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Single-Leg Extension

Unilateral quad isolation — reveals and corrects strength imbalances by training one leg at a time, while providing pure quad focus without balance demands

⚡ Quick Reference​

AspectDetails
PatternExtension (Knee, Unilateral)
Primary MusclesQuads
Secondary MusclesNone (pure isolation)
EquipmentLeg extension machine
Difficulty⭐ Beginner
Priority🟡 Supplemental

Movement Summary​

🎯 Setup​

Starting Position​

  1. Seat adjustment: Position back pad so knee aligns with machine's pivot point
  2. Shin pad: Adjust so pad sits on lower shin, just above ankle
  3. Seating: Sit fully back against pad, back supported
  4. Working leg: Place one shin behind pad
  5. Resting leg: Either off the pad or resting lightly (not assisting)
  6. Grip: Hold handles for stability
  7. Starting knee angle: 90° bend (or as much as machine allows)

Machine Setup​

ComponentPurposeAdjustment
Back PadSupport torsoKnee aligns with pivot
Shin PadResistance pointJust above ankle
Weight StackResistanceStart with 50-60% of bilateral weight
Setup Cue

"One leg only — the other leg does nothing. You'll immediately notice which leg is weaker."

🔄 Execution​

The Movement​

What's happening: Seated with one leg bent at 90°, ready to extend

  1. One shin behind pad, knee bent
  2. Other leg completely relaxed (not helping)
  3. Back against pad
  4. Core braced lightly
  5. Hands on handles for stability
  6. Quad of working leg engaged

Feel: Quads preloaded, ready to extend

Key Cues​

Primary Cues
  • "One leg only — no help from the other"
  • "Squeeze at the top" — full quad contraction
  • "Control the descent" — slow eccentric
  • "Keep your butt in the seat" — no hip extension

Tempo Guide​

GoalTempoExample
Hypertrophy2-1-3-02s up, 1s squeeze, 3s down
Strength1-1-2-01s up, 1s squeeze, 2s down
Endurance1-0-2-0Faster pace, constant motion

💪 Muscles Worked​

Activation Overview​

Primary Movers​

MuscleActionActivation
QuadricepsKnee extension — only action██████████ 95%
- Rectus FemorisKnee extension (crosses hip too)█████████░ 90%
- Vastus LateralisKnee extension (outer quad)██████████ 95%
- Vastus MedialisKnee extension (inner quad, VMO)██████████ 95%
- Vastus IntermediusKnee extension (deep quad)█████████░ 85%

Stabilizers​

MuscleRole
CoreMinimal stabilization while seated
Hip StabilizersPrevent hip movement, keep pelvis stable
Non-working legNone — should be completely relaxed
Unique Benefit

Single-leg training reveals asymmetries — most people discover their dominant leg is 10-20% stronger. Training one leg at a time forces the weaker leg to catch up, preventing compensations and building balanced strength.

⚠️ Common Mistakes​

MistakeWhat HappensWhy It's BadFix
Other leg helpingResting leg pushes slightlyDefeats unilateral purposeKeep resting leg completely relaxed
Using too much weightCan't control descentCheating, less muscle activationReduce weight, control the negative
Partial ROMNot extending fullyReduced quad activationFull extension with squeeze
Hip extensionButt lifts off seatReduces quad focusKeep glutes in seat
Dropping the weightWeight crashes downLess time under tensionSlow, controlled eccentric
Most Common Error

Using the same weight as bilateral — single-leg extensions require significantly less weight (typically 50-60% of bilateral load). Ego has no place here.

Self-Check Checklist​

  • Other leg is completely relaxed
  • Using 50-60% of bilateral weight
  • Full ROM — bent to fully extended
  • Squeezing at top for 1 second
  • Slow, controlled descent (2-3 seconds)

🔀 Variations​

By Tempo​

VariationHowWhen to Use
Standard Tempo2-1-2-0General hypertrophy
Controlled1-1-2-0Building strength

By Position​

VariationHowEffect
Toes PointedPoint toes forwardStandard activation
Toes OutTurn toes outwardSlight emphasis on inner quad (VMO)
Toes InTurn toes inwardSlight emphasis on outer quad

📊 Programming​

Rep Ranges by Goal​

GoalSetsReps Per LegRest Between LegsNotes
Strength3-48-1230-45sControlled tempo
Hypertrophy3-410-1530-60sSqueeze at top
Endurance2-315-2520-30sLighter weight

Workout Placement​

Program TypePlacementRationale
Leg dayAfter squats/leg pressQuad isolation finisher
Unilateral focusFeature exerciseAddress imbalances
RehabilitationPrimary exerciseRebuild single-leg strength
Pre-exhaustFirst exerciseFatigue quads before compounds

Progression Scheme​

How to Progress

Always match reps on your weaker leg. If right leg does 12 reps, left leg must do 12 reps. Add weight only when both legs can hit target reps.

