This leg exercise develops the Quadriceps muscle group. Get the full range of benefits of the leg extension exercise and feel the muscle being worked during the entire movement. For this leg exercise, it is better to go for isolation and squeeze the muscles hard. Leg extension exercise should not be done with heavy weights and if you have any knee problems, should not be done at all.

keep your hips from lifting up as you perform this leg and butt exercise. Then move the lever forward by extending your knees until your legs are straight. This is the only leg exercise that you can lock your knees out without risking damage. After reaching the end of the motion, pause and hold briefly and slowly return the weight to a point near the starting position by bending knees. Repeat.

The best part of this leg muscle exercise occurs at the contraction when your legs are straight. To gain the most out of this best leg exercise, hold the contraction at the top for a few seconds then lower the weight. To hit the outer quads (Vastus Lateralis), lean back, turn your toes out slightly and point them.

Exert outward pressure as your come up. Try to rotate your legs outward at the hip joint, but don’t turn your feet excessively, just about half the range of motion they will go. To hit the teardrop or inner quads (Vastus Medialis), lean over forward, curl your toes back and turn them slightly in. As you come up, exert inward pressure, making an attempt to inwardly rotate your legs at the hip joint.

Keep your legs straight so that you activate of all the quad muscles. To prevent body from rising off the seat, stabilizers are sometimes used during heavy resistances.
This leg and thigh exercise will surely result in slim, tone legs.