Presentation 4: The anatomy of an MSD
Presented by: Dr Gary Dennis (Ergo Enterprises)
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Presentation 4: The anatomy of an MSD
Presented by: Dr Gary Dennis (Ergo Enterprises)
[Start of transcript]
Okay, we might get started.
Welcome everybody to the second session in the Back to Basics stream with our presenter Gary Dennis who will be talking about the anatomy of a musculoskeletal disorder.
Gary consults extensively on ergonomic issues to a wide variety of industrial sectors both within Australia and internationally. Very well qualified, combining a Research Doctorate in Spinal Biomechanics with Engineering and a First Class Honours Degree in Health Sciences, Gary has the ideal combination of an in-depth understanding of biological tissues with engineering-based solutions to effectively address ergonomic issues.
Gary is the Managing Director of both Ergo Solutions and Ergo Enterprises, and also concurrently holds an adjunct academic appointment in ergonomics and biomechanics within the University of Queensland.
Thanks very much Gary.
Dr Gary Dennis:
Can everybody hear me okay with this mic? Is that alright? Okay.
I tend to move around a bit so that's probably not going to suit me.
So I've been asked to come here and talk about anatomy of tissues or injuries, whatever. For me it's really the mechanism of injury. If we're going to reduce injury in the workplace and put participative ergonomics in and consultative processes and all that sort of stuff, that's really important, that's we do day to day, the first step is to know how injury really occurs. So that's what I really want to talk about is the real underlying mechanisms.
But rather than doing that I thought I'd start with a story. I like telling stories. So hopefully you'll deal with it. Now before I start I haven't got a fetish with guys in tight pants or anything. This is just what I happened to be wearing on the day. Alright?
This is many years ago when I was running the PErforM program starting out at the University Queensland and it's part of the PECIVCON, Participative Ergonomics for Civil Construction which eventually became the manual for PErforM training.
This is at the Port of Brisbane and as you can see with this sort of task, the awkward postures that's associated with it, their repetition and what you can't tell is the length of time he's doing that. This guy owns this business and he's laying those concrete pavers at the Port of Brisbane basically as far as the eye can see, all the way up to these containers up there. On the left of him is about five other workers and they all lay about 90 square metres of sandstone, of this concrete paving per day. Ninety square metres is about a third of this room give or take, something like that.
So I was talking to him and I said to him, 'What are you up to today?' blah, blah, blah, and he said, 'Today I'm going to go for my record for the maximum amount of paving I can lay in one day'. I said, 'What's that?' He said, '365 square metres'. It's about double this room. It's an insane amount of paving and I said, 'That's incredible,' looking at the posture and looking at the repetition and everything he's doing. I said, 'Do you ever get injured?' and this is what he actually did to me. He went, 'Strong as an ox mate,' and I went, 'Mate I'm glad. I'm happy'. He's only about this big this guy. I said, 'That's fantastic.'
Part of PErforM is that you engage with the workers, you get to know them and everything else. So we went to smoko, what's now referred to as morning tea I suppose, later on and he started talking about himself and his life and everything else. He started telling me about his boy and he's going to be this greatest rugby league player since Wally Lewis. This is in Queensland. I said, 'That's great, fantastic,' and I said, 'Do you get out? Do you kick a ball with him? He's gonna be this great superstar of rugby league'. He said, 'No I can't do that anymore mate'. I said, 'Why is that?' He said, 'My back's stuffed'. Ten minutes earlier 'strong as an ox'. Now he can't even kick a ball with his kid. Alright. He might have used the word 'stuffed' too by the way, but this is a presentation. I'm allowed to say those sort of words.
It took me a little while to work it out. I'm not that bright but what I've come to realise is when I was talking to him about his job here, he basically said, was thinking, 'Yeah, I go home. I've got a sore back. It's just part of being a big strong Aussie, burly Australian worker. It's part of life. Get on with it'.
