Your body adapts very specifically to the training you perform, you can get very strong in the low rep ranges if you only train with low reps but your high rep strength won't be very impressive because you lack strength endurance. On the other hand you could get very strong in the moderate/high rep ranges if you only train with moderate/high reps but your low rep strength won't be very impressive.I'm sure you've seen those tables that compare different percentages of your 1RM and tell you how many reps you should be able to perform, well it's not that simple because how you've been training alters your scores, for example take a powerlifter with a 1000 pound 1RM squat, going by the charts he should be able to perform 10 reps with 75% (750 pounds), it's obvious to anyone with half a brain that there's no way in the world he's going to be able to do this. Because he's so strong in the low rep range he could probably perform 10 reps with 600 pounds at most (60% instead of 75%).In regards to someone who trains with high reps like me the opposite occurs, you can lift a higher percentage of your max for 10 reps, I can lift 85% of my 1RM for 10 reps and 75% of my 1RM for 20 reps (I should only be able to get 10). This is because I'm very strong in the higher rep ranges but weaker in the lower rep ranges.So as you can see there are different kinds of strength, there's 1RM strength, 10RM strength and 20RM strenth etc. The guy with the highest 1RM isn't necessarily going to also have the highest 10RM and 20RM, the opposite is also true, my 20RM is higher than all of the powerlifters who train at my gym even though they all (well a least the advanced guys) have a higher 1RM than me. For example a weight I can squat 20 times they can only manage about 10 with, the closest any of them have come to matching me is 13 reps, however they blow me out of the water when it comes to 1RM strength. I'm much larger than these powerifters so I think it's rather obvious that you should be training to get strong in the moderate/high rep ranges if hypertrophy is your goal.Performing the split once per week on Mon, Wed and Fri works well too, the only problem is that you'll be training with half the amount of weekly volume (which will still work, it just won't produce results as quickly), that is unless of course you're feeling like a madman and want to try doubling the total number of reps (or at least increasing them as far as you can if you can't double them).To perform the split twice per week with only 4 days of training (upper/lower) you could set it up like this,D1 - D1 + D2D2 - D3D3 - RestD4 - D1 + D2D5 - D3D6 - RestD7 - RestYou can keep D3 volume at 50 reps but you could reduce D1 and D2 volume to 40 total reps or even 30 total reps if 40 is still too much.