Earlier quoted context omitted.
Intense, short loads, with long recovery breaks in between build muscles and strengthen tendons. That's lifting weights at the gym. Writing and typing is continuous, repetitive strain with no recovery. That's the killer.
Wouldn't you describe cardio as continuous, repetitive strain with no recovery? I would've assumed writing is more of an endurance type of strain, but perhaps the strain due to writing occurs at an intensity that is analogous to lifting weights.
Hard labor is continuous cardio. Hard labor isn't healthy. Athletes have specific training regiments with up times and recovery times.
If I go on a 60 mile bike ride, I need at least a few days downtime to recover. If I'm biking 20-30 miles, I'll do that at most every other day. And if I lift weights, I'll alternate upper body and lower body days. And if I'm doing basic calisthenics and stretching, I can do that every day.
(Not that I do any of those anymore, at my age)