A leg extension done one leg at a time. Isolates each quadriceps so a weaker side can't hide behind the stronger one.
Primary muscles
This information is general fitness guidance, not medical advice. Stop if you feel pain and consult a qualified professional.
Main muscles working (thicker lines)
Sit in a leg extension machine with your knees lined up with the pivot and the pad on one lower shin, above the ankle. Let the other leg hang free and hold the side handles.
Extend the working knee until the leg is straight.
Squeeze the quad for a moment at the top.
Lower the pad under control until the knee is bent to about 90 degrees.
Complete all reps, then switch legs.
Breathing: Exhale as you extend, inhale as you lower.
Squeeze at the top
Slow lowering
Hips stay in the seat
Using the two-leg weight
Kicking the weight up with momentum
Twisting the hips toward the working side
Align your knee with the machine's pivot before loading.