Does a low heart rate always mean an enlarged heart size?

No offense, Lui, but you sited two people who simply stated opinions, Wikipedia and a triathletes forum. Not exactly good sources. I am not doubting your personal story at all. I believe that your heart rate was 35. But that is no where close to the norm and I do not think that someone should expect that simply because they excercise alot.

But that is no where close to the norm and I do not think that someone should expect that simply because they excercise alot. What I don't get is why you mention this. My original question was if it is true that the size of an athlete's heart increases if he has a low resting heart rate. I never claimed that people should expect these really low heart rates from exercising. There are elite athletes with a resting heart rate around 50 and others around 30. It doesn't mean one athlete is better conditioned than the other. All I said is that it is not uncommon to have a resting heart rate of 35 after several people denied this. Just google "Elite Athlete Resting Heart rate" and you will find loads of sites that confirm this. Maybe you don't consider Wikipedia as a good source but it just mentioned that the cyclist Miguel Indurain has the lowest heart rate on record which is a fact. Lance Armstrong's heart rate is around 32-34. I used to also do a lot of cycling which might be a reason my heart rate was that low. I always found with mainly swimming my average resting heart rate was slightly higher than with cycling.

Not a doctor here, either so this is from personal experience. No, it does not mean enlarged heart but it can mean heart issues. My dad has a low heart rate. For him it was especially low when he had a major blockage and needed a bypass (which he got). He tracks his with a heart rate monitor. For me, I am the opposite. I have a high resting heart rate and I have a thyroid condition. My heart rate went up when the thyroid condition came. So thyroid can cause either low or high heart rates.

I am not a doctor either, but I believe I could pass in parts of Appalachia as a beloved "fake doctor" whose patients love and defend him even as the authorities are putting him in the paddy wagon en route to trial. With this as preamble, I have written about some of the changes that the heart undergoes in reaction to heavy exercise. The basic concepts are these: Strength training--because of the incredible high blood pressure generated by heavy dead lifts, world class weight lifters tend to develop thicker heart chamber walls, the better to withstand these pressures. People with untreated high blood pressure often get a condition called LVH, for left ventricular hypertrophy--a similar, albeit pathological, thickening of the wall of the left ventricle. It appears that when such thickening occurs in reaction to weight training, it's not harmful. When it results from untreated hypertension, however, it is harmful. Anyhow, wall chamber enlargement is one form of cardiac remodeling. It does not affect heart rate. Aerobic training--endurance sports requires large volumes of blood being pumped through your body to supply the demands of the skeletal muscles that are doing the work. With enough training, endurance athletes can undergo all sorts of adaptations. For instance, they actually increase their blood volume and may undergo the formation of collateral vessels to better supply different parts of the body (including the heart) that need oxygen and nutrients. In terms of the heart itself, the major change is that the interior volume of the pumping chambers can enlarge. This allows the heart to pump more blood per single beat. Pure aerobic athletes don't see the enlargement, or thickening, of the heart walls per se. But they can get more expandable pumps. Thus one reason your heart rate goes down at rest is that a single beat is circulating more blood than it used to, meeting your body's needs. Final note: All of the above notwithstanding, to get a significant remodeling of the heart--either thickened walls or enlarged pumping chambers--requires very hefty amounts of exercise, much more than even dedicated masters swimmers are likely to do. Think Lance Armstrong, who has 2 liters more blood than most of us mortals; or Bjorn Borg in his prime, whose resting heart rate was measured at 28 beats per minute.

Let me throw in my two cents: I would consider myself an expert in this area. I didn’t look anything up in a book or on the internet to write this post. I am just going off years of schooling and experience. Having an "enlarged heart" isn't necessarily a bad thing. For non-athletes, it is typically bad. There can be many reasons the heart gets larger. It can be due to bad stuff like high blood pressure (and many other things) or it can be due to good stuff like exercise. As far as the original question of the heart rate going down being due to an enlarged heart (that is one possible reason): If your heart pumps more blood with every beat (it is larger and/or stronger), it doesn't need to beat as many times per minute. So yes, an enlarged heart, which would not be bad in the case of an athlete, can cause you to have a lower heart rate. Also, as you train athletically, your muscles get better and pulling oxygen out of the blood. This could lead to a lower heart rate as well. As far as what heart rate is too low, I would say that it depends. Your heart just needs to pump blood around to your body. If that happens at 20 beats per minute, that is fine. Heart rate isn't important, blood getting to your organs is.

Final note: All of the above notwithstanding, to get a significant remodeling of the heart--either thickened walls or enlarged pumping chambers--requires very hefty amounts of exercise, much more than even dedicated masters swimmers are likely to do. Think Lance Armstrong, who has 2 liters more blood than most of us mortals; or Bjorn Borg in his prime, whose resting heart rate was measured at 28 beats per minute. Nice explanation. While I was reading your post I was actually thinking of Lance Armstrong. I think he has a heart that is twice the size of an average heart but cycling long distances is a lot different than swimming. I used to work as a bike messenger. That meant cycling about 60-70 miles and 10 hours per day, five days a week. I never had myself checked back then but it would've been interesting to see if my heart was any larger than usual.

I loved watching Borg. He was amazing. A low heart rate does not cause an enlarged heart. You have it turned around. The enlarged heart can be caused by what Jim talked about above, either exercise, and being very fit, the heart is a muscle, it will gain in size, especially the left ventricle, or disease. When the heart you are fit from exercise, there is all kinds of physiological developments that can then lower your resting heart rate. Also there is a genetic component. If you are concerned about it, get checked out by a doctor, especially if you are getting older. If they say you are fine, don't worry about it. As far as being that low, I know many of the teens that my children swim with that have low resting HR. When you are consistently fit, it goes way down.

Borg's physical conditioning was legendary as he could outlast most of his opponents under the most grueling match conditions. He is the most famous athlete known to have bradycardia, with a heart rate at rest reported to be between 30 and 45 beats per minute.

I started swimming and exercising just 4 months ago after a few years off and my resting heart rate went from 90 to 70.