What Muscles Do Straight Leg Raises Work?

Straight leg raises (SLRs)—lifting your leg while keeping the knee completely straight—work your hip flexors and quadriceps, particularly the rectus femoris which crosses both the hip and knee. This fundamental exercise is a cornerstone of knee rehabilitation because it strengthens the quad without requiring knee bending, making it safe even immediately after surgery.

Quick Answer

Primary muscles: Hip flexors (iliopsoas), rectus femoris (quad that crosses hip)

Secondary muscles: Other quadriceps (isometric), tensor fasciae latae (TFL), sartorius, core

What makes it unique: Strengthens the quad without knee movement—perfect for early knee rehab when bending is limited or painful.

Complete Muscle Breakdown

Hip Flexors / Iliopsoas (Very High Activation)

The iliopsoas is the primary hip flexor:

Psoas major: Originates from lumbar spine
Iliacus: Originates from pelvis
Together: Powerful hip flexion
Primary mover: Does most of the lifting work

Rectus Femoris (Very High Activation)

This is the key quad muscle for SLRs:

Unique anatomy: Only quad that crosses the hip
Dual function: Hip flexion AND knee extension
Critical for SLRs: Works as hip flexor when knee is straight
Why SLRs build quad strength: Rectus femoris is part of the quadriceps

How the Rectus Femoris Works Here

When your knee is straight:

Rectus femoris is shortened at the knee
But lengthened at the hip when leg is down
Lifting the leg shortens it at the hip
This works the muscle significantly

This is why SLRs are considered quad exercises even though the knee doesn't bend.

Other Quadriceps (Isometric)

The other three quad heads work isometrically:

Vastus lateralis: Keeps knee straight
Vastus medialis (VMO): Maintains extension
Vastus intermedius: Supports knee lock
No movement: But must stay contracted throughout

Tensor Fasciae Latae / TFL (Moderate)

The TFL assists hip flexion:

Located on outer hip
Assists with lifting the leg
More active with internal rotation
Works with iliopsoas

Sartorius (Moderate)

The sartorius assists:

Longest muscle in the body
Helps flex the hip
Runs diagonally across thigh
Secondary helper

Core Muscles (Moderate)

Your core stabilizes throughout:

Rectus abdominis: Stabilizes pelvis
Obliques: Prevent rotation
Must brace: Especially with heavier legs or added weight

Why Straight Leg Raises Matter for Rehab

Quad Strengthening Without Knee Bending

After knee surgery or injury:

Knee bending may be restricted
But quad strength is critical for recovery
SLRs work the quad without knee movement
Allows early strengthening safely

The Extensor Lag Problem

After knee injury:

Quads often can't fully straighten the knee against gravity
This is called "extensor lag"
SLRs specifically train this ability
Must lock knee before lifting

Post-Surgical Essential

SLRs are typically started immediately after:

ACL reconstruction
Meniscus surgery
Total knee replacement
Most knee procedures

They're often the first strengthening exercise prescribed.

Hip Flexor Importance

Strong hip flexors are needed for:

Walking (lifting the leg)
Climbing stairs
Getting in/out of chairs
Running and sports

Proper Straight Leg Raise Technique

Setup (Supine Position)

Lie on your back on firm surface
Bend the non-working leg (foot flat on floor)
Working leg straight on the ground
Arms at your sides or hands under low back for support

The Movement

Lock your knee completely straight (tighten quad)
Flex your foot (toes toward shin)
Lift the straight leg off the ground
Raise to 45-60 degrees (or match the bent knee height)
Hold briefly at the top
Lower with control back to start
Keep knee locked throughout entire movement

Key Cues

"Lock your knee before you lift"
"Keep your knee straight the whole time"
"Toes toward ceiling"
"Lower with control"
"Press your low back gently into the floor"

The Quad Set Connection

Before Each SLR

Perform a quad set (isometric quad contraction):

Tighten the quad
Press the back of the knee down
This "locks" the knee straight
THEN lift the leg

Why This Matters

If you lift without locking:

Knee may bend
Less quad activation
Defeats the purpose
May stress the knee

Always set the quad first.

