You guys recording 40 or less BPM, do you honestly understand the significance of that? Let me give you two examples:
Miguel Indurain is one of the greatest cyclists of all time and won the Tour de France 5 times. He had a resting HR of ~30bpm. Here are just a few facts about the man's physiology to put it into perspective just how hard to believe that numerous people on here are claiming to have such low resting HR's:
1. A normal person takes in about 3-4 liters of O2 into their blood per minute. A professional cyclist will be able to get about 5.5. Indurain would consistently measure over 7 liters per minute.
2. A pro cyclist can pump about 25 liters of blood through their body per minute. Indurain could pump 50 liters through his.
3. His lung capacity is nearly 8 liters. Yours is probably about 5, with a strong athlete getting near 6.5.
4. Your VO2 max, the amount of oxygen your body can uptake in a given time is one of the best measures of cardiovascular fitness. Your average weight male will be in around 45, while an amateur competion cyclist will measure about 60. Indurain measured over 88.
Here's someone everyone is familiar with: 6-time TDF winner, Lance Armstrong
1. Armstrong, at his absolute peak, had a resting HR of 32-34bpm. During mountain stages, this corresponded to him sometimes reaching his max HR of over 200bpm. That's off the measured scale for professional athletes at 20 years of age. It decreases by about 5bpm every 5 years and Armstrong's prime was in his mid 30s. Less than 100 athletes in the entire world have ever tested even near this range.
2. Lance's VO2 max was highest after recovering from cancer, as was his muscle mass and efficiency. His oxygen capacity is over 40% greater than the average male athlete in his 20s. He was a decade older.
3. When you use your muscles strenuously, you produce stuff which I'm sure you know is called lactic acid. This hampers performance and creates incredible muscle fatigue. His body was so efficient that his muscles produce less than half of what a normal male does.
4. This is the most impressive in my opinion: as an endurance athlete, you need slow-twitch muscle fibers more than fast-twitch ones. If you don't know, fast twitch fibers are used for explosive movements and sports: plyometrics, boxing, etc. He wasn't originally one of the best at muscle efficiency, but because of the reasons above, he was able to convert his muscle fibers into about 80% slow-twitch fibers. A pro cyclist is able to pedal at about 75-80 RPM. He changed his musculature to allow him to pedal at 105 RPM.
Look up the stats for cyclists, cross-country skiiers, rowers, ultramarathoners, etc. While you might not consider tennis an endurance sport, at the pro level where you're given 20 seconds between huge rallies, it definitely is. That's the main reason why Djokovic used to struggle: his lungs aren't efficient enough to keep his body going at the level he needs to in grand slams.