In Praise of Walking: A New Scientific Exploration

by Shane O'Mara

walking is hugely beneficial for our minds, our bodies and our communities. Walking is holistic: every aspect of it aids every

aspect of one’s being. Walking provides us with a multisensory reading of the world in all its shapes, forms, sounds and feelings, for it uses the brain in multiple ways. Walking together can be one of the best experiences of walking. Social walking – marching in concert and with purpose – can be an effective goad for real change in society. Walking is so vitally, centrally, important to us, at both individual and collective levels, that it should be reflected in the way we organise our lives and societies. Our public policymaking needs to fully embrace why walking makes us so distinctively human, and should feed in to our urban and suburban planning. I look forward to the day when physicians the world over write prescriptions for walking as a core treatment for improving our individual and aggregate health and well-being. In fact, GPs in the Shetland Islands have already started prescribing beach walks as a preventative treatment against maladies of brain and body.

I will show how walking makes us social, by freeing our hands for tools, and for gestures – movements that allow us to signal meaning to others. Walking allows us to hold hands, sending out signals of exclusive romantic involvement; walking allows us to provide physical support to each other; marching in protest is a common feature of our free political lives, which is why the prevention of assembly and of marches is one of the first orders of an autocrat. Walking is good for the body, good for the brain, and good for society at large.

The French philosopher Jean-Jacques Rousseau commented that ‘I can only meditate when I am walking. When I stop, I cease to think; my mind only works with my legs.’2

Moreover, recent studies show that walking increases blood flow through the brain, and does so in a way that offsets the effects of sitting around.6 Regularly interrupting prolonged bouts of immobility through the simple act of standing up changes the state of the brain by calling on greater neurocognitive resources, constituting a call to action as well as a call to cognition.

As well as improved cognitive control, it’s clear that walking confers many, many other benefits. We all know that it is good for our heart. But walking is also beneficial for the rest of our body. Walking helps protect and repair organs that have been subject to stresses and strains. It is good for the gut, assisting the passage of food through the intestines.7 Regular walking also acts as a brake on the ageing of our brains, and can, in an important sense, reverse it. Recent experiments asked

elderly adults to participate in thrice-weekly, and relatively undemanding, walking groups.8 In the regular walking group, over the course of a year, the normal ageing of the brain areas providing the scaffolding for learning and memory is somewhat reversed in the walkers, by perhaps about two years or so. An increase in the volume of these brain areas was also found; this is quite remarkable in itself, suggesting that the act of regular walking mobilises plastic changes in the very structure of the brain, strengthening it in ways similar to how muscles are strengthened when worked. One way of interpreting the literature on ageing and walking is straightforward: you don’t get old until you stop walking, and you don’t stop walking because you’re old. Lots of regular walking, especially if conducted at a high tempo, with an appropriate rhythm, forestalls many of the bad things that come with ageing. Walking is also associated with improved creativity, improved mood, and the general sharpening of our thinking. Periods of aerobic exercise after learning can actually enhance and improve recall of the previously learned material. Reliable, regular, aerobic exercise can actually produc...

But walking is special to me as a form of transport. Walking allows me to walk it off, whatever it is. Walking clears my mind, allowing me to think things through. Natural movement brings with it experiences and demands on the body and brain that do not arise from other types of movement. Cars, bicycles, trains and buses all divorce you in different ways from the environment, you are mechanically propelled, sometimes insulated behind glass, travelling too fast, worried about crashing, trying to find that new song on the radio. There is a peculiar passivity to it: you are sitting, yet you are moving at speed. This can never be true of walking: one foot must go in front of the other until you get there, under your own steam. You make your own way, and experience the world close at hand, at your own speed, in your own way. But, how do we know that walking has all these multifarious benefits for our minds, bodies and quality of life? What’s the evidence? The evidence is extensive and, as we will see in the course of this book, shows that walking enhances every aspect of our being, from our physical health, to our mental health, to our social lives and beyond.

