Heart Rate Kinetics

I had an epiphany recently. It wasn't exactly a road to Damascus moment, but for someone obsessed with all things heart rate, it was another step towards my cardiac enlightenment.

For years I've recorded heart rates of each run, and plotted curves looking for trends and signs of improving performance. I'd always tended to ignore the first mile though, as it takes time for the heart rate to stabilise and provide any data that could be interpreted meaningfully.

Then the other day I read an article on VO₂ kinetics, which indicated that maybe that first mile's data may not be so meaningless after all.

I've always understood heart rate to be a good indication of oxygen demand and lactate levels. While heart rate monitors are good for this in aerobic exercise, eg marathon pacing, there are times when they are not so useful, namely high level exercise and short duration events (intervals) where the HR hasn't stabilised.

To see how the HR ramps up at the start of the effort, here's my heart rate plot of the first kilometre of the Eastleigh 10k:


In this plot you can see the HR curve increasing exponentially up to a value of 146bpm by the end of the first km. As there isn't a stable HR level to pace off, I'd always assumed the initial data was best ignored.

However, a Peak Performance article reveals that this data may be useful after all.

The thing I'd never quite understood is that if HR indicates oxygen demand, then why is the oxygen demand apparently less during the opening minutes? Surely if you are running aerobically at an even effort then oxygen demand should be constant? I'd assumed that HR wasn't a reliable indicator of oxygen demand until after a few minutes when you'd warmed up.

The revelation came seeing a VO₂ plot in the pages of Peak Performance of the initial minutes of high intensity exercise. This was a plot of the actual oxygen consumption, measured using a face mask and oxygen analyser, rather than a heart rate plot. The thing was though that it looked the same as the above. Oxygen demand increased exponentially just as HR did. Heart rate wasn't such a bad indicator of oxygen demand after all.

So if the effort is constant, but oxygen demand isn't, what's going on?

It takes a while for the muscles to 'warm up' and for the mitochondria to all come on line and process the much needed oxygen. This means that for the first few minutes of exercise there is an oxygen debt. In the absence of an adequate oxygen supply the muscles are forced to produce energy anaerobically, and this means:

Lactate!

Ah, music to my ears!

Clearly then, the faster the muscles get up to their maximum oxygen uptake the better, as there will be a smaller oxygen debt and less lactate build up. Any build up of lactate early on will have a major impact on subsequent performance as the muscle's efficiency is reduced. The smaller the oxygen debt the better.

This whole process of oxygen uptake is known as 'VO₂ kinetics'.

So how do we maximise our VO₂ kinetics? The most effective way is a good warmup. Allow me to demonstrate.

Here is a HR plot of a treadmill session I did on Saturday. The blue line shows 4 minutes of effort at 14.5kph. It was a little dodgy going straight to 10k pace without a warmup so I did at least do some mobility exercises. I was careful to keep my HR down though so that I was performing from cold. After the first effort I had 5 minutes of standing recovery to get my HR down quickly. The green plot shows a second 14.5kph effort of 4 minutes following the 5 minute recovery. The first effort had thus acted as a warmup. The red dotted curve I'll explain later.



A word of caution: these are HR plots not VO₂ plots. HR will give a fair indication of oxygen uptake as the greater the demand for oxygen the faster the heart has to beat, but HR and VO₂ are not the same - heat can affect HR as blood is pumped to the skin for cooling and causing a restricted oxygen supply to the muscles. Here though, with short efforts and a full recovery, it is likely that both efforts are being done in similar conditions with similar oxygen transport.

Having said that, clearly the green plot shows a faster increase in HR. The slope of the 2 curves is initially similar, with a small lag on the blue curve, but then there is something of a blip on the blue curve and the gap between the 2 increases somewhat. It is a fair bet that the oxygen debt for the second curve is less than the first. Personally, the second effort felt a lot more comfortable. I've noticed this effect too when running fast intervals - it takes 2 or 3 efforts before my legs feel comfortable, despite them all being the same pace.

The dotted red line in the above plot is a mathematical plot - an exponential curve with a time constant of 25s. What this means is that every 25s the gap between the current HR value and the value it is tending towards, 147bpm in this case, is halved. My time constant is thus about 25s.

Clearly the smaller one's time constant the better. The PP article made it clear that a good warmup will influence this, but also that endurance training can improve the time constant. Current research does not show what sort of training might have the best influence though.

I was intrigued to learn that the person with the fastest measured VO₂ kinetics is Paula Radcliffe with a time constant of 8-9s. Whether this is down to training or genetics is unknown. Maybe I should try 140-mile weeks and test again.

Of course my time constant is measuring HR kinetics and not necessarily VO₂ kinetics - they may not be the same, though I can assure you I don't have a VO₂ time constant of 9s!


So what more can I learn from this data? It appears that we have a way of quantifying the effects of a warmup. I wouldn't call 4mins of 10k pace followed by 5mins standing, the best warmup. What I want to do is repeat this test with my usual warmup of 10mins jogging followed by some 30s sprints picking up the pace. In order to influence the oxygen uptake one has to warmup around threshold pace or faster in order to generate a bit of lactate. It's important to have a bit of recovery before the race (or test) though, as otherwise the warmup might impact the performance. I think my standard warmup fits the bill, so it will be interesting to see how it affects the test.

Can I improve the effectiveness of my warmup? With the track season coming up, I hope I can. Watch this space for details.

Reference: Peak Performance Number 245: Oxygen kinetics - start smart for a mean finish!