LOw 50's is good !.. wish I could ge there. Usually our rides in the US require a pulse down of 64 to 60.
Its all about the temperature. Sorry can't do celsius, but anything over about 85 degrees and I am unable to get Bo to pulse down, even when riding at barely finnish speeds.
My second horse emma has several things going for her that make her much better suited. I am just now getting her in shape, Planning on a 25 miler (41km) Limited distance ride in Nov. See how she does. SHe has a much more efficient gait than bo does. Bo has a high stepping rack, emma has a true flat walk/ run walk and can really move out at a shuffle. We shall see. Hopefully I can at least turn her into a happy consistent finisher.
Also not sure what "strap him down" means. Are you using lots of water ? Water on water off, ice water on also, concentrating on neck, whithers and shoulders., Avoid the butt, you can cramp them.