This may seem an obvious point, but when we’re walking our brains are in motion too. In fact, as we shall see, we evolved as a mobile species: we walk about, we move, we seek new sources of information from the world. In other words, we are not just brains locked in a skull, we are minds in motion – we are ‘cognitively mobile’. The study of how we think, how we reason, how we remember, how we read, how we write, is known as the study of cognition. Typically, the scientific investigation of cognition occurs in a laboratory, using carefully controlled experiments and a range of methods and tests that measure cognitive abilities.

walking markedly changes activity in the brain in subtle, important and powerful ways.

(Need to lose weight? Don’t go to the gym; go for a really, really long walk. And do it in nature, over a period of days to weeks. It will be far more beneficial to you.)

Beyond health, walking brings many other benefits for brain, body and behaviour, which we’ll go on to explore throughout this book. We’ll also discuss the many poets and writers who have written eloquently on the wonders of walking as a spur to mood, creativity and thinking. Writers are amongst the best at recognising walking’s essential and intrinsic virtues and rewards. The poet I return to, time and again, is T. S. Eliot. I find Eliot’s poetry has a cadence and rhythm that are remarkable, especially if read aloud. His great modernist poem ‘The Love Song of J. Alfred Prufrock’ (1915) is a journey on foot, and a journey through states of mind. It is a poem set to the rhythm of a long urban walk, undertaken uncertainly as evening falls. The opening lines are an invitation to walk the city: Let us go then, you and I, When the evening is spread out against the sky Like a patient etherized upon a table; Let us go, through certain half-deserted streets, The muttering retreats Of restless nights in one-night cheap hotels And sawdust restaurants with oyster-shells Eliot extends an invitation to an unspeaking, and unseen, other to walk with him in the late evening, to explore on foot the low areas of the city. The journey on foot is essential to the cadences of the poem. Eliot does not ask the unseen other to cycle with him, to take a cab with him, to travel in the train with him. It is an invitation to walk. Walking promises types of experience denied other forms of transport, n...

The larger lesson is clear: brains have evolved for movement. If you’re going to be stuck, unmoving, in one place, with your food all around you, then why do you need a costly brain?

Running and walking are also closely related. Humans are not especially fast runners – we can easily be outrun over short(ish) distances by lots of other species (think tigers and gazelles) – but we are exceptional walkers, possibly the best walkers of all species. And this has been the secret underlying our far-flung dispersion across the face of the earth. We humans are the most dispersed of all animal species, living in the northern and southern extremes of our planet, and at virtually every land point in between. Walking allowed us to probe and extend the edges of our world, and then undertake occasionally risky journeys by boat, travelling to the next island – which we then explored on foot. Furthermore, we first colonised the planet by walking in small migratory groups – at its core, our walking is social. An African proverb reflecting this reality says: ‘If you want to go fast, go alone. If you want to go far, go together.’16 At a slow, five kilometres a day walking – at most a few hours walking for a family group – for 300 days, you can travel 1,500 kilometres. Do that over a few years, and you will cover many thousands of kilometres. The longest land-only distance on the earth is from the west coast of Liberia on the Atlantic seaboard of Africa to the eastern Pacific coast of China, a distance of some 13,589 kilometres – about a nine-year amble. Bring it up to twenty kilometres per day for 300 days, and it’s a little over two years of walking. Over the course of a...

Is regular activity a form of easily self-administered medicine, because movement is intrinsically good for us? The evolutionary biologist Daniel Lieberman of Harvard University offers an important evolutionary perspective on our odd relationship to physical activity: it’s good for us, yet we tend to avoid it, in order to conserve energy, for the simple reason that’s how we evolved. Until recently physical activity was obligatory, and food sources were scarce. As Lieberman puts it, ‘humans evolved to be adapted for regular moderate amounts of endurance, physical activity, into late age . . . because energy from food was limited, humans were also selected to avoid unnecessary exertion’.29 Consider the following example: many national institutes of health and diet recommend an average daily diet of perhaps up to 2,000 calories for adult females and 2,500 calories for adult males.30 Let’s call that 2,400 for ease: that implies the burn-rate of those calories is 100 calories per hour, on average – that’s through both waking and sleeping hours. Those consumed calories must serve all the needs of the body: breathing, walking, thinking, digesting, secreting and so on. The food itself must be digested, nutrients transported around the body from the roots of the hair on the top of your head all the way to the soles of your feet. Digesting the food you eat costs energy (calories) – high-fibre foods with a low energy density are more costly to digest than are high-calorie foods. Your...

