Squats, Leg Press, Bulgarian Split Squat - Do You Need Leg Extensions?

The Rectus Femoris Problem - Why It Matters

• A 2021 PMC study comparing EMG activity found that leg extension showed 24.3% higher rectus femoris activation than the leg press (p = 0.002) - confirming the hip-crossing problem directly.
• A 2026 RCT (Kassiano et al., Journal of Strength and Conditioning Research, PMID: 41379528) compared squat vs. leg extension over 8 weeks in 63 women. Leg extension produced far greater rectus femoris hypertrophy at all three measurement sites (proximal: +11.4% vs +2.0%; middle: +12.3% vs +5.7%; distal: +17.5% vs +7.9%) compared to the back squat. The squat was better for the vastus lateralis at the distal site.

Is Leg Extension Safe?

The concerns that led to the warning:

1. Anterior tibial shear force - At low knee flexion angles (0-30°), the quadriceps pulls the tibia forward, stressing the ACL. This was the original reason leg extensions were banned in ACL rehab.
2. Patellofemoral joint stress - The kneecap experiences compressive force during quad contraction.

What the evidence actually shows:

• Patellofemoral stress: A 2014 study by Powers et al. (referenced in Physiotutors) found that patellofemoral stress is actually HIGHER during squats at 90-45° of knee flexion than during seated leg extensions at the same angles. To minimize patellofemoral stress, leg extensions in the 90-45° range are actually safer than deep squats.
• ACL strain: The peak anterior shear force during a seated leg extension is less than 250N at 10-30° of knee flexion. Normal walking produces over 300N of anterior shear force at ~20° of knee flexion. If walking is safe post-ACL surgery, leg extensions at controlled ranges are too.
• A 2018 systematic review of 10 RCTs found no evidence that leg extension exercise damages ACL grafts or increases anterior knee laxity.
• A 2000 study found that ACL rehab patients who included leg extensions had greater quad strength at 6 months, were more likely to return to sport at the same/higher level, and returned to sport earlier.
• Patellofemoral contact area: A 2012 study (Chiu et al.) found that 8 weeks of leg extension + leg press training actually increased patellofemoral contact area, which spreads force and reduces kneecap pain.

When leg extensions ARE genuinely risky:

• Heavy load at 0-30° of knee flexion in people with existing patellofemoral syndrome, patellar tendinopathy, or tibial stress issues
• Immediately post-ACL surgery at maximum effort before the graft has matured
• People with specific instability patterns their surgeon has identified

Do You NEED Leg Extensions Given Your Current Program?

Summary

1. Rectus femoris is systematically undertrained in squats, leg press, and Bulgarian split squats due to active insufficiency when the hip is flexed. The 2026 RCT confirms leg extension produces significantly more RF hypertrophy.
2. Leg extensions are not inherently dangerous for healthy people. The patellofemoral and ACL concerns originated from poorly interpreted biomechanics research. Performed with moderate load, avoiding grinding through 0-10° of knee flexion under heavy load, they are safe. Squats actually produce higher patellofemoral stress at deep ranges.
3. The orthopedic surgeon's advice may be appropriate if you have a specific knee condition (patellofemoral syndrome, meniscal tear, patellar tendinopathy, early post-op status) - in that context it is good individualized advice. As a blanket rule for healthy people, it is not supported by current evidence and is increasingly being walked back in sports medicine literature.
4. Practical recommendation: If you're training for muscle development and your knees are healthy, adding leg extensions (90° to ~40° of knee flexion, avoiding grinding at full extension under max load) will round out your quad development in a way your current three exercises cannot fully replicate.

Leg Extensions, Chondromalacia, and Patellar Tendinopathy: What the Evidence Actually Shows

The Core Biomechanical Issue

• The patella (kneecap) is embedded in the quadriceps tendon and acts as a pulley. When quads contract forcefully, the patella is compressed against the femoral groove beneath it.
• The patellofemoral joint reaction force (PFJRF) depends on two things: (1) the knee flexion angle, and (2) the force the quadriceps generates.
• In a standard leg extension machine with a fixed pad (constant resistance), the moment arm grows as the knee approaches full extension - meaning the quads have to work harder at 0-30°, creating a spike in patellar compression at terminal extension.
• Patellofemoral contact area is smallest near full extension, so the same force is spread over less cartilage surface - increasing stress per unit area.

Chondromalacia - Is Leg Extension a Cause?

What the research actually shows:

• At 90°, 75°, and 60° of knee flexion: the squat produced significantly HIGHER PFJ stress than both types of leg extension.
• At 30°, 15°, and 0° of knee flexion: the non-weight-bearing exercises (leg extensions) produced significantly HIGHER PFJ stress than the squat.
• Their clinical recommendation: to minimize PFJ stress, squats should be performed from 0-45°, and leg extensions should be performed from 45-90° of knee flexion.

• Heavy loading at terminal extension (0-30°) repeatedly
• Pre-existing patellar malalignment (high Q-angle, patellar tilt, trochlear dysplasia) that causes abnormal tracking and focal cartilage overload
• Training beyond tissue capacity (too much load/volume too fast)

Patellar Tendinopathy ("Jumper's Knee") - Is Leg Extension a Cause?

• The primary risk factors for patellar tendinopathy are: high jump/landing volumes, rapid training load increases, poor quad flexibility, and quad weakness - NOT specifically the leg extension machine itself. Basketball and volleyball players get it from thousands of jump-landings, not from gym machines.
• In fact, the leg extension machine is actively used as a treatment for patellar tendinopathy in evidence-based protocols. A PMC article on clinical management of patellar tendinopathy includes concentric leg extensions as a core strengthening exercise, and isometric leg extensions as an early-stage pain-management tool.
• The Steinkamp (1993) and related studies found higher patellofemoral compressive forces at 50-90° during leg press vs. leg extension at the same range - suggesting that for people with patellofemoral arthritis, leg extensions in certain ranges may be preferable to leg press.

• If someone already has patellar tendinopathy, heavy resisted terminal extension can provoke pain and delay healing. This is why some clinicians restrict it during acute flares.
• Doing very high volumes of leg extension on top of high jump volumes (e.g., a volleyball athlete doing heavy leg extensions AND hundreds of jumps per session) could be an excessive cumulative load on the tendon.

The Summary Table

Practical Guidance

1. Chondromalacia risk from leg extensions is low if you do not grind heavy reps to full lockout. Working in the 45-90° arc keeps PFJ stress comparable to or lower than your squats.
2. Patellar tendinopathy risk is more about total training load than the exercise itself. If you keep volume reasonable, you are not creating unique risk.
3. The orthopedic surgeon's concern is most valid for someone with pre-existing patellar malalignment, trochlear dysplasia, existing chondral damage, or a history of anterior knee pain - in those people, leg extensions at heavy load to full extension are genuinely risky.
4. A variable-resistance leg extension machine (where the cam reduces load near full extension) is significantly safer than a constant-resistance machine, per the Powers 2014 data.