Why do we bob up and down while walking The mystery of Antiquity walking shadow finally deciphered - The Physiological Society Login The Physiological Society Menu Menu Login Physiology News Magazine Download this issue Home Summer 2008 - Issue Number 71 Editorial Board Contribute Current issue Previous issues Past Issues 121-127 Issue 127 Issue 126 Issue 125 Issue 124 Issue 123 Issue 122 Issue 121 Past Issues 111-120 Issue 120 Issue 119 Issue 118 Issue 117 Issue 116 Issue 115 Issue 114 Issue 113 Issue 112 Issue 111 Past Issues 101-110 Issue 110 Issue 109 Issue 108 Issue 107 Issue 106 Issue 105 Issue 104 Issue 103 Issue 102 Issue 101 Past Issues 91-100 Issue 100 Issue 99 Issue 98 Issue 97 Issue 96 Issue 95 Issue 94 Issue 93 Issue 92 Issue 91 Past Issues 81-90 Issue 90 Issue 89 Issue 88 Issue 87 Issue 86 Issue 85 Issue 84 Issue 83 Issue 82 Issue 81 Past Issues 71-80 Issue 80 Issue 79 Issue 78 Issue 77 Issue 76 Issue 75 Issue 74 Issue 73 Issue 72 Issue 71 Past Issues 61-70 Issue 70 Issue 69 Issue 68 Issue 67 Issue 66 Issue 65 Issue 64 Issue 63 Issue 62 Issue 61 Past Issues 51-60 Issue 60 Issue 59 Issue 58 Issue 57 Issue 56 Issue 55 Issue 54 Issue 53 Issue 52 Issue 51 Past Issues 41-50 Issue 50 Issue 49 Issue 48 Issue 47 Issue 46 Issue 45 Issue 44 Issue 43 Issue 42 Issue 41 Past Issues 31-40 Issue 40 Issue 39 Issue 38 Issue 37 Issue 36 Issue 35 Issue 34 Issue 33 Issue 32 Issue 31 Past Issues 21-30 Issue 30 Issue 29 Issue 28 Issue 27 Issue 26 Issue 25 Issue 24 Issue 23 Issue 22 Issue 21 Past Issues 11-20 Issue 20 Issue 19 Issue 18 Issue 17 Issue 16 Issue 15 Issue 14 Issue 13 Issue 12 Issue 11 Past Issues 1-10 Issue 10 Issue 9 Issue 8 Issue 7 Issue 6 Issue 5 Issue 4 Issue 3 Issue 2 Issue 1 Summer 2008 - Issue Number 71 Full issue Why do we bob up and down while walking The mystery of Antiquity walking shadow finally deciphered You may find it trivial to move up and down while walking but this motion is a masterpiece of 4 cm shaped by 7 million years of marriage between smoothness and bounce This vertical movement first noticed by Aristotle and now tracked by satellites is particularly beneficial for loading our muscles to perform a little work efficiently A lesson on how optimization is a relative success story and not perfection is described here Features Why do we bob up and down while walking The mystery of Antiquity walking shadow finally deciphered You may find it trivial to move up and down while walking but this motion is a masterpiece of 4 cm shaped by 7 million years of marriage between smoothness and bounce This vertical movement first noticed by Aristotle and now tracked by satellites is particularly beneficial for loading our muscles to perform a little work efficiently A lesson on how optimization is a relative success story and not perfection is described here Features Firas Massaad, Thierry M Lejeune, Christine Detrembleur Rehabilitation and Physical Medicine Unit, Université catholique de Louvain, Brussels, Belgium https://doi.org/10.36866/pn.71.25 You are watching TV, when you suddenly notice a mass of brainy heads bobbing up and down in a walking crowd. Walking accounts for 20% of their energy budget and brainy heads account for another 20%. So why are these ‘expensive ’ heads moving further? Puzzled, you take a stroll while carefully holding a cup full of boiling coffee, and here with each step the coffee bobs up and spills out of your cup. Unconsciously, you flex your legs to walk smoothly; but why does this bobbing occur when you have the ability to walk flat? Interestingly, this vertical bobbing was noticed 2400 years ago by Aristotle, who had a habit of walking around the lyceum while talking. He curiously stated in the first known written reference to walking analysis in 350 BC: ‘If a man were to walk parallel to a wall in sunshine, the line described (by the shadow of his head) would be not straight but zigzag, becoming lower as he bends, and higher when he stands and lifts himself up’. Ironically, even his school was named later Peripatetic, meaning to walk up and down the shaded walks. Hence, could that be but a shadow? Why do we bob up and down ‘… man, the only erect animal …’ Aristotle again hinted, and indeed it is all in our legs. Humans walk on straight legs. Consequently, our body’s centre of mass (CM) rises as the supporting leg becomes vertical and descends again so as to arc upwards with each step (Fig. 1). This vertical movement enables humans to save energy by reducing the muscle work because we slow down as we rise and speed up as we fall, thus passively recovering kinetic energy into gravitational potential energy and back again as in an inverted pendulum (Cavagna, 1963). In straight walking, the line of action of our weight passes close to the leg joints, which also decreases the need for large forces in our leg muscles. Human walking is unique so even the apes – our closest relatives – bend their legs when walking on two legs, which reduces the rise of the CM (i.e. flat gait) (Alexander, 1992). This is, however, paramount for arboreal locomotion because it enhances stability on branches. Accordingly, scientists postulated that our common ancestors with apes, approximately 7 million years ago (Ma), may also have walked flat as the first transition to terrestrial bipedality (Schmitt, 2003). However, recent