The speed of action of the vestibular system is also apparent during walking: let’s imagine you go out for a walk on an icy day. You’re striding along – and you hit a patch of ice, and immediately try to stabilise yourself. Having managed not to fall, you replay the sequence of events in your mind. You were striding comfortably, then your foot hit ice and you slid. The first thing you will have done is stiffened: all of the muscles of your legs and then your body trunk rapidly acted in concert, and tried to stop your body from slipping or moving further. Then, having successfully righted yourself, you started to walk forward again, this time attending to the ground. What is really striking is that this rapid stiffening happens absolutely automatically and reflexively.8 It precedes the conscious awareness that your foot has slid from under you. How does this work – and within milliseconds? The signals from the extremely sensitive vestibular system result in several types of information: ‘I’m slipping’, followed by ‘Stiffen muscles quickly’, followed by ‘I haven’t fallen’, followed by ‘OK, resume walking – but be careful’

Stable, rhythmic movement is the core of walking, another crucial input is the system that allows foot placement without us paying continuous conscious attention to the foot itself. How does the brain solve this problem of knowing where your foot is, and then placing it against the ground, and levering you on? As we will soon see, the brain possesses an acute sense of extended space, a ‘cognitive map’ that allows you to navigate the world. But it also has an acute sense of the body that it animates. The brain engages in ‘exteroception’ (processing information about the outside world from vision and hearing), and ‘interoception’ (processing information about, for example, hunger, thirst, pain from internal organs). We also have a highly developed sense of the position of our ankle, knee and hip joints in space, and also from signals from the muscles and ligaments: this is our ‘sixth sense’ – proprioception.14 Testing your proprioceptive sense is the subject both of many childhood games and neurological tests. One simple test is touching your nose with your fingertip with your eyes closed. The margin of error here should be less than about a centimetre. Another is walking in the dark or with your eyes closed – in an obstacle-free environment this is surprisingly straightforward. Finally, stand on one leg with your eyes open, and repeat with your eyes closed. As you try to maintain an upright posture you become acutely aware of how visual input is integrated with your joint s...

how the visual sense and the proprioceptive sense can come together to maintain an upright position.

Dead reckoning – estimating where you are likely to be, based on your speed and direction of movement from some fixed and known point – is a process used by mariners and navigators since time immemorial. It is used by ants and homing pigeons and humans, and other species too. In biology, it is known as ‘path integration’ and, if you keep track of your speed and direction of movement, it allows you both to work your way towards a goal and back to your point of origin.1 But it is not a perfect process and errors do occur.

Tolman took a different view, influenced by gestalt psychology, an important undercurrent in psychology during the mid-Twentieth-century which focused on how we perceive the world more or less instantaneously as a whole, rather than as components that have to be built up, bit by bit. Although he disliked the English translation, the famous phrase ‘the whole is greater than the sum of its parts’ derives from the German gestalt psychologist Kurt Koffka.5 It would be fair to say that Tolman might have wondered if rats were gestaltists. Would frustrated maze-running rats, he speculated, perceive the maze situation as a whole, and base their future actions on what they learned from their previous experience of the overall properties of the maze?

To get to where you want to go, two estimates are always necessary: one is a straight line or as-the-crow-flies estimate to the goal, and the other is an on the ground estimate that may feature extra distances caused by the presence of obstacles. Even fairly straightforward destinations may require some degree of circumnavigation, where you follow

a straight line and then make a left turn or a right turn.

Brain cells have particularly distinctive sounds when they are played through a speaker (there are many recordings available online). Some sound like angry bees, going ZZ-ZZZ-ZZZZ-ZZ-ZZ. Others sound like a dying wasp, making a zzzZZZZZZZZZzzz sound, then falling silent before suddenly coming back to life. Listening in to the ongoing neuronal chatter is a sobering and amazing experience. You know you are listening in to a brain cell close to the electrode tip, and it is in conversation with its neighbours near and far: you are experiencing something – hearing something – normally inaccessible from within the deep, dark silence of the brain. Decoding these sounds and signals is the path to understanding what the brain does, cell by cell, neuron by neuron.

