In this article I will review the research that provides validity in training by heart rate. I will also discuss the advantages and disadvantages of training by heart rate. Finally, I will review current research that provides evidence for training by heart rate during long endurance training sessions, versus training by power or pace.

Principles of Heart Rate Monitor Training

The goal of the vast majority of triathletes is to become faster, whether the athlete is a novice triathlete, an elite age-group athlete, or a professional. For those athletes that have limited amount of time, the majority of age-group triathletes, it is essential that the time devoted towards triathlon training is productive. In order to make training time as productive as possible, it is necessary to monitor training intensity and have a means to gauge training progress. In endurance/triathlon training, the means to do this include monitoring pace, power production (specific to cycling), and/or heart rate (HR). In this segment, I will focus primarily on heart rate training and will devote some time to pace. The HR training I will discuss will be limited to cycling and running disciplines.

Validation of HR Training

Using HR to monitor exercise intensity is one of the easiest and least expensive methods of monitoring exercise intensity. The argument against using HR monitors for measuring exercise intensity is that intensity is commonly referred to as a measure of energy expenditure. Heart rate does not directly reflect energy expenditure, in the same way as VO2 (amount of oxygen consumed) or power output on the bike reflects energy expenditure. The two latter measurements are direct measures of energy consumption, but you cannot measure VO2 every time you train (not practical) and power output cannot be measured while running. There is a linear relationship between HR and VO2 (Strath, Swartz, Bassett, O'Brien, King, Ainsworth, 2000) and exercise intensity based on HR has long been considered an effective method of measuring exercise intensity (Fernandez-Garcia, Perez-Landaluce, Rodriguez-Alonso, Terrados, 1999; King and Senn, 1999; Padilla, Mujika, Orbananos, Santisteban, Angulo, Jose Goiriena, 2001).
Heart Rate Zones

Most athletes and coaches train with five HR zones. These are categorized as zones 1 through 5. There are some athletes and coaches that list zone 5a, 5b and 5c, but the reality is that zone 5b and 5c are maximal effort and the intervals are so short the athlete never actually reaches a HR of zone 5b or 5c. Zones 6 and 7 or 5b and 5c are more applicable to training with the use of a power meter. I will be discussing how to determine your HR zones shortly, but for now I will mention they are based on HR at lactate threshold (LT). The zones and percentage of LTHR are listed below:

ZONE Percentage of LT HR
Zone 1 <68%
Zone 2 68-83%
Zone 3 84-94%
Zone 4 95-105%
Zone 5 >106%

There are variances of HR zones based on lactate threshold with different coaching philosophies. In the end, the zones are relatively similar regardless of the percentages based on threshold.

Determinates of Endurance Performance

Now is a good time to briefly discuss determinates of endurance sports performance. A plethora of research has been conducted investigating what makes athletes excel in endurance performance (i.e. triathlons) and three characteristics have been assigned to predict endurance sports performance 1) efficiency 2) lactate threshold and 3) VO2 max. Efficiency is defined as the amount of power output produced for a given energy consumption. Some exercise physiologists will use the term economy instead of efficiency, especially when dealing with running. The difference is economy is defined as movement velocity for a given energy consumption. In both cases energy consumption is measured in VO2 (the amount of oxygen consumed).

Regardless of the terminology, what is known is that the most efficient cyclists and economical runners can often overcome their genetic limiter of a low VO2 max. So what is VO2 max? The true definition of VO2 max is the greatest rate of oxygen uptake by the body and is dependent on maximal cardiac output (amount of blood pumped by the heart) and maximal arteriovenous oxygen difference (amount of oxygen extracted by muscles). When VO2 max is measured in a laboratory, a mask is attached to the athlete and the athlete is required to perform maximal work. When maximal work is achieved the amount of oxygen consumed per minute is measured. VO2 max has always been considered the “gold standard” of fitness. The reason is because VO2 max is dependent of the amount of blood the heart can pump (cardiac output) and the amount of oxygen the muscles can extract from the blood to use for aerobic metabolism (arteriovenous oxygen difference). Historically, it was considered that the higher the VO2 max was, the better the athlete performed in his or her sport. However, this has been shown not to be the case as exercise physiology evolved and numerous studies evaluated the greatest athletes of various sports. What was found was that it is not always the athlete with the greater VO2 max crossing the finish line first. This should come as a relief to many athletes as the ceiling of VO2 max is genetically determined. Once an athlete reaches a high level of fitness, VO2 max cannot be significantly improved. What studies have shown is that more efficient athletes are able to overcome their genetically determined lower VO2 max and perform the same amount of work with less metabolic cost. Slattery, Wallace, Murphy, and Coutts (2006) found it is the velocity at VO2 max (Vmax) that predicts running performance in a 3 KM time trial. What was also found was that some athletes are able to out perform their competition with a higher VO2 max by having a higher lactate threshold. Van Schuylenbergh, Eynde, Hespel, (2004) found that triathlon performance could be precisely predicted by determining swim speed and run speed at maximum lactate steady state levels. To summarize the last two statements, it is the velocity at lactate threshold that determines performance.

