The quadriceps, or quads for short, are the major knee-joint extensors. They are active when you perform common activities -- such as sitting down, standing up and walking -- and when you play sports involving jumping and sprinting. Using dumbbells is an effective option for strengthening your quads. Most gyms have a variety of dumbbells, ranging from 1 to 100 pounds or more, or you can buy your own to use at home.
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Deadlifts are typically performed with a barbell loaded to your desired weight, but you can also use dumbbells. The weights provide extra resistance as your quads contract to lift them off the floor. Place two dumbbells on top of pillows and spread the pillows out on the floor wider than your shoulders. Stand between the pillows with your feet at least shoulder-width apart and toes pointed forward. Keeping your spine straight, flex your hips and knees and grasp the weights with your palms facing inward. Extend your hips and knees to lift the weights to just outside your hips, then drop them on the pillows. Reposition the weights and start over.
Lunges strengthen the quads through a knee extension and flexion range of motion. During the flexion phase of the exercise, the quads contract eccentrically -- while they are lengthening -- to control the speed of the movement. Stand upright with your feet a comfortable distance apart. Hold dumbbells just outside your hips with your palms facing inward. Step forward 2 to 3 feet with your left foot, coming onto the toes of your right foot. Squat down until your right knee nearly touches the floor, then stand up and step back to the starting position. Repeat with your right leg. Continue alternating legs for your desired number of repetitions.
Squats are similar to deadlifts, but you carry the weight though the entire range of motion, not just the knee-extension phase of the exercise. Stand upright with your feet at least shoulder-width apart and toes directed forward. Hold dumbbells by your hips or 1 to 2 inches in front of your shoulders with your palms facing each other. Flex your hips and knees until your thighs are parallel to the floor, then stand back up and repeat. Keep your back straight throughout the movement to prevent a lower-back injury.
Step-ups also strengthen the quads through knee extension and flexion ranges of motion in a stair-climbing-like movement. Stand in front of a 3-inch box or step, holding dumbbells by your hips. Step on top of the box with your right foot, then extend your right leg completely, lifting your left foot off the floor. Step back down with your left foot, followed by your right, then step up with your left foot and extend your left leg. Continue alternating like this for your desired number of repetitions. Increase the height of the box or step over time and/or increase the weight of the dumbbells to make the exercise more challenging.