I just got done reading a great article in Training Peaks on Heart Rate (HR), Lactate Threshold Heart Rate specifically. One of the most common questions I get is "since my Threshold HR changed is my fitness getting better or worse" or "my threshold HR is 175 but so and so's is 185, so he/she must be faster?".
I see this common mistake of athletes associating Threshold HR and changes in HR with decreased or increased mountain bike performance.....don't fall for this!
Heart rate is a great way to improve your training regime. It helps organize the workouts in the training plan into a more readable and easy to follow workout. I.E. Go ride 3 hours at Zones 1-3. Having heart rate zones allows you, the athlete, to quantify "how hard" zones 1-3 is by having Heart Rate zones. That's the BEST part of heart rate.......it is an easily accessible way and affordable way to manage your intensity for a workout. But it is NOT a means to identify current fitness levels, past fitness levels, or to be used when comparing yourself to another athlete.
Changes in HR can indicate changes going on in your body, but without true race data or power data (such as power or Time Trial changes) you have no reliable data suggesting increases or decreases in fitness with only HR.
HR by itself is simply a great means of helping you establish training goals, train more effectively and efficiently, and perform workouts better. You can also use it somewhat for evaluation purposes, but these evaluations are not useful unless you have other data such as power and / or race data associated with it to help out
Read more up from this great article written by Mike Schultz of Highland Training: Does it Matter if Your Threshold Heart Rate is High or Low?