Just about every rider you see in the bunch nowadays has a swanky heart rate monitor with altitude, cadence, heart rate and speed that can be downloaded and produces a lovely graph with lines and curves to entertain you for hours. However do you understand this? And can you apply or interpret it enough for training? Let's have a look at heart rate so those lovely colorful spectacles actually have a purpose.
Heart rate monitors are used in endurance sports such as cycling to determine exercise intensity. They tell us to train harder, train slower or help you are about to blow! Heart rate simply is the rate at which your heart beats, or the rate at which is pumps out blood from the left ventricle of the heart and into the body. When we exercise we need to pump a certain amount of blood to our muscles to deliver oxygen each minute (cardiac output). This is determined by how much blood is pumped out per beat (stroke volume) and how often it beats (heart rate). To increase exercise intensity to deliver more oxygen we either increase our stroke volume or increase our heart rate. Both happen in concert with each other but anatomically our hearts are only so big, so stroke volume can only increase to the maximum size of our left ventricles. Heart rate then continues to increase after stroke volume has reached its maximum, allowing continuous increases in intensity until heart rate is maximized as well.
To make improvements in training we need to constantly increase the stimulus for adaptation – ie. Make training harder. And to do this we can through volume, intensity or frequency of training. The human body is extremely adaptable as shown in the figure. Windtrainer session one and two where conducted 2 days apart – the exact same trainer session. However in session two we can see heart rate is much lower, decreases a lot quicker after an effort and doesn't go as high - all for the same workload. This is why doing the same thing day in day out doesn't improve your performance. That is how adaptable the body is. Likewise with detraining/lack of training, plasma volume reductions are one of the first changes, decreasing stroke volume and therefore increasing heart rate if you were to look at a given cycling workload.
Because of the large effect plasma volume and therefore stroke volume has on heart rate a phenomenon called cardiac drift is essential to look out for. Cardiac drift is an increase in heart rate due to a change in plasma volume. Sweating reduces plasma volume, so heart rate increases to maintain the same cardiac output making the use of heart rate zones ineffectual. So intensity is changing – but not really. It's giving an inaccurate picture of how hard you are working so on long rides and/or under warm conditions it is advisable not to use heart rate to precisely/pedantically as a gauge of intensity in the later stages of your ride. And you can see this in your graphs of a long easy ride in the summer. With heat acclimation this can change as you become accustomed to it, but only to a certain extent – depending on sweat rate, how hot it is and how long you are exercising for. Under extremely warm conditions you can see heart rate rise 10+ beats per ten minutes for the same workload.
Heart rate zones are used to gauge intensity, and therefore used to induce specific adaptations to change performance. Percentages of measured maximum heart rate or age predicted maximum heart rate work extremely well to help endurance athletes who do not have a big training history, however the more trained you are the more difficult it is to induce changes using percentage zones because as you get fitter your maximum heart rate doesn't change but you do. So you are working at a lower rate for a given percentage. That's where sports science testing or changing to a power meter is the next step to help maintain specificity and keep you improving.
Our heart rate is controlled by our sympathetic and parasympathetic nervous systems. So when we are a wee bit nervous on the start line or excited in an event or race our heart rates are all over the show. Don't be nervous if your heart rate is well above normal training heart rates. Using the software packages available now you can see changes like this and monitor responses to workloads, rides or predict fatigue. The Australian Institute of Sport use resting heart rate to monitor training induced fatigue. A resting heart rate of 6 to 12 beats high is indicative of some fatigue in the system, and if its over 12 beats higher then take the day off.
The fitter you are the more quickly your heart rate accelerates to a given effort and this is easily seen when you overlay graphs of similar sessions. When buying a heart rate monitor an important function to look at is sampling frequency. Most swanky ones that make your heart rate go up looking at the price have ranges from 5 to 120seconds. So it is taking one sample every 5 seconds at a minimum. The lower range ones tend to sample less frequently often giving an inaccurate picture of average and even maximum heart rate. A lot can happen in 30seconds, let alone 120seconds, which is why power meters are far superior in this aspect. You can have a lot of fun with your heart rate monitor. Some of them seem quite complicated when in fact they aren't. Nonetheless it is worth taking the time to learn them inside out so you can understand how your body is reacting to training and racing and continue to climb that incline of improvement.
Amy Taylor is an Auckland based Exercise Physiologist with a MSc(Hons) in Sports Science. One of the founders of Kinetic Edge Training Technology (www.kecycling.com) she coaches recreational to World Champion cyclists and has been cycling herself for 15 years and can be contacted on firstname.lastname@example.org
This article orginally appeared in Endurance Sport Magazine.