Within the context of cycling, a powermeter measures the amount of work being done on the bike, while a heartrate monitor measures the impact that the work has on our physiology. Both forms of data are important in maximizing our performance; but because bicycle powermeters are still relatively expensive, the following examples will be keyed to heartrate.
The graph above tracks heartrate during a two hour steady-paced spinning class. The session took place in a room that was poorly ventilated and poorly cooled; as the class progressed, the heat generated by 24 participants raised the room's ambient temperature by almost ten degrees. While every attempt was made to keep the perceived effort the same throughout the entire session, the rise in room temperature resulted in a gradually increasing heartrate. Take away: adequate cooling is critical for optimal athletic performance.
Stochastic Variables and Outdoor Training
In the example above, note the significant variations in heartrate for the two hour session. The intent for this session was to ride at a steady aerobic pace for two hours. Due to the ride's terrain and a gusty wind, it was impossible to ride without spikes in heartrate. As a result, what should have been a simple zone three ride actually became a ride with a series of unstructured intervals: a significant portion of this ride was completed in zone four and five. Because this ride was much more intense than planned, the following day's session was not effective due to the fatigue generated by the prior session.
The term "stochastic" means "random." The randomness of the hills and wind associated with the session above outweighed the session's planned intent. Perhaps the easiest way to limit environmental variables is to ride an indoor trainer or ergometer, which completely eliminates the influence of wind and hills.
The plan for the session above was to complete three intervals at AT threshold. Note the clearly defined curves in the heartrate graph that marks warm-up, interval, recovery, and warm-down efforts. By performing this session on an ergometer, effort and intensity could be carefully controlled to yield maximum benefit for the athlete.