Does Leaning Back During Leg Extensions Build More Muscle? What Research Shows

Leg extensions are one of the most common exercises for developing the quadriceps. Most people focus on the weight, repetitions, and range of motion, but there's another variable that may influence muscle growth: the position of your upper body.

Does sitting upright produce the same results as leaning back during leg extensions? Or could adjusting the seat angle change which quadriceps muscles grow the most?

A study published in the Journal of Sports Sciences in 2025 investigated exactly that question. Researchers compared leg extensions performed with the torso relatively upright against a more reclined position over ten weeks of resistance training.

The results suggest that leaning back during leg extensions may produce greater growth in the rectus femoris, one of the four quadriceps muscles, without significantly changing growth in the vastus lateralis.

Here's what the researchers found, why body position matters, and how these findings might influence your leg training.

Why Does Body Position Matter During Leg Extensions?

The quadriceps consist of four muscles: the rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius. Together, they contribute to extending the knee, but their anatomy is not identical.

The rectus femoris is particularly interesting because it crosses two joints: the hip and the knee. In addition to helping extend the knee, it contributes to hip flexion.

The other three quadriceps muscles cross only the knee joint.

This anatomical difference means that changing hip position can change the length of the rectus femoris, even when the knee performs the same movement.

When you sit upright in a leg extension machine, your hip is typically flexed to approximately 90 degrees. When you recline the backrest, the hip becomes less flexed, placing the rectus femoris in a more lengthened position.

Researchers have proposed that training muscles at longer lengths may provide a greater hypertrophic stimulus under certain conditions. However, the effects can vary between muscles and exercises.

That raises an interesting possibility: could reclining during leg extensions improve rectus femoris growth simply by changing its starting length?

How Did Researchers Test Upright vs. Reclined Leg Extensions?

Larsen and colleagues recruited 30 previously untrained adult men for a ten-week resistance training intervention. Twenty-two participants completed the study and were included in the final analysis.

Rather than dividing participants into separate training groups, researchers used a within-participant design. Each participant trained one leg with approximately 40 degrees of hip flexion and the other with 90 degrees of hip flexion.

This allowed the researchers to compare two positions within the same individual, reducing the influence of differences in genetics, nutrition, and other personal characteristics.

The two positions were:

• 90-degree hip flexion: A conventional upright seated position, with the torso approximately perpendicular to the thighs.
• 40-degree hip flexion: A more reclined position, with the torso leaning backward and the rectus femoris positioned at a longer muscle length.

Participants performed unilateral leg extensions twice weekly for ten weeks, completing three to four sets of 10–20 repetitions per session. Sets were performed to momentary concentric failure, with progressive increases in training load as participants became stronger.

Researchers standardized important variables, including knee range of motion, repetition technique, and training frequency.

Before and after the intervention, ultrasound measurements assessed muscle thickness at the proximal and distal regions of the rectus femoris and vastus lateralis.

This distinction is important because the study measured changes in specific quadriceps regions rather than overall thigh circumference or total leg muscle mass.

What Did the Study Find?

Both positions produced muscle growth over the ten-week intervention. However, the magnitude of growth differed for the rectus femoris.

The researchers reported the following average changes in muscle thickness:

Source: Larsen et al. (2025). Values represent changes in ultrasound-measured muscle thickness, not changes in total quadriceps mass.

The differences in rectus femoris growth favored the reclined position at both measurement sites, with strong statistical evidence supporting the effect.

In contrast, the researchers found no meaningful difference in vastus lateralis hypertrophy between positions.

The main takeaway is that leaning back appeared to improve growth in the rectus femoris, but it did not produce superior growth across every measured quadriceps region.

Why Might Leaning Back Produce More Rectus Femoris Growth?

The most likely explanation involves muscle length.

Because the rectus femoris crosses the hip joint, reclining the torso places it in a more lengthened position before and during knee extension.

Training at longer muscle lengths has attracted considerable research attention because some studies have found greater hypertrophy when muscles are challenged in lengthened positions.

