Leg extension
The leg extension isolates the quadriceps and is the best-supported way to grow the rectus femoris, the one quad head that squats and leg presses barely touch. Several randomized trials show solid growth; it adds to compound lifts rather than replacing them, since it does nothing for the glutes or adductors.
What the science says
- Builds the quadriceps Strong evidence
Kinoshita et al.'s 2026 MRI trial found 12 weeks of knee extensions increased whole-quadriceps volume by 7.1%, comparable to the leg press. Zabaleta-Korta et al. (2021) and Pedrosa et al. (2022) also found significant quadriceps growth in randomized leg extension programs.
- Best option for the rectus femoris Strong evidence
Kinoshita et al. found the rectus femoris grew 13.2% with knee extensions versus 1.1% with the leg press, and Zabaleta-Korta et al. found leg extensions grew all regions of the rectus femoris while Smith machine squats did not. Ema et al. (2013) found the rectus femoris grew proportionally more than the vasti after 12 weeks of knee-extensor training.
- The deep, knee-bent part of the range matters most Moderate evidence
Pedrosa et al. found training the initial range (knee at 100°–65°, quads lengthened) produced more regional growth than training the last part of the range, and as much as or more than full range, in 45 untrained women.
- Does not grow the glutes or adductors Moderate evidence
Kinoshita et al. found the leg press grew the gluteus maximus (+15.4%) and adductor magnus (+6.2%) while knee extensions did not, so the leg extension complements rather than replaces a squat or press.
How to do it
Muscles trained: quadriceps
- Line the knee up with the machine's pivot point; pad just above the ankle
- Start from as much knee bend as the machine allows
- Extend to nearly straight in about 1 s, lower in 2–3 s
- Keep the hips down on the seat and hold the handles
How to apply it
Add it after your main squat or press, use a full range starting from deep knee flexion, and take sets close to failure.
Who should be careful
Use loads you can control through the whole range and lower slowly; people with kneecap pain often tolerate a shorter range better. Adjust the seat so the knee lines up with the machine's pivot. Anyone with knee pain or a recent knee injury should get individual advice from a physician or physical therapist.
Frequently asked questions
Is the leg extension bad for the knees?
Not for healthy knees at sensible loads. Trials such as Kinoshita et al. and Pedrosa et al. used it for 12 weeks in untrained adults to grow the quads; people with kneecap pain may need a shorter range and should get individual advice.
Do I need leg extensions if I already squat?
For the rectus femoris, yes. Kinoshita et al. found it grew only 1.1% with the leg press but 13.2% with knee extensions, and Zabaleta-Korta et al. found Smith machine squats did not grow it.
Should I use full range or partials?
Use the deep, knee-bent part. Pedrosa et al. found that training the initial range grew more muscle in several regions than training only the last part of the range.
Scientific references
- Hypertrophic Effects of Single- versus Multi-Joint Exercise: A Direct Comparison between Knee Extension and Leg Press
- The role of exercise selection in regional Muscle Hypertrophy: A randomized controlled trial
- Partial range of motion training elicits favorable improvements in muscular adaptations when carried out at long muscle lengths
- Inhomogeneous architectural changes of the quadriceps femoris induced by resistance training
More exercises
- Front squat Moderate evidence
The front squat holds the bar on the front of the shoulders, which keeps the torso more upright and loads the knees less than the back squat while recruiting the thigh muscles about as much.
- Bulgarian split squat Moderate evidence
The Bulgarian (rear-foot-elevated) split squat trains one leg at a time and builds the quads and glutes with far less load on the bar than a back squat.
- Romanian deadlift Moderate evidence
The Romanian deadlift is a hip hinge that loads the hamstrings and glutes at long muscle lengths, and trials of stiff-leg deadlifts and similar hip-extension exercises show real hamstring growth, mainly in the semimembranosus and biceps femoris long head.
- Conventional deadlift Moderate evidence
The conventional deadlift trains the whole posterior chain and grip and is a core strength lift, but research on its growth effects is thin: trials either use other deadlift variants or measure strength rather than muscle size.
- Barbell hip thrust Moderate evidence
The barbell hip thrust grows the glutes, and it does so with little growth of the quads or adductors, which makes it a good targeted addition.
- Lunges Moderate evidence
Lunges (forward, reverse or walking) train the quads and glutes one leg at a time and highly activate the glutes.