Using microelectrodes implanted in the human brain, we have learned that we have place cells too.12 They have come to be recognised as the core elements of the cognitive map – they tell you where you are in the world, and they work best, and acquire most information, when we are walking. Place cells can also sometimes code for which way the rat is pointing. In rats, place cells are often recorded on ‘radial mazes’ – an experimental set-up with a central hub, the rat’s task being to walk to the end of the corridors of the mazes and retrieve food pellets. Under these circumstances, place cells have directional preferences: they will fire in one direction but not in the other. Part of the problem when we are lost, whether hillwalking or while in an unfamiliar part of town, is that you are likely to have been walking in a single direction. Your place cells will fire when you are walking in one direction but not the other. And when you realise that you’re lost, it’s because you aren’t getting the constant feed that hippocampal place cells provide.

Here at last we are now starting to assemble the elements of a proper cognitive map in the brain. Place cells code for where you are in your environment; and head-direction cells code for your orientation in that environment. In other words, there are two populations of brain cells that are directly involved in knowing where you are and where you are going. Beyond that, since the early 2000s a staggering range of cells throughout what might be referred to as the extended hippocampal formation system have been described. In the entorhinal cortex, for example, the Norwegian researchers Edvard Moser and May Britt Moser discovered cells which have become known as ‘grid’ cells. These answer the long-standing question of how the brain knows the dimensions of space at all: at least over short distances, entorhinal cortex grid cells appear to provide a metric that the brain uses to code for distance. O’

and Moser and Moser shared a Nobel Prize for their work on the brain’s GPS system.

The brain’s GPS codes for your position in the world, independent of what you are doing in that space, and allows animals and humans to solve problems that are absolutely fundamental to survival: to find and remember secure and safe places for shelter, or to find reliable sources of food, for example. This system is activated by movement, such as walking or running. It has also been co-opted in the case of humans to allow mental time travel, in addition to supporting physical space travel. It even allows one to deal with the problems presented by predators in the environment. You learn where possible refuges are, and you know where the boundaries of those refuges are, because of ‘latent learning’ during exploration; you also learn to deploy that information quickly and effectively in order not to become a meal. Our walking journey so far has taken us through how we find our way, how we get lost, and how we build our inner maps of the world we inhabit. The world we inhabit is increasingly urban, and it is a complicated one. Our urban space is a world of artifice, quite unlike anything found in the natural world we evolved in. Most humans now live, and therefore walk, in a built environment – their village, their town, and particularly their city. Let us now walk the streets of the city, and see how it moulds our walking.

Urban design that fully and properly takes account of the needs of walkers will make cities much more attractive places to live and work. Churchill famously said that ‘We shape our buildings, and then our buildings shape us.’2 Similarly, we first shape our cities, and then our cities shape us. To extend the metaphor, our cities walk us, for the shape of the cities we create determines the shape of our urban walking, for better or worse. There are great potential walking futures in cities for us all. What

we need are acts of imagination fusing the needs of walkers with the expertise of town planners and that of psychologists and neuroscientists. In turn, science, imagination and evidence need to be turned into policy and from that point translated to beautiful, interesting streets of ease, variety and quality. Road-crossing designs, street furniture, the texture and type of footpaths and pavements, the presence of cars and buses – these all act for or against our ability to walk in cities.

‘What works best in the best cities is walkability,’ says Jeff Speck, the renowned urban planner.6 And the best walks in cities, according to Speck, must be useful, safe, comfortable and interesting. For a walk to be useful, according to Speck, ‘most aspects of daily life are located close at hand and organised in a way that walking serves them well’. Walking should be safe: this is self-evident, although sometimes ignored. Pedestrians should not be put at risk by fast-moving vehicles and should be treated by urban engineers with at least the same respect as traffic. (Imagine if we invested as much in walking in our cities as we do in driving in them!) Walks should also be comfortable, and here Speck outlines a powerful idea – that city planners and designers should think about urban streets as being akin to ‘outdoor living rooms’. We, the users and walkers of cities, should find the streetscape welcoming, with entertainment, seating, refreshment and diversion available. Finally, walks should be – must be – interesting. Speck suggests that to be interesting, our streets should have ‘unique buildings with friendly faces, and that signs of humanity abound’

