By: Heather Nielson
It seems like the science and technology around training for just about any athletic pursuit has seen a pretty steep upward curve in the last decade or so with heart rate monitors, power meters, nutrition, recovery, oxygen, economy, technique and more.
Power based training for cyclists came after heart rate based training and has turned out to be an arguably far more useful training tool for many reasons. The range of power numbers within any given training zone is far wider and more variable than any heart rate zone (0-2000 watts or so as opposed to 50-200 heart beats). Exerting power into the pedals and the resulting physiological response and neuromuscular recruitment produces a wattage number almost instantaneously whereas heart rate is almost always lagged. For all the above reasons and more, many competitive cyclists prefer training by power rather than by heart rate. This isn’t to say that heart rate isn’t valuable at all. In fact, one of the main measurements of overall fitness is a cyclists’ FTP, or functional threshold power, which is by definition, very closely tied to heart rate.
So what is the relationship between heart rate and power and why pay attention to heart rate at all anymore?
In a former life, I was a scientist and have an education that is based on biochemistry and nutrition. Without getting too scientific, Functional Threshold Power is the power number, or range, that is associated with an individual’s lactate threshold; which is essentially the point at which lactate begins to accumulate in the blood stream. How and why does that happen? You breathe in oxygen, oxygen is what enables your body to create energy from ATP, ATP comes from your food in the form of carbohydrates, lipids and amino acids. Obviously, your body is doing this all the time. However, when you exert yourself, the ‘system’ is churning much faster than at rest and if go as hard as you can until your energy supply to meet the demands don’t match up, then ‘bottlenecks’ are formed in the metabolic pathway and metabolites build up along the way. Think of a water fountain where one bowl empties into the next; but if the water, or energy requirement, flows quicker than the bowl is able to ‘absorb’ or handle, then the water spills over. Some of the excess water, or metabolites cause you pain, make it so the individual muscle fibers can’t contract anymore, create an electrolyte imbalance, create an osmotic imbalance and any other number of side effects that basically cause you to want to STOP. Your heart rate and stroke volume then are one of the main limiters in how quickly your metabolism can churn.
So if we can’t make our hearts beat 400 beats a minute, how do we get fitter? Fitness is really a question of efficiency. You may have an FTP of 250 at say a heart rate of 175, but how long can you hold that….really? There are many definitions of fitness and one’s ability to hold a maximum effort at or near FTP for longer and longer periods of time is certainly one definition. As you get fitter and stronger then your FTP will probably raise also &/or you will get better at holding that FTP for longer at roughly the same heart rate. In other words: a fairly unfit cyclist may have an FTP of 200 at a heart rate of 175 while a pro tour rider may have an FTP of 350 at the same heart rate. Concurrently, you may also find that you can go harder at a lower heart rate as you get fitter because your system has become that much more efficient. How does your body become more efficient? On the biochemical level, over time, your metabolic system essentially becomes more and more efficient at dealing with higher physical demands by using or creating more mitochondria to deal with more oxygen loads, becoming more efficient at clearing the excess metabolites, becoming more efficient at burning sugar over lipids, or the other way around, and many many other pathways.
Your ability to hold that kind of effort for longer and longer periods of time is one way to measure fitness and training that takes a long time, not just one season or a few one hour bike rides. Consistency is your biggest ally. Realize also, that just because your FTP is 250 one day, does not mean it’s 250 the next! Your threshold is variable depending on your recovery, sickness, motivation, cross training, training program, genetics etc. If you’ve had your FTP tested then you’ve probably been given a power range of 10-20 watts around that number; which is the best way to look at it.It’s just one number and there are many thresholds and many other ways to measure fitness. That being said, your overall level of fitness, efficiency, training level and health is most closely tied to your FTP than any other power number.