Lactate threshold is a term used to describe the work output performed when blood lactate increases above baseline. Lactate is a product of anaerobic metabolism by the muscles. As exercise intensity increases, the amount of oxygen reaching the muscles is not sufficient to allow the muscles to generate energy using oxygen (aerobic metabolism) and the muscles are required to produce energy via anaerobic (without oxygen) mechanisms. As the intensity of exercise increases, the amount of lactate produced increases, and due to changes in blood flow, the removal of lactate decreases. A point is reached in which lactate production increases beyond lactate removal and the blood lactate concentration increases. This is the lactate threshold and the point is similar to the term anaerobic threshold used previously in exercise science. Generally speaking a person’s lactate threshold is the intensity of exercise that can be maintained for an hour. I will qualify the previous statement and state there is a variation in how long an athlete can perform at LT and there are many factors that must be considered when stating how long an athlete can continue at lactate threshold intensity. Regardless, the higher an athlete’s lactate threshold in relation to VO2 max, the higher the intensity of exercise that can be maintained.

Now to bring the three determinants of performance together we can discuss how these determinants can interrelate. Picture the VO2 max as the size of the engine in a car. The higher the VO2 max, the bigger the engine and potential for power. Now picture the lactate threshold as the red line limiter of the RPM. If the engine produces maximal power at 5,000 RPM (or the athlete at 180 beats per minute HR), but redlines at 3,000 RPM (or 108 beats per minute HR), the high VO2 max is not the true limiter in performance. The athlete cannot even come close to VO2 max intensity. Generally speaking, a lactate threshold of 85% of VO2 max is considered good. Where does efficiency come in? In the human body efficiency is developed through economy of movement and partially by the genetic makeup of the body. Economy in running is related to qualities such as high stride rate and midfoot or forefoot running. If two runners have identical VO2 max at 70/ml/kg/mn-1 and lactate threshold at 75% of VO2 max one runner could run at a 5 minute/mile pace and the other runner at a 7 min/mile pace solely due to a difference in running economy. Efficiency in cycling deals with a smooth pedal stroke, pedaling in circles, ankling, and proper cadence for a given power output. Swimming efficiency is the most critical of the three sports and deals with a smooth, streamlined stroke.

Training Zones

• Zone 1 is used primarily for recovery, building resistance to fatigue during long training sessions, and for working on efficiency.
• Zone 2 is primarily used for endurance training. The intensity of the zone is sufficient to produce cardiovascular improvement with training, yet not intensive enough to severely break down the body during training.
• Zone 3 is a higher intensity used for more intensive endurance training. This is the zone commonly used for tempo training. The intensity stimulates more cardiovascular benefits than zone 2, however the volume of zone 3 training should be limited (especially in running) due to the stress imposed on the body with this intensity of training.
• Zone 4 is primarily used for lactate threshold training. The intensity is just below to just above an athlete’s lactate threshold. This is intense training both physically and psychologically and is generally used at least once per week during structured training in the build/precompetition phases of training; however, it should be limited due to the high risk of injury.
• Zone 5 is used to train an athlete’s VO2 max and should be used very sparingly, especially in running, due to the extreme stress. Once an athlete has achieved a high level of fitness, VO2 max intervals should be used scarcely. As I previously mentioned, VO2 max is genetically determined and once the ceiling of VO2 max is reached, it cannot be increased significantly. Training time should be spent elsewhere. The exception, to a degree, is cycling in which we can increase power at VO2 max. A final statement concerning the training zones; as one progresses from zone 1 through zone 5, the percentage of fat supplying body fuel decreases and the percentage of carbohydrates supplying fuel to the working muscles increases.

