The debate over the “best” rep range for building muscle has raged in gyms for decades. Three recent studies shed fresh light on this question, along with whether single-limb exercises beat bilateral movements and which exercises actually grow your quads. The findings challenge several popular beliefs.
Does Unilateral Training Build More Muscle Than Bilateral Training?
A common claim in training circles is that single-limb (unilateral) exercises are superior for hypertrophy compared to their bilateral counterparts. The reasoning sounds plausible: when you train one limb at a time, you may achieve greater neural drive, higher muscle fiber recruitment, and more local muscle fatigue because cardiovascular demand is lower.
A recent study put this to the test. Researchers recruited 54 untrained women and assigned them to train three times per week for eight weeks. One group performed bilateral dumbbell curls, while the other performed unilateral dumbbell curls, training one arm at a time before switching. Both groups completed two sets of 8-12 reps to concentric task failure, with loads adjusted to keep them in that range.
The results were clear:
- Strength: Bilateral curl 1RM increased similarly in both groups. Unilateral right-arm strength favored the unilateral group (likely because they ended up lifting slightly heavier loads per arm over time), but left-arm differences were not significant.
- Muscle growth: Changes in elbow flexor thickness at both measured regions were not significantly different between groups.
The takeaway? Training one arm at a time did not produce greater muscle growth than training both arms simultaneously.
Some might argue that the elbow flexors are a small muscle group, so perhaps the advantage of reduced muscle mass per exercise would show up in larger movements. But other research tells a similar story. Studies comparing squats (larger muscle mass involvement) to hip thrusts (smaller muscle mass involvement) have found similar glute growth between the two. The pattern is consistent: whether an exercise is unilateral or bilateral does not appear to be a meaningful driver of hypertrophy.
When Does Unilateral Training Still Make Sense?
That said, unilateral training is not without practical benefits:
- Correcting size imbalances between limbs
- Working around injuries that limit loading on one side
- Adding variety to your program
The tradeoff is that unilateral training takes roughly twice as long since you train each limb separately. If your goal is pure muscle growth efficiency, bilateral exercises will get you there in less time.
Is There a Best Rep Range for Hypertrophy?
This is the question that refuses to die. Bodybuilding tradition insists on the “hypertrophy zone” of 8-12 reps, while strength-focused trainees gravitate toward lower rep ranges. A new study compared heavier loads (8-12 reps) against lighter loads (20-25 reps) in a well-designed within-subject protocol.
Twenty previously untrained men trained three times per week for ten weeks. Here is the key detail: each participant had one arm and one leg assigned to heavier loads, while the other arm and leg trained with lighter loads. This within-subject design eliminates individual variation as a confounding factor for hypertrophy comparisons.
Both conditions trained the unilateral leg extension and unilateral dumbbell preacher curl for three sets per session, with all sets taken to volitional failure. Loads were adjusted across sets to keep reps in the prescribed ranges.
The results across every measure of muscle growth were not significantly different between heavy and light loads. This included:
- Biceps thickness and cross-sectional area
- Vastus lateralis thickness and cross-sectional area
- Fat- and bone-free lean mass
- Muscle fiber cross-sectional area (both slow-twitch and fast-twitch)
That last point is worth emphasizing. A long-standing theory suggests that lighter loads and higher reps preferentially build slow-twitch fibers, while heavier loads target fast-twitch fibers. This study, along with several others, found no support for that idea. Both fiber types grew similarly regardless of load.
What Actually Matters More Than Rep Range
The critical variable is not the number on the rep counter. It is proximity to failure. As long as you are pushing sets to or very close to failure, a wide range of rep counts produces comparable muscle growth. This has now been demonstrated across:
- Trained and untrained populations
- Upper and lower body muscles
- Multiple measurement methods (ultrasound, MRI, biopsy)
Practical application: Choose rep ranges that suit the exercise, your joints, and your preferences. Heavy compound lifts in the 5-8 range, moderate accessory work at 10-15, and higher-rep isolation or machine work at 15-25 can all coexist in a well-designed program. The best rep range is the one you can execute with good technique while pushing close to failure.
Do Some People Respond Better to High or Low Reps?
The same study explored an intriguing question: could certain individuals experience better growth with one loading scheme over the other?
When researchers ranked each participant’s hypertrophy response to heavy versus light loads, they generally found that people who grew well with heavy loads also grew well with light loads. However, a handful of data points suggested some individuals might respond slightly better to one or the other.