When I was talking to him about his child what he was actually saying is, 'Now that I'm old my back is inevitably going to be damaged'. Now what you can't convince me for a million years is to say doing this job – this is all he does, day in, day out for decades, isn't related to his back being stuffed and I really want to get the point across there's this big difference between chronological ageing and occupational ageing. Too often we've got this aged workforce mentality that because we've got older workers somehow we've got injured workers. You don't get injured – there are some genetic changes that do occur, that's true, but you can't tease those out from what you do every day. We are a reflection of the things we do every day from a physical point of view, from a relationship point of view, from our mental capacity, from our communication skills. Right? What you do every day either is improved or it's degraded and for him, the amount of load that he was doing is obviously related to him being injured down the track.
Alright. Enough of that little preachy bit.
When we normally do these I basically talk about 'What's the injury?' and 'What are we going to do about it?' So there's sort of three questions to pose. I'm going to answer question one in this little bit and then the keynote from Robin Burgess-Limerick, my business partner, an academic colleague will answer the parts two and three if you will.
So the first one is, 'How does musculoskeletal injury really occur?' and then Robin later on will talk about, 'Once you know that, what can you do about it?' and, 'How can you get it as effectively as possible?' Okay? So I've just got one question I want to answer. That's it, and just to let you know there's about 15-16 slides to go, so you know when to wake up. I'm almost done, okay?
Alright, the first bit for me is how does injury really occur? Injury really occurs when the load on tissue is greater than tissue capacity. That's all there is to it. From an engineering point of view this building itself is a perfect example. The I-beam or the structures in the ceiling, good engineer hopefully, would have decided 'What's the weight of the ceiling above it?', 'What's the weight on the nodes?', would have understood those forces and would have designed the structures within this to beyond those forces, to make the tissue capacity or tolerance if you will of that I-beam, to beyond those weight forces.
In a biological system, so if we use this example here, a guy on the railway, again no fetish for guys in tight shorts. This is just what he was wearing. When he's banging away, this is a pig's foot. He's lifting out a dog spike – nice colourful language in the railway - and you can see that he swings harder and harder. On the first repetition he applies a certain amount of load, let's just say to a muscle in the back. Okay? If that force is less than the tissue capacity there's a margin of safety. No injury occurs. Okay?
But as he goes through the little dog spike's not coming up, so he swings harder and harder. If you swing hard enough you get an injury. That's all there is to it, okay? Which is fairly easy.
The difference between the I-beam in the ceiling and biological tissue is the tissue capacity constantly adapts to the load that's placed upon it. So as you do the task even if you don't swing harder and harder, as you do it throughout the day the tissue capacity will start to fall. So what was safe at the start of the day isn't safe at the end of the day. What's safe when you're 20 years old compared to 20 years' worth of injury when your older, aged body, not necessarily aged from chronological reasons but from the work you've done, all of a sudden that same task can cause an injury, right? So it's not just what's the absolute force. It's what that does to the tissues as you do it. Now that can be either positive or negative. It's really tissue adaptation that's actually occurring, particularly with chronic injury.
So it can either go up so what you've actually got is – well, how about this? I often say to people, 'Why do you go to the gym?' and people go, 'To get stronger'. I said, 'So at the end of a gym session, you lift your weights, at the end of an hour can you lift more or less than when you started?' and they say 'Less'. 'So did you get stronger during that hour or you got weaker during that hour? You got weaker didn't you? You deliberately damaged the tissues. That's what you did.'
So you go in on a Monday and you do your bench press or whatever it is. Then Tuesday you have a day off or whatever it is. It's during the recovery period that your muscles adapt to whatever load you've placed upon them. If there's enough period of time they overcompensate and then they're stronger than where they were to begin with. So you've got this slowly upward trajectory to improve capacity or training. Okay?
Conversely injury, chronic injury occurs. All I've done here is put one more repetition in. So you've gone in and you've done more loading periods, less recuperation periods and now you don't have enough time to recuperate and you've got this slow degradation down to a chronic injury over time. That's all that's happened, right.
Now some of the things that can influence that, the first one I've already talked about, it's exposure time, the relative time of loading to recuperation and if that isn't adequate enough and it's not recuperation in terms of having a rest and watching TV. It's doing one task at your work to moving onto a different task rather than doing the same thing over and over again. So that relative time of loading to recuperation can have an effect on whether you go down to injury or upwards towards improved capacity.
The other things that can happen is these green bumps here if you will, rather than being that height, if you've got greater load, what we call exertion, greater forces on the body, that means that red sloped line is going to come down steeper and you're more likely to be in this bottom scenario towards injury. Okay?