Determining HR Zones

The next topic I will discuss is determining your HR zones. The most valid method of determining HR training zones is a lactate threshold test on the treadmill for running and the computrainer in stand-alone mode or cycle ergometer for cycling. The lactate threshold test involves increasing the workload in stages and obtaining blood samples at the end of each stage. There are two points of interest when analyzing the blood lactate to workload relationship. Lactate threshold is identified as the exercise intensity eliciting a 1 mmol·L-1 increase in lactate above base-line values. Onset of blood lactate accumulation (OBLA) is identified as the exercise intensity eliciting a blood lactate concentration of 4 mmol·L-1. It is very rare that a stage corresponds to an exercise intensity eliciting 4 mmol·L-1of blood lactate, thus interpolation between the point above and below 4 mmol·L-1 is performed. It is critical that a lactate threshold test is performed for cycling, as well as running, as there is often a significant difference in HR at OBLA and LT between the two disciplines.

There are numerous field tests that can be performed to estimate lactate threshold for cycling and running; however, the validity and reliability of most field tests has been questioned. If a laboratory lactate threshold test cannot be performed, the best method of determining lactate threshold heart rate is via a 40K time trial on the bike and a 10K run race. To determine lactate threshold HR from these races, measure the average heart rate for the entire event. It is best that these are actual races in order to maximize motivation during the testing.

Once LT HR is determined, training can be focused around the lactate threshold. When the lactate threshold test is performed by a knowledgeable coach, maximum effort is usually reached in order to evaluate maximum HR. Maximum HR corresponds to VO2 max HR, but VO2 max cannot be determined without a metabolic cart. Regardless, one can look at the LT HR and VO2 max HR and determine a percentage. If LT HR is above 75%-80% of VO2 max, but the run velocity is slow and we are dealing with a trained athlete, time should be spent working on run economy. If the athlete is young in terms of training age, training could be focused on VO2 max and running economy. If the LT HR is below 70% of VO2 max HR, then training should be maximally focused on LT. The training objectives should also be considered based on the training season (base, build, competition, etc.) and training age of the athlete.

Monitoring Training Stress

Another benefit of training via HR is effective monitoring of training stress/load. Efficacious training protocols take into consideration volume, frequency, and intensity of training. Training without a HR monitor allows one to easily measure volume and frequency with a calendar and watch, but training intensity can be very difficult, if not impossible, to monitor without a HR monitor. The HR is a measure of exercise intensity. By measuring HR and duration of exercise, training impulse (TRIMP) values can be calculated and used as an integrative marker of exercise load during training and racing (Padilla, Mujika, Orbananos, & Angulo, 2000). A principle of training is progressive overload. Successive blocks of time should be characterized by increasing training load and by monitoring TRIMP; an athlete can ensure the weekly training load (a function of frequency, intensity, and volume) progressively increases. Furthermore, by calculating training load, the overload does not increase too much, minimizing the risk of injury and/or over training. Effectively monitoring training use TRIMP allows one to monitor acute training load (the individual training session) and chronic training load (training stress over time) and furthermore, the TRIMP score allows one to standardize the stress level of athletes. For example a TRIMP score of 150 for one athlete will be similar to a TRIMP score of 150 for another athlete. Finally, using TRIMP scores can help design workouts. For example, if an athlete I am coaching is training using a HR and achieves a TRIMP score of 175 on a five-hour bike ride, I may decide to develop a workout the following week to achieve a TRIMP score of 190. Achieving a TRIMP score of 190 can be achieved by three different types of rides: this can be done by maintaining the ride time, but increasing the intensity, decreasing the ride time, but significantly increasing the intensity, or finally maintaining intensity and increasing the ride time. A TRIMP score of 190 will have the same training stress regardless of which of the three ride options. There would be a slight physiological nuance that will be different between the three rides, but for our purposes here, I will state the training stress will be the same between the three rides.

