March 13, 2019

C.F. Notarius.A, P.J. Millar A,B, C.J. Doherty B, A.V. Incognito B, N. Haruki A, E. O’Donnell A,C, J.S. Floras A

A University Health Network and Mount Sinai Hospital Division of Cardiology, University of Toronto, Toronto General Hospital, University Health Network, 200 Elizabeth St. Toronto ON, Canada
B Department of Human Health & Nutritional Sciences, University of Guelph, Guelph ON Canada
C School of Sport, Exercise and Health Sciences, Loughborough University, Loughborough UK

When activated by stressors such as exercise, sympathetic nerves supplying the heart and muscle blood vessels release a chemical called noradrenaline which causes increases in heart rate and blood vessel constriction. In humans, the firing rates of the noradrenaline-releasing (sympathetic) vasoconstrictor nerves in the calf muscle (muscle sympathetic nerve activity, MSNA) can be measured directly from microelectrodes inserted into the fibular nerve. This technique requires a stationary leg but it is possible to record MSNA from a resting leg while the opposite leg cycles because MSNA is assumed to be the same in both legs.

We know from the work of others that MSNA increases with age but decreases when young healthy individuals perform mild and moderate intensity leg exercise, presumably to allow increases in blood flow. The aim of this study was to compare how much MSNA decreases in healthy middle-aged and young subjects during short duration low intensity dynamic 1-leg cycling. We hypothesized that over the first 4 minutes of exercise MSNA will decrease less in healthy middle-aged individuals than in young subjects.

We studied 36 healthy, medication-free, volunteers: 18 young (mean age 23±1 years; range 18-28; 6 women) and 18 middle-aged (mean age 57±2 years; range 48-72; 6 women). Subjects were seated upright and measurements of MSNA, heart rate and blood pressure as well as ratings of perceived exertion (RPE) were made during 4 minutes (2 minutes at zero load and 2 minutes at 15-20% of the work rate at peak oxygen uptake) of 1-leg cycling.

Contrary to our hypothesis, the decrease in MSNA during both mild and moderate dynamic leg cycling was greater in the older group compared to the young at a similar relative workload, although RPE tended to be higher in the middle-aged group. In addition, the middle-aged subjects had double the increase in systolic blood pressure during exercise compared to the younger group. We attribute these findings to an age-related effect on autonomic reflexes engaged by exercise. Specifically, the greater increase in systolic blood pressure during exercise in the older group would have elicited more arterial baroreflex-mediated inhibition of sympathetic activity.


Practical Implication: This study highlights the different neural adjustments to mild-moderate intensity dynamic exercise in middle-aged adults and emphasizes that their responses cannot be generalized from studies of young healthy subjects.

Summary of Key Points:

  1. The present study is the first to directly examine differences in the contralateral muscle sympathetic response to non-steady state dynamic leg exercise in middle-aged and young subjects.
  2. Contrary to our initial hypothesis, the drop in MSNA during short duration mild and moderate intensity cycling was augmented in the middle-aged participants.
  3. The 100% increase in systolic blood pressure in the middle-aged group amplified the normal arterial baroreflex-mediated drop in sympathetic activity. This demonstrates that reflex responses during mild to moderate intensity exercise can be exaggerated in middle-aged healthy subjects compared to young healthy individuals.

Original Article:

C.F. Notarius, P.J. Millar, C.J. Doherty, A.V. Incognito, N. Haruki, E.O’Donnell & J.S.Flores. Microneurographic characterization of sympathetic responses during 1-leg exercise in young and middle-aged humans Appl Physiol Nutr Metab. Published on the web 31 July 2018., doi/abs/10.1139/apnm-2018-0101

This article is a summary of an article published in Applied Physiology, Nutrition and Metabolism. If you intend to cite any information in this article, please consult the original article and cite that source. This summary was written for the Canadian Society for Exercise Physiology and it has been reviewed by the CSEP Knowledge Translation Committee.