If you push all of those humps together, have a faster cycle pattern, so rather than doing a task where you're lifting a bag of concrete once every minute, if you're doing three or four or five times per minute, you increase the risk of injury because the cycle time is shorter and the tissue capacity comes down faster.
If you have awkward postures, so at the extreme ends of the range of motion you have a decreased ability to generate force, the entire red line is actually depressed and comes down. So again more likely to be in the bottom area and then you have environmental factors, psychosocial stress, heat, cold, all of those sort of things. So they're all the different factors and what I wanted to do now is just go through each one of those.
First it's important to understand the difference between the acute injury and the cumulative injury. Acute injury actually is quite rare. It's either associated with a very high exertion, very high forces or very high speeds, or extreme postures or falls and that sort of stuff. I'm talking about musculoskeletal injury here now.
More often than not it's the cumulative load over time, high exertion, speed, static or awkward postures, repetitive movements or environmental factors that all interact together to have that slow degrade down to a chronic injury or what this fellow just thought was ageing. Okay?
So I'm going to go through each one. The first one is exertion.
Exertion is the internal muscle force that is experienced by the person, not the external weight. So the higher the effort, the higher the internal muscle contractions, the higher the forces, the greater the level of risk and in the next slide I'm going to go through and actually show you the mechanics around that.
As well as the I suppose muscle force it's also the muscle speed. So performing a task, he is knocking out a pin on a dragline in the mining industry, high speed movements require high muscle contraction velocities. High muscle contraction velocities are associated with high exertion and you get an increased risk of injury. The other thing that actually happens as well in this case is we've got this force velocity relationship where the greater the velocity, the decreased ability to generate force and the greater the risk of injury. So again if you're going back to the gym, if you were lifting a 10 kilogram weight doing a bench press, you'd be able to do it very quickly. But as you put more and more load on it, you'll find you'll get slower and slower and slower. You have a decreased capacity at higher speeds as well as increased contraction velocities.
The other thing that happens in terms of exertion is eccentric contractions. So often we hear this lifting sort of risk factor, okay, which is a concentric contraction. The muscles are shortening as they're generating force when you're lifting a load or when you're swinging the sledgehammer there. Okay? But when you're actually lowering the load or catching the sledgehammer as it's falling, the muscles are lengthening as they're generating force and you've got a massively increased risk of injury. It's a bit of a generalisation but I tell my clients often, 'It's not the lifting that'll hurt you, it's the lowering,' particularly if you're lowering very accurately so you've got to control the position where you're actually lowering a box or a weight or whatever into a container or whatever it is. It's the eccentric contractions that are most associated with injury. They're also the ones that generate the most amount of tissue capacity in terms of muscle growth and development. If you find, if you ever see extreme athletes plyometric training or weightlifters or what not, they'll do a lot of negative reps, a lot of eccentric muscle contraction to actually get that increased amount of injury or I suppose degradation of the tissue which means they've got to recover greater.
The other one is smaller tissue size. So the cross-sectional area, CSA, every square centimetre of muscle can generate about 30 newtons of force, irrespective of what muscle it is or even if it's human tissue or baboon tissue to be honest, okay. So the greater the size of the muscle the greater capacity it has to generate force, so the decreased risk of injury that it would actually have. So larger muscles in your legs or your back can handle more load than smaller tissues in your forearm for example. So it's not an absolute number. There is no magical number about below which you're safe. It's really how much force the person is experiencing and the best person to judge that is the person themselves. So as part of this assessment we just heard in the last presentation, it's interacting with the workers. It's their perception of the exertion that is most important.
So that exertion can either result in acute tissue damage or it can have some sort of micro trauma that leads to chronic injury later on.
I said I was going to talk about this internal muscle force. So I'm going to give you this example here. This is a mate of mine, well a guy who became a mate out of these sort of projects, Dick. He's lifting a screamer pump. That's a 68 kilogram, high volume, high flow rate pump in the mining industry. It's used to deflood mines and you can see he's lifting with quite a stooped posture but I've given a diagram there that's got more of a squat or semi squat sort of position to show you it doesn't make any difference.