HR Training Shortcomings

I will now discuss some drawbacks to training by HR. The first drawback is that HR is not always a valid indicator of exercise intensity. For example, on a training run at an 8-minute per mile pace an athlete’s HR may be 140 on a given day. On another day, an 8-minute per mile pace may yield a HR of 150. This can be a good measure of training intensity, or it can be misleading. If a person is sick, dehydrated, injured, etc. the training HR is a valid measure of stress and should be used to monitor exercise intensity. It would be very likely in the athlete’s best interest to modify the workout or call it a day and focus on recovery. Alternatively, it can be an invalid measure of stress, For example, if the athlete is racing and the excitement of the race is “artificially” driving up the HR due to hormones, the HR should be ignored and other measures of monitoring exercise intensity should be implemented, i.e. rating of perceived exertion or pace. It is for the latter reason, that I have my athletes go into a race with multiple methods of monitoring exercise/race intensity. Finally, a largely debatable topic when it comes to monitoring heart rate in endurance training/races is the topic of cardiac drift. The definition of cardiac drift is an increase in HR over time at the same exercise intensity. I can confidently state that every athlete reading this article has experienced cardiac drift. A common example is the 60 + minute training run. At the start of the run lets say you are running 9-minute miles with a HR of 155. You decide to maintain the run pace at 9 minutes per mile, but at mile 6 you notice your HR is 160. Has your exercise intensity increased? Based on pace, no, but in terms of physiology, it depends on who you ask from the exercise physiology field. Historically, the underlying reason for cardiac drift was considered to be dehydration. However, recently it has been shown that cardiac drift cannot be prevented by sufficient hydration. Wingo, Lafrenz, Ganio, Edwards, and Cureton (2005) found that hydration does not prevent cardiac drift and cardiac drift is associated with a decreased VO2 max. The implications are that HR should be used to monitor relative training intensity. If one maintains the same power output on the bike or the same run intensity, with disregard to training HR, the athlete may be training or racing in a non-desired zone (i.e. above lactate threshold) and the training/racing duration progresses. Of course, there are training sessions in which a certain pace or power intensity is desired with disregard to HR, but those training sessions have specific goals. Other factors that can affect HR are caffeine, altitude, and hormones. Depending on the circumstances, training by HR when under the influences of the above mentioned variables, may or may not require adjustment of training zones or using other methods of monitoring training intensity.

Training by Pace

I will finish this newsletter by briefly addressing the topic of training by pace vs. HR. Training zones based on running pace can be calculated based on recent race results, laboratory lactate threshold tests, and various field tests. At the initial phase of a training session HR and running pace correlate very closely; however, as previously mentioned, as the training session continues cardiac drift will occur and maintaining the same running pace will yield a higher HR. The Wingo, et al. (2005) study suggests that HR should be monitored, as that is a relative indicator of metabolic intensity. However, there is considerable debate surrounding this topic. In cycling, research has supported even pacing for optimum time trial performance (Foster, Snyder, Thompson, Green, Foley, Schrager; 1993). There is a paucity of studies concerning pacing in run events. The two most popular methods of pacing in run events are HR and run pace. The common denominator of both these methods is pacing based on lactate threshold. Renowned marathon coach Bob Glover suggests running at a pace of 95-97% lactate threshold. Jack Daniels has developed running paces for different events based on time trials. The most common piece of advice by veteran marathoners and experience running coaches is to run even splits. Running slightly faster than goal pace (banking time) is usually a disastrous approach to running a marathon. When running based on pace vs. running by HR a word of caution is warranted. If the implications of the Wingo, et al. (2005) study are true, then running at a pace of 97% threshold pace could lead to a poor performance in some marathoners. Let’s say your marathon goal pace is 7:10/mile and your lactate threshold is 167 bpm. At the start of the marathon you are running at a HR of 162 and holding the pace perfectly. The racecourse warms up, blood flow is redistributed in the body, and cardiac drift begins to occur sending your HR over 167 and you cross the lactate threshold sending your performance in a downward spiral. Until future studies are performed to examine the relationship between cardiac drift and relative metabolic intensity I would suggest caution when training and racing based on pace alone. I suggest using multiple methods for monitoring training/racing intensity. In my coaching practice I generally prescribe training/racing by run pace for those with a very large base of training, for specific workouts to mimic race pace, and when the training environment is known (flat). When racing by pace, it is critical that one has a solid base of training behind him or her to attenuate the effects of fatigue. This last point emphasizes the importance of long zone 1 and 2 runs to build a solid endurance foundation. I will lastly mention a very important benefit of training by pace. When training by pace, the athlete is require to achieve the desired pace, thus economy is reinforced. A common mistake I see athlete make when training by HR is achieving a desired HR due to inefficiency. This is easily seen when the pace at two different training zones are identical or at least very similar. Training by pace circumvents this potential training error.

