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Max VO2 intervals: Timing the length?

Fri, Apr  30, 2010 - By Bryan Fish

Thanks to SkiPost for letting us use this article.

Q: In doing maximum intensity intervals of say 3 minutes I start with a heart rate of around 100 or 120 and struggle mightily to bring my heart rate up to 175 or 180 in just 3 minutes, so that only a fraction of the 3 minutes is actually spent at goal (my observed max is 186). Naturally after I'm at the third or fourth interval its a bit easier but still only a part of what I'm counting as the 3 minutes is spent near maximum heart rate. Is that correct or should one be spending the whole 3 minutes at goal and not even count the time leading up to it?

A: This is a very common question.
I think the first step is to look at the overall goal of VO2 max intervals.  I am going to call upon Dr Sue Dobson's (former head exercise physiologist for the US Ski Team) explanation and general assessment of why we do VO2 max intervals.  She states that the aim is to stimulate a steady state at a maximal load.  The idea is to maximize time at the appropriate load.  Max stroke volume is the key to max VO2.  Max stroke volume cannot be challenged at less than max load, so time must be spent at maximum to improve max stroke volume and max VO2.

It is important to look at the overall perspective of what is happening to the body when we do such intervals.  Technical and neuromuscular adaptations happen simultaneously as well.  Keep a strong sense of perceived effort to assist with improving pace and technical form.  Monitoring your heart rate can be an asset or a liability depending on how you use this information.  It is critical to focus attention on your pace and perceived exertion and utilize the heart rate monitor to provide valuable feedback.  The heart rate monitor should not dictate your training however.

It generally takes the heart rate about 1.5 to 3 minutes to stabilize.  It is possible to build up to a max heart rate quicker, but excessively high lactate accumulation will occur.  This is counter-productive.  Therefore the general length for max VO2 intervals is between 3 and 6 minutes in length.  This develops a balance between allowing the body to ramp up to achieve maximum stroke volume and not too long that you need to slow the pace to lower than a max stroke volume pace. 

This heart rate lag is also witnessed when training effort is decreased after the interval.  The ramp-up of the heart rate during intervals is individual.  The ramp-down of the heart rate after exercise is dependent on the individual and the individual's present fitness.  A high level of fitness will allow for a more rapid recovery rate.  Other variables can affect this - for example altitude and hydration.  Altitude, for example, will require more recovery time between intervals.

I encourage you to keep an eye on pace and effort of each interval.  The goal is to maintain the same distance each interval or possibly go further each interval. Focus on strategies that will improve your pace, yet maintain the same intensity (heart rate or lactate).  For example, modify technique or cadence.  Your heart in combination with the pace can provide valuable feedback as to whether the modifications improved your performance.  Also watch your heart rate recovery rate between intervals.  Are you fully recovering?  Remember there are numerous reasons why this is changing.  A decrease in recovery rate may mean an improvement in performance or the simple fact that less lactate was produced in the last interval.  Less lactate could mean better biomechanics or more consistent pacing.  An increase in heart rate may display inconsistent pacing or dehydration to name just a couple of possibilities.

Remember that heart rate monitoring is a means to monitor, but certainly not the "ends."  There are numerous debates questioning whether muscle fatigue dictates heart rate or heart rate dictates muscle fatigue.  The debates between the traditional studies by AV Hill on monitoring VO2 versus the more contemporary studies by Dr Tim Noakes on the Central Governor Theory (brain and nervous system) are on-going.  The reality is that both compliment one another and ONLY looking at heart rate for your training will likely stifle performance more than enhance it.  However, using heart rate as a tool and keeping heart rate in context to the numerous parameters that can affect performance can enhance performance.

In short, do intervals that are 3-6 minutes in duration with complete recovery.  The pace should be such that your first interval is as fast as your last. 
Bryan Fish, CXC Team Vertical Limit Head Coach

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