Common Mistakes

Knee Bending During the Lift

The knee must stay locked:

Any bend reduces quad work
May stress healing structures
Focus on quad contraction throughout
If knee bends, quad is too weak (need more quad sets first)

Lifting Too High

Higher isn't necessarily better:

45-60 degrees is adequate
Going higher reduces hip flexor challenge
Quality over quantity

Using Momentum

Controlled movement is key:

No swinging the leg
Lift and lower with control
2-3 seconds up, 2-3 seconds down

Arching the Low Back

Keep spine neutral:

Don't let low back arch
Press gently into floor
Bend non-working leg to help
May need hands under low back for support

Forgetting to Lock the Knee

The quad set is essential:

Tighten quad before lifting
Maintain throughout
This is what makes it a quad exercise

Programming Straight Leg Raises

For Post-Surgical Rehab

Sets/reps: 3 sets of 10-15 reps per leg
Frequency: 3-4 times daily
Hold at top: 2-3 seconds
Start: As soon as cleared (often day 1)

For Quad Strengthening

Sets/reps: 3 sets of 15-20 reps per leg
Frequency: Daily initially, then 3-4x per week
Progression: Add ankle weights when easy

For Extensor Lag

Sets/reps: Multiple sets throughout day
Focus: Perfect knee lock before lifting
Goal: Eliminate any bend during lift
Duration: Until lag is resolved

For General Fitness

Sets/reps: 2-3 sets of 15 reps per leg
When: As warm-up or accessory
Purpose: Hip flexor and quad maintenance

Sample Knee Rehab Day

Quad sets: 3x20 (10-second holds)
Straight leg raises: 3x15 each leg
Terminal knee extensions: 3x20
Heel slides (if cleared): 3x15
Ankle pumps: 3x20

Straight Leg Raise Progressions

Bodyweight SLR (Start Here)

No added weight
Master form first
Build up to 3x20 easily

Ankle Weight SLR

Add 1-5 lb ankle weights
Progressive overload
Increase when reps become easy

Resistance Band SLR

Band around foot, anchored
Provides constant tension
Different resistance curve

Hanging Straight Leg Raise

Hanging from bar
Much more challenging
Core dominant variation
Different exercise really (more core than quad)

SLR Variations

Supine SLR (Standard)

Lying on back
Standard version
Most common for rehab

Side-Lying SLR (Hip Abduction)

Lying on side
Targets gluteus medius
Different muscle emphasis
Common companion exercise

Prone SLR (Hip Extension)

Lying face down
Lift leg behind you
Targets glutes/hamstrings
Different exercise

Seated SLR

Sitting on edge of surface
Extend knee and lift thigh
More hip flexor emphasis
Good variation

Standing SLR

Standing, lift straight leg forward
Balance challenge
Functional variation
More advanced

Who Should Do Straight Leg Raises?

Essential For

Post-knee surgery patients (often day 1)
ACL reconstruction rehab
Meniscus surgery recovery
Total knee replacement recovery
Anyone with extensor lag

Great For

Hip flexor strengthening
Quad maintenance when knee is limited
Older adults (functional strength)
Core activation (secondary)

Safe For Most People

SLRs are very low risk:

No knee stress
Controlled movement
Easy to modify
Appropriate for most conditions

Use Caution If

You have acute hip flexor strain
You have low back pain with leg lifts
You have hip joint pathology (check with provider)

The Bottom Line

Straight leg raises work your hip flexors and quadriceps—particularly the rectus femoris—without requiring any knee movement. This makes them one of the most important exercises for knee rehabilitation, allowing quad strengthening even when knee bending is restricted.

The key is locking your knee completely straight before lifting. This turns a hip flexor exercise into a quad exercise, maintaining the muscle strength that's critical for knee recovery. Simple, safe, and effective—SLRs belong in virtually every knee rehab program.

Build quad strength safely with targeted programs at FoundationalRehab.