Does the thought of lower-body exercise make you weak in the knees?

The knee is one of the strongest joints in the human body, responsible for supporting such daily activities as running, walking and even standing. Workouts that target the knee joint are vital for increasing total body strength and stability. However, if you suffer from sore knees, you're more likely to avoid these exercises entirely due to a fear of injury.

While people with wonky knees should practice caution when working out, the real training tip is to know which exercises to embrace and which to ditch.

Here are the top three moves to nix, and the best substitutes to support lower-body strength:

Avoid: Full arc extensions. You've likely been using the leg extension machine since you bought your first gym membership. But this potentially hazardous piece of equipment is most likely a contributing cause to your sore, stiff knees.

The machine puts the knees in a very dangerous position, as the joints are not designed to carry heavy weight in the forward direction. That much force can also jeopardize your ACL — the ligament that stabilizes the knee joint. Plus, most of us don't know how to properly adjust the seat to ensure that the equipment engages the correct muscle group. When this occurs, your leg is aligned for one thing: injury.

Try this instead: Lying leg lifts
Low-impact, equipment-free and knee-friendly, the lying leg lift offers more muscle toning opportunities than the standard seated extension. This move is unique in that it improves core and back strength, as well.

  • Start in a supine position, with your legs bent and your feet firmly planted on the floor.
  • Imagine pushing your back into the floor. You should not be able to slide your hand between the small of your back and the floor. This test ensures that your core is fully engaged.
  • Extend your right leg fully but do not lock the joint, keeping the ankle bent at a 90-degree angle. Slowly lift the right leg off the ground until it reaches the height of the left knee. Hold for two breaths, then lower it until it is about 1-2 inches from the ground. Keep tension on the leg by never allowing it to fully rest on the floor.
  • Repeat 20-25 times on each leg.

Avoid: Lunges. I love lunges. They are an important exercise for burning calories and building strong glutes. However, when performed incorrectly, the lunge can put unnecessary stress on the knee joint.

So if you have healthy knees and practice proper form, go lunge the day away. On the other hand, if you suffer from sore knees, opt for modified moves to ensure that your form is correct and your joints are stable.

Try this instead: Wall sits
The wall sit is an isometric, quad- and glute-strengthening exercise. It is safer for the knees because the body is in a fixed position with added support from the wall.

  • Begin by facing away from the wall.
  • Lean the back of your body against the wall and walk your feet out one to two feet from the wall.
  • Slide your back down the wall until your thighs are parallel to the ground. Hold for 30 to 60 seconds. Repeat three times with a 60 second rest between sets.

Avoid: Hurdler's stretch. This move is more of a stretch than an exercise but I'd be remiss not to include it on my "caution list" as so many people practice it before and after their fitness routine.

The hurdler's stretch is the one for which you lock out your knees and awkwardly try to reach your toes with your fingertips. But locking the joints can cause hypertension and stress to the knee. In fact, you should never lock a joint, especially if you have any pre-existing knee troubles.

Try this instead: Supine hamstring stretch
Healthy hamstrings prevent lower-back pain, improve physical fitness, and reduce your risk of injury. Keep your back safe by practicing this stretch in a supine position. For best results, use a large towel or resistance band for an assisted stretch.

  • In a supine position on the floor with legs extended, wrap a towel or resistance band around the center of your right foot. Keep tension on the band/towel by grasping the ends with both hands.
  • Keep the left leg extended on the ground as you slowly raise the right leg up in the air. Continue moving the leg up until your feel a deep stretch in the right hamstring. Hold for 30 to 45 seconds. Repeat on the opposite leg.

Want more knee-friendly exercises? Include these three moves into your next routine:

  • Hip thrusts
  • Step ups
  • Squat sits

Get a leg up on your workout by being kind to your knees.