So we've got a force in the hands of that 68 kilogram pump multiplied by that lever arm there is actually creating a flexion torque, pulling him forward if you will. Okay? It's a function of the weight in the hands and how far it is away from the body.
To counteract that obviously you've got a contraction force in the muscles of the back to prevent you falling forward and they've got a much smaller lever arm. In fact for males that lever arm, that red one there is about six centimetres and for females it's about five centimetres. It's mostly height related.
So in this situation if we've got 68 kilograms of force in the hand and let's say this lever is, just to make it easy, 60 centimetres, so 10 times bigger than the lever in his back which is about right, about that position there, 60 centimetres away from his L4 L5 joint. If the lever in the back is 10 times smaller, in order for everything to be balanced, the force, the red arrow has to be?
This is the interactive part of today's proceedings.
Ten times greater? Yeah. 68 kilograms. 680 kilograms. That's just the weight in hands. You've obviously got the weight of the upper body, 40 or 50, maybe for Dick a few more kilos than that in the upper body, right? So that's 680 kilograms, could even go to 750-800 kilograms. That's just to stay still. If you want to lift up the red ones have to be bigger than the black ones. So that's 900. If you want to do it quickly which in this case you do because it's in a flooding situation, it can easily be 1,000 kilos or 10,000 newtons for those of you mechanically related. So that's basically the pressure of a tonne sitting on your back and not only does that extend the back but it also compresses all of the spine together, all of those spinal segments, okay, that compressive force which I'm sure you've heard, pushing upon the vertebral bodies and the intervertebral discs.
Then you may have seen a picture something like this. Has anybody seen that? Yeah, yeah. Your disc is a marshmallow shock absorber or whatever, that sort of stuff? To quote Donald Trump, 'Wrong'. It's not what happens at all. That is just complete rubbish. The disc – as a biomechanist this is what we do for a living, the disc doesn't absorb shock. It isn't the shock absorber of the body. The disc itself is basically made of fluid and fibrous materials. For those of you who know hydraulics. Liquids are basically incompressible. The disc basically goes under a hydrostatic pressure and it pushes up into the vertebral bodies itself. This is what actually happens. So as you put pressure on the disc the endplates, the surfaces of each of the vertebral bodies actually get pushed up superiorly and inferiorly. So you've got the distension.
It's a lot like I suppose, a little trampoline if you will. You've got this frame on the outside of the vertebral body and then the mat is like the endplate that's moving up and down, which for a one off scenario mightn't have a problem but you jump again and again on the trampoline, you apply this force again and again and again, and eventually you can have those fractures of the bones, those endplates. Do you know about endplate fractures? So you have a herniation of the disc either superiorly or inferiorly.
I don't want to go onto it too much. What I really want to point out, it's his perception of that red arrow that's really important. His perception or the group of people doing that saying, 'This feels difficult' is the best assessment of the level of risk. Okay, not the external weight, whether it's 68 kilos or 20 kilos, whatever. If you're holding 60 kilos very close it's going to feel much easier than holding say 40 kilograms further away from your body, and you know that to be true.
The next one is exposure, so tasks that involved loading for prolonged periods of time without periods of recuperation and they're very important in terms of tissue size. So the example at the top is an underground mine. Loaders or shuttle cars, whatever you're driving, you typically drive in by looking to the left, right, driving down the development road and where you're driving out by, out of the mine, you look to the right. So you constantly have your neck in either this position or that position. The rest of your body is static, sitting down the whole time. But it's the smaller tissues in the neck that are more associated with injury and pain in this task than the larger muscles of your back or whatever.
The same thing for the one down the bottom. So what he's doing there is clipping up Trak-Lok 2 clips, these C clips that hold basically the rail to the sleeper and it's the forces in the wrist. Even though there's a lot of force in the back and the legs as you're driving down, they're much bigger structures. It's the forces in the wrist that they really complain about holding on to that clipping bar.
Those exposure factors are really associated with chronic rather than acute type of injuries.
One of the things that we often talk about when we talk about exposure is how long you're actually doing something without a break, without recuperation and yet within ergonomics we often talk about designing the workplace to suit the worker. So I wanted to give an example of that. So here we have Homer who's actually, it says, 'Ergonomics helps to adapt jobs to the people who perform them,' which sounds good, right? Ergonomics – designing the workplace to suit the worker rather than the worker having to fit in with an inadequate workplace. That's what we're trying to do.
