i had a pretty hard brick workout on sat and on sun did a 16 mile run. towards the end of the run(around 12-13 miles) i bonked pretty hard, even slowing to a walk at a point. i checked my heart rate and to my surprise is was only 80-90, not the 130 or so which i would expect in such a longish run. so for all you docs and experts out there, is this a normal reading for someone whos bonking or is there a deeper problem which i may be encountering? any help would be appreciated, thank you in advance.
Not a doctor, nor an expert but bonk often enough to share. Everytime I bonk, my heart rate drops like a rock. I’m guessing it’s cause you cannot push hard(or push at all for that matter) and so your heart rate drops.
Same for me whenever I’ve bonked. Usually happens on longer bike rides where I go out too hard the first 20mi.
I can tell when the tank is running low when the HR starts going down. By the time it drops from 155 to 135, its probably a bit too late.
same here, I always figured that I just don’t have the juice to produce any power. Luckily, it dosen’t happen too often, but thinking about the times that it did, always a low HR.
I’m not a doc, but I’m pretty sure that I read somewhere on Gordo Byrn’s blog (also not a doc, but a very well informed athlete) that running out of carbs will produce a lower HR and subsequent bonk, whereas dehydration will increase HR. So if you were not properly taking on carbs during your run a low HR and bonk can be expected.
…im sure other people familiar with English colloquialisms looked in to this thread with a smaile on their faces only to be disappointed…
When you bonk, your work rate drops - like you said, you were walking. Just because it feels hard doesn’t mean that it is hard work for your muscles. Because you are walking/slowing, there is no real need for high blood flow anywhere, hence low HR. Muscles likely have adequate fuel, so it’s not hard for them, it’s your brain that is feeling the pinch. Remember, a proper bonk is empty liver, not empty legs.
i had a pretty hard brick workout on sat and on sun did a 16 mile run. towards the end of the run(around 12-13 miles) i bonked pretty hard, even slowing to a walk at a point. i checked my heart rate and to my surprise is was only 80-90, not the 130 or so which i would expect in such a longish run. so for all you docs and experts out there, is this a normal reading for someone whos bonking or is there a deeper problem which i may be encountering? any help would be appreciated, thank you in advance.
When you are exercising hard you are relying primarily on glucose for energy (~80% from muscle glycogen, and ~20% from blood glucose from liver or exogenous sources) because you oxidize it faster than you oxidize fats. IOW, the rate at which you can produce energy is greater if using carbs than if using fats. When you bonk it is because blood glucose levels drop but this typically occurs after prolonged exercise when you’re also depleting muscle glycogen heavily. Consequently by the time you’re bonking much of your working muscle is relying on fats for energy and because this is a relatively slow process it doesn’t require the same rate of oxygen delivery. I’ve seen it stated that you can only work at about 50% of VO2max if relying solely on fats as opposed to carbs for energy. That is probably why you see the HR drop.
When you are exercising hard you are relying primarily on glucose for energy (~80% from muscle glycogen, and ~20% from blood glucose from liver or exogenous sources) because you oxidize it faster than you oxidize fats.
he was on a 16 mile run after a (presumably) long bike - likely not exercising hard, and quite possibly already oxidizing alot of fat
IOW, the rate at which you can produce energy is greater if using carbs than if using fats. When you bonk it is because blood glucose levels drop but this typically occurs after prolonged exercise when you’re also depleting muscle glycogen heavily. Consequently by the time you’re bonking much of your working muscle is relying on fats for energy and because this is a relatively slow process it doesn’t require the same rate of oxygen delivery.
You can conceivably be burning alot of carbs while bonking, the only reason your usually burning fat is for the reason you pointed out, but doens’t mean they’re causatively linked. Doesn’t matter what fuel is being used by muscle, if the liver runs out and can’t make more fast enough, so does your brain run out.
I’ve seen it stated that you can only work at about 50% of VO2max if relying solely on fats as opposed to carbs for energy. That is probably why you see the HR drop.
Most people won’t get out of bed relying solely on fats. In exercise, depends on training and diet - some can exercise at >70% VO2max with a great deal of fat (total and %) for fuel under certain conditions.
Can I make this any more simple - he was walking!!! The metabolic demand to walk is exactly what you might think it to be - not very much! You brain detects falling blood glucose, you get the signal to slow down. Since the energy cost of walking is small (eg <25% of VO2max), you don’t need much blood flow to the working muscles, regardless of how you feel!!!
…im sure other people familiar with English colloquialisms looked in to this thread with a smaile on their faces only to be disappointed…
Hmm…my immediate reaction before reading the thread was “I have never worn a heart rate monitor whist bonking, but heh, maybe I’ll give it a go”.
yeh…exactly what i thought too!!! hahahaha
I am always bonking hence the four kids!
Unfortunately the other type also sometimes happens when racing and each time it does my rate rate drops 15 to 20 beats. I also feel really strong for about 5 minutes just before the bonk as if my body is saying this is the final push and then no more.
If anyone has yet to experience this type of bonking register for this years Titan HALF Ironman which is Europe’s (and possibly the Worlds) toughest 70.3. Find details at www.triatlontitan.es
This will certainly test your limits and is an amazing event.
When you are exercising hard you are relying primarily on glucose for energy (~80% from muscle glycogen, and ~20% from blood glucose from liver or exogenous sources) because you oxidize it faster than you oxidize fats.
he was on a 16 mile run after a (presumably) long bike - likely not exercising hard, and quite possibly already oxidizing alot of fat
IOW, the rate at which you can produce energy is greater if using carbs than if using fats. When you bonk it is because blood glucose levels drop but this typically occurs after prolonged exercise when you’re also depleting muscle glycogen heavily. Consequently by the time you’re bonking much of your working muscle is relying on fats for energy and because this is a relatively slow process it doesn’t require the same rate of oxygen delivery.
You can conceivably be burning alot of carbs while bonking, the only reason your usually burning fat is for the reason you pointed out, but doens’t mean they’re causatively linked. Doesn’t matter what fuel is being used by muscle, if the liver runs out and can’t make more fast enough, so does your brain run out.
I’ve seen it stated that you can only work at about 50% of VO2max if relying solely on fats as opposed to carbs for energy. That is probably why you see the HR drop.
Most people won’t get out of bed relying solely on fats. In exercise, depends on training and diet - some can exercise at >70% VO2max with a great deal of fat (total and %) for fuel under certain conditions.
Can I make this any more simple - he was walking!!! The metabolic demand to walk is exactly what you might think it to be - not very much! You brain detects falling blood glucose, you get the signal to slow down. Since the energy cost of walking is small (eg <25% of VO2max), you don’t need much blood flow to the working muscles, regardless of how you feel!!!
I think you misinterpret a lot of what I wrote. I also think it’s hard to imagine too many situations where the decrease in work rate when bonking is purely central as opposed to also contributed to significantly by glycogen depletion in muscle/decrease blood glucose to muscle.
I read an interesting article a year or so ago that might have bearing. The author’s argument was that if you exercise above anaerobic threshhold too many times per week (something like 4 or more), you can tire your heart out. Once that happens, it essentially refuses to work hard. He also argued that triathlon training was only sport that regular training was likely to have you working too hard too many days per week. I have personally experienced this once, I think, and it was much as you described.
I understand your points, but am trying to illustrate that everyone seems to be overthinking this. The clear cut answer to firedude’s problem is that he had to walk - this was because his blood glucose could not be maintained by his liver, but that’s not the point, regardless of fuel mix/source, that’s why his HR was low. Sure, you often have to slow down due to depleted muscle glycogen, but you don’t “bonk”, in the true sense of the term.