I'd like to point out one thing here to those of you who are concerned about heart health, but first, I commend Jay for trying to tease out the scientific literature. It is not an easy task to complete a jigsaw puzzle with so many pieces missing.
The only comments we can make about physiology come from interpretation of scientific research, which itself is limited by the technology used. In his book, about the literature, he states:
“The lack of matched control groups in some of the studies looking at improvements in cardio-vascular function following x-number of weeks of training is highly problematic, as we actually do not know if the improvement observed is significantly different because of the effect of the training or the variation across subjects and time.
This is not to be mistaken for a review of the current literature, as I have not set any criteria in terms of research methodology to be included in the listing. The reason for this is that for some of the categories, the research is fairly limited. The listed studies are more meant as examples of what has been done in the research of cardio-vascular exercise and specific activities. Some of the studies are on the lower end of the quality scale, but most are of a high or very high quality, which makes attacking the lower quality studies somewhat easy. Nevertheless, the studies (high and low quality both) confirm basic physiology concepts. I have, however, tried to find studies that include as many participants as possible. One major problem with research in exercise and training (strength training, cardio-vascular training) is the numbers of participants are usually on the low side, making variation with the group significant.”
Excerpt From: Kenneth Jay. “The Cardio Code.” iBooks.
The Cardio Code by Kenneth Jay on iBooks
I applaud him for being transparent... most authors take a more vested interest in their interpretations and conclusions. This shows that even if you have a sound study, it only means that this particular piece of the puzzle is "less blurry". It doesn't change the fact the puzzle is still missing more pieces than you actually have in front of you... many of which might be too blurry to place.
W/r to the cardiac physiology, short of butchering live athletes and controls, the only way to gain insight on heart adaptations is through echocardiograph. In reference number 12, they did just this... and what they conclude agrees with Jay's commentary in the book.
But when you reference this study... and luckily is was available in full text for free... you'll find two simple charts that nicely report the findings in a very easy manner to read. The two charts report that LV thickness (the thing everyone seems afraid of) is highest in cyclists and swimmers, then strength/power athletes, then basketball players, then paddlers and runners. Note that only the very high results are "statistically different" from the controls (the couch potatoes). You need to read up on statistical significance as used in statistics... it will make you a non-believer. Anyway, the mean control LV thickness is 0.364 as compared to 0.418 in strength/power athletes, and 0.383 in runners. I could not find the units used, but unless this is yards or meters, I'm not sure this so much of difference to be concerned about. Moreover, this was estimated using an average difference in dimensions, as the heart is beating while they are watching it. And further, since the authors admit to body size affecting heart size, they performed additional mathematics to even out the formula.
The chart reporting LV diameter is even more interesting: strength/power athletes and basketball players were only slightly lower than the controls at 37.6, 37.2, and 37.9 respectively. Runners came in at 41.1 and paddlers at 39.4. Again, if the units were yards and meters, I'd admit a large difference.
So, the numbers support Jay's commentary. But if I were you, I'd just do some exercise, as we know it is better than doing none at all. Choosing something you enjoy will lead to compliance, which is probably more important than KBs, C2 rowers, running, or DLing.
You heart is going to fail you one day anyway... my goal is just to be able to wipe my own a#@ on that day.... YMMV ;]