One question our coaches are frequently asked is, “What is the ideal HR zone for the Bike & and Run portion of a full IM?” Even for someone just hoping to finish an Iron distance event this is an important question! Many people think the answer is high Zone 2 or Zone 3 but training and/or racing at that level would be quite tough, even for an Elite athlete. Let’s take a look at the considerations:

When training for an Ironman race most of our training should be done in Zone 2, otherwise known as the ‘Endurance Zone’ – a heart rate which can be sustained over a very long period of time. Why? Because in this zone, where our heart rate is about 20-30 beats below Race Pace HR or Lactate Threshold, the body will utilize the biggest resource we have for fuel: fat! We have a virtually unlimited supply of fat stores that if trained properly would allow us to run back to back marathons or even longer! When we cross over into Zone 3, we tap into more of our glycogen stores for fuel – and the body can only operate using stored glycogen for two to three hours, tops!

Zone 3 efforts should be reserved for the final miles of a Half Ironman or even a marathon, and only for a well-conditioned athlete. Going into Zone 3 in an Ironman ‚Äì for an athlete who is racing longer than 11 hours, would be risky, setting yourself up for a tough run! I‚Äôd rather see an athlete avoid depleting their stored glycogen and keep moving in Zone 2, burning as much fat as possible! Z2 may seem ‚Äòeasy‚Äô, but stick with it and over time you‚Äôll see an improved pace at the same HR! 

Another consideration for most us mere mortals is cardiac drift: a condition that is likely to occur when you are getting back to normal training levels or taking on a new training regime. This is where the HR rises into Z3 although your effort remains in the Z2 range ‚Äì a perfectly normal occurrence, but one that should be avoided as much as possible. Even if it means ‚Äì gulp -walking, in order to keep the HR in Z2! I do allow my athletes to experience this once per week when they are starting up training but I make sure that they know to back right down and stay within the set HR parameters indicated in their training plan. 

Once you have a solid endurance base, you shouldn‚Äôt see much in the way of cardiac drift unless you are dehydrated. If you are training or racing in the heat, you may need to let your HR drift and instead measure your effort by RPE (Rate of Perceived Exertion) . But only do so if you have taken into account the extra calories your body will use trying to keep cool . 

To put all of this into perspective, let’s take a look at an imaginary athlete, Joe. Joe’s LTHR (lactate threshold heart rate) on the run is about 160-163. His Zone 1 ends at about 137 bpm (beats per minute) or 84% of his LT. When training in a Zone 1-2 workout, Joe should be running ‘easy’ which is something I like to refer to as ‘guilty pace’! For him, that might be around 135 bpm, which would put him near the top of Zone 1. When running ‘Steady’ (which is also called Aerobic Threshold or AeT), Joe is around 140-145 bpm, which puts him about the middle of his Zone 2. This would be about right for an IM effort. After 112 miles of cycling, if Joe can maintain his ‘steady’ running HR for 26.2 miles, he would, with all other factors being equal, be running the same pace as he had on his long training runs. And that would lead to a pretty solid IM marathon time!

Training your body to work efficiently in Zone 2 will help you keep your HR and pace even throughout an Ironman. This theory holds true for both the bike and run portions although most athletes will see somewhere around an 8-12 beat difference between their bike and run LTHR. Let’s explore LTHR on the bike continuing to use our athlete ‘Joe’ as an example:.

First of all, how does Joe apply ‘Steady’ heart rate to cycling? Let’s say that Joe’s LTHR on the bike is about 155 and his Zone 1 HR ends at about 125 bpm or 81% of LTHR. When Joe trains on his bike and is riding ‘easy’ his HR is in Z1 – right around 125 bpm. When he’s biking ‘Steady’ (AeT) he’s around 135-140 which puts him in the middle to the top of Zone 2. With the exception of climbing hills, he shouldn’t reach over 140 bpm in training when the workout calls for a ‘Steady’ effort. There are times that he might see 145bpm on a steep hill, but that would otherwise be rare. Once again, these are his IM pacing efforts. In his IM race he will still have to run 26.2 miles after this 112 miles of cycling, so he won’t be punching the accelerator at all on the bike if he wants to run well (and who doesn’t?)!

When writing an iron distance plan, I’ll give an athlete a directive of ““Zone1-2” workout. The athlete is expected to find the happy medium and train at a pace they can sustain all day. When I review a log, I sometimes see entries such as “…I went out too hard, and well I bonked….” or “…I was much faster on the first hour of my ride and then I kind of faded…” This is often an indicator that the athlete is training too hard and that I need to do a better job of explaining what output I want to see from that athlete and ‘why’ .

If you can follow the above recommendations, and practice the discipline of simply training ‘Steady’ when your coach assigns you those ‘Zone 1-2’ workouts, over time you will improve your running and cycling efficiency and pace at less of an effort! And these kind of improvements will pay big dividends on race day!

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