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The Duo-Extension: A New “Arthur Jones” Machine in 2019

Half a century ago Arthur Jones, Nautilus inventor, conceptualized a machine, dubbed the 'Leg and Back', that would combine knee extension, hip extension and lumbar extension. While Nautilus never ended up producing the machine, Mike Petrella, owner of STG Strength and Power and prolific exercise machine collector has finally built it, realizing Jones' vision for this legendary device. We had the pleasure of visiting Mike and trying out the 'Duo-Extension', and to put it simply it feels like an exceptionally satisfying contraction and stretching of the entire body.

Half a century ago Nautilus inventor, Arthur Jones’ mind was focused on attempting to solve yet another challenge to improve exercise: how to make the squat safer and more effective. When the solution to doing just this became apparent to him, he referred to it in his IronMan magazine column of November 1970 as “The Final Breakthrough”.


What’s the problem with barbell squats that inspired Jones to look to better such a classic exercise?

Squats are a relatively high-skill exercise, some people’s knees don’t get on with them and those with low back issues may well find squats uncomfortable and more of a liability than a benefit. Just as significantly there is also the issue of spinal compression from vertical loading through the shoulders. Loaded squats may work for some people, some of the time but they are certainly not for everyone.

These issues were key drivers for Jones in his quest to engineer a machine capable of upgrading the squat. A machine that would provide a superior stimulus to more muscle mass than any other exercise, one that could target the quadriceps, glutes and lumbar musculature in one HIT. Jones referred to his concept machine as the Leg and Back, designed to combine knee extension, hip extension and lumbar extension, effectively a triple movement. The knee joint and the hip would be the main axis points of the exercise just as they are with squats. To target all these movements the machine would need to have the torso move from around 12 o’clock toward 3, whilst simultaneously the lower legs would move from 6 toward 9 o’clock. The whole body would end up in pretty much a horizontal position.


The simplest way to imagine this concept machine is to picture performing a leg extension and a Nautilus Low Back… at the same time.

You would be experiencing direct rotational resistance with Nautilus strength curves for the targeted musculature. Jones noted two fundamental requirements for this machine to work: effective restraint around the upper thighs to stop the hips lifting-up and away from the seat, and a mounted track to enable a large part of the machine to move for proper set up for each individual trainee.

Jones’ IronMan article suggests that he had solved these issues, in theory at least. However, for one reason or another Nautilus never ended up releasing the Leg and Back machine, practically speaking it has never existed it the wild. Until now, nearly 50 years later, thanks to Mike Petrella.


Enter Mike Petrella

Mike Petrella is the owner of STG Strength and Power based in Paris, Ontario, a man with an abundant collection of exercise machines, ranging from first generation Nautilus pieces through to Pendulum devices of today. Think of any Nautilus or MedX piece and Mike probably has it, sometimes in duplicate. Perhaps it was the fact that the fabled Leg and Back machine would never be part of that collection that drove Mike to take up where the IronMan article had left off, perhaps he saw it as unfinished business. What’s for sure is that his passion and dedication along with three years of work (and many more ruminating) has resulted in a fully functioning one off piece, realizing Arthur’s vision for the Leg and Back machine. Or as Mike calls it, at least currently, the “Duo-Extension”… effectively a new “Arthur Jones” machine in 2019.

The Duo-Extension, leg extension and low back fusion

At its core the machine is a fusion of two Nautilus machines: a 2nd generation Leg Extension and a second-generation Low Back. Featuring two 400lb weight stacks, heavy duty bracing reinforcement, an adjustable pad for the knee extension and a linear slide system to ensure the low back fits the trainee correctly.

The result is a machine that does what Jones intended- effective targeting of the quadriceps, glutes and lumbar muscles all in one. I was lucky enough to experience this machine first-hand when I visited Mike at STG Strength and Power in Ontario.

My first thought as I sat into the Duo Extension was- this is going to feel a little complex performing a low back and knee extension at the same time! What bubbled up, as I started out with some initial trial reps was the thought “Wow, is an incredibly instinctive and natural-feeling movement to perform”. The physical experience of the Duo Extension is like a satisfying contraction and stretching of the entire body.

It did take a handful of reps to get into the groove of the exercise as the range of movement for the low back is a little longer than the range of the leg extension in this current iteration. Initially, I was finding that I had completed the ROM for the knee joint whilst I still had a little way to go with the glutes/low back. Some mental refinement and focus over the ensuing reps soon balanced the movements out.

Mike pointed out to me that he would like to rebuild the machine in the future with range limiters so that the distance travelled on each movement can be fully tailored to the individual. Nevertheless, even without the range limiters it did not take much time at all to figure it out as a user.


Overall impressions: the machine and the movement feel awesome.

It is such a rewarding movement to perform: lighting up the quads, glutes and lumbar muscles in one cohesive movement in a way that ends up feeling natural. It is of course effectively an integration of two “isolation” type exercises producing a resultant compound movement, a case of the whole being greater than the sum of its parts.

I asked Mike how he fits the machine into a client’s workout and the answer is a simple one, it will typically be placed at the front end of a workout replacing a leg press or squat. It would also eliminate the need for a separate lumbar movement in most all clients, excepting those who may need the true isolation of a medical lumbar device.

Mike points out that the machine can also be used to perform two separate exercises: the leg extension can be performed by itself, as can the low back. He notes that you can get creative with protocol too, for instance the lumbar portion can be performed isometrically whilst the leg extension is performed dynamically. It is then a machine with flexibility.

The Duo-Extension may not be a time machine, but Mike Petrella has gone back to the future to bring Jones’ vision to life.


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