Are Leg Extensions Good or Bad, Safe or Dangerous, Effective or a Waste of time, Functional or NonFunctional? – Exercise Expert Brad Schoenfeld has the Surprising Answers!

I’m very excited to bring you today’s interview revealing the TRUTH About Leg Extensions with Fitness Pro, National Presenter and Author of 7 fitness books: Brad Schoenfeld.


This interview came about because despite all the negative talk about leg extensions, which have been circling around the fitness world for some years now, I’ve always been a fan of leg extensions for certain clients and for my own personal workouts. In all the years I’ve  been using leg extensions and been around the Physique sports world, which uses leg extensions regularly – I’ve never heard or seen anyone who has experienced any negative side effects related to this exercise. Not only have I NOT seen any negative side effects from the leg extension exercise, I’ve seen positive strength and hypertrophy gains. So, I wanted to get to the bottom of this Leg Extension controversy, by separting fact from fiction, science from opinion and fantasy vs. reality. My man Brad Schoenfeld, as I expected, was just the right man for the job!


If you’re looking for the non-biased, no BS, scientifically based and real-world approved TRUTH About the Leg Extesion exercise – This interview delivers BIG!!!






Who are you and why should we listen to you?

I’m one of the few practitioners who bridges the gap between academia and personal training. My primary area of expertise is in optimizing body composition (increasing muscle development and decreasing body fat), but I have diverse interests that span a wide array of fitness topics. I’m known for my evidence-based approach to training and spend a couple hours a day on average poring over peer-reviewed journals. Guess that makes me a bit of a research geek :)


Education-wise, I received my Master’s Degree in exercise science from the University of Texas. I’m currently pursuing my PhD at Rocky Mountain University, where my research focuses on the mechanisms of muscle hypertrophy and their application to resistance training. I’ve published over 20 peer-reviewed journal articles and I serve as an associate editor for the NSCA Strength and Conditioning Journal. I’m an adjunct professor in the exercise science department at Lehman College and in the natural science department at Mercy College.


On the consumer side, I’ve been a personal trainer for almost two decades. I own a private one-on-one training facility in Scarsdale, NY and also do a lot of consulting with physique athletes. I’ve published nine books (my tenth will be released in 2012), have appeared in hundreds of magazine articles and television shows on various fitness topics, and have lectured around the world for many of the popular fitness organizations. You can read my full bio here:



Do you use the leg extension in your training programs? if so, why?

Absolutely! I’ll add, though, that I use dozens upon dozens of different exercises when carrying out my training programs. Exercises are simply tools to achieve a given fitness goal. Like almost every exercise, leg extensions have certain advantages and certain disadvantages. As such, I employ them when I feel it’s appropriate for the goals and abilities of the individual client. To paraphrase one of my mentors, the late Mel Siff, “There are virtually no such thing bad exercises, only poor application and performance of a given movement.”



I’ve read articles by several individuals from both the fitness and rehab worlds claiming that using the leg extension machine could be dangerous to your knee joint? What’s your take – Is it dangerous – Any conclusive research showing its dangers?

This is partially true. There are a couple of issues with the leg extension that can be problematic. For one, loading is applied perpendicular to the long axis of the tibia—a fact that creates shear force at the knee joint (alternatively, loading during multi-joint movements such as the squat is mainly compressive, with forces applied parallel to the long axis of the tibia). Since a joint is better able to withstand tensile forces from compression as opposed to shear, it therefore follows that leg extensions place increased stresses on the knee joint compared with multi-joint lower body exercises.


What’s more, leg extensions tend to heighten stress to the anterior cruciate ligament (ACL). During performance of the leg extension, the quadriceps reacts to the movement by pulling the tibia forward (a phenomenon called tibial translation). The ACL in turn opposes the quadriceps by trying to prevent translation of the tibia. These two antithetic actions place a considerable amount of stress on the ACL, and can potentially injure the ligament. It should be noted that in closed chain movements (i.e. squat, leg press, etc) the hamstrings are activated as co-contractors and exert a counter-regulatory effect on the pull of the quadriceps. The co-contraction of the hamstrings and quads help to neutralize tibial translation, alleviating stress on the ACL.


That said, the aforementioned factors should not have a detrimental effect on someone with healthy knee joints provided the exercise is performed properly. I’ve seen no evidence of an increased injury risk to those with healthy knees from performing leg extensions. I could even make a case that it might help to maximally strengthen these structures to a greater extent than other exercises, as tissue adaptation is specific to the degree of stress.


On the other hand, I would say that leg extensions would be contraindicated for those with existing knee problems, particularly when they involve the ACL. I know some physical therapists continue to use the leg extension as a rehabilitation exercise, but based on the research I’ve seen this practice remains questionable.



Has anyone actually been injured while doing a leg extension machine exercise?


People get injured all the time performing pretty much every exercise imaginable, and that would certainly include the leg extension. But that isn’t an indictment of the exercise, only poor application/execution. I will say that under my supervision spanning many hundreds of clients, I am not aware of any of my clients ever having been injured from performance of the leg extension.



People say the leg extension is “not functional”. However, in the video below you discuss why that may not be true – Can you add any more research to your points made in the video.



