How long should I train? How hard should I train? How often should I train and what will the consequences of my training be in terms of successful performance gains or inevitable signs of fatigue. These are important questions an athlete must ask themselves during every phase of training and planning. The summation of these questions is: what training load am I placing on my body, and how much do I need for optimal development? Training load simply stated is the cumulative amount of stress placed on an individual from a single or multiple training sessions over a given period of time; and is represented as a product of two factors: 1) training duration and 2) training intensity. Perhaps, the most widely cited model to address training load in a practical sense is Eric Banisters training impulse or TRIMP model (1991). The model applies training intensity and training duration metrics to assess training load in terms of both performance and fatigue. It is evident that both duration and intensity are relevant and should be monitored. I would recommend a review by Shona Halson (2014), for more insight into all the possible metrics, and a series of papers by Peter Hofmann on the relevant effect of varying both intensity and or duration on training load (2017, 2022). Duration and intensity are both measurable parameters. While by no means, less relevant and often under evaluated (see the 2022 paper by Birnbaumer, Hofmann et al.), duration is generally a much more straightforward measure. Deciding the number of minutes or hours to train can of course be challenging, but the metrics are simple – seconds, minutes, hours. Intensity on the other hand is not as straight forward. It is self-evident that intensity effects performance and fatigue. Simple evaluation of a power or speed duration curve illustrates the non-linearity of the relationship, and thereby the effect of both intensity and duration on performance and fatigue. But how should you measure intensity? A general split can be made using either internal or external metrics. External meaning physical output, for example power or speed. Internal meaning physiological work or input to generate a physical output, for example heart rate or ventilation, or as per subject of this article muscle oxygenation (SmO2). External metrics are objective and extremely useful. They are the measurement that defines success. To clarify, the winner of a running race is the athlete who can run the fastest for the duration of the event – cover the distance in the shortest amount of time. In other words, speed is the defining performance metric. However, the problem with external metrics is that the human body does not function like a machine, and performance can vary significantly from day-to-day, or between environments. This is true for competition, but also for daily training. If I define my training intensity solely on external metrics, for example watts using my power meter, it will function irrespective of my individual physiological state today. My 200-watt intensity ride will always demand 200 watts, but my internal load may vary significantly when attempting to generate the same 200 watts. Coupling this very important external load metric with an individual internal load metric can help us understand our performance (the external load) and potential fatigue. An internal load, understood in context to external load, will optimise intensity control and thereby training load.
