A question that people who don't use prosthetics often ask is 'are prosthetic arms stronger than biological ones?'. They are usually thinking about the prosthetics from fiction like the Six Million Dollar Man (which, fun fact, popularised the term 'bionics'!). Of course fiction doesn't have physics to contend with, but real life prosthetic technology is just as exciting!
If you've thought about prosthetics before but thought they wouldn't be durable enough for your lifestyle, the second part - 'are prosthetic arms stronger than they used to be?' - will assure you that the times have changed.
Are prosthetic arms stronger than biological ones?
If you're getting a replacement, you might as well get an upgrade at the same time, right? Well, it's best to be upfront - you're unlikely to lift weights with your prosthetic that are much heavier than you could without it.
To lift a heavy weight, it's not just the arm that has to work. The whole body - your shoulder, back and leg muscles - all have to work together. This means that the prosthetic can only boost your strength by so much, if at all, and your other muscles have more to do with how much you can lift.
If your limb loss is due to amputation or injury (as opposed to congenital), you've already practised using the muscles in your residual limb to lift and carry. This will make the rehabilitation process of learning to use your prosthetic easier than it would be if the prosthetic was the first experience you had moving weights with that arm. Prosthetic users with congenital limb difference can of course train their arm to do the same things, but it will take more practice, and might require the help of a physiotherapist.
It's important to understand the limits of what your prosthetic arm can put up with. Some prosthetics can handle heavier weights, others less so. Depending on the design of your prosthetic, there are a few problems you could run into while trying to lift heavy objects.
You're likely to run into trouble with the socket of your prosthetic while trying to carry weights. Anyone who's used a myoelectric arm will tell you that sweat on your stump can interrupt the connection between your arm and the prosthetic device, meaning you lose control of the arm. This is a fairly common issue during everyday use, let alone when you're exerting your body to move all that weight!
Even on body-powered prosthetics, sweat can cause the socket to slip and make the arm uncomfortable (if not impossible) to use. Body-powered prosthetics are often recommended for rugged environments because of the mechanical cable controls. However these cables are designed for delicate movement as much as lifting and carrying. This means they aren't designed to hold heavy loads, and if made to do so they can be damaged and snap. Of course these are replaceable parts for exactly this reason - but having to do repairs often can be really frustrating!
It's also important to think about the weak points in your prosthetic. Joints in particular are where pressure will be felt the most, depending on how you approach lifting the object. For instance, if your limb loss is below the elbow and you try to lift a heavy bag up in front of you, the wrist of your prosthetic will experience a lot of torque. Joints are designed to move, and while the design ought to take pressure like this in mind, there are always compromises between delicate movement and rugged durability. Over time this force can damage or break components of your prosthetic, even in the most hard-wearing designs - and again, these can be annoying to repair or replace.
When you have to move a heavy object, you might find it easier to adapt your approach. For tasks like moving furniture, some users find it easier to remove their prosthetic, and instead use a 'bear hug' to lift and carry large objects. Using as many of the muscles in your body as possible will distribute the weight, making the furniture much easier to move around!
Alternatively, some prosthetic users replace their hand with an activity-specific attachment. An attachment like the TRS Dragon, a flat rubber attachment originally designed for contact sports, could be useful for pushing heavy things. This also gets around many of the wrist problems mentioned earlier, as the force is felt all the way through the arm - no one joint is taking the strain.
A myoelectric prosthetic hand's grip can be really strong if you want it to be. Some people use this as a party trick - 'want to see me crush a can with my bare hands?' - but in everyday life this strength can be more of a hindrance than a help. Without the feedback to tell you how strong your grip is, it can be difficult to find the balance between too much and too little, so the grip strength is generally tuned lower than the maximum possible.
The forefront of bionic technology is trying to move the needle on this, giving the sense of strength back with neural feedback. Some modern myoelectric designs are built with systems like this in mind too - using vibrations or other means of feedback to the user.
Are prosthetic arms stronger than they used to be?
Recent years have seen a huge shakeup and continuous innovations in prosthetic technology and quality. Newly accessible materials like carbon fibre, titanium and Kevlar have been used in prosthetics to make them stronger and more lightweight (and therefore easier to use and wear for long periods of time).
Prosthetics are older than many people think. Some of the first functional prosthetics were made during the middle ages, when replacement hands were forged from metal. These were built to be as durable as possible, but were really heavy and uncomfortable as a result. Many were made for knights, so they could hold a shield or ride their horse, but examples of prosthetics used in daily life were much rarer.
The most famous example of these old prosthetic designs is Götz von Berlichingen's 'iron hand'. As a soldier, his prosthetic was made of steel plate, but it was so heavy he had to attach it to his suit of armour with leather straps. It was definitely designed with battle, and not everyday life, in mind!
The focus of modern prosthetic science has largely been improvement in the function of myoelectric arms, and developing 'true bionic' technology (i.e. moving prosthetics with thought). Prosthetics using this technology need many more internal components than passive or body-powered counterparts. This made the prosthetic arms pretty unwieldy, so engineers needed to use more lightweight materials on the outside.
Of course there's no perfect material, and they all have unique benefits and drawbacks. For instance 3D printable materials are lightweight and can be manufactured to fit each user perfectly at a much more accessible cost. This is why the ExpHand is 3D printed! Carbon fibre is incredibly strong relative to its weight, and can be really hard-wearing, but it's still expensive relative to materials like steel (and not all carbon fibre is created equal). Often prosthetics will use a mix of these new materials for different components, to try and make the most of each material's properties.
When engineering a prosthetic, it's important for scientists to research the everyday life of the average user. Most modern prosthetics are durable enough to put up with the stresses of daily wear and tear. This durability is becoming more of a focus as the technology for myoelectric and bionic control becomes more widely available.
If you'd like more information about prosthetic arms, check out the FAQ section of our site here. If you're in the UK and want to sign up for our beta trial, please get in touch with us here!