🔄 Alternatives & Progressions​

Exercise Progression Path​

Regressions (Easier)​

ExerciseWhen to Use
Bilateral Leg ExtensionBuild base strength first
Partial ROM Single-LegWork up to full ROM
Assisted Single-LegNeed support initially

Progressions (Harder)​

ExerciseWhen Ready
Tempo VariationsAdd time under tension
Bulgarian Split SquatAdd stability demand
Single-Leg SquatFunctional unilateral strength

Unilateral Alternatives​

AlternativeWhen to Use
Bulgarian Split SquatMore functional, compound
Single-Leg PressMore loading potential
Step-UpAthletic carryover

🛡️ Safety & Contraindications​

Who Should Be Careful​

ConditionRiskModification
Patellar tendonitisDirect stress on patellar tendonMay need to avoid or reduce ROM
Knee arthritisJoint stress under loadUse light weight, full ROM
ACL injury recoveryShear forces on kneeCommon in rehab — follow PT guidance
Meniscus issuesLoaded knee flexion/extensionAssess tolerance carefully
Stop Immediately If
  • Sharp pain in knee (especially patellar tendon area)
  • Grinding, clicking, or popping
  • Swelling after exercise
  • Pain worsens with each rep

Rehabilitation Use​

Single-leg extensions are commonly used in knee rehab for:

  • Rebuilding quad strength after injury
  • Addressing imbalances post-surgery
  • Controlled loading in safe environment
  • Isolating quad without balance demands

Always follow physical therapist guidance for rehab applications.

🦴 Joints Involved​

JointActionROM RequiredStress Level
KneeFlexion/extensionFull (0-90°+ bend)🟡 Moderate
HipStabilized (no movement)Neutral position🟢 Low
Joint Safety

Leg extensions place shear force on the knee joint. This is not inherently dangerous for healthy knees, but those with patellar tendonitis or recent ACL injuries should consult a professional before performing.

❓ Common Questions​

Why do single-leg instead of both legs at once?

Single-leg extensions reveal and correct strength imbalances. Most people have a dominant leg that's 10-20% stronger. Training both legs together allows the strong leg to compensate. Single-leg training forces each leg to work independently, building balanced strength.

How much less weight should I use compared to bilateral?

Typically 50-60% of your bilateral weight. For example, if you do bilateral leg extensions with 100 lbs, start with 50-60 lbs for single-leg. Adjust based on what you can control properly.

Are leg extensions bad for my knees?

For healthy knees, leg extensions are safe. They do create shear force on the knee joint, but this isn't inherently harmful. However, if you have patellar tendonitis or recent knee injuries, consult a professional. Leg extensions are actually commonly used in knee rehabilitation.

Should I train my weaker leg first or second?

Train your weaker leg first while you're fresh. This ensures you give it full effort. Then match those reps with your stronger leg. If needed, do an extra set on the weaker leg.

My right leg can do 15 reps but my left can only do 10. What do I do?

Do 10 reps on both legs (matching the weaker leg), or do an extra set on the left leg to accumulate more volume. Don't let the right leg get further ahead.

📚 Sources​

Biomechanics:

  • Knee joint mechanics and leg extensions — Tier A
  • Unilateral training research — Tier B
  • ExRx.net — Tier C

Rehabilitation:

  • ACL rehabilitation protocols — Tier A
  • Physical therapy quad strengthening — Tier A

Programming:

  • Hypertrophy research — Tier A
  • Unilateral training for strength balance — Tier B
For Mo

When to recommend this exercise:

  • User reports strength imbalances between legs
  • User wants pure quad isolation
  • User is in knee rehabilitation (under PT guidance)
  • User wants to address lagging quad development
  • User prefers machine-based training

Who should NOT do this exercise:

  • Acute patellar tendonitis without PT clearance
  • Recent ACL surgery without PT protocol
  • Severe knee arthritis with pain during movement
  • Anyone experiencing sharp knee pain during the movement

Key coaching cues to emphasize:

  1. "Weaker leg sets the standard — match those reps"
  2. "Other leg does NOTHING — completely relaxed"
  3. "Squeeze hard at the top for 1 second"
  4. "Control the descent — don't let weight drop"

Common issues to watch for in user feedback:

  • "One leg is way weaker" → This is normal; program extra volume for weaker leg
  • "My knee hurts during this" → Assess: sharp or dull? Location? May need to reduce ROM or avoid
  • "Should I use the same weight on both legs?" → YES, absolutely
  • "Can I do both legs together to save time?" → Defeats the purpose; use bilateral leg extension instead

Programming guidance:

  • 3-4 sets of 10-15 reps per leg
  • Rest 30-60 seconds between legs
  • Place after compound leg work
  • Can be used 2-3x per week
  • Match reps on both legs; add extra set to weaker leg if needed
  • Progress only when BOTH legs hit target reps

Imbalance correction protocol:

  • Identify weaker leg (usually non-dominant)
  • Always train weaker leg first
  • Match reps on stronger leg
  • Optional: 1-2 extra sets on weaker leg
  • Reassess after 4-6 weeks

Last updated: December 2024