A recent study on training muscles at longer versus shorter lengths sparked heated debate across the fitness world. The controversy mostly stems from people lumping all “long length training” research into one bucket — when in reality, the evidence breaks down into distinct categories with different conclusions. Here is what the science actually says, and how to apply it to your training.

Does It Matter Where an Exercise Is Hardest in the Range of Motion?

This is the question at the heart of the newest research. The study recruited 20 previously untrained individuals to train twice per week for 10 weeks. They performed chest flies, reverse flies, hip extensions, and lateral raises on machines that could vary the resistance profile.

One side of the body trained with a lengthened bias — peak torque at the start of each rep where muscles are longest. The other side trained with a shortened bias — peak difficulty near the end of each rep where muscles are shortest. Both sides moved through the exact same range of motion. Only the point of peak difficulty differed.

The result? No significant differences in muscle growth between conditions at any measurement site.

This finding aligns perfectly with two earlier studies in the same category:

  • Cable vs barbell preacher curls (2020): Cables provided peak difficulty at a shorter biceps length, barbells at a longer length. Biceps growth was similar.
  • Cable vs dumbbell lateral raises (2024): Cables were hardest at longer side delt lengths, dumbbells at shorter lengths. Side delt growth was similar at both proximal and distal regions.

All three studies isolate the variable of resistance challenge — same exercise, same range of motion, different resistance curves. And all three find that where peak difficulty occurs does not meaningfully impact muscle growth.

Are Lengthened Partials Better Than Full Range of Motion?

This is a separate question entirely, and the answer looks different.

When comparing shortened partials (the top half of a curl, for example) to full range of motion or lengthened partials (the bottom half), the evidence consistently favors more range at longer muscle lengths. A meta-analysis across three muscle regions shows that full range of motion and lengthened partials produce roughly 4.76% more growth at the distal region compared to shortened partials. That is an additive percentage — if shortened partials grew a muscle by 4%, the full ROM or lengthened partial group grew it by nearly 9%.

What about full range of motion versus lengthened partials head-to-head? Six studies have made this comparison:

  • Three studies on untrained individuals all found more overall hypertrophy from lengthened partials
  • Two studies on trained individuals found largely similar growth
  • One study on trained individuals found similar growth at the mid-region but slightly more distal growth with lengthened partials

The takeaway: lengthened partials appear to be at least as effective as full range of motion, and possibly superior in some contexts. This is a meaningfully different finding from the resistance challenge category — and conflating the two is where most of the online confusion originates.

Why Do Biarticular Muscles Respond Differently to Exercise Selection?

Biarticular muscles cross two joints, which means different exercises can place them at dramatically different lengths depending on joint positions. This creates a third, distinct category of long-length training research — and it has the most consistent findings.

Here is what the evidence shows for each major biarticular muscle group:

Hamstrings (3 of 4 heads are biarticular)

  • Seated leg curls involve ~90 degrees of hip flexion, lengthening the biarticular heads more than lying leg curls
  • One study found greater growth of the biarticular hamstring heads with seated leg curls
  • The short head of the biceps femoris (not biarticular) grew similarly between both

Rectus Femoris (biarticular quad head)

  • Leaning-back leg extensions place this muscle at a longer length than standard leg extensions
  • One study found greater rectus femoris growth with the lengthened position
  • The vastus lateralis (not biarticular) grew similarly between both

Gastrocnemius (biarticular calf heads)

  • Standing calf raises lengthen them more than seated calf raises
  • Three studies found greater gastrocnemius growth with standing calf raises
  • The soleus (not biarticular) grew fairly similarly, with one study slightly favoring seated raises

Triceps Long Head (biarticular)

  • Overhead extensions place it at a longer length than pushdowns
  • The stronger study design found greater long head growth from overhead extensions
  • Even the weaker study showed some measures favoring overhead extensions

Biceps (biarticular)

  • Incline curls (shoulder extended) place the biceps at a longer length than preacher curls (shoulder flexed)
  • Five studies compared these positions with mixed results, but the stronger study designs favor shoulder-extended curls for biceps growth

The pattern is remarkably consistent: when you can position a biarticular muscle at a longer length through exercise selection, it tends to grow more. The non-biarticular muscles in the same movements grow similarly regardless.

