A new study found that overall quadriceps growth from squats and leg extensions is similar when sets and proximity to failure are matched, but regional hypertrophy differs: leg extensions grow the rectus femoris (middle quad) more, while squats grow the vastus lateralis (outer quad) more.
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Strength gains are skill-specific—doing squats made people stronger at squats and not at leg extensions, and vice versa, even at the same muscle mass.
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Leg extensions are less systemically fatiguing than squats, while squats also stimulate hamstrings, glutes, and lower back; using both exercises is optimal for maximizing quadriceps development.
Protocols
Concrete recipes — what, when, how much, and why
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Combine compound and isolation exercises for balanced quadriceps growth
WhatInclude both squat-pattern movements (e.g., barbell back squats) and leg extensions in your leg-training program to stimulate all regions of the quadriceps.
WhenWithin weekly leg sessions; the exact split (same day, different days) wasn’t specified, but the recommendation is to consistently use a mix of both types.
DoseNot specified directly, but the study used hard sets taken near failure, so following standard hypertrophy guidelines (e.g., 2–4 hard sets per exercise per week) with appropriate proximity to failure is implied.
For whomAnyone whose goal is maximal quadriceps hypertrophy, particularly if they want to avoid lagging outer or middle quad development.
WhySquats preferentially grow the vastus lateralis and also engage the posterior chain; leg extensions preferentially grow the rectus femoris and impose less systemic fatigue. Using both ensures regional development across all quad heads and addresses fatigue management.
CaveatsSquats are more systemically fatiguing, so volume and frequency must be managed to avoid overtraining. Leg extensions can be added as a lower-fatigue option to increase total quadriceps stimulus. Strength specificity means that if maximal squat strength is a goal, squats must still be practised.
The speaker explains that the quadriceps does not respond uniformly to all exercises. Regional hypertrophy occurs, likely due to differences in muscle activation patterns and biomechanics across the four quad heads during various exercises, though the exact mechanism isn’t elaborated. Evidence from between-subject and within-subject studies confirms that squats and leg extensions produce a complementary growth pattern. Leg extensions are a knee-dominant, open-chain movement that isolates the quadriceps without significant contribution from the hips or lower back, making them less neurally and metabolically demanding. Squats are a closed-chain, multi-joint movement that loads the spine and requires coordination of many stabilizers. The speaker explicitly advises using variety—‘use a variety of exercises, both compound and isolation’—as the optimal strategy. This clashes with a one-note approach of only doing heavy squats or only doing machine isolation work. The recommendation is backed by the contrast in regional hypertrophy, the fatigue profiles, and the goal of maximizing all quad heads. This isn’t about one exercise being superior; it’s about strategic combination.
If you want to maximize muscular hypertrophy, probably use both, which is why I tell people use a variety of exercises, both compound and isolation, in order to get the best results.
Also said
“Leg extensions are less overall fatiguing than squats. So, taking a squat to the same proximity of failure as a leg extension is more overall fatiguing. ... squats target the quads, the hamstrings, the glutes, the lower back. There are a lot more muscles involved in the quads, which is also why it's more fatiguing, but they also stimulate other muscle groups.”— Provides the fatigue trade-off and broader muscle activation rationale behind combining the exercises.
“Again, what that says is variety is the spice of life, use both.”— Caps the argument with a memorable push for exercise variety.
What's new
Personal practice updates, fresh positions, predictions
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Regional hypertrophy in quadriceps differs by exercise
A new study and supporting within-person trials show that squats and leg extensions build overall quad mass similarly, but each exercise preferentially grows different regions of the quadriceps—rectus femoris for leg extensions, vastus lateralis for squats.
Why this matters: Confirms the 2000s gym-bro belief that you can target specific parts of a muscle with different exercises, contradicting the oversimplified view that exercise selection doesn’t matter for regional development.
Background
Prior conventional wisdom often held that any compound or isolation movement would grow a muscle uniformly, and that trying to ‘shape’ a muscle was largely pseudoscience. Bodybuilders have long claimed otherwise by emphasizing leg extensions for the front of the thigh and squats for outer sweep.
