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I'm currently training to run in a specific HR zone (135 +/- 2 at the moment), in order to build up aerobic endurance. I'm observing that the pace with a given HR is constantly increasing during the workout - if during first 5-10 minutes of the workout I'm able to keep pace around 7:50, closer to the end it falls to around 9:30. I have tried to split running into several 15-20 minute blocks with 5 minutes of walking in between, the performance is still the same.

Are there any techniques or workout plans, apart of just keeping running in a zone for extended time, that may help to get to stable performance while running in a heart rate zone?

Thanks in advance for your answers!

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What you are experiencing is a phenomenon that is related to cardiac drift - whereby if you were to run at a constant pace, you would see your heart rate increase over the run. This is due to the stress experienced by the body with prolonged exercise. However, the alternative is that if you keep a constant heart rate, then you will see the pace come down in order to keep the effort the same.

As you run, you start to deplete your muscles of glycogen, up to a point at which you may notice your heart rate increase markedly - signalling depletion. You can improve your performance, resulting in heart rate stability, by running aerobically - that is, at an easy, comfortable pace (less than 75% HRMax). This allows for your muscles to adapt, resulting in more economic running - which is reflected by a faster pace for a given HR.

So, to an extent, you will always see a tail-off in pace, or an increase in HR, however consistent aerobic training will have a marked effect on stabilising your pace.

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In the first minutes of training, the body simply warms up, so you run fast with a low heart rate. Upon reaching a certain level, the pulse stabilizes. But then it starts to increase, and you have to slow down. This is explained by the fact that as the readily available carbohydrate stores are used, the blood becomes less "sweet", and the body becomes increasingly difficult to supply muscle with energy. Therefore, to maintain the desired pace, the heart has to pump blood faster - the muscles receive less sweet blood, but it comes more. There is no way to avoid this process altogether. You can only improve your performance. Longer training sessions - they develop metabolic efficiency, accustom the body to extract energy from fats faster. Improve the technique of running to run more economically. And store more carbohydrates before the race. (google translated)

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