Does Muscle Length During Exercise Matter for Growth?

Why Exercise Position Matters More Than You Think

Imagine you have a rubber band. If you stretch it out long before you snap it, the band experiences a different kind of stress than if you let it go slack first. Your muscles work in a similar way. Depending on your body position during an exercise, a muscle can start in a stretched (lengthened) state or a shortened state before it contracts.

In recent years, fitness enthusiasts and researchers have become very interested in what is called lengthened-state training (LENG-thend stayt TRAY-ning), which means exercising a muscle while it is in a more stretched position. The idea is that muscles might grow bigger when they are challenged at longer lengths.

But does this actually work? And does it work the same way for every muscle? Two recent studies tackled these questions, one looking at the biceps in the upper arm and the other at the hamstrings in the back of the thigh. Their findings paint a nuanced picture that is worth understanding if you care about building muscle effectively.

What the Research Shows

Biceps: Stretched vs. Shortened Position Produced Similar Growth

A 2025 study in the European Journal of Sport Science compared two versions of a cable biceps curl in 15 young men who had not trained for at least six months. Each person trained one arm with a Preacher curl (shoulder flexed forward, which shortens the biceps) and the other arm with a Bayesian curl (shoulder extended back, which stretches the biceps). This is called a within-subject design, meaning each person served as their own comparison, which removes a lot of individual variability.

The researchers were very careful about something that earlier studies had overlooked: they matched the resistance profiles (ree-ZIS-tuhns PRO-files) of both exercises. In plain terms, this means the weight felt equally hard at the same points in the movement for both curls. Previous studies that compared Preacher and incline curls used dumbbells, which have different resistance curves depending on arm position, making it impossible to isolate the effect of muscle length alone.

After 10 weeks of training twice per week (3 to 5 sets of 8 to 12 reps), the results were clear:

Both exercises produced meaningful muscle growth and strength gains. The Bayesian curl showed slightly higher numbers for the biceps itself, while the Preacher curl showed slightly more brachialis growth, but none of these differences reached statistical significance.

The brachialis (bray-kee-AL-iss) is a muscle that sits underneath the biceps. It also flexes your elbow, but unlike the biceps, it only crosses one joint. This matters because when the biceps is shortened (as in the Preacher curl), the brachialis may pick up more of the workload, and vice versa.

The researchers noted an interesting post-hoc observation: when they subtracted brachialis growth from biceps growth, there was a small effect favoring the Bayesian curl for biceps-specific growth. This was not statistically significant, but it hints that shoulder position may shift which muscle does more of the work, even if total elbow flexor growth ends up being similar.

Hamstrings: Stretched Position Clearly Won

The story was quite different for the hamstrings. A 2024 study in Medicine and Science in Sports and Exercise compared two eccentric-focused training methods in 42 young men over 12 weeks (34 sessions):

• Lengthened State Eccentric Training (LSET): A seated leg curl machine modified so the hip was held at 120 degrees of flexion. This kept the hamstrings in a deeply stretched position during every repetition.
• Nordic Hamstring Training (NHT): The classic exercise where you kneel and slowly lower your body forward. The hip stays relatively neutral, so the hamstrings work at a shorter length.

A third group served as a no-training control.

This study used MRI scans to measure the volume of seven individual knee flexor muscles, which is a more detailed and accurate method than ultrasound.

The results showed a clear advantage for training at longer muscle lengths:

LSET produced roughly 64% more total hamstring growth than NHT. The difference was even more dramatic for the biceps femoris long head (BYE-seps FEM-or-iss), the hamstring muscle most prone to strain injuries in sprinters. LSET nearly quadrupled its growth compared to NHT (19% vs. 5%).

But the results also revealed something fascinating: the pattern of which muscles grew depended on the exercise’s functional demands. LSET was 2.2 times more effective for muscles that both flex the knee and extend the hip (like the biceps femoris long head and semimembranosus). NHT was 1.9 times more effective for muscles that only flex the knee but do not extend the hip (like the biceps femoris short head and gracilis).

In other words, hypertrophy was specific to the functional role of each muscle within the exercise being performed.

Why the Biceps and Hamstrings Responded Differently

This is the most thought-provoking part of these two studies. Training at longer muscle lengths produced a clear advantage for the hamstrings but not for the biceps. Why?

The researchers in the biceps study offer a plausible explanation. Using biomechanical modeling, they showed that changing shoulder position from flexed to extended only shifted the biceps’ operating range on the force-length curve (FORS-length kurv) by a small amount. The force-length curve is the relationship between how stretched a muscle fiber is and how much force it can produce. Each muscle has a sweet spot where it generates the most force.

The biceps brachii, it turns out, may not be able to reach the very steep, descending portion of its force-length curve through normal shoulder position changes. In contrast, the hamstrings, which are much longer muscles that cross both the hip and knee joints, can be placed at substantially different lengths by changing hip position. The LSET protocol pushed them well into that stretched zone.

