For decades, heart rate (HR) monitoring has been a cornerstone of training for athletes across various disciplines. It provides a direct measure of the cardiovascular system's response to exercise intensity, allowing athletes and coaches to tailor training programs. However, as training methodologies evolve and become more sophisticated, relying solely on heart rate data has its limitations. This is where metrics like Intensity Factor (IF) and Training Stress Score (TSS) come into play. These advanced tools offer a more comprehensive understanding of training load and intensity, helping athletes optimise their performance and avoid overtraining.
Where HR-based Training Works
Heart rate-based training is effective. There's no doubt that athletes - and beginners alike - benefit tremendously from monitoring their heart rates before, during and after activities. The popularity of HR as a training tool, the (mostly) accepted and agreed-upon methods between coaches worldwide, and the widespread availability of scientific literature on heart rate monitoring, and the penetration of HR measurement on the finger, wrist, arm and chest, have made HR-based training the gospel for lifestyle, recreational and competitive athletes. In its simplest form, HR monitoring helps in:
Establishing Training Zones:Â HR data is used to set different training zones (e.g., recovery, aerobic, anaerobic), which can guide athletes in maintaining the desired intensity during workouts. This forms the basis of most, if not all, training programmes for any endurance sport across the world.
Tracking Fitness and Recovery:Â Changes in resting heart rate and heart rate during exercise can indicate improvements in cardiovascular fitness over time. Add HRV (heart rate variability) to the mix and the trifecta is complete with recovery indicators also included. By monitoring HR and measuring individually across a spectrum of activity types, athletes can ensure they are not consistently pushing themselves too hard, which helps in managing recovery and preventing burnout.
Complement to Pace: Road and track running is all (or mostly) about meeting time targets. With enough sports science backing this philosophy, achieving target paces at prescribed heart rate(s) is the key to achieving target race timings. Other endurance athletes like swimmers, cyclists - and triathletes - swear and live by heart rate numbers.
Where HR-based Training Doesn't Work
Despite its benefits, HR-based monitoring is, at best, an indicator of trends rather than actual data. Even if we look past factors such as inaccurate or skipped readings, usability issues around chest straps, debates on which part of the body can give the most realistic HR data, using HR alone as the measure of training has certain notable limitations:
Delayed and Non-linear Response:Â Heart rate lags behind the actual effort, especially during high-intensity intervals. This delay can make it difficult to gauge intensity accurately in real-time. Also, at higher intensities, the relationship between heart rate and power or pace is not linear. This makes HR less reliable for measuring intensity in high-intensity interval training (HIIT) and other demanding workouts.
Influence of External Factors:Â Factors such as temperature, hydration, stress etc. can affect heart rate independently of exercise intensity, leading to potential misinterpretations of effort. Add to this, the over-reliance on numbers, which renders even an extra 1bpm as a 'higher' effort, which can further add to mental stress. The alternate method, which is RPE-based, is too subjective and requires a robust understanding of one's body and ability to be accurate.
Trail and Mountain Running:Â Ultrarunners don't like anything easy. Which is why when there's trail or mountain running in question, unlike the terrain, HR-based training falls flat. Any irregularity in the terrain or surface can end up affecting heart rate, and skew the measurements drastically. Also, the 'time in zone' prescriptions become less relevant once an athlete crosses a training run duration of 2.5-3 hours or more.
Power Metrics in Running - IF and TSS
To address the shortcomings of HR-based training, athletes and coaches often turn to power metrics, which provide a more direct measure of training intensity. Power metrics give more 'immediate' data that's unaffected by external factors that can otherwise influence heart rate.
Intensity Factor (IF)Â and Training Stress Score (TSS)Â are two of the most popular power metrics. These were developed by Dr. Andrew Coggan and popularised by platforms like TrainingPeaks. These offer a more nuanced view of training intensity and load, and are considered more 'universal' as they're an independent measure of performance output compared to heart rate or derivatives of it.
Power metrics have a catch, though. These metrics were originally developed to be used for cyclists, as power meters could simply be installed at the wheel(s) which would give the output in Watts. One of the most crucial metrics for cyclists is the Functional Threshold Power (FTP), which is a measure of the highest power that can be sustained for an hour. As such, this does not apply to running. However, with advancements in wearable technologies, an almost like-to-like equivalent can be derived for running, as today's sports watches are able to use a combination of pace, vertical oscillations, grade and even local wind conditions to determine running power. This like-to-like equivalent, while sharing the same abbreviation, is called the Functional Threshold Pace - or just threshold pace - and is the basis for calculating power metrics for runners.
Intensity Factor (IF)
IF is a measure of the intensity of a workout. Measured relative to a runner's threshold pace, it's simply a ratio of the Normalised Graded Pace (NGP) for an activity and the runner's threshold pace.
IF = NGP / FTP
The NGP is a weighted average pace that reflects the physiological cost of the variability in running pace, adjusted for elevation changes, wind conditions etc.
An IF of 1.0 corresponds to a training run at an athlete's threshold, while values above 1.0 indicate efforts above FTP, and values below 1.0 indicate sub-threshold efforts. There are universally accepted guidelines for types of efforts corresponding to IF for a training activity.
Training Stress Score (TSS)
TSS quantifies the overall training load of a workout by considering both intensity and duration. TSS is a bit more complex than IF, as there can be multiple models of variables going into scoring the training stress for an activity. However, TSS calculations are based on a model called TRaining IMPulse (or TRIMP), developed in 1975 by Dr. Eric Bannister. TRIMP, in its earliest version(s), was an attempt to quantify the training load for an activity by using various metrics like heart rate reserve (HRR), duration and mileage, and applying a weightage or a scaling factor based on another set of variables.
TRIMP gave rise to more widely accepted and widely used models for calculating TSS. For runners, especially, the TSS is denoted as rTSS - or Running Training Stress Score - and is calculated as:
rTSS = ((Duration × NGP × IF) / (FTP × 3600)) × 100
Where Duration is the workout duration is seconds and NGP, IF and FTP have the same meanings as in the IF calculation above.
The rTSS provides a single value that represents the training load, allowing athletes to compare workouts of different durations and intensities.
Conclusion and The Catch
Power metrics are great. They succeed in overcoming a lot of limitations posed by training using heart rate alone. To a large extent, the very nature of 'relative' calculations automatically makes the numbers more accurate by wrapping them in their own respective contexts. But how useful are these in the context of training for a trail or mountain race?
On one hand, power metrics eliminate the 'lag' that heart rate has. Yet, power metrics end up emphasising pace, which is quite irrelevant in trail running. On one hand, NGP takes into account variables like elevation/grade and weather conditions that makes it closer to a trail-like setting. Yet, it fails to capture the complexity of the terrain itself. And perhaps the clincher - despite all the advancements in technology, Rating of Perceived Exertion (RPE), which is nothing but a measure of 'feel', remains the most popular and widely accepted training metric for trail runners. On the other hand, IF and rTSS are ready-to-use metrics that coaches can use to assess how hard an athlete is training.
The key here is to look at everything in totality. The simple logic of 'if it can be seen, it can be improved' is what stands out as the key message. As with any monitoring paradigm, it is extremely important to look at how numbers are trending, and where they sit with reference to a baseline. Regardless of what metric you adhere to, it eventually comes down to performance improvement over time.
And if you need help with making sense of the vast oceans of training data, we at IronStride are happy to help!
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