Our heavenly, humble human hand

They have one of the most enormous jobs in our body, so when one doesn't work, it is complicated to make a new one, writes Lana Hart.

They are always there: waiting, responsive, available to do our bidding whenever we want. They caress and tap and grasp at our command. They are designed to be stronger than they look and can convey some of the deepest of human emotions. A gesture of one can change the meaning of our very words or cause the most severe of offence. Only primates have them. Most of us have two. When they don't work properly or are injured, it profoundly changes our world.

Of all the incredible organs in the human body, our hands have one of the most enormous jobs. We need their many abilities in nearly everything we do - work, eat, show emotions, create and fix things, communicate, clean ourselves.

Our understanding of these loyal appendages is fairly patchy. Did you know that your fingers have no muscles? Or that – so complex are our many hand movements - the largest part of our brain's motor cortex, the area that controls all muscle movement, is responsible for directing only the hands? How about this one: happy and stress-reducing hormones surge when we feel a friendly hand on our body?

Opposable thumbs, not unique to humans, evolved for primates' common ancestors to better grasp branches and support their entire body weight while swinging. Over time, claws disappeared, replaced by flat fingernails and wider fingertip pads. Almost all primates have the ability to exercise both strength and precision in their fingertips. Humans, for example, can both grasp a hammer tightly and delicately steer a pen to write.

Muscles in the palm and the forearm, not the fingers themselves, work together to give fingers their strength and nimbleness. Like puppets on a string, fingers are moved by the short, bulky muscles in the palm and the long, stretchy muscles in the forearm, connected by flexible tendons.

Try this: Bend the small, ring and middle fingers one at a time. Notice the finger next them moving a little too? The tendons that move these fingers are controlled by the same muscle in the forearm. The adjacent fingers move with the finger you're trying to flex because they share the same muscular source of control.

The complexity of the human hand becomes even more clear when we try to replicate it robotically.

"Prosthetic arms and hands can't feel pressure," says Alison Wilding, a musculoskeletal physiotherapist hand therapist at In Touch Hand Therapy, Christchurch.

"Human hands treat an egg differently to a tennis ball. Artificial limbs don't have the sensory perceptions to know the difference in order to send the right messages back to the brain."

There is a lot of interest in this area at the moment, she explains, especially involving the use of muscle contractions in the forearm and upper arm to control the prosthetic hand. Trouble is, she says, "they break very easily. Prosthetics aren't yet strong enough to stand up to repeated use that is always going to be useful to the user."

But robotic hands are providing great promise to amputees thanks to today's 3D printing capabilities and newer materials such as silicone and PVC.

David Lovegrove and the design team at 4ormfunction in Christchurch designed a world-leading bionic hand and are now creating a much smaller version for children.

The Taska hand is strong enough to crush a tennis ball but delicate enough to grasp an egg without breaking the shell.

Designers spent thousands of hours improving the hand, which has a tiny motor, gearbox and clutch for each finger and two for its thumb. It's worth $35,000.

Dunedin sculptor Gavin Wilson, who lost his hand six years ago after he put his arm in a shredding machine he believed was turned off, has one.

Since the accident, Wilson has had a number of prosthetics but told the Weekend Herald in September the robotic sensors in the Taska hand allowed him to do "nearly everything".

One of the questions that has been raised in prosthetics is: do we need our replacement hand to look like the other one?

In fact, two different robotic hands, each one designed with different abilities, might be a more useful tool for a double amputee. For example, one hand could be designed for gross motor skills like gripping and turning, while the other would be used for fine motor movements like writing and picking up small objects.

"Society says yes," says Wilding, "that we should have symmetrical limbs. But two different limbs would probably work better than two of the same design. Prosthetics gives us the opportunity to get more creative in our thinking about hands and limbs."

"Artificial intelligence and prosthetics are coming rapidly at us. How we choose to use this technology is going to be interesting to watch."

But the real thing has a connection to the nervous system that is difficult to replicate.

Hands host some of the densest areas of nerve endings in our bodies; skin on the fingertips have evolved to be highly specialised in the perceptions of pressure, temperature, pain, and touch. This increased sensitivity means hands are closely associated with not just our physical world, but our emotional one as well.

When we touch or are touched in a supportive and friendly manner, the brain releases oxytocin, that feel-good hormone that brings about feelings of trust, bonding, and affection. The stress hormone cortisol is also reduced by positive touching and acts as a psychological buffer in stressful situations, which is probably why we hug a loved one when they're stressed or touch a colleague's shoulder to wish them well before a presentation.

Research from the University of California demonstrated how people can convey emotions through touch alone. Dr Dacher Keltner's experiment separated participants by a barrier so they could not see or talk to each other. Touch recipients received a touch to the forearm, then were asked which of 12 emotions – such as anger or compassion - the toucher intended to convey. Participants guessed correctly more than half the time, with increased accuracy for the more positive emotions.