In the last decade a growing number of athletes have shifted to using power to monitor and pace their bike training sessions and races. Training by power has a lot of benefits. However, my professional opinion is that training by power should not be done exclusively and is most effective only when incorporating heart rate (HR) based training and monitoring by rating of perceived exertion (RPE). In this newsletter I will provide the benefits and limiters of each type of training and then provide what I see as a balanced training approach of combining power, HR, and RPE into the training paradigm. The majority of this article will pertain to cycling training. However, I will briefly mention the pros and cons of monitoring run training by use of the HR monitor and run pace.

Benefits of Power-Based Training

There are many benefits of training by power. One of the most practical reasons for training by power is that you can dial in short intervals and immediately insure you are training in the proper training zones. An example is zone 5 intervals where one is aiming at improving VO2max/Peak Power Output (PPO). If an athlete is training by HR alone, one may under-estimate or over-estimate the intensity of the short training interval due to the fact that HR does not instantaneously increase when power is increased (aka HR lag). Oftentimes one is nearly complete with the interval before HR reaches steady state (which can take 3+ minutes) and the interval is complete before the athlete realizes he or she has over or under exerted the interval. This is also particularly apparent when training for sprint power/neuromuscular power where training by HR is impossible due to the short intervals. Furthermore, power-based training can be used to calculate training stress and insure training is progressively adding adequate stress to an athlete and more importantly insuring adequate recovery.Caution is warranted, as training stress via the current TSS score formula that has been proposed (1) may lose validity beyond one hour of training. The reason for the decreased validity of the power-based training stress score is that power output (4) and VO2 max (4,8,9) decrease during prolonged exercise sessions. Another benefit of power-based training is that it is very easy to track fitness improvements. One can compare average power during the interval and compare the power to average HR (and vice versa). With proper training, power output will be increased at a given HR. This is critical with cycling performance as tracking speed at a given HR is highly variable (due to temperature, course variations, wind, etc.). On a related note, training by power allows you to record your efforts. For example, you may race on a particular course and note the average power output for that course. At a later time you race the exact same course and have a different finish time. Did you race better or worse? By evaluating the power data, you can decide how you progressed. HR data alone will not tell you this. With power data you can evaluate your strengths and weaknesses.

Explosive Power and its Benefit to the Endurance Athlete

There is recorded data from athletes where you can compare your power output for a given length of time (i.e. 5 second, 5 minute, 20 minute, etc.) to average power output in a given category of cyclists (i.e. Cat 1/2, Cat 3, etc.) It may seem odd for a triathlete to compare power out at any length of time other than one hour or beyond to other athletes, but it is becoming more evident that sprinting ability is actually quite important for triathletes. A triathlete will likely never sprint in a triathlon; however, the concept of power reserve is very important for a triathlete, as power reserve has been shown to be a better predictor of performance than VO2max in time trial performance (7). One of the deficits I frequently encounter with triathletes is the ability to "hammer" the pedal. Power-based training allows one to track the progression of peak power for different time intervals. An analogy of how increasing power reserve can help a triathlete's time trial performance may help. Two athletes are performing the squat exercise with weights. One athlete has a ten-repetition max of 480 pounds and another athlete has a ten-repetition maximum of 280 pounds. Both athletes can squat 250 pounds 20 repetitions; however, one can speculate that squatting 250 pounds 20 repetitions will be more stressful for the athlete with the one repetition max of 280 pounds compared to the athlete that has a one-repetition maximum of 480 pounds. This can be generalized to bike performance. If two athletes have an identical threshold power output of 315 Watts, but one athlete has a peak 30'' power output of 500 Watts and the other athlete has a peak 30'' power output of 600 Watts, one can speculate the athlete with the greater 30'' peak power output will be less "stressed" while racing at threshold (assuming the same training volume/paradigm).

