If you have spent any time on fitness TikTok in the last fortnight, you will have seen the clip. A young American lifter, shirt off, demonstrating dumbbell curls by only doing the bottom half of the rep. He holds the bottom position for a beat, half-rises, drops again. Caption: "lengthened partials. The secret the 70s guys knew. Change your training."

Eight million views and counting. Predictable pile-on in the replies: "broscience", "you are a shill", "this is new science", etc. The actual question worth asking is whether the claim is right, whether the 70s guys really did know it, and whether you should change what you do on Monday morning.

What are lengthened partials

A "full range" rep takes a muscle from one end of its length to the other: biceps curl goes from arm straight (muscle long) to arm bent (muscle short). A "lengthened partial" stays in the bottom portion of that range: arm straight, arm half-bent, arm straight. The muscle never fully shortens. It stays at or near its longest position for the entire rep.

The research on this has moved fast. Wolf et al. (2023) in the International Journal of Strength and Conditioning ran a meta-analysis suggesting that hypertrophy gains from training in the lengthened position are meaningfully larger than from shortened-position work, at least in the lower body. Follow-up work by Maeo et al. and others has generally supported the finding for biceps, triceps, quads, and hamstrings.

So the underlying science checks out. Training a muscle under tension while it is long produces more hypertrophy per set than training it under tension while it is short.

Did the 70s guys actually know this

Here is where the TikTok clip gets a bit loose. The golden-era bodybuilders did not sit around with biomechanics textbooks talking about "sarcomere-stretch-mediated hypertrophy". They did not write papers. They did not even use the phrase "lengthened partials".

What they did do, consistently across the tapes we have reviewed for this site, is train with a heavy emphasis on the stretched position of their target muscles. Specifically:

Flyes, not presses, for chest work in the final sets of a session. Flyes put the pec at its longest position under tension. Almost every Mr Olympia from 1970 to 1985 built their chest primarily on flyes.
Incline dumbbell curls, not standing curls. Incline curls stretch the bicep further at the bottom than the standing version. Arnold made these famous, but Columbu and Draper were doing them earlier.
Stiff-leg deadlifts and Romanian deadlifts for hamstrings, rather than leg curls.
Pullovers as a chest and lat exercise. This lift loads the lat in a very long position and is almost extinct now. It was standard in 1977.

If you look at any two-page photo spread of Frank Zane, Franco Columbu, or Mike Mentzer in their training phase, you will see them in bottom positions of movements that load the muscle long. They did not have the vocabulary we have, but they had the intuition. The TikTok clip is correct on the big picture.

Where the clip goes wrong

Two things, both minor:

He implies this is "the one secret". It is a useful technique but it is not the whole game. Strength in the shortened position matters too, for injury resilience and for sports transfer. Training only lengthened partials would be silly.
He demonstrates it with dumbbell curls in a way that looks close to a half-rep with momentum. The research protocols that support lengthened partials are careful about the range. They are not "half reps"; they are reps within a carefully controlled 40 to 60% of the range at the stretched end. The difference sounds pedantic but matters. Done sloppily, the technique is just cheating. Done strictly with time under tension, it is what the research measured.

What the Monday-morning version looks like

If you take the good part of the TikTok clip and apply it to your programme, the change is small but specific:

One accessory movement per muscle per session run as lengthened partials. Keep the rest of your training as it is.
Example for biceps: incline dumbbell curls, 3 sets of 12 in the bottom 40% of the range, controlled tempo, no swinging. Arm extends fully at the bottom, curls to about 45 degrees, extends again.
Example for chest: dumbbell flyes at a slight incline, 3 sets of 10 to 15 in the bottom 50% of the range. Stop before the dumbbells meet at the top. The point is not the squeeze at the top; it is the stretch at the bottom.
Example for hamstrings: Romanian deadlift to below knee, back to upright without a pause. Keep the knees slightly bent. Control the eccentric.

Four weeks of this added to your current programme will produce noticeable changes, particularly in the biceps and hamstrings.

The wider point

The lesson is not "the 70s guys knew everything". They did not. They had worse drugs, worse nutrition science, worse injury care, and worse recovery tools. Modern bodybuilding training is objectively better-informed on almost every dimension.

What they did have was the physical intuition that comes from training a single discipline for eight to twelve hours a day for fifteen years. They arrived at techniques that later researchers would eventually measure and confirm.

When a TikTok clip tells you that a golden-era trick is backed by modern science, there is about a sixty per cent chance the clip is right. For the other forty per cent, you get viral nonsense. Lengthened partials happen to be in the sixty.

Watch the clip if you have not. Then watch a Franco Columbu flye sequence if you want to see the technique done at the level only a two-time Mr Olympia can do it.