We have come a long way in the development of our urban environments. The streets of our cities and towns a century and a half or so ago presented a considerable public health risk. Open sewers were common, indoor toilets were few and far between, and human waste and effluent handling were of an especially poor standard. Observing acts of necessity that would appal us now, John Gay’s 1716 poem, ‘Trivia, or The Art of Walking the Streets of London’ warned of the dangers posed by the emptying of chamber-pots when walking underneath London windows, where ‘dropping vaults distil unwholesome dews / Ere the tiles rattle with the smoking shower / And spouts on heedless men their torrents pour’.11 The effluvia and miasma of the time must have been dreadful. House design reflected this: boot-scrapers were commonly placed at doorways and are often still present at the entrances of Georgian and Victoria-era houses.

Focusing on cities as diverse as Dublin, Hong Kong and San Salvador, researchers measured walking speed (how fast it took people to walk sixty feet in two differing downtown areas), postal speed (how quickly you could buy a stamp in the major post office), and finally clock accuracy. Other information was gathered from publicly available data sources on climate, economic indicators, measures of individualism, size of population, coronary heart disease, levels of smoking and subjective well-being. These were all combined to create an overall pace-of-life index. Switzerland was measured as having the fastest pace of life, closely followed by Ireland (then in the middle of

major, decade-and-a-half-long economic boom), followed by Germany and Japan. (Italy, England, Sweden, Austria, the Netherlands and Hong Kong completed the top ten, in that order.) Mexico came in last. On the larger global scale, it was shown that Japan and the non-ex-Soviet bloc of Western European countries had the highest pace of life, with people in Ireland clocking the highest individual walking speed. Switzerland lived up to a stereotype, ranking number one for clock accuracy.

In a busy, bustling city, where we’re competing for rewards, a major physical obstacle is of course other people. To avoid collisions, we must be able to quickly and accurately estimate the walking speed of others. When we walk together in pairs or in triples, we fall into step with each other naturally and unconsciously. We moderate our pace so that we

can keep to the same speed: we coordinate our walking speed with that of others.

What gives our cities their vitality, attraction, upsides and downsides? How do walkable cities acquire their sociable character? What is it about cities that attracts people to live in them, despite their downsides? There is a density of social interaction afforded by crowding people into compact urban spaces. Cities do change you – and you won’t even know it. The life you have, the way you think about the future and the past will change if you are living in the city. Greater competition for resources subtly alters our behaviour in ways that we are hardly aware of, even the pace at which we walk. People leave the city to enjoy a slower pace of life, and go to the city in order to be stimulated by it. I have often walked the towns and cities of Italy and have always been struck by that wonderful tradition of the passeggiata – the sociable, evening stroll along the streets, greeting and chatting with neighbours and friends. This habit of walking the city sums up everything good about urban walking and shows how entirely achievable this is as a daily activity, if only our cities were built for it. I offer the acronym EASE to assist our city designers. To enable the passeggiata, our cities should be easy (to walk); accessible (to all); safe (for everyone); and enjoyable (for all). The steps to make and remake our cities walkable are straightforward. We just need to take them.

Hippocrates famously claimed that ‘walking is the best medicine’. Yet in our modern world, most of us spend all day indoors sitting down, which can have terrible consequences for our health and well-being. We spend less time outdoors than ever before. One major study in the USA showed that people spent 87% of their time in the artificial environment of offices, houses, shops and other buildings.1 Some have even claimed (only somewhat exaggeratedly, in my view), that ‘sitting is the new smoking’. The sentiment behind this statement is straightforward: our bodies are built for regular movement, and profit from it. Sedentary life is fundamentally unhealthy, leading to decline in muscle volume and strength.2 Moreover, long periods of inactivity produce not dissimilar changes in the brain. One interesting study has recently found that lack of activity is even associated with personality change, and by this I mean change for the worse.3 Overall, lower levels of physical activity were associated with changes in three of the ‘Big Five’ factors of personality (these are openness, conscientiousness, extraversion, agreeableness and neuroticism, easy to remember as OCEAN).4 Lower levels of physical activity were associated with declines in openness, extraversion and agreeableness, suggesting a ‘detrimental’ pattern of long-term personality change. Even minimal levels of activity were found to have a moderating effect on personality change. Those individuals who were the most inactive ...

