When embarking on a program to improve running performance, the three metrics that are most frequently used for workout prescription and monitoring are pace, heart rate, and rating of perceived exertion. All three metrics can be used to guide workout intensity, but there are circumstances that can arise in a workout that renders each metric unreliable. Each metric can be used alone to effectively design and monitor a training program, however understanding and making use of all three metrics together will yield a more applicable plan. The benefits and circumstantial limitations of each metric will be discussed below, along with tips for switching between metrics within a workout.
Since the advent of consumer heart rate monitors in the early 80’s, heart rate has been used as a way to prescribe exercise intensity within a workout. A fitness enthusiast’s training zones are often set up based on a percentage of maximum heart rate, while competitive runners often develop training zones based off of their lactate threshold heart rate. Proponents of heart rate training believe that heart rate is the best predictor of overall stress on the body by encompassing not only training stress, but all other environmental stressors as well. Opponents of heart rate training believe that too many variables such as temperature, elevation and hydration can alter heart rate too dramatically for it to reliably gauge exercise intensity.
Monitoring the pace of a running workout is the gold standard for competitive runners. Since all competitions and physical tests are based on speed of completion and not level of effort, it is hard to argue that there is a more important metric than pace. Proponents of pace prescription believe that it is the most relevant and meaningful way to guide training. Pace based training can be an effective way to progress towards faster training split times in hard workouts. It can also serve as a great governor of intensity when necessary, since running fast too frequently is a common detriment to performance. Much like heart rate, there are circumstances such as running into a strong head wind, or encountering an uphill section of your running course that will make hitting a target pace unreasonable.
A rating of perceived exertion (RPE) involves using a numerical scale to subjectively rate a level of effort. The popular Borg scale of perceived exertion ranges from 6-20 with a 6 being an extremely easy level of effort to a 20 being a maximal level of effort. Other scales have been adapted to range from 0-10 for simplicity. RPE scales used in conjunction with physical exercise have been validated in research many times over, however users of the scales do need to undergo a familiarization period to ensure their subjective feelings of effort match the levels of true physiological stress. Opponents of perceived effort based workouts feel that a subjective rating of performance is not as reliable as more objective measures like heart rate or pace.
A truth in performance training is that there are very few absolutes. There are more training variables than can be mentioned that go into exercise physiology and performance training, so to use any one training variable to guide the training process is a very myopic approach. A more powerful approach would be to understand the strengths and weaknesses of each metric and learn to apply the most appropriate metric at the most appropriate time within a workout. Below is a list of many of the circumstances that will make transitioning between metrics advisable.
It may be impossible to run at the same target pace uphill compared to flat ground, or the intensity of running would need to be drastically increased to maintain a target pace which changes the objective of the workout. Pace based training on a hill will likely be the least practical metric to follow. Heart rate and RPE are both good metrics to transition to when you approach a hill. Aim to keep your heart or RPE at a similar value while running on the hill as you were holding during flat ground running.
Especially at faster running speeds, drag created from running into a strong headwind greatly increases energy expenditure to maintain a target run pace. Much like with uphill running, this will negate the efficacy of pace leaving heart rate and RPE as better options.
Extreme Heat and loss of hydration
Running in hot and humid conditions causes a pronounced increase in heart rate due to thermoregulatory stress and water loss. Our bodies have to work harder to keep us cool as our core temperature rises, and the heart has to work harder to pump more viscous blood as we become dehydrated. Heart rate can easily be elevated by 10-20 beats per minute above normal. A target pace may also be unattainable due to a higher heart rate at slower speeds, so RPE may be the ideal metric in hot conditions.
A Change in Running Surface
If your target paces were prescribed based off a field test performed on asphalt, it is unlikely that you will be able to meet target paces on a softer and slower running surface like grass or trail. In the event that your running route leads you onto a soft running surface, you may need to disregard pace and rely on heart rate or RPE for guidance.
It is inevitable that at some point in a training run technology will fail. Whether it be a short period of time that a heart rate strap ceases to recognize a heart rate or the battery in your GPS watch dies, there will be a period of time where you will be without knowledge of heart rate and/or pace. If technology fails, having an understanding of the RPE expectations in your current workout will help you stay on track.
Short Intervals, Early in workouts, Late in workouts
These are three instances where heart rate can be misleading. During short interval sprints (30-90 seconds), heart rate may not respond to your rapid increase in pace and effort quickly enough to provide a true representation of your effort. There is often a lag time in heart rate response during abrupt changes in output that could take 20-60 seconds to catch up to your given running pace. For short interval sprints, pace or RPE should be utilized. Just like other muscles, the heart takes time to warm up and become most efficient. In the early stages of workouts prior to being thoroughly warmed up, heart rate may be slightly lower than anticipated at a given pace. In the first few minutes of a workout it may be better to rely on pace until your heart rate response normalizes. Heart rate at the end of long workouts can be higher than normal. Cardiac drift is a phenomenon where heart rate steadily begins to climb without a concomitant improvement in running pace. Researchers believe cardiac drift is a result of fluid loss during a workout. If you notice cardiac drift happening in the later stages of a long run, fall back on your running pace to guide you through the finish of the workout.