Exercise guide · evidence-graded

Single Leg Extension

The single-leg extension is the seated leg extension machine worked one leg at a time: you straighten one knee against the shin pad, then bend it back under control. It isolates the quadriceps of each leg separately, so neither leg can take load from the other.

Also called: Unilateral Leg Extension, One Leg Extension, Single-Leg Leg Extension, Unilateral Knee Extension · Equipment: machine · Pattern: squat

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How to do it

Step by step

  1. Adjust the back pad so the middle of your knee lines up with the machine's pivot point, and set the shin pad on the front of your shin, just above the ankle.
  2. Sit back fully against the pad with the back of your knees close to the seat edge, and hold the handles at your sides.
  3. Place your working leg behind the pad and let the other leg rest relaxed beside it or tucked under the seat.
  4. Brace your trunk and straighten your working knee in a smooth movement until the leg is fully extended, keeping your thigh pressed into the seat.
  5. Hold the straight-leg position for a moment and squeeze the front of your thigh.
  6. Bend the knee back under control to the start position, about 90°, without letting the weight stack touch down between reps.
  7. Complete the set on one leg, then switch legs and match the reps on the other side.
Coaching cues
  • Squeeze the front of the thigh at full extension
  • Control the weight back down
  • Thigh pressed into the seat
  • Sit back against the pad
  • Match the reps on both legs

In VALDA, these cues are one tap away on the exercise while you train.

Muscles worked

What it trains

MuscleRoleEvidence
Quadriceps: vastus lateralisPrime moverA
Quadriceps: vastus medialisPrime moverA
Quadriceps: vastus intermediusPrime moverB
Quadriceps: rectus femorisSecondaryA

Evidence grades: A = strong research base · B = good · C = limited · D = coaching consensus.

Common faults

What goes wrong, and the fix

Swinging the weight up

Fix: Straighten the knee smoothly and hold the top for a moment.

Letting the stack crash down

Fix: Bend the knee back slowly and keep the plates from touching between reps.

Partial reps short of full extension

Fix: Straighten the knee fully and squeeze before bending it again.

Hips lifting off the seat

Fix: Hold the handles and keep your thigh pressed into the seat.

Knee out of line with the pivot

Fix: Adjust the back pad so the middle of your knee sits level with the pivot.

More reps on the stronger leg

Fix: Match reps and load on both legs, starting with the weaker side.

Programming

What the research says

Rehab & prehab · first choiceMuscle growthGeneral fitnessEndurance

  • One leg at a time — Research on the bilateral deficit finds two limbs working separately usually produce more combined force than both together, which is one reason lifters use single-leg work to load each leg fully.
  • Load and effort — Research finds muscle growth is similar across a wide range of loads when sets are taken close to failure, and true failure adds only a small extra benefit. Common practice is about 10–15 controlled reps per leg.
  • Volume — Research links more weekly sets for a muscle to more growth, with diminishing returns. Training each leg separately takes about twice the time for the same total volume.
  • Range of motion — In a knee-extension training study, reps in the deeper, long-muscle part of the range grew the quads as much as or more than full range, and more than short top-range partials.
  • Transfer — In a 6-week trial, squat training improved squat strength and jump height more than leg-extension training, so the machine is commonly used alongside squats or presses rather than instead of them.
Alternatives · VALDA FLEX

Swap it mid-workout

With VALDA Pro, FLEX offers alternatives like these when you swap an exercise mid-workout.

Log the single leg extension in VALDA, with these cues one tap away while you train and your history for it in one place.

General information about exercise technique, not medical advice. If you have an injury or a health condition, your healthcare provider is the right person to ask.