As we explored in the last chapter, as more and more of us live in towns and cities, green spaces will only become more essential for our well-being. Building design, especially in northerly and more inclement regions, has, in some respects, historically taken account of this fact. Cloisters in university buildings, monasteries and other locations, allow people to walk outdoors while protected from the elements. Cloisters are sometimes referred to by their ritual and processional purpose – deambulatorium, obambulatorium, ambitus – the solemn, Latin descriptors of the architectural elements of a monastery, all derived from the root ‘ambio’ – ‘I walk in a circle’.5 They are also often called, appropriately enough, ambulatories. And, of course, cloisters are usually constructed around a garden – ensuring a tamed element of nature is at the centre of the walk.

But why does walkable green space matter so much for our well-being? What is it about nature that makes us feel better? Walking in the woods is something we humans have done since time immemorial. Some cultures venerate this experience: the Japanese, for example, have the glorious tradition of ‘forest bathing’ (shinrin-yoku): the practice of absorptive, enveloping walking in deep forests for the soothing properties of being connected to, and fully immersed in, the sights, sounds and feel of

nature.9 Forest bathing is an important manifestation of something that appears to be a universal in

human experience – a veneration of nature as foundational to our lives, from early pantheistic theories which imagine that spirits inhabit trees, woodland brooks, stones, and the like, through religions that worship the Earth Mother or deities (like the Inca goddess Pachamama), to the present-day idea of ‘Gaia’ – scientist James Lovelock’s contention t...

There’s an important difference between a mild and transient down-in-the-dumps feeling, to which we are all subject, and major depressive disorder (MDD). MDD has been defined as ‘a depressed mood or a loss of interest or pleasure in daily activities consistently for at least a two-week period. This mood must represent a change from the person’s normal mood; social, occupational, educational or other important functioning must also be negatively impaired by the change in mood.’16 The World Health Organisation regards MDD as one of the greatest hazards to health and well-being over the coming decades, with the lifetime risk of suffering an episode of MDD at up to 15% in Western populations.17 This form of depression usually requires psychiatric care, cognitive behavioural therapy and/or drug treatment.

An unappreciated way walking boosts mood is through the pleasure derived from resting after extended physical exertion – in a warm bath, or simply sitting in a comfortable chair.22 The great British philosopher Bertrand Russell remarked that ‘I used, when I was younger, to take my holidays walking. I would cover twenty-five miles a day, and when the evening came I had no need of anything to keep me from boredom, since the delight of sitting amply sufficed.’23 While the relationships between mood and walking are not straightforward, the emerging findings do suggest that regular walking has acute and chronic effects on mood, boosting feelings of well-being both in the moment and long-term.

marked difference to the structure and function of the brain. Aerobic exercise facilitates the brain in populating one particular region, critical to learning and memory, with new brain cells. Moreover, aerobic exercise supports the widespread production of key molecules that act to keep the brain in good working condition. Running as an aerobic exercise is a powerful method of inducing these changes, but running comes with disadvantages (special shoes, preparation, changing clothing, showering). More than these small inconveniences, the risk of injury rises with the distance run, whereas injury risk stays approximately the same with walking, irrespective of distance walked.29 One small-scale study concludes there is lowered injury risk for walkers compared with joggers.30 To get the maximum health benefits from walking, speed should be consistently high over a reasonable distance – say consistently over 5 or 5.5. kph, sustained for at least thirty minutes, at least four or five times per week.31 Walking offers the supreme advantage of needing nothing more than a decent pair of shoes, perhaps a raincoat, and little else. If undertaken in regular doses during the day, it provides the small, cumulative and significant positive changes for lung, heart and especially brain health. Brain health here is to be understood in its most general sense. Just as the purpose of the heart is to pump blood, and the purpose of the lungs is to allow respiration, the purpose of the brain is t...