That video was from my presentation titled “The Functional Fitness Continuum” given at the 2011 NSCA Personal Trainer Conference. It was based on an article I wrote for the American College of Sports Medicine where I challenged many of the conventional beliefs about functional training. In the segment you posted, I discuss the classic research study by Fiatarone back in 1990 that evaluated the effects of 8 weeks of leg extension training on functional capacity in nursing home patients (average age: 90 years old). Subjects trained 3 days a week performing 3 sets of 8 repetitions. The results? Subjects improved their functional scores on tests of walking and balance by 48% and lower body strength increased 175%. Two participants were actually able to walk without the assistance of their canes! Now I bring this up only to show that the commonly held notion that leg extensions are “non-functional” simply isn’t true. The fact is, all modalities can elicit functional improvements largely mediated by increases in strength. Machines may facilitate functional gains early on through simplification and in later stages can aid in targeted strengthening of weak muscles.


The take home message (and the whole point of my presentation) is that functional training exists on a continuum, where machines have functional benefits but greater functional transfer is seen with movements taking place in three dimensional space such as free weights and free form equipment (i.e. cable-based machines). A recent study by Spennewyn in 2008 sheds light on this fact.  The study compared the effects of fixed-plane machines (including the leg extension) vs. free form equipment on measures of strength and balance in untrained subjects (mean age = 49 years) over 16 weeks. Results showed functional strength increased 57% in the machine training group–that’s pretty impressive, right? But the subjects in the free form group saw a 115% increase–double that of the fixed machine group. Moreover, balance improved 49% in the fixed machine training group versus 245% in the free form group–a fivefold difference. So on the functional fitness continuum, free form equipment elicits greater functional transfer than fixed machine movements.



I’ve heard folks say that the quads don’t extend the knee in function. So, the leg extension doesn’t work the quad muscle in a productive manner. Your take on this statement?

This goes back to my previous comments about the functional fitness continuum. Look at the studies I cited–the leg extension clearly can develop the quads in a productive manner. It’s also important to understand that when it comes to exercise selection, it’s not necessarily a “one or the other” scenario. You can selectively employ machine-based movements and not lose any of the increased functional benefits associated with performing free weight movements. Adding in a fixed machine movement to a free weight protocol could potentially produce greater benefits, depending on the individual client.


On a related note, I always get a chuckle when people say bodybuilding is “non-functional.” First off, bodybuilders generally perform tons of free weight and cable exercises during the course of their training regimens, including squats, lunges, rows, and presses. Those are all highly functional movements on the functional fitness continuum. To this end, show me a bodybuilder who can’t lift a package or push a car–I’ve never seen one. Better yet, take a look at the following video– – Fast forward to the 1:10 minute mark. How many people can perform that type of move?



Who would the leg extension be a good exercise option for?

I generally use the leg extension as a supplement to closed chain movements, as it elicits different recruitment patterns of the quad muscles. Studies show it to be particularly effective in targeting the rectus femoris. If nothing else, variety helps to facilitate more complete quadriceps activation and thus better muscular development. This is particularly beneficial for those whose goal is muscle hypertrophy (bodybuilders, general fitness enthusiasts).



Who wouldn’t it be a good exercise option for?

As I mentioned previously, anyone with knee pathology would generally be contraindicated for leg extensions. I also rarely employ leg extensions (or most machines, for that matter) when I train athletes. It just doesn’t provide sufficient cost/benefit to warrant inclusion in their routines. Then again, I look at each client individually, so if there is a need, I don’t preclude using any and all exercises to achieve a given outcome.



If some one is currently avoiding the leg extension machine for what ever reasoning. What would you say to convince them otherwise?

I would simply say to consider your goals and abilities. There might be legitimate reasons why a leg extension should not be part of your routine. That’s fine. But no exercise should not be dismissed outright simply because someone says it is “non-functional” or “dangerous.” These are gross generalizations that, based on the research, are inherently flawed in terms of practical application.



Any other research on the leg extension or cool tid-bits of knowledge you think we should know about the leg extension?

There has been some interesting research about altering foot position during the leg extension to elicit greater activation of selected quadriceps muscles. Specifically, medial rotation (i.e. turning the legs inward) causes a greater activation of the vastus muscles (i.e. vastus lateralis and medialis) while external rotation (i.e. turning the legs outward) produces greater activation of the rectus femoris. While this seems cool on the surface, the practical benefit is questionable. In my view, altering leg position can cause the knee to track improperly and thus potentially increase the potential for injury. I personally feel it’s not a good risk/reward ratio. My general suggestion is to keep the legs in neutral position for optimal safety.



Take Home Points

– If you currently have an exsisting knee problem, the leg extension exercise may not be the best option for you unless otherwise recommended by a medical professional.

– If you’re goal is to increase the muscle mass of your Quadriceps, the leg extension offers benefits that other exercises may not.

– The Leg extension exercise can be part of a comprehensive muscle building / bodybuilding workout program without any negative effects to your knees

– Athletes may still benefit from performing leg extension due to the strength carryover into real life movements.



Check out Brad Schoenfeld‘s books, Newsletter and Fitness Blog at




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