IRSST - Institut de recherche Robert-Sauvé en santé et en sécurité du travail

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Manual Handling: Towards a More Precise Understanding of Physical Demands


Manual Handling: Towards a More Precise Understanding of Physical Demands

Researchers experimented with a system to measure, with an acceptable degree of error, material handlers’ movements while they perform their tasks. Ultimately, such a system will enable them to quantify the effects of ergonomic interventions to reduce workers’ physical exposure.

Manual handling of loads causes a great deal of physical stress on the human body. Handlers are subject to many risk factors associated with musculoskeletal disorders (MSDs) such as excessive repetition, awkward postures and heavy lifting. Their backs are particularly vulnerable. It is therefore not surprising that these workers’ claims represented nearly one injury in five (17.5%) accepted by the CNESST from 2009 to 2013. During that period, the Commission paid out more than $125 million to compensate victims of this type of injury. Cases of that nature constituted 16.5% of total payouts.

For more than 15 years, the IRSST has been conducting research on handling. The goal has been to develop a new prevention program to reduce the incidence of MSDs, and has resulted in the Integrated Prevention Strategy for Manual Handling (IPSMH), developed and implemented by researcher Denys Denis.

“We have identified eight principles that are skills indicators,” explains André Plamondon, biomechanics researcher at the IRSST. Among these principles, for example is that of the lever arm principle, which advocates bringing a load closer to the centre of one’s body mass to reduce the effort of handling.

The effectiveness of these theoretical principles in describing the rules of handling have, however, never been demonstrated outside of laboratories because of a lack of reliable tools. Fortunately, with technological advances in recent years, it is now possible to quantify workplace movements with inertial sensors. Once fitted to the bodies of handlers, these sensors, which are only a few centimetres in size, record the movements of their various body segments (arms, legs, etc.), and then re-create them in 3D.

However, there are concerns about their reliability, cautions André Plamondon. “At first glance, it is tempting to think that any system will do. However, the data that the inertial sensors generate are not necessarily reliable, which could be due to magnetic disturbances that can affect the sensors,” he stresses. That is why the research team tested a measurement system of this type, the Xsens, composed of 17 inertial sensors, in order to quantitatively estimate the physical demands of the work of handlers, to assess the effectiveness of prevention approaches, among others.

Laboratory and Field Tests

Manual Handling

Ten handlers at two distribution centres were observed as they carried out their regular tasks. Their various movements were captured with the Xsens system and by video camera and then compared to verify the concordance of the two sequences obtained.

First step: Validate the reliability of the system in the laboratory, first without the presence of magnetic disturbances, then with them. The measurements produced by the Xsens during handling tasks were compared with those of optoelectronic cameras, commonly used in research to generate measurements of human movement in the laboratory.

The first result indicated that the Xsens system could estimate the joint angles of the body within an acceptable error threshold of 5 degrees. Moreover, “we noted that the magnetic disturbances negatively influence the Xsens results, making them less reliable,” explains André Plamondon.

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André Plamondon, researcher at the IRSST, explains the advantages and the limitations of a system that will provide quantitative estimates of the physical exposure of material handlers and evaluate the effectiveness of prevention approaches. (In french only)

The next step: to evaluate the Xsens in a real work context, i.e., in two distribution centres. Ten handlers were observed as they carried out their regular tasks. Their various movements were captured both by the Xsens system and a video camera; then compared again to verify the concordance of the two sequences obtained.

The good news is that 68% of the data gathered were deemed acceptable under conditions of normal use in the field. The bad news is that almost one third (32%) were not—they were totally wrong. According to André Plamondon, these losses were mainly due to the environment, i.e., the presence of magnetic disturbances from the pallet trucks used by the handlers.

“When the workers used a pallet truck, it took from 30 seconds to one minute before the data become correct again,” he explains.

Note that the error rate was specific to the two distribution centres studied. “As the level of magnetic disturbance may vary considerably from one place to another […], there is no reason to worry that 32% of the data were rejected. It could be more, or it could be less, according to the report entitled Développement d’un système de mesure et d’un protocole de mesures permettant de quantifier l’exposition physique des manutentionnaires (Development of a measurement system and protocol for quantifying the physical exposure of material handlers).

In the Near Future

Despite the somewhat mixed outcome of the study, André Plamondon is encouraged by its results. “We’ve come a long way: at first, simply passing a pair of scissors in front of the sensors produced a great deal of disturbance. Happily, this is no longer the case today,” he states.

According to the researcher, it is only a matter of a few years before the use of inertial sensors spreads. “In my opinion, if the technology continues to progress as it has over the past years, in perhaps five years it will be ready to be used by occupational therapists,” he thinks. The price of the system, which is currently quite high, should fall and thus become more widely used.

However, we must be aware of the limits of this technology, which is still under development. “Magnetic disturbances constitute a source of interference. They must be taken into account when reading the data, either by questioning certain measurements, or by introducing algorithms to detect measurement errors,” warns André Plamondon. It is essential to verify and cross-check these data.

In the near future, the system could be used to provide feedback to handlers about how they work in order to reduce the amount of effort they expend. The IRSST is also planning to use it to improve awareness and prevention measures regarding IPSMH. “Thanks to it, we will be able to provide advice to handlers during occupational health and safety training. Does the level of loading on the back change between the beginning and the end of training, for example? Has there been progress or not? Are they working in a way that lessens physical effort? Can we improve the work environment to decrease physical demands?” asks André Plamondon.

Maxime Bilodeau

This article was originally published in French in the magazine Prévention au travail

Prévention au travail