One proposed explanation involves the relationship between muscle length and force production. Changing joint position can alter the mechanical conditions under which muscle fibers generate tension.

In the Larsen study, the reclined position may have provided a more favorable mechanical stimulus for rectus femoris growth.

The vastus lateralis, however, does not cross the hip joint. Changing hip flexion therefore does not alter its length in the same way.

That anatomical difference is consistent with the study's finding that vastus lateralis growth was similar between the two positions.

Importantly, the researchers did not directly establish which physiological mechanism caused the additional rectus femoris growth. Muscle length is a plausible explanation, but changes in muscle architecture, force production, and regional loading may also contribute.

Should You Lean Back During Leg Extensions?

If your goal is to maximize quadriceps hypertrophy, particularly growth of the rectus femoris, this study provides a reasonable argument for considering a more reclined position during leg extensions.

However, that doesn't mean everyone should immediately change their training technique or assume that upright leg extensions are ineffective.

Both positions produced measurable muscle growth, and the study's findings apply most directly to previously untrained men performing a specific leg extension protocol.

There are also practical considerations.

First, not every leg extension machine allows meaningful adjustment of the backrest. Some machines are designed around a relatively fixed seated position.

Second, changing your torso position should not compromise stability, knee alignment, or the ability to perform controlled repetitions through an appropriate range of motion.

Third, the study does not establish that reclining as far as possible produces the best results. Researchers compared approximately 40 degrees with 90 degrees of hip flexion, not every possible seat angle.

Finally, exercise selection is only one component of effective hypertrophy training. Appropriate training volume, progressive overload, recovery, and consistency remain important regardless of seat position.

For a broader discussion of progression, read Does Progressive Overload Always Require Adding Weight to the Bar?

What Are the Study's Limitations?

Although the findings are interesting, several limitations should influence how broadly we apply them.

The study included only 22 previously untrained men in its final analysis. We cannot assume that experienced lifters, women, or older adults would experience identical results.

The intervention lasted ten weeks, which was sufficient to observe changes in muscle thickness but does not establish whether the differences would persist over longer training periods.

Researchers also measured only two quadriceps muscles at specific locations. They did not directly measure growth in the vastus medialis or vastus intermedius, so the findings should not be interpreted as a complete assessment of all four quadriceps muscles.

Additionally, participants in the reclined condition did not all train at precisely 40 degrees of hip flexion because of equipment limitations.

These limitations do not invalidate the results, but they mean the findings should be treated as promising evidence rather than a universal rule for leg-extension technique.

The Bottom Line: Does Leaning Back Build More Muscle?

A 2025 study found that performing leg extensions in a reclined position produced greater rectus femoris hypertrophy than performing the exercise while sitting upright.

Growth in the vastus lateralis was similar between positions, suggesting that the effect was specific to the muscles influenced by hip position.

For lifters interested in maximizing quadriceps development, adjusting the leg extension backrest may be a useful consideration, provided the machine allows it and the position remains comfortable and stable.

However, the evidence does not establish that reclining is necessary for building large quadriceps or that a particular seat angle is optimal for everyone.

The practical takeaway: If your leg extension machine allows you to lean back comfortably, a more reclined position may provide an additional stimulus for rectus femoris growth. But consistent, progressive training remains far more important than chasing a perfect seat angle.

For more information about how exercise selection, progressive overload, and recovery work together to support long-term muscle development, explore The Foundation of Long-Term Strength, Muscle, and Performance.

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References

Larsen S, Kristiansen BS, Swinton PA, Wolf M, Fredriksen AB, Falch HN, van den Tillaar R, Sandberg NØ. The effects of hip flexion angle on quadriceps femoris muscle hypertrophy in the leg extension exercise. Journal of Sports Sciences. 2025;43(2):210–221. https://doi.org/10.1080/02640414.2024.2444713.

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About the Author

AFT Fitness Coaching has spent more than 20 years coaching adults to build muscle, improve body composition, and train with confidence. We share evidence-based guidance on strength, hypertrophy, recovery, and nutrition, built on the same principles behind our coaching.