How does strava calculate fitness and training load based on heartrate for cyclists?

Hello, 

I’m wondering how Strava calculates the two above mentioned metrics.
I performed exactly the same endurance training, separated by 8 days.  Suprisingly, in exactly the same time, so exactly the same average speed, denivelation (1060 m), distance (131 km).  However, heartrates of course were different.  First and second workout are summarized below.

1. AVG HF : 120.  4 min in zone 2, 232 min. in Zone 3, 143 min in zone 4, 2 min in zone 5.  It resulted in training impulse of 388, and an fitness increase from 136 to 142 
2. AVG HF : 12 min in zone 2, 152 min in zone 3 and 66 min in zone 4. It resulted in training impulse of 304 and increase from 136 to 142

How are the time spent in the different zones factored in?  Is the AVG HF factored in?  And are increase/decrease of fitness derived from the training impulse values?

Thanks for clarifying!