Has Power Killed Heart Rate in Cycling?

Power Meters on bicycles first appeared in the 1990s but the technology only really became accessible to serious recreational cyclists around twenty years later. Now, when cyclists compare performance they very rarely quote heart rate. It has become all about a rider’s ‘power profile’. Their sustainable power across different time frames from one minute to an hour. The two most significant determinants of performance or fitness and form are the five minute power (VO2) and the one hour power (Functional Threshold Power or FTP).

Cyclists with a plan will use these numbers to determine the intensities of their training sessions in much the same way as their counterparts, or for many, the way they themselves used heart rate before the turn of the century. Many cyclists have switched completely and don’t use HR at all, relying completely on power as a guide and analytical tool for their training and racing. So, like TV did to radio, has power killed heart rate?

There are some obvious advantages to using a power meter in training. The most significant is the instantaneous feedback. When we increase our intensity at the start of an interval, our power reflects that effort right from the moment we stamp harder on the pedals. HR has a lag. Our hearts respond more gradually to an increase in effort. A response which results from the need for the body to increase the amount of oxygen transported to the muscles via our bloodstreams. This need is not immediate, no matter how much harder we go. The increase in oxygen supply is required because it is essential for energy metabolisation. The more energy our effort requires, the more oxygen is needed. The body can however power the muscles for a short time without oxygen and this built in delay allow us to suddenly increase effort without our hearts immediately bouncing out of our chests.

So, starting an interval or longer effort with a desired HR range as a goal, we have to take into account that our HR will gradually rise, only reaching our target range after a while, even though the effort we are putting out is higher than what our HR monitor indicates. So, we have to know how it feels to run at our goal HR range, in order to start our intervals at the correct pace and effort. In all interval training, starting harder than required and then backing off to stay within a certain range, is definitely not the ideal scenario. The shorter the intervals in our set, the more things lean in favour of the power meter as a means of controlling output.

In modern times our heart rate monitors definitely still have a role however, and the most significant is that of assessing training and racing performance. Comparing HR and power outcomes after the fact will help determine our level of improvement, recovery and freshness/fatigue.

Comparing HR and power in a set of intervals, a test or even a race will help us establish if we, performed to our potential, if we have improved or, if we are training too much without enough recovery and building up too much fatigue. Simply put, if our muscles are fatigued they will not be able to push up our HR. If we are unfit, our cardiovascular efficiency is low and our HR will rise rapidly without significant power being put through our pedals.

Same power / lower HR = improvement.

Higher power / same HR = improvement.

Same power / higher HR = we need to reconsider our training because we are putting in more physical effort for no improvement in mechanical results.

During interval training, the power meter is the undisputed king because of it’s instantaneous readings but, for more steady-state, endurance type efforts the heart rate monitor still has a big role to play. Currently, Zone Two training is all the rage in the cycling World. This is a level of effort which is sustainable for long periods of time but is harder than the effort that we would classify as a recovery session. The responsiveness of power meters can make them quite frustrating to use to control this sort of training, anywhere other than on the indoor trainer, and the reality is that it is what our hearts are doing at this intensity level that produces the outcomes that we are looking for. Zone Two training has our hearts beating at a level which allows aerobic energy to fuel our muscles’ work. It is a level where the body is meeting the oxygen needs of the muscles and has time to utilise a high percentage of fats in our fuel mix. Easing off the pedals for an intersection or powering over a small incline to maintain momentum will move us out of Zone Two power but, as long as these intervals are relatively short, our HR will not be affected much and maintain a fairly constant rhythm throughout.

Yes mechanical power is sent through our chains from our pedals to our wheels, which results in the speed that we are travelling forwards but we are not mechanical engines. A power meter shows us the results of our effort but a heart rate monitor monitors the organism that produces those results. It definitely has a role to play and leaving it in the drawer with the rest of our unused gadgets is missing out on an important piece of the training puzzle.