We all have a variety of ways to judge performance during outdoor cycling. Power output is certainly the most concrete way, since it is a direct measure of workload. But you are also taking in environmental cues, like your speed on your favorite climb, the gearing you are using on a specific stretch of road, or how your breathing changes when you reach a certain point on a hill. On an indoor trainer many of these cues are either absent or different, so here are some good ways to evaluate efforts indoors.
Field Test to Establish Training Ranges
Whether you are going to use heart rate or power output on an indoor trainer, it is important to start your training program with a field test so you can establish your baseline performance and set training ranges. I use and recommend the CTS Field Test: Two 8-minute time trials separated by 10 minutes of easy spinning recovery. Record the average heart rate and average power output for each 8-minute effort. If you are using heart rate, use the higher of the two heart rate values to set your training ranges. If you are using power, take the average of the average power outputs (Effort 1: 300 watts. Effort 2: 310 watts. Use 305 watts to set ranges). What’s the difference between this test and a 20 minute time trial test, and why do we use this one? Here’s an article that explains that. Once you get your field test results, you can use these calculations to establish your heart rate and power training ranges.
The Problem with Heart Rate
Heart rate training has been around for 30+ years and although power is a better training tool, that doesn’t mean heart rate training is useless. Training with heart rate is fine, you just need to be aware of its limitations so you can view your heart rate data in context.
Heart rate is an observed response to the work you are doing, rather than a direct measure of that work. As such, it can be influenced by many factors unrelated to your workload, like caffeine, hydration status, core temperature, stress, lack of sleep, anxiety, prescription drugs, and many others.
Cardiac drift is one of the important considerations to make when training indoors by heart rate. As your core temperature increases and you lose fluid through sweating, your heart rate at any given workload will gradually increase. This means that during a 3x10minute lactate threshold interval set at 250 watts, your average heart rate for the first interval might be 160bpm, then 164 for your second interval, and then 168bpm for your third interval. If you are only using heart rate to guide your workout and you stuck at 160bpm for all three intervals, your second and third intervals would have been done at lower power outputs. This diminishes the effectiveness of the workout. However, since we never know exactly what changes in heart rate we can attribute to cardiac drift, we can’t say athletes should increase target heart rates across a series of intervals. The best way to monitor training in this instance is through a combination of information: heart rate, rating of perceived exertion, and speed/cadence/gearing if available.
Indoor vs. Outdoor Pacing
Athletes ask us all the time if there is a difference between power training ranges indoors compared to outdoors. The question comes because indoors, athletes have a harder time achieving the power outputs they can achieve outdoors. Often this is reported as about a 5% drop in sustainable power output (lactate threshold power output) on an indoor trainer. But to our knowledge no one has shown a significant difference caused by the trainer. It appears to be mostly a difference of perceived exertion. Without the environmental cues associated with speed outdoors, achieving that same output indoors feels harder. Put differently, when you’re indoors and reach the perceived exertion you associate with lactate threshold, you may be 5% below your actual lactate threshold power.
Dialing in your pacing on a trainer takes some practice. You can be a lot more precise with the time you spend at specific power outputs on the trainer because you don’t have to contend with the variability of road or trail conditions. But when you start intervals it’s a good idea to give yourself 15-30 seconds to ramp up to the prescribed interval intensity, rather than attempting to jump up to that intensity in a few pedal strokes. Similarly, when an interval ends it is a good idea to shift into an easy gear or reduce the resistance dramatically but continue pedaling lightly rather than coasting altogether.