This page contains information on how to track heart-rate data.
If you don't have a GPS logger that records your heart-rate at each point, you can still record average and maximum heartrates against logs manually (i.e. based on a non-logging heart rate monitor strap.)
To switch heart rates on manually:
Once you have heart rates recorded against log files (either extracted automatically or entered manually) they will show up in various summary and comparison tables and the graphs for routes and tags.
You can view a heart rate histogram and heart rate vs time graph on the log detail page if your logger supports it.
If your logger records heart rates, or you key in averages as described above, you can view graphs of your heart rate on each date for each route and tag.