One question our coaches are frequently asked is, "What is the ideal HR zone for the Bike & Run portion of a full IM?" Even for someone just hoping to finish an Iron distance event, this is an important consideration! Many people assume the answer lies in high Zone 2 or Zone 3, but training or racing at that intensity could be quite challenging, even for an elite athlete. Let's delve into the considerations:

When training for an Ironman race, the majority of our training should occur in Zone 2, also known as the 'Endurance Zone'—a heart rate sustainable over an extended period. Why? Because in this zone, roughly 20-30 beats below Race Pace HR or Lactate Threshold, the body predominantly utilizes fat for fuel, which we have in abundant supply. Properly trained, this allows us to sustain prolonged efforts like back-to-back marathons or even longer races. When we venture into Zone 3, we start tapping into glycogen stores for fuel—a limited resource lasting only two to three hours at most. Zone 3 efforts are typically reserved for the final miles of a Half Ironman or marathon, and even then, only for well-conditioned athletes. Pushing into Zone 3 during an Ironman, especially for athletes racing longer than 11 hours, can lead to a challenging run due to glycogen depletion. It's preferable to stay in Zone 2, maximizing fat burning.

Another consideration for most athletes is cardiac drift, a phenomenon where heart rate rises into Zone 3 despite maintaining effort in Zone 2—an occurrence to be minimized whenever possible, even if it means adjusting pace, perhaps even walking, to keep heart rate in Zone 2. While I permit my athletes to experience cardiac drift once a week during initial training phases, it's crucial they reduce intensity to stay within prescribed heart rate zones.

Once you've established a solid endurance base, cardiac drift should diminish unless dehydration is a factor. In hot conditions, you might allow heart rate to drift while monitoring exertion through Rate of Perceived Exertion (RPE), considering the additional energy needed for thermoregulation.

To illustrate, let's consider an imaginary athlete, Joe. Joe's Lactate Threshold Heart Rate (LTHR) on the run ranges from 160-163. His Zone 1 ends at about 137 bpm (84% of LTHR). During Zone 1-2 workouts, Joe aims for an 'easy' pace, around 135 bpm, placing him near the upper end of Zone 1. For a 'steady' effort (Aerobic Threshold or AeT), Joe targets 140-145 bpm, placing him in the middle of Zone 2—a suitable intensity for an Ironman race. If Joe can maintain this steady effort throughout the marathon after completing the bike leg, he'd maintain a similar pace as during his long training runs, resulting in a solid marathon time.

Training the body to efficiently operate in Zone 2 helps maintain consistent heart rate and pace throughout an Ironman. This principle applies to both the bike and run portions, although athletes may observe an 8-12 beat difference between bike and run LTHR.

Let's now explore LTHR for cycling, continuing with Joe as our example. Joe's LTHR on the bike is approximately 155, with Zone 1 ending around 125 bpm (81% of LTHR). During easy rides, Joe maintains a Zone 1 heart rate, around 125 bpm. For 'steady' efforts, he targets 135-140 bpm, placing him in Zone 2. With the exception of hill climbs, Joe should avoid exceeding 140 bpm during 'steady' rides. These efforts mirror his Ironman pacing strategy—he conserves energy on the bike to perform well on the run.

When prescribing workouts for an Iron distance plan, I often specify 'Zone 1-2' intensity. Athletes are expected to find a sustainable pace, avoiding overexertion. Occasionally, athletes may struggle with pacing, reporting instances of starting too hard and subsequently 'bonking' or experiencing performance decline. This suggests they're training too intensely, and it's my responsibility to clarify expected output and reasoning behind the prescribed intensity.

By adhering to these guidelines and consistently training 'steady' during 'Zone 1-2' workouts, athletes can enhance efficiency and pace with less effort over time. These improvements translate to significant performance gains on race day!

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