Ok, I wasn't going to say anything, but...
"I politely disagree (If I may). Strength is important but endurance must be there. Strenth is part of the picture. I did not tell him to stop "strenth" training, no, only to include high rep sets."
Why must the endurance be there? You have to make a choice - a maximally strong muscle, a maximally endured muscle, or a little of both. A maximally strong muscle CANNOT be maximally endured.
"Muscle does grows as it becomes stronger, but not that much. MOST of the size that can be gained is in slow twitch (endurance fibers). Ask yourselfm powerlifters do heavy sets all the time. They can move more weight than bodybuilders do. If powerlifters are so strong, why aren't they so damn big?"
Muscle itself does have to grow to become stronger, but you don't have to gain muscle to gain strength. You can increase neuromuscular efficiency, tendon strength, exercise technique, etc. Most of the size gained is NOT ST (slow twitch) muscle fiber growth! FT (fast twitch) fibers account for approx 80% of muscle fiber volume, ST is only 20%. ST fiber growth can benefit you because training ST fibers will cause sarcoplasmic growth of the Type IIA (medium twitch) fibers. Sarcoplasmic growth means more energy producing parts ofm a muscle fiber - increased mitochondria number and density, capillarization, etc. But, FT fibers are what cause the most growth, because they have a much larger diameter than ST fibers. Additionally, training for FT growth will cause more myofibrillar (sarcomere) growth, which means more actin and myosin (contractile proteins), which means more strength. OK, the reason Powerlifters do not have as much muscle mass as bodybuilders is because power lifters do not train for sarcoplasmic hypertrophy nor Type IIA hypertrophy. Power lifters also have a nervous system optimized for maximal force production, while bodybuilders have a nervous system optimized for repeated efforts. FT fibers have the most impact on strength, these fibers are "all business" meaning - they all contribute to strength. Powerlifters don't have any significant sarcoplasmic hypertrophy or ST hypertrophy. This all equates to more pound for pound strength.
"By doing heavy sets all the time you are hitting only SMALL PORTION of available muscle fibers. So only 'SMALL;' portion of the muscle grows. If you include higher reps, you are hitting MORE fibers, and thus more fibers grow."
By doing heavy sets all the time you are increasing neuromuscular efficiency to a LARGE degree, which means you can recruit MORE muscle fibers and consequently MORE growth. I don't know where you got the idea that ST fibers are the primary contributors to muscle growth - but that is dead wrong. ST fibers have such a small effect on total muscle mass that they can be completely ignored by the average lifter, and trust me - you'll get big anyway.
"If you beleive in doing ONLY hjeavy sets, then switch to powerlifting. If you want MAX size then do higher rep sets, like bodybuilders do!"
This statement is just stupid. Just because i prefer to maximize growth of the FT fibers, which account for 80% of total muscle mass, and increase my neuromuscular efficiency so my nervous system can recruit more muscle fibers, which all leads to more muscle - why the hell should i switch to powerlifting?
"Hopefully you understood what I have just said."
I understood it all right, and it was a load of shit.