computer modelling suggested that the australo-pithecines (represented by the 3.2 million year old ‘Lucy’) already walked straight and moved up and down. Therefore, our bobbing is more ancient than we previously thought. We make war so that science may live in peace Our CM kept out of the limelight until after the Second World War, when masses of people were lumbering with stiff straight legs, bobbing extensively and were out of breath. Suddenly, the realisation dawned that although relatively straight legs may save energy, the straighter we walk, the more we bob and the more energy we consume. Hence, a landmark article postulated that if humans walked flat like the CM of a wheel (Fig. 1), muscle work and energy consumption would be minimized because bouncy walking with excessive vertical bobbing would waste energy to redirect and raise the CM against gravity (Saunders et al. 1953). However, recent human experiments showed that flat walking costs more energy than normal walking (Ortega & Farley, 2005). Recent breakthrough in computer modelling also concluded that work requirements would be minimized by normal bobbing (Srinivasan & Ruina, 2006). Why is our natural bobbing optimal Therefore, we asked healthy adults to walk on a treadmill (1 to 6 km h–1) while they were provided with their CM displacement feedback (Massaad et al. 2007). For each speed, they walked normally, with minimum vertical CM displacement (flat walking), and with maximum displacement (bouncy walking). We measured the total mechanical work provided by their muscles (Wtot), their oxygen consumption as energy cost (Cnet), their muscle efficiency (η=Wtot/Cnet), and the EMG activity for their leg muscles .We also calculated their vertical CM amplitude (Av) and the ‘recovery’ percent of mechanical energy exchanged between gravitational potential and kinetic energies of the CM. The subjects were able to decrease Av in flat walking. However, Wtot was normal, Cnet nearly doubled with a sharp decrease in η (Fig. 2). In bouncy walking, Wtot and Cnet dramatically increased but η remained normal. In both flat and bouncy walking, the recovery decreased and muscle co-contraction timing increased (i.e. when antagonistic muscles are working against each other). The optimal combination of ~3 cm in Av at ~ 4 km h–1 minimized Cnet, corresponding to normal human walking. In bouncy walking, Cnet increased slightly with a quite large increase of Av from the optimum, whereas in flat walking, Cnet was highly sensitive to reductions of Av (Fig. 3). Wasteful flat exuberant bouncy Thus, the energy cost increased in both flat and bouncy walking. This increase was unexpectedly sharpest in flat walking where muscles provided little work but wasted energy. In bouncy walking, muscle provided extra work but conserved energy. Indeed, walking flat is like an experienced old man driving a rusty car using too much fuel although he drives straight, while bouncy walking is like a reckless teenager with a well-oiled car using the necessary fuel, but going through exuberant zigzags. Despite the prediction of Srinivasan & Ruina’s model that normal walking would minimize work, we found no significant difference in work requirements between flat and normal walking. However, our muscles are inefficiently working in flat walking in opposition to bouncy walking. Therefore, not only do humans bob up and down in normal walking to save energy via a pendulum-like mechanism, as commonly thought since the 1960s, but they also load their muscles more efficiently this way. Efficient locomotion is when most of the metabolic energy input is transformed into mechanical work. Flat walking like a wheel (the most efficient tool of motion) is inefficient probably because energy losses may have occurred due to muscle co-contraction and the need for high muscle forces due to flexed legs. However, in bouncy walking, the muscle co-contraction did not affect muscle efficiency. Hence, it seems that efficiency determinants remain a mystery because it is not yet possible to measure the force and the energy consumed by individual muscles during walking. Would another war resolve this mystery? Efficiency was settled first Even after probably 7 million years of ancestry with apes, we are still close to a flat walking (Fig. 3). This suggests that reducing our bobbing is indeed important, but to certain limits. Nature may ultimately have chosen the best compromise between safe flat locomotion that requires little work and bouncy ‘risky’ locomotion that turns walking into a ‘series of falls’ but improves muscle efficiency. Straight legs are efficient. This was probably a primary criterion likely achieved if the short ‘Lucy’ already walked straight. The increase in leg length seen in H. erectus 1.8 Ma would have made it possible to cover longer distances more effectively at high speeds. Thus, nature may have maximized muscle efficiency (rusty to well-oiled) millions of years before reducing energy consumption at higher speed which was probably vital. This recalls Peter Drucker’s famous quotes: ‘Efficiency is doing better what is already being done’… ‘Efficiency is doing things right; effectiveness is doing the right things’. He was obviously not the first father of management! Carefree of age our CM is heading the future Mankind