We Told Them to Lift with Their Legs, but They Just Won’t Listen!
I am reposting what we feel is a new look at Manual Materials Handling techniques towards back injury prevention Dr. Peter Budnick, PhD, CPE
In this study, the authors hypothesized that people whose back strength is greater than their total knee strength are more likely to adopt a back-lift strategy, and vice versa, and devised an experiment that considered the relationship between a person’s back and total knee strength, gender, self-knowledge of their strength, and the load weight on lifting strategy.
The authors review some of the scientific literature surrounding lifting strategies, such as bending, stooping or squatting (i.e., lifting with the legs). They note the following:
We don’t really know what factors drive a person’s lift strategy;
We focus primarily on back compressive force when evaluating the risk of a particular lifting strategy;
There’s no clear evidence that knowledge of the weight to be lifted influences a person’s choice of lifting strategy;
There is evidence that an individual’s perception and knowledge of his/her own strength can impact lifting strategy;
There is evidence that personal factors impact lifting strategy;
As muscle fatigue increases (during prolonged periods of lifting), lifting strategy shifts from leg lift to back lift strategy;
People with low back pain tend to try to limit back flexion and overall movement in an attempt to limit spinal loading, yet fail to do so because of muscle coactivation;
Stronger people tend to choose a heavier load per carry strategy, resulting in fewer carries per batch, and that those who are more aware of their strength capabilities “became more emboldened in load handling”;
Back strength is poorly correlated with back injuries;
The physiological/metabolic demands are much higher for leg lift strategies than for back lift strategies; and
The size of a load impacts strategy (i.e., larger items do not fit between the knees, making a leg lift strategy less likely);
Primary Findings
There was no significant difference in back strength between those found to have back-stronger-than-knee-strength (BSTK) and those found to have knee-stronger-than-back-strength (KSTB), but there was a significant difference between knee strengths between the groups;
Those with BSTK mainly used a back preferred lift strategy;
Those with KSTB used variable lift strategies;
However, the leg-preferred lift strategy was exclusively adopted by those with KSTB;
“It can be concluded that participants with back strength greater than their total knee strength tended to use a back preferred lift strategy, whereas those who used a leg-preferred lift strategy all had total knee strength greater than their back strength”;
There was a significant gender effect, with male subjects predominantly selecting back-preferred strategies, and female subjects predominantly selecting leg-preferred strategies; and
Knowledge of strength feedback did not show a significant effect on lifting strategy.
The Bottom Line — How this Applies to Our Practice with OPC Clients
We’ve all heard it, and some may even be strong proponents of it: “lift with your legs, not with your back!” The problem is, many or most people simply do not do so. This study looks deeper into why that is, and concludes that total (combined) knee strength vs. back strength is a significant predictor of how someone will choose to lift. Add this new knowledge to previous research that demonstrates that the physiological/metabolic demands of leg-lift strategies are far greater than those for back-lift strategies, that many items are too big/bulky to effectively use a leg-lift strategy, and consider the effects of knee function and injury, it’s no wonder few people follow the conventional wisdom advice.
The authors suggest that these results can be used to justify training programs that focus on increasing knee strength. While such an approach, under strict conditions, may produce a sustainable change in lifting behavior, there is still no clear evidence that such behavior changes will translate into a reduction in back injuries, which is yet another challenge to the “lift with your knees” approach to ergonomics.
However, rather than spending a great deal of time, money and resources trying to change people, which may or may not be successful, why don’t we focus on designing the system to better fit human capabilities, which will be successful? Why don’t we just get to the root of the problem? Why don’t we match the work to the people, vs. matching the people to the work? Why don’t we design our process so that lifting naturally occurs in the “power zone,” or the “strike zone,” meaning lifting close to the body near waist height? After all, this is also the most effective lift, reducing the time it takes to conduct the lift (reducing “movement waste” in Lean terminology) and reducing the likelihood of error (dropping, colliding with other equipment or people, etc. — “do it right the first time” in Lean terminology).
If you have any questions or would like OPC to conduct MMH training courses for your employees feel free to contact us 416 860-0002 or j.sleeth@optimalperformance.ca
Reference
Li, Kang; Zhang, Xudong, Can Relative Strength Between the Back and Knees Differentiate Lifting Strategy?, Volume 51, Number 6, December 2009 , pp. 785-796(12).
