Active Voice—Exercise Intolerance in Individuals with Intellectual Disability: Is It the Heart, Lungs or Muscles?
By Thessa Hilgenkamp, Ph.D., and Bo Fernhall, Ph.D., FASCM

Thessa Hilgenkamp, Ph.D. Bo Fernhall, Ph.D., FASCM
Individuals with intellectual disabilities are at a high risk for lifestyle-associated health problems. An intellectual disability (ID) is characterized by a combination of limitations in intellectual and adaptive functioning. Most individuals with ID require some form of assistance or support with daily activities. Additionally, they demonstrate higher prevalence rates for many health conditions. Some of these relate to an underlying genetic syndrome (i.e., Down syndrome), while others relate to an unhealthy lifestyle. Over the past decades, recognition of the importance of physical activity for this population has grown exponentially, and many initiatives have been launched to improve their physical activity.

Unfortunately, current initiatives have not produced substantial improvements in physical activity levels in this population. Both fatigue and motivation appear to be the main barriers. Individuals with ID frequently report they are too tired, activities seem too hard, or they find a given distance too far to walk. We suspect these perceptions likely are related to very low aerobic capacity levels.

Deconditioning, however, may not be the only explanation of lower tolerance to activities and exercise. Earlier studies have revealed specific physiological limitations in subgroups of this population, such as a lower maximal heart rate in individuals with Down syndrome. Understanding the physiology behind the extremely low aerobic capacity in this population is necessary, so that physical activity programs may be adapted or designed to allow these individuals to be more successful.

To increase our knowledge of how individuals with ID cope with exercise, we conducted an in-depth analysis of the cardiopulmonary profile of individuals with ID during maximal exercise testing. As described in our research paper, published in the September 2019 issue of Medicine & Science in Sports and Exercise, we conducted a cross-sectional study using data for young (mean age approximately 29) adults with ID (N=27) and controls (N=35) without ID, but having similar ages and physical characteristics. Our subjects performed maximal exercise tests on a treadmill. We looked at peak oxygen uptake (V̇O2pk), peak heart rate (HRpk) and several other cardiopulmonary exercise test measures, such as peak ventilation (Epk), oxygen uptake efficiency (OUES), O2 pulse and the V̇E/
V̇CO2 slope.

With one exception, individuals with ID exhibited reduced treadmill on all outcome measures. With deconditioning, we would indeed expect a much lower V̇O2pk and a lower OUES, but with a normal HRpk, normal (high) values for Epk and somewhat higher than expected values V̇E/
V̇CO2 slope. However, for the individuals with ID, these outcome measures were not in the normal range. They demonstrated a lower HRpk, likely resulting in reduced cardiac output and possibly related to cardiac autonomic dysfunction. Individuals with ID also showed a higher V̇E/ V̇CO2 slope which, in combination with the low OUES, could indicate skeletal muscle hypoperfusion. Since OUES is related to forced expiratory volume and because we concurrently saw a lower-than-expected Epk in subjects with ID, these responses suggest a ventilatory limitation in which a perfusion mismatch may be involved as well.

Recognition of these potential physiological limitations should help establish directions for future activity-related research for individuals with ID. Deconditioning likely is not the only explanation of their exercise intolerance and concomitant experiences with difficulty with participating in exercise. Our results show that individuals with ID likely are asked or expected to exercise at higher intensity than suggested by exercise prescription guidelines based on estimated maximal heart rate. This circumstance may lead to reduced motivation and exhaustion. It also may play a pivotal role in adherence to a physical activity program or attaining an active lifestyle. Therefore, it is critical for individuals with ID to adjust their physical activities to an appropriate intensity level, both in free-living and structured exercise settings.

About the authors:
Thessa Hilgenkamp, Ph.D., was trained in physical activity epidemiology with a specific focus on individuals with intellectual disabilities, earning her Ph.D. from Erasmus University Medical Center Rotterdam in the Netherlands. During her post-doctoral studies at the Integrative Physiology Laboratory of the University of Illinois in Chicago, she gained expertise in cardiovascular exercise physiology and autonomic function in special populations. She currently is a tenure-track assistant professor in the Physical Therapy Department of the University of Nevada, Las Vegas. Dr. Hilgenkamp is a member of ACSM.

Bo Fernhall, Ph.D., FASCM, earned his Ph.D. from Arizona State University in Tempe, Arizona. He served as an associate editor of
Medicine & Science in Sports & Exercise for 12 years and is currently an associate editor for Exercise & Sport Sciences Reviews. He is a professor of kinesiology and dean of the College of Applied Health Sciences at the University of Illinois at Chicago (UIC). He founded the Integrative Physiology Laboratory at UIC, and his research is focused on cardiovascular exercise physiology in people with disabilities including individuals with ID and Down syndrome.

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