Leg extensions are the go-to exercise for bodybuilders looking to build massive quadriceps—but that's about all they're good for. Not only do they not build any sort of functional strength (when would you possibly need to lift something heavy attached to your lower leg?) but they also can be hard on the knees.
If you want to build thigh strength and size with exercises that have real-world applicability and won't kill your knees, you've got plenty of options.
Squats are one of the most effective exercises for targeting the quadriceps. You'll also target your glutes, hamstrings, calves and core muscles—leg extensions can't do that.
HOW TO DO IT: Take a shoulder-width stance holding dumbbells or with a barbell across the backs of your shoulders. Bend at the knees and hips, sending the butt back and down and keeping the chest open, torso upright.
Come down until your thighs are parallel or slightly below. Press through your heels to rise back up to standing, forcefully contracting your quads at the top.
2. Bulgarian Split Squat
If squats are good, split squats are even better. This version takes the quadriceps burn up a notch or ten, isolating each leg for an extra challenge.
HOW TO DO IT: Hold dumbbells or position a barbell across your shoulders. Stand a few feet in front of a box or bench. Lift your left foot up and back, placing the tops of the toes on the box or bench. Contract your core muscles and squat down into the right leg, keeping the torso upright and shoulders back.
Come down until your right thigh is parallel, then press back up to full extension. Squeeze the quad at the top.
How much more functional can you get? With enough weight, you can also build some pretty good size and strength in your quads doing step-ups.
HOW TO DO IT: Hold dumbbells in both hands at your sides. Place your right foot on a step, box or bench, transfer the weight into your right leg and extend through the knee and hip to stand up tall on the bench. Step down with the left foot and repeat.
4. Reverse Lunge
Any type of lunge works the quads and is safer for your knees than leg extensions. However, the hard stop of stepping into a forward lunge can exacerbate existing knee problems if you have them. Stepping back into a reverse lunge is easier on the knees.
HOW TO DO IT: Hold dumbbells at your sides in both hands. Take a big step back with your right foot, landing softly on the ball of the foot and bending the knee as you land. Lower down until both knees form 90-degree angles. Keep your torso erect and chest open. Ensure your front knee does not travel forward of your front toes. Push off the back foot to come back to your starting position.
5. 'Natural' Leg Extensions
If you absolutely can't live without leg extensions, try this 'lite' version of the exercise that uses only your body weight.
HOW TO DO IT: Start on your knees, with your knees and feet parallel about hip-distance apart. Contract your core muscles, glutes and quads and begin to lean back. Reach your arms out in front of you for balance. Lean back as far as you can while maintaining good posture. Return to the starting position using only your quad muscles.