So the takeaway is not that lengthened training “doesn’t work.” It is that the benefit depends on how much you can actually change the muscle’s working length, and that varies from muscle to muscle.

Who This Information Is Most Useful For

People Focused on Arm Size

If your main goal is bigger biceps, the Preacher curl and Bayesian (or incline) curl appear to produce very similar results when performed with comparable resistance profiles. You do not need to stress about picking the “perfect” biceps exercise based on shoulder position alone. Choose whichever feels most comfortable and allows you to train consistently.

That said, there is a small signal that shoulder position may influence whether the biceps or the brachialis does more of the work. If you want to cover your bases, doing both types of curls over the course of a training week is a reasonable strategy.

Athletes Concerned About Hamstring Injuries

Sprinters, soccer players, and other athletes who are at risk for hamstring strains should pay close attention to the hamstring study. The biceps femoris long head is the most commonly injured hamstring muscle, and LSET produced nearly four times more growth in this muscle than NHT. LSET also increased the size of the aponeurosis (ap-oh-noo-ROH-sis), the connective tissue sheet inside the muscle. A larger aponeurosis may help distribute force more evenly and reduce injury risk, though this has not been directly proven.

However, Nordic hamstring training still produced meaningful growth in other knee flexor muscles, so it should not be dismissed. The two exercises appear to complement each other rather than compete.

Beginners

Both studies used untrained participants. This is important because beginners tend to respond to almost any training stimulus. It is possible that differences between exercises become more apparent in experienced lifters who need a more specific stimulus to keep growing. We do not know this for certain from these studies.

How to Apply This in the Gym

For Biceps Training

1. Pick curls you enjoy and can perform safely. Whether it is a Preacher curl, Bayesian cable curl, standing curl, or incline dumbbell curl, all of them will grow your biceps if you train hard enough.

2. Use 3 to 5 sets of 8 to 12 reps, taken close to failure. This was the protocol in the biceps study and reflects standard hypertrophy guidelines.

3. Train biceps twice per week. This matches the frequency used in the study and aligns with broader research on optimal training frequency.

4. If you want to hedge your bets, rotate exercises. Do a Preacher-style curl one day and a Bayesian or incline curl another day. This may slightly shift the emphasis between the biceps and brachialis, giving a more complete stimulus.

5. Consider resistance profiles. Cable exercises maintain tension throughout the full range of motion. Dumbbell curls are hardest at the midpoint and easier at the top and bottom. Using cables or machines may provide a more consistent stimulus.

For Hamstring Training

1. Include at least one exercise where the hamstrings are stretched. A seated leg curl with your torso leaning forward (increasing hip flexion) or a Romanian deadlift both train the hamstrings at longer lengths.

2. Don’t abandon Nordics. They grew muscles that the lengthened training did not emphasize as much. A balanced program might include both.

3. Progress gradually into deep stretch positions. The hamstring study progressively increased the range of motion over the first five weeks. This is wise because muscles are more vulnerable to injury when loaded at unfamiliar long lengths.

4. Emphasize the eccentric (lowering) phase. Both exercises in the hamstring study focused on slow, controlled lowering over about 4 seconds. This is a well-supported strategy for building muscle and strength.

Sample Weekly Approach

What We Know and What We Don’t

What the evidence supports:

• For the biceps brachii, changing shoulder position (flexed vs. extended) during cable curls does not produce a meaningful difference in muscle growth or strength when the resistance profile is matched. Both positions work.
• For the hamstrings, training at longer muscle lengths (with the hip flexed) produces substantially more overall growth, particularly in the biceps femoris long head, compared to shorter-length eccentric training like Nordics.
• Different exercises emphasize different muscles within a muscle group, even when those muscles technically do the same joint action.
• The benefit of lengthened-state training appears to depend on how much the exercise actually changes the muscle’s operating length.

What we still don’t know:

• Whether these results hold up in trained individuals with years of lifting experience.
• Whether longer study durations (beyond 10-12 weeks) would reveal differences that weren’t apparent in these timeframes.
• Whether women respond the same way. Both studies used only young men.
• Whether slowing down the eccentric tempo in the biceps study would have given the Bayesian curl an advantage, as the researchers speculated.
• Whether the small biceps-vs-brachialis recruitment shift observed in the biceps study has practical significance over months or years of training.
• Whether the aponeurosis growth seen in the hamstring study actually translates to reduced injury rates in real-world athletic settings.

It is also worth noting that both studies were relatively small (15 and 42 participants). The biceps study, while well-designed, had only 15 participants, which may have limited the ability to detect small differences. The hamstring study was larger and used MRI, which is the gold standard for measuring muscle volume, giving those results a bit more weight.

Quick Reference: Key Studies

Last updated: June 2025

This article synthesizes findings from peer-reviewed research. It is for educational purposes only and does not constitute medical advice. Consult a healthcare provider before starting any new regimen.