Our expressive hands can help tell a story as we gesticulate, turn outward towards a listener to express humility, and demonstrate our confidence in ourselves by steepling and pointing. In his work with mock juries, Joe Navarro, in a Psychology Today article 'Body Language of the Hands', found that lawyers and witnesses that hide their hands while speaking are perceived to be less open and less honest by the jurors.

And of course, our hands show affection and care by caressing, handholding, hugging, and connecting with our loved ones.

That emotion was played out in the 2014 RoboCop remake. Detroit-based police officer Alex Murphy suffers burns to most of his body after a car explosion.

In the original, the character has only his face and brain salvaged and is transformed into a cyborg. In the remake, Murphy has one of his hands salvaged, while the other is robotic.

The Detroit Metro Times revealed a deleted scene which shows on the DVD version.

"Can you save his right hand?" says Michael Keaton's character Raymond Sellars. "My father always said you can tell a lot about a man by his handshake."

And at last week's Consumer Electronics Show in Las Vegas - the world's biggest technology fair - a pet robot, complete with fuzzy teddy bear arms, was a hit.

Designed as a loyal companion for lonely elderly people, the Lovot is packed with sensors to respond to human touch and coos when its owner strokes it. When it wants to be cuddled, it waves its arms in the air, and will trail around adoringly behind its owner on wheels. It will even fall asleep in their arms if offered a cuddle.

Researchers at Cleveland's Laboratory for Bionic Integration have recreated the feeling of kinesthesia - the intuitive sense of knowing where your limbs are, as well as the positions they're making.

Finely-tuned vibrations were sent into the skin of the upper arms and shoulders of six arm amputees.

The approach improved their ability to feel and control their prosthetic arms when performing actions such as gripping and pinching.

"When she holds my hand it feels really nice. But not having the ability to respond by squeezing her hand is hard for me. She has learned to put her hand on the top of my hand, cupping it from above while my hand rests on the arm of my wheelchair."

"We can sometimes do the traditional palm-to-palm handhold and this feels more rewarding because I can participate, using the fingers that still work to squeeze so it's more reciprocal and mutual. This is more emotionally charged for me."

Ward no longer has the use of one of his hands. The other hand still enjoys the partial movement of three of his fingers and, on good days, a compliant thumb. Muscle atrophy, he says, has occurred over the past 18 months mostly in his palm and top of his hand, rather than his fingers.

Despite the partial use of one hand, he says his hand movements are "clumsy, not refined. Hands work best if fingers and thumb are healthy. They rely on each like an orchestra. With each finger losing strength and movement, the hand becomes even more redundant."

Think, for a moment, about the electrical triggers of your motor cortex right now. If you are reading this story on your phone, four fingers are shaped in a soft but firm curve around the back of your phone, while the thumb strokes the screen with just the right amount of learned pressure to scroll according to your reading speed.

If you are reading on a computer, one hand is touching a mouse and sparking the muscles in your palm and forearms to point the index finger to scroll or click. The other hand may be resting on your thigh or scratching your chin.

Or, if you are reading in hard copy - maybe while reclined on a beach towel in the sand or on a soft chair under a tree - your hands will be busier than your mind; holding the pages steadily, unconsciously securing something as thin and delicate as a piece of paper. Your fingers somehow know the exact dimensions of the paper so that it can grasp it without tearing the page. Then, your hand will turn the page by flicking a wrist – also part of the ecosystem at work here – to command the paper to rest without complaint on the page opposite. Magic.

So why did evolution deny fingers autonomy from the rest of the hand, leaving them without muscles? Wilding says that this may have to do with the size of the hand muscles.

"The fingers need to bend and when a muscle contracts, it bulks up," she explains. "There isn't room for muscle contraction in the fingers without losing fine motor movement. Fingers have the ability for delicate motor control because they are slender and fine, not bulky."

Muscle-less fingers also need to be able to adapt to different shapes and textures, says Wilding. "Hands must be mobile and mould around so many different things: a doorknob, a baby's finger, a steering wheel."

"Unlike the foot whose main role is to bear weight, hands are non-weight-bearing, so you can have the movement, manipulation, and the sensations, as well as being strong," Wilding says.

These wide-ranging abilities require the hands to make an enormous combination of movements. Given the complexity of this task, it's no wonder that a disproportionate amount of the brain's motor cortex is devoted to operating the hands.

Next time you're holding someone's hand and squeeze, be grateful for the symphony of bones, neurological pulses, muscles and hormonal surges that serve you, mostly without even thinking.

Where is your palmaris longus?

• Fourteen per cent of the world's population don't have a palmaris longus muscle, which is visible in the inner forearm when pressing together the ring finger and thumb. Since this muscle is not needed, it is sometimes removed by surgeons and used for tendon grafts in the person's wrist.

• Our palms and fingertips have no hair and do not tan.

• It takes up to six months for a fingernail to grow from root to tip.

• The hand has 27 bones, 29 joints and at least 123 named ligaments.

• Six per cent of all men and 9.9 per cent of all women are left-handed.

• Women tend to have longer index fingers than ring fingers. For men, it is the opposite.

• Despite many myths about it, there is no scientific correlation between the size of a man's hands or fingers and the length of his penis.