specialization is basic as vertical bobbing appeared since quadrupedal terrestrial locomotion emerged ~400 Ma (i.e. since legs were adopted). However, early sprawling vertebrates like salamanders lumbered and bobbed excessively than cursorial animals that move gracefully like dogs; hence, our perception of lumbering exists (Reilly et al. 2006). Modern humans, however, take advantage of both flat and bobbing walking. They shortcut millions of years of evolution in a few seconds to carefully hold their beloved coffee and walk flat to achieve impressive walking speeds in competition. Our natural bobbing is also used to generate electricity to power portable devices and is now tracked by satellites to assess human walking in daily life. Recent humanoid robots also rival human walking by bobbing with simple control of actuators while some state-of-the-art robots still use sophisticated motor control to walk flat with excessive energy consumption. This echoes what we have found as physiological determinants of optimal bipedalism. In conclusion, our results showed that our natural bobbing loads our muscles efficiently. This may have a direct implication in robotics to walk flat no longer and rehabilitation of pathological gait to bob excessively no longer either. You may now wonder how disastrous it would be for future humans to walk while holding their coffee if they were heading towards further head bobbing. However for now, 2400 years after Aristotle, we can eventually assert that our bounciness is not but a walking shadow (Fig. 4). Still, like Shakespeare, we say ‘Life is but a walking shadow.’ Figure 4. Antiquity walking shadows. Figure adapted from ‘the school of Athens,’ one of the most famous paintings by the Italian renaissance artist Raphael, painted in 1511. You can see Aristotle on the right talking with his mentor Plato while they walk up and down the shaded colonnades at the Greek ‘university’. Plato is pointing upwards to the source of higher inspiration, the realm of ideas ‘Look to the heavens if you want to know. ’ Aristotle, on the other hand, is gesturing downwards, towards the starting point of all the natural sciences ‘Look around you if you want to know.’ Indeed, Plato’s ideas notably the myth of the cave shadows (that humans are like men sitting in a cave seeing shadows on the wall, as symbolised by Aristotle’s shadow) would have inspired his student Aristotle to step outside and walk in the sun. Aristotle, by looking around him, noticed the bobbing of our head shadow while walking in the sun (as symbolised by Plato’s shadow). Only 2400 years later, we were eventually able to understand the physiological mystery of their head bobbing in the painting laying down the stairs, whereas satellites are now tracking our up and down movement to assess walking in daily life (figure conceived by Firas Massaad and designed by Marianne Roegiers [[email protected]]). no longer either. You may now wonder how disastrous it would be for future humans to walk while holding their coffee if they were heading towards further head bobbing. However for now, 2400 years after Aristotle, we can eventually assert that our bounciness Acknowledgements This work was funded by the Belgian Fonds National de la Recherche Scientifique and the grant of a ‘Coopération au Développement ’ fellowship from the UCL. We thank M Roegiers for her skilful help in graphic design and S Hatem for her insightful reading of this paper. We are also grateful to R McNeill Alexander for his precious comments on our article in The Journal of Physiology. References Alexander R McN (1992). Exploring biomechanics: animals in motion. Scientific American Library, New York. Cavagna GA (1978). Aspects of efficiency and inefficiency of terrestrial locomotion. In International series on biomechanics, Vol 2A, pp 3–22. University Park Press, Baltimore. Cavagna GA, Saibene FP, Margaria R (1963). External work in walking. J Appl Physiol 18, 1–9. Massaad F, Lejeune TM, Detrembleur C (2007). The up and down bobbing of human walking: a compromise between muscle work and efficiency. J Physiol 582, 789–799. Ortega JD & Farley CT (2005). Minimizing center of mass vertical movement increases metabolic cost in walking. J Appl Physiol 99, 2099–2107. Reilly SM, McElroy EJ, Odum RA & Hornyak VA (2006). Tuataras and salamanders show that walking and running mechanics are ancient features of tetrapod locomotion. Proc Biol Sci 273, 1563–1568. Saunders JB, Inman VT & Eberhart HD (1953). The major determinants in normal and pathological gait. J Bone Joint Surg Am 35, 543–558. Schmitt D (2003). Insights into the evolution of human bipedalism from experimental studies of humans and other primates. J exp Biol 206, 1437–1448. Srinivasan M & Ruina A (2006). Computer optimization of a minimal biped model discovers walking and running. Nature 439, 72–75. The Physiological Society Menu Menu Login Site search Filter Content Type Latest News Upcoming Events Blog Magazine Pages Team Members Obituaries Honorary Members PDF Video Audio Filter Content Type Latest News Upcoming Events Blog Magazine Pages Team Members Obituaries Honorary Members PDF Video Audio Sign up to our newsletter Sign up to our newsletter Name First Last Email* Your data will be processed in accordance with our Fair Processing Notice. CAPTCHA