The exoskeleton’s biomechanical impact

Mélissa Moulart, a biomechanical engineer at the Japet science centre, explains the exoskeleton’s biomechanical impact.

Photo Melissa Moulart

How can the biomechanical impact of an exoskeleton be analyzed ?

To measure the biomechanical impact of an exoskeleton, there are mainly three parameters to study: muscular impact, skeletal impact and impact on energy cost.

– The measure of the muscular impact, is generally done through EMG sensors (electromyogram, NDLR). The study consists in comparing muscle activity with and without an exoskeleton for similar movements and then analyzing certain parameters such as averages, standard deviations, quartiles, peaks, etc..

There are other capture solutions under development such as MMGs (mechano-myography, NDLR), which consist in listening to muscle noise, which aren’t used as much, because of the post-treatment need. Other tests are also possible on the long run, such as an endurance test or a strength test with an isokinetic machine.

– The measure of skeletal impact, most of the time consists in recording the movement using, for instance inertial imagers (IMU) or near-infrared cameras, then integrating this data into a simulation software, allowing the loads repercussions in each joint to be calculated by inverse dynamics.

– On the other hand, the measure of the energy costs impact, is more subtle. It is interesting to study the balance between the physiological cost (oxygen consumption, heart rate, etc.) and the biomechanical cost (related to kinetics, motion). There is, naturally, a balance between these two energies, and the exoskeleton must not disturb this balance.

Do exoskeletons limit exposure to biomechanical risk factors ?

The real benefit of exoskeletons is to relieve highly stressed joints. From a biomechanical point of view, one must be careful to consider the body as a whole. One of the advantages of exoskeletons is their capacity to reduce peaks of muscular activity, sometimes linked to false movements.

However, it is impossible to make generalizations about the exoskeleton impact since it depends on the design. Each exoskeleton is specific but it is possible to give trends on sets of products (lower limbs, upper limbs, back).

We still have a little hindsight on the long-term impact of exoskeletons, can you give me your opinion ?

The INRS and AFNOR are working a lot on the subject to create a normative framework for the exoskeletons use.

There are several studies in progress but it is difficult to ignore a real biomechanical study to understand this impact. For products such as ours, we need to prove that in the long term the impact is transparent on healthy people, and that it is beneficial (less pain while maintaining correct posture) for pathological subjects. To do this, studies must be carried out taking all types of people and generating sufficient data.

It is necessary to conduct studies regularly in the coming years in order to have hindsight and verify that the benefit/risk ratio is good. In 5 or 10 years, the hindsight will be sufficient.

It is up to the manufacturers to keep in touch and check that there are no problems with the users. The risk is that a user will no longer wear the device (because it does not suit him) but that the information does not reach the exoskeleton manufacturing companies.

Exoskeletons are like medicine, clinical evidence is needed. As a medical device, Japet has had to validate its claims through clinical studies and a follow up with its users in order to control the risks related to the use of its products.

Why can’t exoskeletons carry more weight?

The exoskeleton is there to relieve a part of the body. If we increase the load, we find ourselves in a case equivalent to the initial problems, and the exoskeleton no longer plays its role of relieving the joints but then takes on the role of external muscle.

Except in the military case, which is a specific case, it is an aberration to want to carry more weight, because it only displaces the problem instead of solving it. For example, a person with MSDs from carrying 10kg loads will trigger MSDs if he or she carries 20kg loads with an exoskeleton. The problem will always be the same: that person has MSDs.

What are, the points of vigilance, from a biomechanical point of view, exoskeletons ? 

For exoskeleton manufacturers, it is a question of making studies of the whole body, in order to get a global vision of the impact of their device, and to avoid focusing on a single joint in particular. It is quite complicated to perform global studies, because they require a large implementation. When we offer people to carry out measurements, everything has to be done in a maximum of one hour, from the installation of sensors to the de-installation… if we want complete studies, it necessarily takes more time.

On their side, companies must verify that users use the exoskeleton in the right conditions, respecting a duration of use. The follow-up must be regular and frequent at the beginning, and a follow-up every year will be sufficient thereafter. If a company realizes that its employees stop using the exoskeleton, or conversely, if they want to use it much more than recommended, there are questions to ask.

It is also important to involve the occupational medicine, which can anticipate a problem or detect it as the exoskeleton is used.

For the user, one should not live with an exoskeleton but learn how to use it when one really needs it.

In your opinion, how do exoskeletons find their place in companies ?

Exoskeletons are innovations that were created because there was an actual need.

There are workstations that cannot be adapted for various reasons (machine clutter, confined spaces, etc.). Retirement is coming later and later; people are working longer and not always in good conditions. Exoskeletons can really bring a lot, but it has to be done intelligently. Putting on an exoskeleton is never insignifiant.

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