The study in question took untrained individuals through months of training, with groups performing either squats or leg extensions. Both groups were standardized to the same number of hard sets and proximity to failure—the two main drivers of hypertrophy. When total quadriceps growth was measured, the groups didn’t differ. However, when the quadriceps were divided into individual heads, a clear pattern emerged: the rectus femoris (middle quad) responded more to leg extensions while the vastus lateralis (outer quad) responded more to squats. This regional effect isn’t a fluke; another set of studies used a within-person design where one leg was trained with a compound movement and the other with an isolation movement, controlling for genetic and lifestyle differences, and the same results appeared. The practical takeaway is that no single exercise optimally develops all heads of the quadriceps. Additionally, strength is highly specific: the squat group got stronger on squats but not leg extensions, and vice versa. Thus, if your goal is to be strong in a particular lift, you must practice that lift. Finally, the fatigue trade-off matters: taking a squat to failure drains more systemic recovery than taking a leg extension to the same relative effort, making isolation moves useful for adding volume without excessive systemic fatigue. Conversely, squats engage the hamstrings, glutes, and lower back, making them more time-efficient for overall lower-body development. Using both types of exercises yields the most complete hypertrophy.
The gym bros from the 2000s were right. Different exercises can target muscle growth in different places.
Also said
“Overall quadriceps growth was not different between the groups. And again, they standardized them to number of hard sets and proximity to failure. ... However, there were differences in what we call regional hypertrophy. So, the rectus femoris, the middle quad, grew more with leg extensions, whereas the vastus lateralis grew more with squats.”— Directly describes the study’s main finding of distinct regional growth.
“Now, this study doesn't stand in isolation. There are other studies showing this too now. And they've even got studies where people train one leg in a compound movement, and the other leg with an isolation movement, leg extension, and they looked at differences in hypertrophy, and they found the same thing.”— Highlights that this pattern is replicated with more powerful within-subject designs, strengthening the claim.
“Leg extensions are less overall fatiguing than squats. So, taking a squat to the same proximity of failure as a leg extension is more overall fatiguing. ... On the other hand, squats do not just target the quadriceps like leg extensions. Squats target the quads, the hamstrings, the glutes, the lower back.”— Explains the practical pros and cons of each exercise beyond regional growth.
Recommendations
Products, supplements, and tools mentioned in the episode
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Incorporate both squats and leg extensions in leg training
Practice
For maximizing overall quadriceps hypertrophy and ensuring balanced development across all heads of the muscle.
The recommendation emerges from a synthesis of new research showing regional hypertrophy differences between squats and leg extensions. While overall quad growth is similar when volume and effort are matched, relying on only one exercise leaves specific regions under-stimulated. Leg extensions grow the rectus femoris (middle quad) more, while squats grow the vastus lateralis (outer quad) more. Additionally, squats provide the benefit of targeting glutes, hamstrings, and the lower back, while leg extensions allow extra quad-focused volume with less systemic fatigue. Therefore, a balanced program that includes both compound and isolation moves is superior for complete leg development. This directly counters the advice to simply do whichever exercise you prefer or to believe that a single ‘king of exercises’ covers all needs.
vs alternatives
Compared to doing only squats, adding leg extensions improves rectus femoris development. Compared to doing only leg extensions, adding squats improves vastus lateralis development and provides whole-body strength and posterior chain stimulus. Using both also manages fatigue better than adding extra squat volume alone.
This isn't saying squats are best, leg extensions are best. What this is saying is that if you want to maximize muscular hypertrophy, probably use both, which is why I tell people use a variety of exercises, both compound and isolation, in order to get the best results.
Also said
“Now, when it came to strength, surprise, surprise, people who did squats got stronger on squats than people who did leg extensions and vice versa.”— Underscores that strength is specific; if your goal is general quad development, you still need practice on the specific exercise you care about.
Lines worth pulling out — contrarian, specific, or perfectly phrased
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The gym bros from the 2000s were right. Different exercises can target muscle growth in different places.
A direct, provocative acknowledgment that popular ‘bro science’ turned out to be supported by controlled research, challenging academic orthodoxy.
Strength is skill specific. If you practice the specific skill to get stronger at, you will get stronger than people who do not practice that specific skill even at the same level of muscle mass.
Encapsulates the principle of specificity in a clear, actionable sentence that applies far beyond just squats and leg extensions.
Variety is the spice of life, use both.
A memorable, pithy closing advice that ties the entire research discussion into a simple training heuristic.
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Educational summary of the cited expert source — not medical advice. Open the source recording linked above and consult a qualified physician before acting on any protocol.
