Experienced Torso Weight Reduction

Introduction

On this page, you will find a support specification of the Laevo FLEX exoskeleton expressed in kilograms.

Yes, Laevo hopes that one day, every exoskeleton on the market will provide a Torque Curve as a standard specification for support. Haven't you learned about Torque Curves yet? Please read our introduction Learning Center page first. It also explains some of the terminology used on this page. If you are just here to look at kilograms, please proceed to Chapter 1.

Laevo, and many others, see that Torque Curves are an excellent specification and tool to match specific exoskeletons to specific tasks, which is the most essential aspect of a specification: To base your buying decision on a qualitative and reliable specification, comparable to horsepower for cars.

This definitely is not the case in the current exoskeleton market. The most popular support specification used by exoskeleton manufacturers still seems to be a single 'kilogram' value. Many of these 'kilogram' specifications are calculated or measured differently, ranging from the most simple theoretical calculations to extensive scientific research measuring muscle activity in actual use over an extended time. And the worst thing is: many manufacturers do not even mention how they specified it. Because there is no agreement or regulation on how to specify this kilogram value, these specifications actually tell us nothing.

Despite Torque Curves being the preferred exoskeleton support specification, Laevo understands that in some situations the 'kilogram’ unit fits better because it is easier to relate to. Laevo has therefore thought of a way to express support in kilogram, based on Torque Curve data. For Laevo, it is important to specify the number of kilograms in the most universal way possible.

The following chapters will tell you how and why we did it this way:

1. Experienced Torso Weight Reduction

2. How is it calculated?

3. What can we learn from the table?

4. Why is it calculated this way?

5. Conclusion

1. Experienced Torso Weight Reduction

Let’s get right into the results! Below we present the ‘Experienced Torso Weight Reduction'. Below each predefined, common bending posture you can find the corresponding support values expressed in kilogram for each Laevo FLEX Spring Strength. The values express how much lighter your torso will feel while bending down using the Laevo FLEX exoskeleton.

So, there they are! Kilogram values! For example, we can read an experienced torso weight reduction of up to 15kg in Posture 3. Now, is 15kg a lot? The weight of the complete upper body including arms and head is 65-70% of our body weight. So, if you are 75kg, your complete upper body will weigh around 50kg. Knowing this, some might ask:

And this is why support specifications still do not say much. You have to physically experience 15kg of support to know this is really substantial support. It's on the edge of what can be comfortably supported on the human body. Imagine laying on your back with a 15kg bag of sand on your chest, because that is the weight the exoskeleton will apply on your chest in order to support you using our Ultra Strong gas spring. However, using the Laevo FLEX Vest with its large and soft chest surface area and available in 3 different shapes, we can still guarantee a comfortable user experience.

So, a support specification does not say much when you have no experience with exoskeletons. But, how cool would it be if you experienced, for example, a single 30Nm torque exoskeleton and you really liked the amount of support, but only wished it supported you a bit earlier in your bend? If all exoskeletons had torque curves as a specification, you could find an exoskeleton that fits your support need, just by looking at its specification, instead of endlessly trying out different exoskeletons.

Are you considering buying back-supporting exoskeletons and are you curious to experience different amounts of exoskeleton support? Contact us to find out how we can arrange this.

2. How is it calculated?

Let’s look at how these kilogram values are calculated. We will do an example calculation for all 3 postures using the Laevo FLEX Ultra Strong Spring strength.

Example calculation for Posture 1

The average-sized user in Posture 1 (drawn above) is bending forward with a back angle of 45 degrees which places the center of mass of the complete upper body about 28cm in front of the hip joint.

Using the Laevo FLEX Torque Curves we can find what support torque we can expect at this 45-degree bending angle. In the Torque Curves graphs shown below, especially for these examples, dashed vertical lines are shown at 45 and 90 degrees. We can find that a 45-degree bending angle for an Ultra Strong gas spring (blue curve) provides 58Nm while bending down. This torque value, and the other gas spring's torque values used for the Torso Weight Reduction Experience, are shown in the table on the right side of the Torque Curve graph below.

Dividing the 58Nm torque by 28cm, we get a force of 207N, which converts to a rounded 21kg. You can find this 21kg value in the Experienced Torso Weight Reduction table shown at the top of this page for Posture 1 and Ultra Strong. If you are unfamiliar with these units or calculating a Torque (Nm) using a Force (N) and Torque Arm (m), please read our Learning Center page ‘Introduction to Torque Curves’.

Example calculation for posture 2

The difference between Postures 1 and 2 is the bent knees in Posture 2. Although the torso angle relative to the vertical remains 45 degrees, the exoskeleton is bent 45 degrees more because of the knee angle. This evaluates to a total exoskeleton bending angle of 90 degrees, while the Torso arm remains 28cm.

Looking at the Torque Curves below we find a 57Nm torque on the blue Ultra Strong curve at a 90-degree bending angle. Dividing 57Nm by 28cm, we get a force of 204N, which also converts to a rounded 21kg. Again, you can find this 21kg value in the Experienced Torso Weight Reduction table shown at the top of this page.

Example calculation for posture 3

Posture 3 is similar to Posture 1, where the exoskeleton and torso have equal bending angles. Because of the higher (90-degree) bending angle, the center of mass of the upper body is now further (40cm) in front of the hip joint. We already looked up the supporting torque at a 90-degree exoskeleton bending angle for Posture 2, which is 57Nm. Dividing 57Nm by 0,4m evaluates to 143N, which converts to a rounded 15kg.

3. What can we learn from the table?

Because the Experienced Torso Weight Reduction table is so easy to read, we can easily recognize some of the amazing properties of the design of the Laevo FLEX Torque Curves. The table is shown again below.

