Leg Extensions and Knee – Tips for Personal Trainers and Physiotherapists

The issue of the safety of the leg Extension exercise is a very hot topic and one that will surely raise some discussion – however here is a simple summary of the issues with this exercise.

1. Shearing Forces

In the majority of ground based leg exercises – such as squats and lunges – the resistance to movement runs through the knee joint – basically compressing and stabilizing the knee itself. However – in an exercise such as the Leg Extension the resistance is at 90 degrees to the shin – as this is where you are pushing against to raise the shin pad. 

The resistance is at 90 degrees to the long axis of the bone (the tibia) – which has the effect of producing shearing forces across the knee – compared to a squat where the resistance runs along the long axis of the bone.

2. No Co-contraction:

In conventional squats and lunge type movements there is contraction of the quadriceps ( to extend the knee) at the same time as there is contraction of the hamstring group ( to extend the hip ) – this is termed co-contraction.

In the Leg Extension exercise there is contraction of the quads with limited hamstring involvement – again promoting shearing forces across the knee and reducing muscle involvement.

3. Functional Transfer:

The actual motion of the Leg Extension exercise produces limited functional carry over to real life sports and daily tasks – compared to squatting and lunging movements that are common patterns in everyday life.

4. Reduced Muscle Involvement:

The majority of PT clients are time poor and are better off doing an exercise involving a large number of muscle groups compared to smaller isolated groups – simply from a time management and results perspective.

That being said – I think the actual risks of doing the leg extension exercise are somewhat exaggerated however it comes down to time efficiency and getting your client the best results in the shortest possible time.

For a full analysis of the Leg Extension and knee rehabilitation in general I suggest you check out my “Knee Injury Prevention and Rehabilitation for Health Professionals” which can be previewed at:


You can get full acces to the full presentation and more than 50 hours of other exercise rehabilitation and injury prevention videos at www.PhysioProfessor.com

Paul Wright

One thought on “Leg Extensions and Knee – Tips for Personal Trainers and Physiotherapists

  1. Knee ExercisesMy right knee has aawyls sprained easily and had other such maladies. My doctor told me I need to do exercises to strengthen it but didn’t tell me which ones. I don’t belong to a gym, so what type of home exercises can I do to strengthen it? Several blood relatives have had knee replacements in their 60 s and I don’t want to look forward to that in 20 years.If you can buy a stationary bike for your home, do it. Best investment I ever made (got mine on sale through amazon). It’s a lifetime thing, protecting tricky knees, so I totally think it’s worth it. I once had to undergo a lot of physical therapy for my knees. Every knee problem is different, and every physical therapy routine is tailored to the individual, but my guy uses the stationary bike with almost everyone. We also did a variety of stretches, massages, resistance band exercises, and exercises with some heavy ankle weights, but I would not recommend doing these without careful research, if not help from a professional. But stationary bike you can do right away. Try this: while you’re cycling, really think about those muscles on the lower part of your thigh, above the knee from time to time as I cycled, my physical therapist would put his hands on the top and sides there to keep me focused on working those muscles. Expect soreness, it’s a workout. But if you ever start feeling any PAIN (actual joint-related pain), slow down and stop (no sudden stops), readjust and realign yourself, and slowly start up again. You do need a good amount of resistance on your bike, enough that it’s hard and sweaty work (if it’s too loose, you’re more likely to jerk around and use momentum as opposed to your muscles, leading to further injury). Hard resistance, firm and steady pedaling, NOT fast cycling. Don’t forget careful stretching after your workout and no bouncing through the stretches!Eventually, strengthening those muscles protected my knees and kept my kneecaps better aligned, and the pain went away and flexibility returned. (Obviously, our problems are different, but this might be helpful anyway.) I don’t know what your fitness level is, but probably stay off the weights for now, or go very easy. And I’d say do NOT do squats until your knees are in better shape and your leg muscles feel stronger from the bike work. I absolutely love squats and they’re one of my favorite exercises now that my knees are better. But you gotta go slow and steady with something as tricky as knees, and you don’t want them to get worse by straining them before you’re ready. Good luck!