But in Homer's example here he's got his foot massage little spa going on, he's got his head massage, he's got his little coffee. Everything is designed perfectly for him, right? So we're designing this workplace to suit the worker.
What's the one thing Homer won't do?
Move. Why would you get out of your chair? I wouldn't if everything was like that.
What it's really about is saying, 'We need to design this workplace to suit the worker in the worker's best interest'. So sometimes – you obviously know this about office ergonomics. It's not about how comfy the chair is or how good the desk is or whatever else. It's how often you actually get out of your seat and that's what these sit to stand desks or planning having different meeting rooms is all about. Now of course that's just one little example of Homer and whatnot. You say, 'Well that's not real'. Well, yeah it is. Okay? This is a chair that you can buy online. If you put 'ergonomics office furniture' I bet you it will come up in one of the top listings somewhere. It's sold as an ergonomic chair and whilst all the body positions are pretty good, 120 odd degrees at the hips and all the rest of it, again the one thing you're never going to do is get out of the chair. It's going to take you 10 minutes to get in and out of it to start with I would think. Alright?
Is it motorised?
Dr Gary Dennis:
Sorry? Is it motorised? I wouldn't know. I'm not about to buy one to find out.
So really exposure is about how long you do things without a change and it's how the environment is designed so that change naturally occurs. Not telling people 'take a break when you need it' because they won't, as you know. If anybody works in an office, sits at your desk, needs to get done. The next thing it's five hours later and I haven't moved.
Two more. Awkward postures. So tasks that require an unnatural posture to be maintained and there's a force length relationship here. It's an inverted 'U' and it means around the normal range of motion, right, you actually don't have much of a change in risk, but at the extreme ends the risk goes up exponentially. Okay? In fact it goes up so much that right at the very end of the range of the motion the risk is off the charts and I want to give you a real simple one for this so you can actually see it for yourself. So everybody without hitting anybody else, put your left arm out for me please. Go full extension, full flexion, okay, all the way in, just as much as you can. Grab your right hand, put it on your left wrist and you should be able to push it in a tiny bit further. Can you feel that, you get – so if I let go that's about the angle and if I push in it goes a bit further in. That's called the active range of motion for muscles and this is the passive range. It means even at the end of a range of motion you've still got some range that's associated with the passive structures and the ligaments and whatnot, and at that point the risk goes up massively.
So if you have external forces that are applied to you, catching or holding an object underneath something that's low or awkward, the bottom ones there, and you get into the passive range, the risks goes up massively. It's also dependent upon tissue size. So the smaller tissues in the shoulder up the top there can't hold that position for as long as larger ones in the back and legs.
You're going to get either acute injuries if the posture is extreme enough and it's right at the end of the range of motion. I see too many people go, 'We're bending over a little bit here and there'. It actually makes very little difference in terms of the risk if it's done for a short period of time. But if you have those awkward postures combined with other factors you can often have these cumulative injuries creep in.
I'm just trying to check to make sure that I'm going for time.
So I want to again, give you one more example of that. So this is a task that's done in an aluminium smelter at Gladstone and what they're actually doing here, that's a table top. So the aluminium comes in the table top and gets poured into big billets which are about eight metres long and then they pull them apart, chop them up and sell them off.
Some of the excess aluminium remains in the top area and it's worth a lot of money. So these guys go through and pull out that excess basically with crowbars to recycle them if you will. This task might take sort of 20-odd minutes. It's changed since then, okay?
The little figure on the top left here is the lumbar spine, say L3, 4 and 5. In between each of these posture response processes you've got these little interspinous ligaments if you will. In that position as it is there with that lumbar lordosis those ligaments are nice and relaxed. If you're bent over though, that spine starts to curve around and those spinous ligaments start to stretch, okay? As they stretch they start to creep and one of the best ways I can think of explaining this, can you remember you had those slinky springs as a kid, older people in the room, yeah like me, and invariably you play with the slinky spring for a bit down the stairs and you get a bit bored with it. You give it to your mate and you'd go across the room. 'Let's see how far this thing can stretch?' Do you remember that? We all did that. Then you let it go and what happens to the slinky spring? It's stuffed, right? It doesn't come back to its resting length and that's fine. So if you stretch it far enough you can damage it.
