Plasticity of the heart in response to changes in physical activity

Abstract

The human heart is very adaptable, with chamber size, wall thickness and ventricular stiffness all modified by periods of inactivity or exercise training. Herein, we summarize the cardiac adaptations induced by changes in physical activity, ranging from bed rest and spaceflight to endurance exercise training, while also highlighting how the ageing process (a long-term model of inactivity) affects cardiac plasticity. Severe inactivity during bed rest or spaceflight leads to cardiac atrophy and ventriculo-vascular stiffening. Conversely, endurance training induces eccentric hypertrophy and enhances ventricular compliance, and can be used as an effective countermeasure to prevent adverse cardiac changes during prolonged periods of bed rest or spaceflight. With sedentary ageing, the heart undergoes concentric remodelling and irreversibly stiffens at advanced age. Specifically, older adults who initiate endurance training later in life are unable to improve ventricular compliance and diastolic function, suggesting reduced cardiac plasticity with advanced age; however, lifelong exercise training prevents age-associated cardiac remodelling and maintains cardiac compliance of older adults at a level similar to those of younger healthy individuals. Nevertheless, there are still many knowledge gaps related to cardiac remodelling and changes in cardiac function induced by bed rest, exercise training and spaceflight, as well as how these different stimuli may interact with advancing age. Future studies should focus on understanding what factors (sex, age, heritability, etc.) may influence the heart's responsiveness to training or deconditioning, as well as understanding the long-term cardiac consequences of spaceflight beyond low-Earth orbit with the added stimulus of galactic cosmic radiation.

Keywords: ageing; astronaut; bed rest; endurance exercise training; inactivity.

Figure 1. Comparison of left ventricular (LV) mass index relative to a healthy, untrained adult (white bar)

Cross‐sectional comparisons between adults with spinal cord injury (de Groot et al., 2006) and elite runners (Milliken et al., 1988) reveal an adaptive range for left ventricular mass index of ∼75%. Longitudinal changes in mass index are also presented following 2, 6 and 12 weeks of bed rest (peach bars) (Perhonen, Franco, et al., 2001), and at 3 month intervals during 1 year of endurance exercise training in previously sedentary adults (blue bars) (Arbab‐Zadeh et al., 2014). Comparable reductions in left ventricular mass index to men were observed in women following 60 days of bed rest (peach bar with ♀) (Dorfman et al., 2007).

Figure 2. Summary of cardiac adaptations to bed rest, endurance exercise training, spaceflight and ageing

Figure 3. Comparison of Starling (left) and pressure–volume curves (right) before and after 15 days of head‐down bed rest or acute hypovolaemia

Reductions in stroke volume for a given filling pressure were exacerbated with bed rest compared to hypovolaemia, and a leftward shift in the pressure–volume relationship with a reduction in the equilibrium volume was only observed following bed rest. LVEDV, left ventricular end‐diastolic volume; PCW, pulmonary capillary wedge pressure; S, stiffness constant that describes the shape of the curve; V₀, equilibrium volume (i.e., the left ventricular volume when filling pressure is 0 mmHg). Reproduced with permission from Perhonen, Zuckerman, et al. (2001).

Figure 4. Conceptual illustration depicting how the left ventricular pressure–volume relationship changes from being on Earth (blue) to in space (red)

Removal of external constraint from around the heart in microgravity increases left ventricular filling despite the filling pressure being reduced. PCWP, pulmonary capillary wedge pressure.

Figure 5. Comparison of pressure–volume curves from young adults (black triangle), older sedentary adults (black circle) and Master's athletes (white circle)

Older sedentary adults have a leftward shift and a steeper curve than young adults and older adults who performed lifelong endurance exercise training, reflecting their increased left ventricular stiffness and reduced diastolic function. Importantly, Master's athletes’ pressure–volume curves were superimposable with those of younger adults, demonstrating the benefits of exercise for maintaining ventricular compliance throughout the lifespan. Reproduced with permission from Arbab‐Zadeh et al. (2004).