@jesuslovesme The load is absolutely a variable, alongside countless other variables. In general, lighter weights/higher reps are friendlier to tendons than heavier weights, but endurance training can absolutely still induce tendon breakdown. We can see this clearly in the running population, wherein overuse injuries are the most common type (for example, Plantar Fasciitis), something I myself received in the last year from running too hard and too much for my own good.
In fact, running wisdom is very clear that a runner should be careful when increasing mileage - only doing it at a slow rate and never running through pain.
To be clear, none of these things are certain to cause injury. An experienced individual doing a few pull-ups a day in the "grease-the-groove" training style may not receive any injury, but an unexperienced individual going to failure every day is much less likely to be physically capable of taking that training load. Additionally, you cannot understate the impact of lifestyle variables such as nutrition, sleep, and stress levels. Things that an inexperienced, underweight teenager is likely not taking much care of.