What most people don't realise is if you get the slinky spring, you move it out a foot, let it go, it comes back to its resting length. No problem. But you get the slinky spring, you put it out to a foot and you clamp it there overnight, same distance, but you leave it overnight. When you come back in the morning you unclamp it, it doesn't come back to its resting length. It's not just a function of how far you go and how extreme the posture is. It's how long you hold it for. As you hold it for a longer period of time the tissues start to change and creep. So with this task being bent over for 20 minutes, not only are you at a risk of injury during the task, but you're probably at a risk for the other half an hour after the task is finished because you've got a decreased, I suppose, stability of the lumbar spine at that point.
The one on the bottom left is just a disc herniation and I suppose I wanted to just tell you the mechanism so that people know what actually happens because often it gets wrong. I get pulled into court all the time for this one.
In order for you to have a herniated disc you must have pre-existing annulus fibre tears. The tears in the annulus fibre for the nucleus to go through. That's mostly associated with rotational type of movement is where those tears come from. At that point you go through as what's happening here, a period of prolonged flexion. As you are flexing for a longer period of time, slowly but surely that nuclear material starts to creep through the cracks in the annulus fibres if you will, okay?
Then after you're flexed over for 15, 20 minutes or whatever it is, you have a period, you have a compression force and rapid extension at the same time. So you're bent over, you're working away for 20 minutes. Then you've got a piece of aluminium that's very difficult and you try and reef it out very quickly. You've got a lot of muscular force compressing the spine, preventing the nuclear material coming back to its original position, rapid extension, the posterior section of the vertebral body impinges upon that part of the nuclear material and spits it out into the posterior lateral sections.
You must have exactly that mechanism for the event of a disc herniation to occur. That's just mechanics.
That's the work of Bogduk and McGill.
Movement patterns, the last one. So you have injuries that increase with static postures particularly if they're held for longer periods of time. So the one up the top here, this is welding up a dragline cable. You can imagine, I suppose it's a bit of that tissue creep that's going on, that static posture. You've got static contractions, decreased muscle supply, you have neural drive decreasing with technique because you've got a static position etc.
It also increases with repetitive movements particularly if they're over very short cycle times. So this is an example of a guy who's cutting the 50 strand signal cables on the railway with a pair of manual cable cutters.
Out of interest can anybody see something that's not quite right with this? Don't worry if you won't because – yeah. It's funny. I showed this video for years, honestly, showing repetition and all the rest. I was in a workshop one time and one of the workers went, 'He's an idiot'. I went, 'What's going on?' He went, 'The cable cutters are upside down'. Normally when you use a pair of scissors or whatever else, the static bar which is the one that's closest to his body if you will, is on your thumb and you do that sort of thing, right? But if you look at it what he's doing, he's got the static bar on his fingers and he's pushing there, like that, which does seem a bit stupid. But I know for a fact because I took this video, that he's been doing this for a long time and I'm pretty sure he knows how to use a pair of cable cutters.
I'd love to go back and ask him because I can't get to him now, why he was doing it? I would lay my bottom dollar that he's been doing this for that long that now he's got an injury, now he's turning it the other way around because it feels better. That's why he's doing it. It's not this training, let's teach people how to do it. They're doing it the way that feels best for them but of course in this case I feel fairly confident that he's going to head to the next injury. If you asked Robin later on, he'll actually show you video footage of a library in Brisbane where the librarian, where they sort the books on the table and then they take them out to the various areas, yeah, you know the sorting table? Whenever there's a large, wide group of books this distance, he grabs it between the thumb and the finger like we all would. Okay? But as soon as it gets down to a small, like just one or two little books, he can't grab it that way because all of the force has gone out from the strength in his hands and he actually grabs it like that and sorts it. Not because they think that's a fun way to do things but because for that person it feels better.
Okay. So smaller tissue size again, the smaller muscles in the hand are more associated with injury. Then all of those can be associated with chronic injuries.