Can You Separate Resistance Challenge From Range of Motion Effects?

This is the critical nuance most people miss, and it explains much of the online controversy.

Consider lengthened partials on leg extensions. Leg extensions are typically hardest at the top (shortened position). So when you compare full ROM to lengthened partials, two variables change simultaneously: the range of motion and where peak difficulty occurs. You cannot isolate which variable drove any differences in growth.

The same issue appears in the biarticular muscle research. Incline curls lengthen the biceps but provide peak difficulty at a different point than preacher curls. One study found that the brachialis and brachioradialis (neither biarticular, trained at the same length in both exercises) grew slightly more with preacher curls — where those muscles experienced greater difficulty at longer lengths. That might suggest resistance challenge matters, but it could also be some other uncontrolled property of the exercise.

To truly isolate the resistance challenge variable, you need the same exercise, same range of motion, and only different resistance curves. That is exactly what the three studies in the resistance challenge category did. And they all found no difference.

The isometric literature adds another wrinkle. Some evidence shows isometrics at longer muscle lengths produce more growth than at shorter lengths. But comparing static contractions at different lengths is not physiologically identical to comparing dynamic exercises with different resistance profiles. The isometric data deserves its own category.

Does Training Experience Change the Results?

This is an open and important question. Many of the key studies were conducted on previously untrained individuals. Some have suggested that extra growth from longer muscle lengths may be a specific type of adaptation that plateaus quickly, making it relevant mostly for beginners and less so for experienced lifters.

The evidence behind this idea is not as clear-cut as some suggest. Additionally, the small sample sizes in many of these studies mean they may lack the statistical power to detect real differences. A study finding “no significant difference” with 20 subjects is not the same as proving no difference exists.

It is also worth noting that we cannot confidently say all muscle groups respond the same way. Much of the range of motion research focuses on the quads, while the resistance challenge research has mostly examined the side delts. Whether findings from one muscle group transfer to others remains unconfirmed.

How Should You Actually Program Your Training?

Based on the current evidence across all categories, here are the practical recommendations:

For resistance challenge (where the exercise is hardest):

  • Do not stress about it. Whether a cable or dumbbell version is harder at the top or bottom of the rep does not appear to meaningfully affect growth.

For range of motion:

  • Full range of motion is an excellent default. It consistently outperforms shortened partials.
  • Lengthened partials are worth experimenting with. They perform at least as well as full ROM in the research and may offer slight advantages in some scenarios.

For biarticular muscle exercise selection:

  • Include at least one exercise that trains each biarticular muscle at a longer length:
    • Seated leg curls for hamstrings
    • Overhead extensions for triceps long head
    • Incline curls for biceps
    • Standing calf raises for gastrocnemius
    • Leaning-back or sissy squat variations for rectus femoris

Above all else: The muscle length you train at is not the most important factor for growth. Training with sufficient effort, accumulating adequate volume, and showing up consistently matter far more. Exercise selection is a meaningful detail worth optimizing, but it will never be the make-or-break factor for your results.

Frequently Asked Questions

Do I need to use special resistance curves or cam-based machines for more muscle growth?

No. The research consistently shows that where peak difficulty falls within the range of motion does not significantly affect hypertrophy. A cable lateral raise and a dumbbell lateral raise, performed through the same range of motion, produce similar growth despite very different resistance profiles. Pick the variation that feels best and lets you train hardest.

Should I replace full range of motion reps with lengthened partials?

Not necessarily. Full ROM remains an excellent default with strong evidence behind it. Lengthened partials perform at least equally well in research and may be useful as an occasional tool — particularly if joint issues limit your full ROM on certain movements, or if you want to add variety. There is no need to overhaul your entire program around them.

Effective training means making smart choices about exercise selection, range of motion, and effort — then staying consistent long enough for it to matter. If you are tracking your sets and looking for a faster way to log workouts, Splitt is built for exactly that.