Bending the knees has a minor influence on Torso support

As mentioned, the only difference between Postures 1 and 2 is the bent knees. The torso angle relative to the vertical remains the same. Therefore it’s logical the support the torso requires also does not change. This is one of the reasons why Laevo Torque Curves are shaped the way they are.

An exoskeleton using a linear torque curve, like all passive soft exoskeletons that we know of, will have higher support in Posture 2 compared to Posture 1. Laevo thinks this is a less natural feeling. If you ever picked up a pen from the floor while wearing a passive soft exoskeleton you might remember the ever-increasing support force which can make it difficult to even reach the floor with your hands. Picking up a pen from the floor while wearing a Laevo feels natural and easy because of the Laevo-designed Torque Curves which actually decrease torque while bending deeper.

torso Support decreases again as the bend becomes deeper

You can see that the kilogram support values for Posture 3 are less than for Postures 1 and 2. Laevo designed the Torque Curves on purpose to be like this, as the human body itself will provide more resistance to bending when making deeper bends, therefore requiring less support. Making deeper bends, muscles will start to reach their maximum length, and tendons and ligaments will start to prevent further movement. Of course, this resistance to bending is very personal, but by iterative tweaking of our Torque Curves for many years with many users, we feel we created a universal solution that feels natural to almost all users.

4. Why is it calculated this way?

Read below why Laevo thinks the Experienced Torso Weight Reduction is an acceptable way to express support in kilograms.

Defined to be as Universal as possible

Torque Curves are a good specification because no assumptions have to be made: a torque curve measurement can be done anywhere, anytime, by anybody, and it will give the same result. To specify a kilogram support value, torque has to be converted to force or weight. To do this, you need to make an assumption: the torque arm. In this case, the torque arm is the horizontal distance from the center of mass of the torso, to the hip joint. Ideally, Laevo would not like to make an assumption on this distance because it makes the support specification dependent on the size of the user. A specification should be universal.

But to be able to express the support in a kilogram value, we have to make an assumption. Though, using only one assumption for specifying a kilogram support value already is a really good result compared to many other kilogram support specifications used in the current market. Laevo has used the length of an average-sized torso and made sure to clearly show the reader we did this below all applicable tables and figures.

exoskeleton sizing has no influence

Using the center of mass of the upper body for this specification removes the need to take exoskeleton sizing into account. Yes, the forces on the body interfaces will change based on exoskeleton sizing and will have a big influence on experienced comfort, but the upward force on the center of mass of the upper body will remain the same while the torque around the hip joint remains the same too. Laevo is sure we can always find comfortable-sized parts for your body using the Laevo FLEX modular part system.

No statements about the reduction of externally lifted weight

Another choice in making the kilogram specification as universal as possible is making no statement about the load that is lifted using the arms and hands. Lifting a load close to the body will result in different kilograms of support compared to lifting a load further away from the body because of the difference in Torque Arm. The torso center of mass has a more predictable position relative to the hip than the location of a lifted load.

A table of values, not a single value

Laevo would like potential buyers to know that it is impossible to specify exoskeleton support as a single number. A single support value only says something if we know at what exoskeleton bending angle this support value occurs. More information is needed to know what kind of support the exoskeleton provides. Torque Curves show the amount of support for every angle which can be too much information. In that case, narrowing the amount of information down to 3 different postures should be easier to read and digest.

Predefined, recognizable bending postures

Laevo understands that some potential buyers do not have the time and energy to get to know Torque Curves and how they can be universally used to calculate support force values for every user and every situation. That's why we want to make it as easy as possible for you. No skills for reading graphs are required, you just find a posture that is most recognizable for your work and read the value.

no human was harmed during setting up this kilogram specification

Joking aside, Laevo thinks it is not a good idea to use a measurement done on a human to set up a support specification. For example: Yes, it is very clever to measure EMG (muscle activity) in back muscles during bending without the exoskeleton, and then try to find an equal amount of muscle activity while wearing the exoskeleton but lifting a certain amount of additional kilograms, and claim the exoskeleton relieves the back for that amount of kilogram.

This would be a very nice study, but the result will be highly variable based on the participant. Humans are physically very different. You cannot measure one person and make a claim this is the result for everybody. Next to that, EMG is a very sensitive measurement technique. It will be hard to duplicate results using different researchers and participants in different places in the world. Besides, you may lift with a different posture with the exoskeleton compared to without the exoskeleton, which will influence the EMG results. This does not sound like a good specification, right?

5. Conclusion

By no means, Laevo proposes to use the Experienced Torso Weight Reduction as a standard exoskeleton specification. Laevo understands that Torque Curves are not for everybody right now. While Torque Curves can be a bit overwhelming at first, they do provide a truly universal and reliable support specification and contain much more information about the support characteristic than 1 or 3 kilogram values could ever give. For expressing support in kilograms, assumptions have to be made, making a kilogram specification less universal. On this page, we described a way to define kilogram values in the most universal way we can think of. Laevo hopes the Experienced Torso Weight Reduction table can be of value to those who are looking for something easier to read than Torque Curves.

Why do you think other exoskeleton manufacturers are still not specifying Torque Curves despite many of them being involved in scientific research where torque curves are a common specification? Or why they are not fighting for another kind of standard support specification? We think they might be afraid to be compared on paper. Why would you fight for a way to be compared? Comparing can only cause losing customers, they might feel.

As exoskeletons manufacturers we need to understand that we need to work together to get exoskeletons truly integrated into society. To do this, we need to match specific exoskeletons to specific tasks, and a Torque Curve specification is definitely one of the good tools for achieving this. We have seen people lose interest in exoskeletons completely because their first experience with exoskeletons wasn’t good, while the right exoskeleton might have been great for them. Many exoskeletons on the market are not even competing with each other, because they excel at different use cases.