Ok Nerds!! Help me out here! (Science!!)

[Oramac]

So I was reading this article and came across the following quote from a neurophysiologist named Kenneth Jay:

 

 

 

With an increased HR to VO2 relationship it will never be as good as typical cardio exercises. It is simple physiology really. Increased heart rate decreases the time available to fill the left ventricle of the heart, which means that the left ventricle will contain and eject less blood per contraction. This means that the 'stretching' of the heart wall, which is necessary to increase your stroke volume and your VO2, does not happen. It’s the Frank-Starling mechanism in full effect and it’s basic cardiorespiratory physiology. Moral of the story: stop thinking you can “get your cardio in†by lifting weights—no matter how fast you lift them!

 

Now, ignore the whole "lifting faster isn't cardio" thing.  That's not my question.

 

My question is in regards to this part: "Increased heart rate decreases the time available to fill the left ventricle of the heart, which means that the left ventricle will contain and eject less blood per contraction."

 

Now, I'm no doctor, but it appears to me that the heart (and our whole blood-flow system) is analogous to a closed-loop hydraulic system.  And in such a system, moving one valve faster doesn't cause any kind of decrease in volume.  I understand the size of the pipes (blood vessels) has bearing on flow rates, but I don't see how his statement can possibly be true.  In a closed hydraulic system like I'm used to seeing, opening a valve faster will fill that space faster, assuming the fluid lines can flow enough volume to keep up.  Also, since it is a closed system with one-way check valves, the pump (also the heart, in this case) would create pressure to force the fluid into the newly opened area.  If the valve opening faster is accompanied by a similar increase in pump output, there shouldn't be any loss in fluid, even if the time frame is shorter.  

 

Any physicists/physiologists out there want to comment?

[The Stranger]

I'm not a doctor, but my observation is that my heart beats pretty damn fast when I run - almost certainly faster than when I do strength work (I do bodyweight, not weights, but the principle is the same), though I admit I don't measure it.   Do other people lift so intensely that it elevates their heart rate more than running a few miles?  Even if heart rate does spike at the end of a tough set, it seems like it comes right back down, no?

 

I'm also curious about the claim that cardio gains are dependent on stretching the heart wall - is that a widely known thing that I just haven't encountered before?  If that's the case, I can imagine that a super-elevated heart rate could lead to a "shallower" heartbeat (akin to lifting without full range of motion, perhaps).  As you say, the limiting factor would be the "pipes."  But it seems like strength work is unlikely to get your heart rate that high, unless I've been doing it wrong.

 

I know, I only talked about the "not your question" part.  But I guess that's my question.

[Jolt]

The Frank-starling mechanism is a genuine hemodynamic concept. In diastole the blood returns to the heart. With elevated heart rate time in diastole decreases meaning less return of blood. This return of blood is also called preload. Generally speaking stroke volume (the amount of blood sent "forward" per heart beat) is dependent on preload. How much so depends on a lot of variables i.e. volume status, heart valve disease, ischaemic heart disease and others.

The mechanism can be likened to the bounce out of a deep squat. Streching the cardiac muscle fibres allow more force to be generated in contraction thus increasing stroke volume and in turn cardiac output (stroke volume x heart rate).

Real world example: anaesthetising someone with aortic valve stenosis. They are preload dependent because they have to generate higher pressures in the left ventricle to overcome the pressure gradient of the narrow and stiff valve. When you anaesthetise people you always lose some preload and some afterload. If you lose too much or they are "too shallow" their heart rate increases -> preload drops further and you risk entering a downward spiral. This can be a challenge at 3.30 when the surgeon wants to get the ruptured colon out the nice old lady who's septic as &€%#&.

Also the analogy of the cardio-vacular system as closed doesn't work. It's more like two systems that are leaky and can change pipe size at will...

Hope that helped..

/ Jolt, M.D. in anaesthesia

Edit: Ps the above is simplified somewhat as i'm typing on a tablet.

[Machete]

You NEED Long Duration, Low Intensity Cardio

[TaraTheMiddle]

You NEED Long Duration, Low Intensity Cardio

 

That was a very informative and interesting article.  

[krusteh]

difficult to unravel the point being made in that qoute out of context. cardiac output is purely the product of the stroke volume of the heart ( the amount it pumps each beat) and the heart rate. oxygen delivery is dependent both on the cardiac output (the volume of blood pumped each minute) and the amount of oxygen carried in that volume of blood ( which is dependent on how much red blood cells you have and how well you are getting oxygen into the blood). in excercise most folks increase their hearts output through a combination of increased stroke volume (thru various mechanisms like increased return of blood and increased contractile force) and heart rate.

if we are talking about maximizing cardiac function efficiency there is certainly a sweet spot regarding heart rate. increasing heart rate increases cardiac out put and therefore oxygen delivery and VO2. at some point increasing heart rate further decreases filling time and thereby the amount ejected from the heart with each beat. at some point the system becomes less efficient as the energy used to drive greater cardiac output falls. when we have to augment cardiac output following cardiac surgery in someone who has a low heart rate we some times pace them at a higher rate. most often that rate is about 90 or so that seems to be the sweet spot for those patients in that context.

I come back to the point being made though. excercise is a stress provided to stimulate adaption. if he means that weight training exercises have an undue effect on raising heart rate in relation to vo2 ( oxygen delivery) how does that impact what we do in training? if your heart must beat faster to do a given amount of work....might that not provide even a better stimulus for adaption? adaption in the hearts capacity to do work ...maybe less efficient if the goal was caloric expenditure.

I would not focus on the scientific comments qouted in that article to make a decision about whether crossfit is a good or bad form of training for you. thats like trying to decide what to do when a tiger is chasing you and someone tells you its orange. a tree in the forest.

the author makes some valid points in her overall arguement though. There are certainly many positives too. crossfit certainly seems like it blends cardio and some degree of strength training in compact episode. I think it probably also builds a mental toughness that might better help one meet their challenges in training whatever they might be. it might be better for some folks than more traditional cardio activities who might otherwise neglect strength training. I havent done crossfit myself. i used to be a wrestler though and i see many similarities in that style of training and what we would do which was mostly but not all body weight oriented.

Krusteh, MD Cardiac Surgery, Surgical Critical Care