Everyone looks at the sled as a brutal conditioning tool, but it's also the single best joint-friendly strength training. Just about everyone who has a bad back or cranky knees can do very heavy sled pushes with zero pain. The sled places little to no extra load on the spine, and it involves far less knee bending than squats and lunges.

For people who experience back or knee pain from traditional lower-body lifts, the sled becomes a staple strength training tool. That means you don't just use it with light loads for longer durations like you would for conditioning.

You load the sled up heavy and push it a much shorter distance, which is analogous to doing heavy squats or deadlifts for sets of 5 or so reps. (If you're looking for a quad burn, use less weight on the sled and push it for a longer distance.)

Quick tip about arm placement:

Here's how I program sled pushes based on total distance pushed per set, relative to more traditional rep ranges:

• Heavy Weight, Low Reps: 20 to 30 total yards pushed for 5 sets
• Medium Weight, Medium Reps: 40 to 60 total yards pushed for 3 to 4 sets
• Light Weight, High Reps: 80 to 100-plus total yards pushed for 2 to 3 sets

Rest 2 to 4 minutes between sets.

Let's say I want an athlete to do a set of heavy lunges for 5 reps per leg, but lunges give him knee pain. I'd load up the sled with a weight he can't push farther than 30 yards without needing a rest between sets. Then he'd do 5 sets of 20 to 30-yard pushes.

Note: If only have 20 yards or turf to push the sled, just turn around and keeping going until you've completed the total distance of the set.

Check out 5 Weight Sled Drills You've Never Tried for more ways to use the sled.

Modified Backward Sled Drag

This is modified simply because you don't perform it with your knees bent at 90-degrees as if you were sitting in an invisible chair. Instead, you sit your hips down by bending your knees only as much as you can without feeling pain, then drag the sled that way.

You don't even have to be as low as Susy Natal is in the video because, even if there's hardly much bend in knees, your quads still have to work hard to drag the sled. The nature of walking backwards is basically a standing leg extension.

Use the same weight-to-distance range provided above. Just make sure you find the heaviest weight you can't pull much farther than the target distance. So if you're going for the sled drag equivalent of a medium-rep range, use a weight that doesn't allow you to move the sled much more than 60 total yards (or so) on each set.

It's important to note that both of these sled drills involve other muscles than your quads. And yes, they certainly get your lungs burning and heart racing. I'm including them here because they're extremely quad intensive, especially at longer distances. Be ready for a quad burn!

Riding a bike is very quad-focused. Outdoor cycling or taking an indoor cycling class a few times per week is great knee-friendly quad training.

However, many lifters aren't interested in spending that much time on a bike. So here's how to use the stationary bike (upright or recumbent) for knee-friendly quad strengthening in a way that fits into a strength workout.

Heavy Weight, Low Reps: Set the pedal resistance to where you feel it challenges you at about an 8 out of 10, with 10 being the heaviest. Perform intervals where you pedal as hard and as fast as you can (you won't be moving very fast) for 20 to 30 seconds, with 2 to 3 minutes rest between sets. Do 5 to 6 sets.

Medium Weight, Medium Reps: Set the resistance to a level where you feel it challenges you at about a 6 out of 10. Cover as much distance as you can in 40 to 60 seconds, with 2 to 3 minutes rest between sets. Do 3 to 4 sets.

Light Weight, High Reps: This is basically endurance work. So, in this case, you'd do it at the end of strength workout. Set the resistance to a level to where you feel it challenges you at about a 3 out of 10, and ride for 3 to 5 miles in as little time as you can. Do 1 set.

Airdyne Bike: Quad-Builder Protocols

Some people only have access to an Airdyne-style bike that also allows you to use your arms, which is what I have at home. Since the Airdyne bike doesn't have tension adjustments, here's how to do the above protocols:

Heavy Weight, Low Reps: Perform intervals where you pedal as hard and as fast as you can for 20 to 30 seconds with 2 to 3 minutes rest between sets. Do 5 to 6 sets.

Medium Weight, Medium Reps: Start without your hands on the handles so you're only using your legs to move the pedals. Cover as much distance as you can in 40 to 60 seconds. If your legs get too fatigued before you reach the target time of each interval, simply grab onto the handles and use your arms to help finish the set. Rest 2 to 3 minutes between sets. Do 3 to 4 sets.

Light Weight, High Reps: Starting without your hands on the handles. Ride for 3 to 5 miles in as little time as you can. Grab onto the handles anytime to give your legs some help. Perform 1 set.

Note: Many people like to grab the back of the bike seat when doing hands-free riding because it keeps them very stable. Give it a shot.

This is a great at-home, knee-friendly training option that's surprisingly a quad killer.

Don't let the small range of motion fool you! Your quads will be burning. You can use a rubber Superband, but I'm using NT Loop bands because they're far more comfortable and stable. You can also do this holding dumbbells. Just place the horizontal band around the back of your working side knee.

Note that your back foot isn't weight-bearing, but it's still touching the ground as a "kickstand" to stabilize you.

Make sure to walk back as far as you can to maximize the tension on the horizontal band. Also, grab the vertical band in a way that takes out the slack and provides enough resistance to challenge you so you can't do any more than 20 to 50 reps per sets.

Do for 3 to 4 sets with 1-minute rest between sets.

Do these exercises as a workout finisher by performing them back-to-back without rest:

• Straight-Leg Walk: 2 x 45 seconds
• Low Linear Walk: 2 x 45 seconds
• Reverse Nordic: 2 x 10-15

Perform 2 to 4 rounds with 1-2 minutes between rounds.

Each of these exercises can certainly be done on their own. However, if you're fairly strong, doing them in this series will provide you with a more appropriate challenge. Plus, doing all three moves makes things more interesting.