Here's part of a piece I wrote a while back...
The idea that bodyparts could be worked effectively just once per week was introduced by a guy name Arthur Jones at about the time he began to tout his Nautilus equipment. This was the advent of the machine age of bodybuilding. Gyms no longer needed to be cluttered with untidy barbells and plates and Jones made a large fortune moving the bodybuilding world away from free weights.
Along with his Nautilus equipment came his theories on High Intensity Training or HIT. No longer need people spend Arnoldesque hours in the gym; they could perform short single sets with maximum weights to absolute muscular failure forcing maximal muscular growth. He was also fortunate enough to have Casey Viator who was an extreme physical specimen often noted for his participation in the Colorado Experiment. Using only Nautilis equipment, Viator gained 45.28 pounds in 28 days while dropping bodyfat. His fatloss was measured at 17.93 pounds and thus his lean gain over the 28 days was 63.21 pounds. This was done allegedly drug-free and Jones had his marketing on a plate.
Thus was ushered in the dark ages of weights training. Barbells disappeared from gyms. Weider began his empire selling supplements to dissatisfied trainees who couldn't hope to emulate the workouts of the adonises gracing the pages of his glossy advertising magazines. Of course, there was nothing wrong with the workouts, after all, they were the workouts of champions. The trainee's failings were down to failing to work hard enough and not buying enough supplements. No-one mentioned the copious amounts of drugs.
Drug dosages were known to increase through this time. With poor training stimulus the only recourse to fuel growth is greater dosages. Bodybuilding inevitably moved away from any real concerns with training and focussed ever more on diet and drugs. The classic flowing lines of Zane, Arnold and Franco honed with frequent workouts, often including AM and PM sessions, centered around free weight compound exercises, stepped aside for the bulky looks we have today. Muscles worked in isolation with little consideration of groups and systems. Underlying it all, though, was the ideology of training your bodyparts once per week. We had pre-exhaustion, drop sets, negatives, super-sets, all to blast a muscle into absolute submission so that it could be left as a quivering jelly for another week.
As a beginner, pretty much anything will fuel growth, including a poor training stimulus. Members would join gyms, marvel at the shiny equipment, get some good progress and inevitably stall. At that stage they turn to supplements or quit the gym for six months. Those that turn to supplements experience a little more progress and then they turn to drugs or quit the gym for six months. A few die-hards will plug away for years on end thinking that they just have one of those physiques that doesn't change much over time but they enjoy working out anyway.
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