Before you rush to declare yourself a “high-rep responder,” there are important caveats. Determining genuine individual differences in training response is statistically complex, and a sample of 20 participants is too small to draw firm conclusions. Natural variation and measurement noise can easily create the appearance of individual differences where none exist.
For now, the safest recommendation remains: train across a variety of rep ranges and push hard regardless of the load on the bar.
Are You a High Responder for All Muscle Groups?
Another fascinating finding from this study relates to individual growth potential across different muscles. Researchers ranked participants by how much their arms grew and compared that to how much their legs grew.
There was a moderate positive correlation. People who were high responders in the arms tended to be high responders in the legs as well. However, roughly 50% of the variance remained unexplained, meaning that some people may indeed grow certain muscles more easily than others.
This is preliminary data from a single small study measuring primarily the biceps and vastus lateralis. But it aligns with what many lifters report anecdotally: that some muscle groups seem to respond to training more readily than others. Future research will hopefully clarify whether this reflects genuine biological differences in regional growth potential or simply differences in exercise execution and effort distribution.
What Are the Best Exercises for Quad Growth?
A study of 63 untrained women compared Smith machine back squats to machine leg extensions, both performed for three sets of 8-12 reps to momentary muscular failure, twice per week for eight weeks. Quad growth was measured across three regions of the rectus femoris and vastus lateralis.
The findings paint a nuanced picture:
- Rectus femoris: Leg extensions produced significantly greater growth across all three measured regions.
- Vastus lateralis: Growth at the proximal and middle regions was similar between exercises, but the distal region grew more with back squats.
This pattern has now been replicated across multiple studies, including research on trained lifters comparing leg presses to leg extensions. The explanation lies in the anatomy of the rectus femoris.
Why Squats Underperform for the Rectus Femoris
The rectus femoris is a two-joint muscle. It crosses both the knee and the hip, meaning it is involved in knee extension and hip flexion. During a squat, both knee extension and hip extension occur simultaneously. If the rectus femoris contracted forcefully during a squat, it would partially fight against the hip extension component. The nervous system appears to minimize rectus femoris involvement during compound hip-and-knee movements to avoid this conflict.
Isolated knee extension exercises like leg extensions do not have this problem. The hip is stationary, allowing the rectus femoris to contract fully through its knee extension function. Other isolated knee extension movements like reverse Nordics and sissy squats likely offer similar benefits, though direct evidence for these is still limited.
The Optimal Quad Training Strategy
For complete quad development, the research consistently points toward a combination approach:
- Squats, leg presses, or lunges for the vastus lateralis, vastus medialis, and vastus intermedius
- Leg extensions, reverse Nordics, or sissy squats for the rectus femoris
Relying exclusively on compound movements will likely leave your rectus femoris underdeveloped. Adding at least one knee extension isolation exercise covers this gap.
It is also worth noting that squats and leg presses grew the vastus lateralis as well or better than leg extensions, despite involving a larger total muscle mass. This is further evidence against the idea that smaller-muscle-mass exercises are inherently superior for hypertrophy.
Frequently Asked Questions
How many reps should I do to build muscle?
Research consistently shows that any rep range from roughly 5 to 30+ reps can produce similar muscle growth, provided you train to or near failure. The traditional “hypertrophy zone” of 8-12 reps works, but it is not uniquely effective. Choose rep ranges based on the exercise (heavier for compounds, lighter for isolations), your joint health, and your personal preference.
Should I do leg extensions or squats for bigger quads?
Ideally, both. Squats and leg presses are highly effective for the vastus heads of the quadriceps, but they underperform for the rectus femoris due to its two-joint anatomy. Leg extensions directly target the rectus femoris. A well-rounded quad program includes at least one compound knee-and-hip movement and one isolated knee extension exercise.
Putting It All Together
The latest research reinforces a theme that has been building for years: the fundamentals matter more than the details. Train hard, push close to failure, use exercises that target your muscles through a full range of motion, and do not overthink whether you should be doing 8 reps or 20. Both work.
Where exercise selection does matter is in covering all heads of a muscle group. For quads, that means pairing squats or leg presses with leg extensions. For most other programming decisions, including rep ranges and unilateral versus bilateral execution, personal preference and practical constraints should guide your choices.
If you are tracking your sets and reps to monitor progressive overload across different rep ranges, a streamlined gym tracker like Splitt can help you stay consistent without the friction of spreadsheets or bloated apps.