‘Use it or lose it’ is a primary rule that muscle cells obey, and the same is true of brain cells. The body cannot afford to waste energy building, supporting and managing muscle cells or other cells that do not contribute to the overall life of the body. The best signal that the body gets to tell it that muscle is required is when the muscle is placed under regular (though moderate) stress and strain. If an organ is working, it is being used, and if it is being used, it needs to be maintained. The muscles of people who are relatively sedentary for long periods of time start to change, with fatty deposits being laid down in their inactive muscle. These deposits have been found in the muscles of astronauts, the obese and the elderly, who do not get enough of the type of exercise that places positive strain on the muscles of the body.

Being sedentary is bad for you, even if you are young and fit: your muscles will decrease in volume, quickly and easily, if they are unused. Moreover, loss of muscle mass is also associated with a loss of the production of molecules important for supporting new brain cells in the few regions of the brain that continue to produce new brain cells through life. As your muscles deteriorate, your brain is also deteriorating. Other malign changes occur too – in personality, in mood, in the very structure of the brain. And yet, we have this wonderful, in-built correction mechanism, a form of self-administered medicine, one without adverse events: movement.

I love walking for all sorts of reasons – but near the top of the list is that I find it the best way to clear the clamour of the day from my head. Walking gives me the freedom to think things through; to have a quiet dialogue with myself about how to solve a problem. The problems may be mundane, but are nonetheless important to me. And I’m not alone. Since antiquity it has been recognised that a good walk is an excellent way to think problems through. The school of peripatetic philosophy in ancient Greece was famous for conducting its teaching largely on foot – indeed the root of its name means ‘walking up and down’. The philosopher Friedrich Nietzsche went so far as to say that ‘Only thoughts

reached by walking have value.’1 In a similar spirit, the writer Henry David Thoreau observed that the moment my legs begin to move, my thoughts begin to flow, as if I had given vent to the stream at the lower end and consequently new fountains flowed into it at the upper. A thousand rills which have their rise in the sources of thought burst forth and fertilize my brain . . . Only while we are in action is the circulation perfect. The writing which consists with habitual sitting is mechanical, wooden, dull to read.

huge areas of ground are covered for what feels like minimal effort and great enjoyment in comparatively short periods of time. This state is known as ‘flow’, and it can apply not only to walking but to a huge range of activities, to work, to sport, to specialised performance of all varieties. Sometimes referred to as the psychology of optimal experience, flow is a central psychological concept, first developed by Mihaly Csikszentmihalyi.23 Flow is the subjective experience of concentration and deep enjoyment accompanying or arising from skilled performance. Feelings of control, of oneness, of immersion, of being in the zone, are all characteristic of flow. The experience here is very different to the elation of overcoming difficult terrain and sitting down for a well-earned rest; rather, it is in the freeing of the mind and brain from the moment-to-moment control of walking, while at the same time covering considerable distance.

What we see here, as we do throughout this book, is that walking brings us to a place of clearer thinking. We can think of ourselves as being able to walk away from a problem by getting to a place where a solution is possible. It is a peculiar and wonderful creative problem-solving state comparable in many ways to that which can arise on the edge of dreaming, and even during dreaming itself. The phrase ‘sleep on it’ is an everyday testament to sleep’s generative and creative powers, and writers through the ages have attested to sleep’s great problem-solving properties. John Steinbeck wrote that ‘It is a common experience that a problem difficult at night is resolved in the morning after the committee of sleep has worked on it.’25 Scientists have found the same: one famous example is August Kekulé having a dream of a snake eating its own tail, and realising that the molecular structure of benzene must be a ring-shaped.26 Beyond anecdote, though, experimental studies support this connection. In one study participants were given a difficult cognitive task requiring the learning of stimulus-response sequences. Hidden within the task was a shortcut – an abstract rule that allowed quick solutions. Exposure to the problem followed by eight hours of sleep more than doubled the probability that participants would solve the task, underlining how emphatically sleep provides time for offline problem-solving and memory consolidation.