Dr.Benjamin Levine (1) and colleagues at the Institute for Exercise and the Environment performed extensive physiologic studies on 12 such subjects ( aged 29 +/- 6 years) and provided valuable insight into the functional and structural change in their hearts over one year.
The training program was intensive and progressive and was divided into four 3 months periods or segments. The third quarter included 2 hour long runs and 4 th quarter involved 7 -9 hours per week with 3 hour long runs and interval training.
The cardiovascular system of the trained endurance athlete differs in a number of ways from the untrained person.These include:
1.increased red blood cell mass and blood volume
2.increased numbers of mitochondria and capillaries in leg muscles.
3.lower peripheral arterial resistance
4.lower systolic and diastolic blood pressure during exercise.
What distinguishes the elite endurance athlete's heart from other equally well trained athletes is the very large stroke volume which in turn depend on a very large end diastolic left ventricular volume (LVEDV). A very compliant left ventricle is the key.
The maximal 0xygen uptake increased from 40.3 =/-1.6 to 48.7 =/-2.5. (The 02 max for elite marathoners is typically 70 to 80 plus). Maximal stroke volume increased from 98 to 113 ml.
A key finding was that both right and left ventricular mass increased to levels similar to those seen in elite athletes but the LV volume did not change until six months of training. In the first 6 months of training when training did not include significant high intensity training the left ventricle remodeling was concentric and eccentric remodeling ( i.e. increased LV volume) did not occur more intense exercise was part of the regimen.The right ventricle began "eccentric" hypertrophy early on. Question -is the eccentric pattern dependent on the addition of some HIT or interval training in addition to the moderate intensity exercise.
Cardiac catherization data derived measures of LV compliance improved but did not approach those typically observed in elite athletes. The "Starling Curves" which plot pulmonary capillary wedge pressure (PCWP) which is an index of left ventricular filling pressure on the x axis versus stroke volume on the y axis shifted up and to the left suggesting an improvement in left ventricular compliance, i.e a ventricle more easily filled.
Their morphology measurement which were done by cardiac MR ( generally thought to be more accurate than echocardiographic measurements) did not conform with the Morganroth hypothesis (1975) which stated that endurance exercise lead to eccentric hypertrophy which is a balanced increase in wall thickenss and ventricular volume while strength training leads to concentric hypertrophy with an increase in wall thickness with no significant change in cavity size.
Levine's subjects first had a LV concentric pattern and only after more intense ( volume and intensity) exercise was part of the program did the classic endurance athletes eccentric pattern become evident. A certain level of intensity of exercise seems to be necessary for aerobic exercise to cause eccentric hypertrophy. This seems to run contrary to the notion that endurance exercise is simply a "volume overload event".
Levine's group has also reported on a similar project (2) involving older ( age 68-74) subjects and although their training program was vigorous it was less intense than the young subjects.The 02 max increased on average by 19%,arterial elastance decreased, LV mass increased with no change in the mass volume ratio ( i.e physiologic remodeling) but the Staring curves did not indicate a more compliant left ventricle.So good things happened but improved LV compliance was not one of them.
The third publication (3) in Levine's hat trick involves similar measurements of heart function and structure in middle age subjects over a two year period.The details are complex and interested readers can find details in ref 3 which has entire text without firewall.
The two year training program involved at least 30 minute session of moderate exercise 4-5 times per week with at least one high intensity exercise session ( the Norwegian 4x4).
The authors were able to show an improvement in compliance using the techniques ( The Starling curves) mentioned above .The data offer the hope that "middle age" is not too late to start .
Levine suggests that sedentary aging effect of the heart has 3 stages; 1) loss of relaxation 2)loss of compliance or stiffening of the myocardium and 3) remodeling.This sedentary aging may predispose to heart failure with preserved ejection fraction (HFpEF) perhaps when confronted by another "hit" such as hypertension,obesity,and diabetes. Levine's data suggests that some doable amount of endurance exercise might retard or mitigate the process . (Whether high intensity exercise is a necessary component is still an open question)
1) Arbab-Zadeh, A "cardiac remodeling in response to 1 year of intensive endurance training.
Circulation 2014, 130 (24) 2152
2)Fujimoto,N Cardiovascular effects of 1 year of progressive and vigorous exercise training in previously sedentary individuals older than 65 years of age. Circ. 2010, 122 (18), 1797
3) Howden EJ et al Reversing the cardiac effects of sedentary aging.A randomized trial.Circulation,2018 137; (full text available on line without firewall)