The general training paradigm for coaching and training endurance athletes has been to focus on volume and maintain training intensity below threshold (anaerobic, ventilatory, or lactate threshold depending on the coaches or athlete's nomenclature). However, research is beginning to show that this may not be the optimal training paradigm (2,7) and that adding explosive strength training and sprinting to the training paradigm can yield better endurance performance, perhaps via increased efficiency and increased threshold power (5), and I would also speculate that this is also due to improved power reserve. A relatively recent research article has found that replacing a portion of endurance training with explosive style weight training (while maintaining overall training volume) can lead to improved time trial performance (7.9% improvement in the explosive training group vs 5.9% in the endurance-only training group). Furthermore, the time trial performance was significantly greater at four weeks compared to baseline in the explosive training group, but not in the endurance-only training group. Finally, explosive training led to a significant improvement in peak power compared to the endurance only group (7). One cannot monitor and track sprint training by HR alone; however, using a power meter can greatly assist sprint training. Finally, a major benefit of training by power vs HR occurs when an athlete is taking certain medications. There are a number of medications (i.e. beta blockers), which make training by HR impossible and training via a power meter is the viable alternative.

Benefits of Training by HR

The benefit of HR training includes the ability to factor what is taking place in the athlete's whole-body. Two examples will help illustrate this case:

Example 1: Let’s say an athlete normally relies on power for pacing training intervals and the athlete based the power intervals from a testing performance on an ideal day where the temperature was 70-degrees and humidity was 40% and the athlete felt spectacular. However on a different training day it is 95 degrees with 90% humidity. It will be very difficult for the athlete to be able to maintain the same performance on this hot and humid day compared to the ideal conditions day. HR monitoring will help take into account the training performance for the different training days based on the athlete's condition and the environmental conditions.

Example 2: There is increased VO2 at a constant work rate combined with a decreased VO2 max that occurs with an endurance training session (4,9). Without going into the details of exercise physiology, this means relative workload increases with prolonged endurance performance, even when the absolute workload (Watts) remains constant. An example will help illustrate this. A common method of determining functional threshold power (FTP) for power-based training is the 20-minute maximum effort test. The basic protocol for this test includes a 20-minute all-out boutand the average 20-minute power output is multiplied by .93, .95 or .97 and the derived number is considered FTP. The FTP is then used to develop training zones (1). However, there is a dilemma for ultra-endurance athletes. Is the number, which is derived by a sub-hour test valid 4 hours into a ride? It is highly unlikely that an athlete can produce the same twenty-minute max effort power four hours into a training session as the athlete can with non-fatigued legs. A recent study illustrated this (4) where trained cyclists pedaled at a fixed power output for 2.5 hours and then performed a 5-minute max effort test, which was compared to a baseline five-minute effort. There was a decrease in five-minute power output following the 2.5-hour ride. There was also an increase in oxygen consumption and RPE over time, suggesting a relative increased work rate (the athlete was working harder from an aerobic standpoint later in the ride compared to earlier in the ride). Monitoring training and pacing via HR allows the athlete to adjust the exercise intensity to maintain the same relative intensity. Furthermore, repeated threshold testing is not required when training via HR zones as HR at lactate threshold remains stable throughout the racing season (6), unlike power-based and pace-based training which require frequent testing in order to adjust power and pace zones to account for fitness changes.

Calculating Training Stress by Power, Pace. vs HR

There is a method of calculating stress based on HR known as TRIMP, which takes into account maximum HR, resting HR, training HR, and training/racing time (3). I have modified the calculation of TRIMPS to compare to a 1 hr performance at threshold pace, thus the numbers derived by the modified TRIMPS score can be compared to the commonly used training stress score (1), which can be easily calculated using WKO + software. A major benefit of training by HR is that heart rate is an indicator of cardiac stress (5). There are a variety of conditions (i.e. hot environment, altitude, transition run, etc.) where HR may be elevated due to increased stress. If one is training by power alone; maintaining a given power output that would normally lead to positive training adaptations in a mild environment, may lead to overtraining in a hot environment (5).