Okay. You also have and I haven't got time to go through this in heaps of detail, but various environmental factors that also play into those whether they're heat that causes fatigue or cold that increases muscle stiffness, time pressures which forces you to work at a rate that's faster than normal so you've got high contraction velocities if you will, lack of control in say an abattoir or meat processing facility where you have to work at the rate that the chain is on. Most production lines in fact you have to work at a very set rate. So you can't change your exposure level to be a little bit quicker now, a little bit slower later on, whatever else, which we're naturally adapted and evolved to do.
There's stress and cognitive overload and underload, that's mental workload that are correlated with injury. We can talk about that ad nauseum if you want to come talk to me later on, pinch points and vibration, whole body and localised. So I just want to pick one of those and show you how it sort of works.
In terms of vibration, injury or the risk of injury increases with increasing amplitude. So when if you're basically going over say the ruts in a road, the bigger those ruts are, the bigger the potholes if you will, the greater the risk of injury. It increases with the frequency. So if there are higher frequency movement patterns it increases the risk and it also increases with resonance. So all of your structures within your body vibrate with a certain frequency and that frequency is determined by the length of the object. So if you go to a playground and you take your nieces or nephews and you put them on the swing, and you push them back and forth on the swing, they will go back and forth at a set rate, say two seconds, one second that way, one second that way for example. It doesn't matter how hard you push them, that's the natural frequency of the swing and it's determined by the length of the chain. The shorter the chain the higher the frequency. The longer the chain, the slower the frequency. If anybody has those old metronomes, you know the ones where you put the weight on, you know that's all you're doing is just lengthening or decreasing the length.
For the spine it's actually the distance across the vertebral body and the resonant frequency is typically between four and five hertz, what it naturally bounces up and down at. Unfortunately the frequency of vehicles is a multiple of that frequency which increases the amount of deflection of the endplate. So what we've often got to do is put seats in that are dampening those specific frequencies. So whole body vibration often associated with lower back pain and endplate damage.
Conversely if you have localised vibration, so this is tamping. This is basically a modified jackhammer with a splayed bit that they're using to vibrate the ballast underneath the rail to support the structure and you can see the amount of vibration that's going on the hands. But whether they're grinders or sanders or rattle guns or whatever else, those sort of vibrations again, amplitude, frequency and resonance can damage the neural system. So you get that tingling sensation or you have vascular damage, so decreased blood supply, increased rates of fatigue and if it's bad enough you can get white finger syndrome where the top of your fingers doesn't get adequate blood supply and literally turns stark white.
Oftentimes people ask or they used to ask us, 'Well how much frequency is good, bad or indifferent?' So Robin did this project as part of MISC, Mining Industry Safety Health Commission at the University of Queensland to develop an app that's free, WBV. It's an app that you can download. It's only on the iOS system, sorry, but you can go and get an iPod Touch for about $200, download it, tell it how long you're driving a truck for and then you just sit on that phone or iPod Touch and it gives out readings like this. They're converted into X, Y and Z, so the three axes if you will, red bad, green good sort of scenario. Okay? You can come and talk to us if you want about that.
Have I got just five minutes left on that? Three or five minutes. Okay.
So just to put a summary of all of that sort of stuff and I've tried to keep it simple really today but I just want to summarise and make a really critical point here at the end. This is the bit where I'm saying you can wake up for this bit. I know you've been all asleep.
So some of the risk factors. One of them is exertion and as you can see with the guy at the top there, that jerky movement, it's not so much the weight that he's actually going through. It's the speed of movement and the contractual velocities of the back. There's going to be a high risk, well a high amount of exertion in those muscles and if those forces go beyond the tissue capacity you're going to get an injury with the graph on the far right.
Likewise if you have a task that's very repetitive. This is a recycling centre and I want to pause really quickly to make something that's become a passion to me recently. We were at this recycling centre and they're basically recycling the plastics and the bottles and all the rest of it. As we did it, not on this line but another one, a giant TV come out over top and was sorted along on the line. This is one comes that out of the yellow recycling bins at home. I went, 'You're kidding me? A whole TV? CRTV?' 'Yeah, yeah. That happens,' blah, blah, blah, blah. 'You should see what else comes out.' We moved the TV and honestly within 10 or 15 minutes later, a dead kangaroo came out over the top of it and I don't want to make it too graphic for you, but it had been there a little while. So when it hit the…
So put the things in the yellow bin that need to be in the bin people. I'm sort of a bit passionate about it because these people have to suffer this at the other end and it wasn't pleasant for anybody.
