Input vs output vs intensity

One thing that has been quite variable throughout sport has been the definition and interpretation of intensity. There are a few scientifically defined  intensities, such as lactate threshold, and a few accepted scales, such as Borg scale of perceived exertion. There are numerous devices now available to measure heart rate, blood lactate, speed, power, cadence and other outputs for the exercising athlete. A major issue over the years has been how to communicate the “same” meaning of intensity over a range of variables. One persons easy is not the same as another’s, and the varying definitions for degrees of hard are also numerous.
A number of years ago, John Hellemans brought this issue up at a coaching seminar, and promptly recommended everyone use the definitions for intensity that he was using, so that when athletes did change coaches, that there was a semi-standard intensity definition. A few years later, he produced “the training intensity handbook”, which tries to align the various intensities used (a good little manual). I certainly agreed with the philosophy of aligning the definitions of intensity, but I had been using my version of intensity scale, which I had developed  from various sources over the years, and to change what I was using was not going to be easy. I am sure many other coaches were in the same boat! Most training systems use 5 levels of intensity – and most are similar, just slight variations and different terms. It is important to understand what the various levels mean in a practical sense.   
There has also been a number of trends and perhaps fads over the years on measuring intensity for exercise. Many years ago perceived effort was it. If you asked a cyclist what there training metrics were for a session, it would be “a few hours pretty easy” or “quite long with heaps of hills that were hard”. They left after breakfast and got back sometime after most others had had lunch, and that was all they knew – as well as how tired they were after. Now you can have exact splits, GPS data, HR data, power data, normalized power ISS, TSS, surges, climbing data, and any number of other metrics, without even having to go to the lab!  
I think some people are after a single metric that tells them how hard they have gone. The reality is that the human body is a very complex machine, and no single metric can tell you this. Many of the available metrics have their own strengths and weaknesses, and only tell part of the story. You need to look at a few pieces of data to start to construct a picture of what is going on. In some circumstances, some metrics are more appropriate than others. For example, in my opinion, speed is a good metric swimming and running for the harder sessions, and power for the bike is the equivalent. HR and to a  certain extent how that interacts with speed/power is more appropriate with easier sessions.  Perceived effort needs to be an important attribute at all levels, so that you do get a feel for what is going on in your body. Also if the toys fail – you need to be able to fall back on the basics!
Adding to the complexity, and perhaps showing that a single metric is not the solution, is that if you do use a speed or power and a HR (or lactate or other physiological measure), the training zones often do not line up when you look at the data after a training session (assuming you have set your training zones correctly). Most good training software will tell you the time is each training zone for a particular training session (or as a weekly/monthly breakdowns). And seldom do the profiles look the same, unless it is a very easy session or very hard one. My feelings are that the way the actual output (speed/power and form) varies with physiological output (HR, lactate levels, breathing etc) is a very important indication of where you are at. The absolute ratio and the trends through a session or between sessions shows how tired you are (or recovered), how your current endurance is, and how you are adapting to the training. Some also feel that the trends are an indication of a need to change training phases (or ability to move to the next phase).

this ride might indicate fatigue - as the HR profile is further to the right of the power pofile

So what does it all mean? When you go out the door to do a training session, you should have an idea of what you want to achieve in that session. Intensity is an important component of that, and how you are going to measure or assess that you are at the appropriate intensity. If you have a coach – you also need to communicate intensities – both in prescribing a session and reviewing sessions.  Some people have an extremely good perception of effort – either naturally or through years of experience. A good swimmer should be able to swim 100m at a variety of paces, and be able to tell you what time they have just swum, within a few seconds (or better). Some need feedback on effort to give guidance on effort – and HR has been a common tool, and pace or power is the other standard. Physiological measures give no indication of how fast you are actually going or your real performance. They given an indication of the stress the body is under. If you are very fatigued, you may be stressed at a relatively slow speed. Outside stress, apart from exercise intensity also affect these measures (altitude, heat, general stress etc). I also see that a number of athletes that have used HR as their base measure of intensity can tend to make an effort harder by subtle means that doe not increase pace. If you make the body tense, or more inefficient (eg bad technique) , you can get the HR up higher without having to go faster! Not quite what the performance athlete is after though. This is where pace and power come into play. If you are doing a key session with target pace/times, and you cannot make therm – why? Are you too tired and should just go home and try to recover, are the targets too hard, or has your technique gone and you are just very inefficient on this day? HR (and other physiological measures) also has a delay – so you put the effort in and the HR response maybe seconds or even minutes (to equilibrium at least) behind the effort. By the time you have gone over the limit, particularly for shorter efforts, it is often too late to adjust.
With any measuring device – you need to be aware of the accuracy of data, and any averaging that is going on. You do not want to put a lot of faith into inaccurate data, and if there is significant averaging – you need to consider how that affects your effort and response to the information. Not all data is equal. Two exercises with the same average may be very different in the detail. If you compare two runs that both average 4min/km, one is a steady 4min/km all the way, and the other is alternating 1min at 5min/km and 1min at 3min/km. The averages maybe the same – but one is definitely more difficult and more stressful than the other!
Don’t be blind to a single number – look at the bigger picture, and gather all the information that is useful to you. For some, this maybe disregarding information that is not useful. Food for thought hopefully…