterogeneity in the responsiveness to aerobic training, assessed by the change in maximal oxygen consumption (VO2max) or peak oxygen consumption (VO2peak), has been observed even in highly standardized training programs (Bouchard and Rankinen, 2001, Hautala et al., 2003a, Hautala et al., 2006a). Mean improvements in VO2max have been about 25%, with a range from 0% to 40% compared with the baseline (Bouchard and Rankinen, 2001).

The physiological background for the wide range of responses to aerobic training is discussed in the present review. We focus on the contribution of ANS status beyond the individual aerobic training-induced changes in VO2max or VO2peak. Furthermore, we examine the measurement of ANS activity as a practical tool for evaluating the physiological condition for determining an appropriate aerobic training stimulus on a daily basis. Finally, we suggest future directions for research on the effects of aerobic exercise on the ANS.

Section snippets

Methods for assessing autonomic nervous system activity

Assessment of the ANS has played an important role in elucidating the centrally mediated neural mechanisms in diverse clinical and physiological settings. The most widely used methods involve measurement of an end-organ response to a physiological or pharmacological provocation. In this context, we will briefly introduce two methods; HR variability (HRV) and muscle sympathetic nerve activity (MSNA), which are both objective and reliable tools for assessing ANS functioning.

HRV refers to the

Association between the autonomic nervous system and aerobic exercise

The effect of age (Craft and Schwartz, 1995, Lakatta, 1993, Lipsitz et al., 1990), gender (Huikuri et al., 1996, Kuo et al., 1999, Ryan et al., 1994) and circadian profile (Huikuri et al., 1994, Peckova et al., 1998) on the ANS has been observed in many studies. Thus, it is clear that responses to aerobic exercise involve integration of these determinants to acute and chronic adaptations of ANS activity. Furthermore, several randomized trials have shown that aerobic training causes reductions

Determinants of individual differences in response to regular aerobic exercise

Marked individual differences in the response to regular aerobic training, in terms of individual changes in VO2max have been observed in healthy subjects after highly standardized exercise programs (Bouchard and Rankinen, 2001, Kohrt et al., 1991, Lortie et al., 1984). Mean improvements in VO2max have been about 10–15% of the baseline values, but the training-induced changes have ranged from almost none to a 40% increase. In line with the previous studies, we have also found considerable

Individually tailored aerobic training prescription

Based on the fact that a standardized exercise training program results in heterogeneous responses of aerobic fitness, and that the known factors behind individual training responses, such as previously mentioned age, gender, race, and genetic factors, cannot be affected by any treatment, an individually tailored exercise training program may remain as the most practical tool for optimizing exercise training responses at the individual level. From this point of view, the main challenge is to

Future directions and general conclusions

The incidence and prevalence of cardiovascular diseases and diabetes have reached epidemic proportions in the western countries. A sedentary lifestyle is the most important risk factor for these diseases. However, the physiological and psychological variables that may lead to a physically passive lifestyle are not known. Since voluntary physical activity and exercise training can favourably influence brain plasticity by facilitating neurogenerative, neuroadaptive and neuroprotective processes (

Acknowledgments

The work reported in this review has been supported in part by the Academy of Finland (Helsinki, Finland), the Ministry of Education (Helsinki, Finland), the Finnish Funding Agency for Technology and Innovation, TEKES (Helsinki, Finland), Polar Electro Oy (Kempele, Finland) and HUR Oy (Kokkola, Finland).

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