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I would arbitrarily consider any movement High Rep if you can perform over 10 in a row. With or without weights, any exercise over that begins to fall into the Calisthenics category or the Warm-up category in my book. Sure, anything can feel heavy after a while, but high-rep is a different sort of exercise unless you are a professional.
Thus if you can barely do 6 pull-ups, it's strength-building Resistance. If you can do 20, it's more like Calisthenics. Likewise, 20 body-weight squats are Calisthenics, 4 Barbell squats to failure are Resistance.
It matters because High Rep exertions, even if reaching the breaking point, do not effectively improve strength. A useful thing about High Rep is that they douse your existing strength and help maintain. In other words, good for general fitness.Other examples are Jumping Jacks and Jump Rope which overlap Cardio and Cali categories but do not build strength.
Different sorts of muscle fibers are used. From here:
High reps develop Type 1 muscle fibers (“slow twitch”) that are endurance based and slow to fatigue. Lower repetitions activate Type 2 muscle fibers (“fast twitch”), which have greater power but fatigue quickly.
Getting stronger is mainly about stressing and damaging those fast twitch muscle fibers. You can do High Rep deadlifts all day long and still never be able to lift a 200 lb box of rocks.
Thus the general idea is that, for strength, Low Rep with very heavy (whatever that is for you). For general fitness maintenance, High Rep Calisthenic-type exercise. For overall conditioning, I do both, plus Cardio. Almost everybody can find time for this, especially if you do it at 5 am. Why not?
Virtually every strength training coach I know would tell you that the advice on bodybuilder.com is generally correct. If your son is benefiting from low-rep, strength building workouts then it suggests that your son was not strong enough in the beginning to compete at his chosen weight class.
A better plan for him would be low-rep, high weight lifting in the off season to build the strength required to compete in his weight class, then switch to high-rep lifting (8-15 reps x 3 sets) to exhaustion during the season in order to build the muscle endurance required for a wrestling match.
IMO, unless you plan to enter powerlifting competitions there is little benefit to be had from low-rep, high weight workouts, assuming you have attained an adequate level of strength. Low-rep workouts make you big, bulky, and strong, but they do very little for cardio fitness, muscle endurance, or whole-body fitness. I know a lot of guys who can deadlift and squat 500 pounds but are dying within the first 10 minutes of a cross-fit style workout. Most athletes will be better served by a whole-body fitness routine that mixes strength training with cardio intervals.
I read a study a couple weeks ago from McMaster University where half of the test subjects were lifting heavier(up to 90% of 1 rep max) and lower reps while the other half lifted lighter(up to 50% of their 1 rep max) after 12 weeks the muscle growth was the same for both groups. The study says that muscle growth occurs when the muscles are worked to fatigue.