The number of athletes using software such as Training Peaks WKO+ is increasing. The software uses power and run pace to calculate training stress for cycling and running, respectively. Under perfect conditions, this is an easy and valid method to keep track of training stress. The benefits of WKO + and tracking training stress by WKO+ are numerous. Under "normal conditions" the training stress score developed by WKO+ and the score developed via the scaled TRIMPS score are very similar. However, problems arise, particularly for tracking running stress under less-than-optimal or unique situations (i.e. transition run, heat, recovery from illness, hills, etc.). The run training stress score calculated by software such as WKO + involves comparing the training session pace to the athlete's threshold run pace. If an athlete is running a hilly course, into the wind, with the wind, is training under extreme environmental conditions, is sick, recovering from illness, etc., the formula of tracking training stress via run pace may no longer be valid.

Let’s look at a couple examples of this.
Example 1: I recently had an athlete perform a run workout following a week of illness. According to the training stress score based on pace, the workout yielded a mild training stress score (based on WKO + software) of 88. However, at the given pace, the HR of the athlete was exceptionally high (likely due to blood plasma volume loss that occurred during the illness) and the HR training stress score was 144. Using the HR value for monitoring training stress, I was able to take into account whole-body stress (5) and realize the workout was not a mild workout, but a rather strenuous one (the equivalent of running roughly 1.5 10K races). The subsequent week's workouts were then modified.

Example 2: Another example of where calculating run stress by pace may not be valid is the transition run. I recently had an athlete perform a 25-minute transition run following a 2.5 hour bike. Calculating run training stress via pace yielded a value of 13.2, which is a very low stress score; however, using HR (which takes account overall body stress: cumulative fatigue from previous training days and the current bike ride, environmental conditions, blood volume loss due to dehydration, etc.) to develop the training score yielded a run training score of 30.4. This is a 230% difference. Though the actual numerical value is low in terms of a training stress score, the difference will be additive when evaluated over several days, weeks, and months when tracking cumulative training stress.

Benefits of RPE

RPE has been used since before the advent of heart rate monitors and power meters. Granted training by RPE could be viewed as less specific as training by HR and power. Nonetheless, RPE should be used under certain circumstances. For example, during the early stages of an Ironman race, HR may not feel in-line with the effort due to the excitement experienced by the athlete. In this instance, RPE may be a valid method of pacing. For a number of athletes that are taking medications that affect HR, training by heart rate is not possible. For these athletes that are taking these medications and do not have a power meter, training by RPE is the only alternative for monitoring training. Furthermore, for the runners taking certain medications, RPE is a useful alternative when conditions are present that will affect pace (i.e. hills, wind, environmental extremes, etc).
Benefits of Training by a Combination of HR, Power, and RPE

I propose a method of combining power based training, HR training, and rating of perceived exertion. There are many coaches out there that propose power-based training as the "holy grail," however this goes against exercise physiology principles on many fronts. Power based training is truly valuable when combined with HR and RPE training. For short intervals power based training insures the athlete is achieving the proper zones. Furthermore, progress can be assessed by evaluating how long an athlete is maintaining a given power output (i.e. perform repeat sessions at various power output to determine critical power and evaluate fitness improvement) and comparing power output to HR (proper training leads to increased power output at a given HR).

Heart rate training allows one to take into account external and internal training variables. Heart rate training also allows one to take into account cardiac drift, which power based training lacks. There are many times when HR and power-based training do not adequately fit the training and racing situation and the athlete needs to go by feel (rating of perceived exertion). This can be addressed in terms of training stress, but is not as precise as gauging stress by TRIMPS or TSS. However this does not discredit the validity of perceived exertion.
Conclusion

A valid training program should not be based on training by power or heart rate exclusively, but should take into account all three: power, heart rate, and perceived exertion. It is acknowledged that not every athlete has a power device at his or her disposal. Cyclists have been training for over a century without power training devices. What is more important is that one takes full utilization of their training devices, whether it is a power meter or HR monitor. Simply looking down at their power meter or HR monitor will not lead to fitness improvement.

Coach Brett can be reached at coachbrett@petersenperformancelab.net
www.petersenperformancelab.net

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