So repetitive sort of movements, decreased tissue capacity and then finally awkward postures. So this is one. What he's doing there is putting a clear paint, a covering on the train so that when people invariably graffiti the trains it washes off nice and easily. Unfortunately in this case we changed this to be on with the spray.
So with those tasks, so exertion, repetition, posture, all of them are associated with risk. What's really important is none of them are associated with risk of their own. It's actually the interactions. The top one there, the exertion, if I go to the gym and I do a shoulder press or something, that's not associated with a lot of injury because I typically only do it once, I've got controlled posture, and I don't do it for prolonged periods of time, like I'll just do three sets of 10 for example.
In the middle one this repetition, so walking. Walking is a repetitive task. Why isn't that associated with a lot of risk? Because we don't do it for excessive periods of time and it doesn't associate particularly awkward postures, right? I'm talking about the legs now.
The one down the bottom, if you went to a golf course and you picked a golf ball out of the hole, right, again yes, you might have an injury from that one-off event but not likely and it's because whilst it's an awkward posture, it's only done once. So again very short exposure times, very low forces. It's really the interactions of all of those factors that play the most important part and that's what I want to really sort of finish off and get you to really understand.
So whilst exertion is force and speed, exposure, duration, recuperation, posture, how awkward it is particularly if it's at the very ends of the range of motion and that exponential risk, repetitive or static movements, environmental factors, it's how those factors interact. This is what we try and do with Ergo Analyst, the risk assessment tool we've developed. So whilst you've got exertion, exposure, posture and movement patterns, it's the interaction of those risk factors that are going to have the biggest impact on either the acute or the cumulative risk. They're different depending upon which risk factor, for how long, multiplied by what other factor, multiplied by what body part and that's the key.
To turn it on its head for just one second, I've talked about exertion, exposure, posture and movement pattern as causing injuries, almost this whole time. Apart from that first slide where I talked about improving capacity in training, what we've also got to realise is each of those risk factors – I've got literally one minute – are associated with health. You actually need a certain amount of exertion on your body for your tissues to get better and stronger. You need a certain amount of exposure, you need repetitive movements, you need a certain range of motion, as long as they're not too extreme.
So for the example that I gave of doing a bench press or the shoulder press, right? So each one of these body parts is just for a different task. I'll just give this for an example. So you go to the gym and the exertion on the shoulders is quite high, right? Not absolutely extreme but getting right up there, but you only do three sets of 10 or whatever it is. So exposure level is quite low. You only do it once every second or third day. The posture isn't to the extreme ends of the range of motion and whilst you're doing three sets of 10 that's not the same repetition as doing hundreds of repetitions in the workplace.
Even though the exertion is high, the risk, yeah you might have a little bit in acute risk but overall not much.
For the back, if we have a person picking a golf ball out of the ground we have an awkward posture there for the orange. The rest are all green. No level of risk. Walking – relatively low levels of exposure, highly repetitive movement patterns. Again no risk.
So I want to leave you with the fact that each one of those risk factors whilst they can cause injury are also associated with positive health if they're done in the right way. I know that people don't always have access to all of these type of risk assessment tools. This is one that we developed for Sanitarium recently that basically said out of those exertion, exposure, posture and movement patterns, if we're able to lower the force of speed, vary the work tasks, work within normal range of motion, or what was the last one? Vary the comfortable postures and the movement patterns, we're going to come with a better outcome.
So for us the question I really wanted to ask at this point and obviously Robin will do Section two and three, how does musculoskeletal injury occur? Well really it's tissue adaptation. If the load is greater than tissue capacity you get an injury. If you modify it with the right amounts of level of each of those you get an improved worker health. I've really pushed that through.
Thank you so much Gary. Are you staying for lunch?
Dr Gary Dennis:
So we haven't got any time for questions but feel free to ask Gary some in lunch.
We'll be regrouping at 1:30 in the main room there for your colleague…
Dr Gary Dennis:
…Robin Burgess-Limerick. So meet you back at 1:30 in the main room.
[End of transcript]
- Last updated
- 10 December 2019