News

  • Why does cycling power increase for a given heart rate on hills?

    In yesterday’s blog post I tried to explain that a cycling time trial on an uphill course will produce more power output for the same average heart rate than the same duration of time trial on a flat course. I used a running analogy, but that specific analogy wasn’t a good one. Let me try again.

    Human power

    The engine in your car does not physically change when you drive. It is a fixed mechanical system that is powering a fixed mechanical chassis. There are certainly changes with aging, but from one week to the next or from a hilly course to a flat course your car is for the most part mechanically stable.

    Some people like to think of cyclists as similar to a car. We are not.

    The bicycle is, for the most part, mechanically stable from one week to the next. Similar to a car, it does change with age over time (there is wear of components that eventually affects performance). The engine powering the bike – you – is not a set mechanical system.

    For example, your body position on the bike affects power output and speed. It is well known that an aerodynamic position during a flat time trial can increase speed, while decreasing power demands.

    You change

    Because you are a human engine, you are dynamic and changing all the time. Your body chemistry, hydration status and glycogen stores are just a few examples of things that are in constant change.  In addition to your internal system status or physiology changing, you change your position on the bike while you ride. You move forward or back on the saddle and that changes the angles of your body in relation to applying force on the pedals.

    Hill climbing recruits more muscles and effects technique

    One study1 looked at three grades affecting efficiency and technique. When comparing riding at zero-, four- and eight-percent grades, the study found that , “The onset of muscle activity for vastus lateralis, vastus medialis, gastrocnemius lateralis, and gastrocnemius medialis occurred earlier with increasing gradient.” Additionally the study found that riding hills increased the overall muscle activity levels.  

    Making some assumptions here, let’s say you can average 200 watts riding on a flat time trial course. If you are using more muscles to produce 200 watts on a hill, I believe heart rate load would decrease. This means if you only look at watts, heart rate should be less.

    To look at an extreme, let’s say you use one leg to produce 200 watts and average a certain heart rate. If you pedal with two legs and aim to produce 200 watts, don’t you think the heart rate or biological cost would decrease?

    Wouldn’t it be best to aim for a constant power output on a time trial?

    If you are aiming for the one-hour record on in a velodrome, perhaps having a constant power output is a good strategy or perhaps aiming for a negative-split power output is a better strategy. But, one study2 found that for rolling course, a variable power strategy improved performance.

    The study used 20 experienced cyclists to complete four,  4k time trials over an undulating course. For two of the time trials, they attempted to produce a constant power output of 253 watts. For the second two time trials, they varied power in response to terrain – producing more power on the climbs and less on the downhill sections.

    The variable power strategy improved course performance by nearly three-percent, which is significant. The participants noted that attempting to apply constant power was really difficult and it ended up reducing average speed in the end.

    A second study3 did find the same result, that variable power outputs result in faster times, and it also noted that the amount of time saved is highly course- and athlete-dependent. This study found that cyclists that weighed the most benefit the most by increasing work rates on uphill sections and reducing work rates elsewhere.

    Applications to training

    If you are using, or plan to use hills in training and you use a power meter as one of your measurement tools, expect power outputs on climbs to be higher than on the flats - relative to heart rate. If the only power number you have is a flat time trial, expect to calibrate that number when you do intervals on a hill.

    How much to increase your power output goal during the interval session depends on the hill grade and the length of your interval.

    Unless you’re doing intervals on a constant gradient hill, even an uphill time trial that determines functional threshold power will need to be adjusted if you’re doing power repeats on a hill with a different grade.

    Repeating yesterday’s recommendation, use a combination of power, heart rate and perceived exertion to be sure you are getting the training response you seek when doing triathlon, cycling or mountain bike training.

    **Disclaimer. I must tell you that I am using the current scientific research and the results I see from my athletes, and my own, power files to draw these conclusions. I believe power is an excellent training tool, but it cannot be used without references to other items such as course profile, heart rate, wind, heat, and specific rider to name a few.

    References

    1.       Arkesteijn, M., et al, “Effect of gradient on cycling gross efficiency and technique,” Medicine and Science in Sports and Exercise, 013 May;45(5):920-6. doi: 10.1249/MSS.0b013e31827d1bdb.

    2.       Cangley, P., et al, “The effect of variable gradients on pacing in cycling time-trials,” International Journal of Sports Medicine, 2011 Feb;32(2):132-6. doi: 10.1055/s-0030-1268440. Epub 2010 Dec 16.

    3.       Boswell, G.P., “Power variation strategies for cycling time trials: a differential equation model,” Journal of Sports Sciences, 2012;30(7):651-9. doi: 10.1080/02640414.2012.654397. Epub 2012 Feb 20.

  • ← Next Post Previous Post →

  • Leave a comment

Home / News / Why does cycling power increase for a given heart rate on hills?