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Cable and Pulley Ratios

This can be sort of a tricky subject so we will try to keep it simple. Some people ask from time to time how the resistance from a certain cable machine equates to barbell or dumbbell weights they lift/pull or what the numbers on the weight stack equal in real pounds of resistance. There are many ways to route cables through pulleys which can give what is called mechanical advantage or mechanical disadvantage. Something you should know right off the bat, anytime you obtain a mechanical advantage/disadvantage using cable/pulleys there is a exchange. You are trading one benefit or disadvantage to gain/lose another.

Lets take one example; We want “fast cables”- that is to say cables that can be moved very quickly out and back in without a lot of slack accumulating in the system. We achieve this through mechanical advantage (a mathematical formulated routing of pulley positions that increases our cable ratio to something like 2:1- some people flip it around and call the same thing 1:2- as long as you understand the principle you need not be confused). The cable end we pull will travel 2 feet for every one foot the weight stack travels upward). Because the inertia of the stack is less in this scenario, we end up with more responsive cable. The “cost” for doing this is total weight available. Ten pounds of weight is now effectively 5 LBS. This is also an advantage for certain individuals. For example those doing rehab who only want small incremental weight changes and who do not require large amounts of weight (think athletes needing small resistance changes and those doing rotator cuff exercises for example). A 2:1 ratio gives the best overall benefits without sacrificing in other areas. 4:1 can be good for speed work but has the downside of making a 200 pound weight stack only equal to 50 pounds of resistance, a problem if you intend to do lat pulldowns for example.

Let’s look at another example. If the lat pulldown handle you are pulling down on has a cable that goes up through one pulley and directly back down, attaching to a weight stack system, this is a near 1:1 ratio. For every one foot pulled on the pulldown bar, the weight stack will also travel one foot upward. A ten pound weight plate being hoisted on one end of the cable will be felt as resistance at the other end as nearly ten pounds. The reason we say “nearly” ten pounds is because anytime you introduce pulleys, you also introduce friction. If you only have a few pulleys the friction is negligible. When you enter many pulleys it starts to increase the total resistance.

The most common pulley configurations in standard “lat pulldown” machines is 1:1 (a ten pound plate produces 10 pounds of resistance and the weight plate travels the same distance as the cable you are pulling) or 2:1 (a ten pound plate produces 5 pounds of resistance and the weight plate travels half as far as the cable you are pulling). The 1:1 ratio is ideal for an exercise where a lot of resistance will be required and the movement will not be very fast such as lat pulldown or leg press. A 2:1 ratio still gives a fair amount of resistance while allowing more travel with slightly better responsiveness.

Again, the 2:1 ratio is good for the now popular “functional training” exercises where you want a “fast cable” adequate travel. This will allow you to (as an example) attach the high speed cables to a wearable vest to work on stability and agility muscles moving around on your feet. A 1:1 ratio would not be well suited for this type of application because the total travel of the cable would be too limiting and inertia of the stack weight plates would be unable to reverse direction quickly enough to be effective.

With the Bodyforce X9 we achieve both a 1:1 and 2:1  on one machine. This gives a lot of flexibility to personal trainers wanting to match just the right cable resistance and travel to clients workout objectives for that day. Furthermore, Bodyforce has designed the X9 Hybrid Gym to utilize what we call “tethering”. Our stack plate cable system works in concert with our heavy duty carriage system providing up to 500 LBS total resistance from our 2:1 ratio low position arms and over 1000 LBS total resistance when performing seated leg press. Again, the best of both worlds.

Learn more about pulley ratios and mechanical advantage here.