In this issue of training principles, 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.

Please send questions and comments to Coach Brett via e-mail at coachbrett@petersenperformancelab.net

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