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Why Springs?

Updated: Jul 1, 2020

Whether you’re a Pilates newbie, a seasoned pro, or have only seen pictures of those strange “medieval torture chamber” machines, you’ve probably wondered, “what’s the deal with all those springs?”

(Anyone else hear Jerry Seinfeld? Just me? Cool.)

To my knowledge, Pilates is the only form of exercise that uses springs to provide the resistance force. Thera-bands and tubes come close but they aren’t quite the “workhorse” a spring is.


Muscle responds to the load placed on it. A load is the term used to mean the force exerted on a body. For simplicity’s sake, let’s compare lifting free weights to using a spring. Heck, let’s get really specific and compare a bicep curl to a kneeling biceps curl on the reformer. While not a “traditional” exercise, it will allow for a more “apples to apple” comparison which we can then expand upon.


The force required to lift a free weight at a constant velocity (ie NOT accelerating because that actually makes a difference and we’re just keepin’ it simple here) is equal to the object's weight. Curling 10 pounds? You need to exert 10 pounds of force.


Disclaimer (before my fellow physics and biomechanics peeps get up in...arms):

Technically, we should be talking about torque because as you perform a biceps curl, you’re rotating your forearm about an axis (your elbow). This rotation means the lever arm is constantly changing as you go through the motion, making the muscles work harder at different points of the exercise (that’s why the weight feels “heaviest” when your elbow is at 90 degrees). I’m just gonna go ahead and assume that for the two exercises we are comparing, that the joint follows the same path and range of motion.


Keepin’ it simple, remember?


So to recap, 10 pound free weight, 10 pounds of force.


Now springs on the other hand, follow Hooke’s Law.


No, not that guy. This guy:

Hooke’s Law states that the restoring force of a spring is directly proportional to both the spring constant (k) and the distance the spring has moved from equilibrium (aka how much you’ve stretched it, x)


In other words:

F = -kx

(The negative sign just means when you pull the spring to stretch it, the spring pulls back in the opposite direction, trying to “restore” it to it’s relaxed length)


What this means is the amount of force you need to stretch the spring increases as you stretch it out. The spring’s restoring force actually gets stronger the more it stretches! So when you first start to stretch the spring, it doesn’t take that much force but towards the end of the movement when the spring is reaching its maximum length, it takes more force to continue stretching it.


This is why closing the springs is just as much work as opening them.

The restoring force is at its maximum when you change “directions” and you have to resist the strength of the spring. This provides your muscles with an eccentric contraction without having to think about it. You CAN get that action with free weights but you have to know what you're doing. No "mindless repetitions" like Joe warned against! Having the springs gives you no option but to utilize the eccentric contraction, otherwise you're just along for the carriage ride.


This is also why strap length is so important.

Imagine you’re on your back getting ready for 100 on the reformer. If your straps were too short, in order to get your hands over your shoulders, there would be tension on the springs already. You’d curl forward, stretching the spring even MORE which means you have to exert more force to hold it in that stretched length.


On the other extreme, maybe your straps are too long and the springs don’t begin to stretch until half way into your 100 curl. Now there is too little force from the springs and you may find the carriage “bouncing” as you pump your arms.


Even with your straps the correct length, you can "play" with the strap length to make an exercise easier or harder. For example, Shave and Hug seated versus Shave and Hug kneeling. When you come up to a kneeling position, the straps are in a totally different place than when you were sitting. This means the springs already have tension on them when you come into the start position for kneeling Shave and Hug. It's like grabbing a heavier weight AND increasing the balance challenge at the same time!


There’s another factor at play and that’s the spring constant (k).

The spring constant refers to how “stretchy” the spring is. If you’re familiar with Balanced Body Springs, the green ("heaviest") would have a large k value while the yellow ("lightest") would have a smaller k. In fact, you’ve probably seen graphs representing Hooke’s Law if you’ve ever purchased equipment springs before. Check out this graph from Balanced Body’s website:

This shows that the amount of force delivered by the spring (in pounds) increases with stretch length (in inches). A steeper slope indicates a higher k-value, meaning a stiffer spring. If you ever need to compare spring strength across brands, looking at graphs like this would help you do so.


Let’s go back to the 10 pound biceps curl comparison.


When I do a kneeling biceps curl, it’s on one Contrology spring (which has a k-value close to their green springs). Using the graph above, I can hit 10 pounds of resistive force in about 1.5 inches of stretch. While I’ve never before taken a ruler to my springs during an exercise, for this post (and for SCIENCE), I did!


My carriage moved about 7.5” during the bicep curl action, which equates to about 17-18 pounds of restoring force at the maximum point.


Traditionally, this biceps curl action can be done as a variation of Rowing 2, seen below. Adding the hinge means the springs start already elongated, then when you curl, they stretch even MORE meaning an even bigger restoring force.


(Not to mention, all this is done while you’re stabilizing your entire body on a moving platform. Take that, gym bros!)

So there you have it. Why springs provide a truly unique workout experience and yet another reason Joe was such a genius.


Ready to feel the magic of the springs for yourself? Schedule a private session with me at The Restoration Space! Click HERE for availability.

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