{"id":251,"date":"2016-04-28T20:34:42","date_gmt":"2016-04-28T20:34:42","guid":{"rendered":"http:\/\/labs.la.utexas.edu\/gilden\/?page_id=251"},"modified":"2016-04-28T20:58:42","modified_gmt":"2016-04-28T20:58:42","slug":"energy-expense-in-gait-produces-a-time-scale","status":"publish","type":"page","link":"https:\/\/labs.la.utexas.edu\/gilden\/energy-expense-in-gait-produces-a-time-scale\/","title":{"rendered":"Energy Expense in Gait Produces a Time Scale"},"content":{"rendered":"

Energy Expense in Gait Produces a Time Scale<\/em><\/strong><\/p>\n

The final chapter of this story has to do with causation.\u00a0 What is there about people or the environment that creates limits on what can be linked together in the mind?\u00a0 Why can\u2019t we hear music when the notes arrive at intervals longer than 2 seconds?\u00a0 Why can\u2019t we dance to 30 bpm?\u00a0 Why do concepts become inactive after a few seconds? Somehow we ended up with this limitation and there has to be a reason.<\/p>\n

Coming up with an account of why we have this or that ability is a fairly subtle undertaking in psychology.\u00a0 Consider for example our ability to see.\u00a0 We see because we have visual cortex and eyes.\u00a0 We see because we have learned to see over our life span: appropriate developmental experience is required for seeing to occur in any given individual.\u00a0 We see because our ancestors evolved from primitive slimy things with skins that sensed heat.\u00a0 Heat sensitivity evolved into eye cups and then into eyes.\u00a0 And finally we see because we live in a world that is illuminated by light and filled with solid reflective objects.\u00a0 All of these levels of explanation are relevant to the question of how it is that we can see.\u00a0 However, the final level, the recognition that we live in light has the most motive power.\u00a0 The light drives all other forms of explanation.\u00a0 Animals see because their interactions with environment are aided by the ability to extract reflectance information.\u00a0 And each animal sees in its own way in congruence with the environment that it lives in, its ecology.\u00a0 Frogs for example, do not see objects.\u00a0 They cannot see a fly unless it is moving.\u00a0 What they see is movement of dark specs, moving shadows, reflectance edges, and overall dimming.\u00a0 That is it, but it suffices for a small amphibian that lives at the water\u2019s edge.\u00a0 Human seeing is similarly constrained by our ecology and by the goals we have at any given moment of time.\u00a0 Seeing is about the acquisition of information\u00a0 not about the acquisition of images.\u00a0 The same ideas are relevant to hearing.<\/p>\n

The same kind of level splitting certainly applies to the question of why our thoughts remain active for only a few seconds, why we cannot experience rhythms that are slower than 30 bpm and we cannot construct melodies from music that is played slower than our metronomes.\u00a0 Although they are not presently available, neuroscience will eventually derive chains of neuronal interactions that mediate auditory and motor responses on the time scale of a few seconds.\u00a0 Presumably the locus of these neurons will\u00a0 be in the thalamus where perceptual and motor pathways intersect, but this is merely a guess.\u00a0 There must, however, be something about our interactions with the environment that create this time scale in the first place.\u00a0 Our sensory systems have the properties they do precisely because they are tuned into our ecology.\u00a0 And the two most fundamental facts about our ecology is that we live in a gravitational field and that there is a solid surface of support beneath us.\u00a0 Interaction begins with movement which in humans has evolved into primarily waking and running.\u00a0 Walking and running are rhythmic motions and everything we sense occurs on board a body that is generating these rhythms.\u00a0 The auditory system evolved along with a body that was generating rhythm.\u00a0 Perhaps these rhythms became part of the way our cognitive processes incorporated time.<\/p>\n

Anybody who has run for exercise probably knows that calorie consumption is determined by how far you run, not by how fast you run.\u00a0 If you run a given distance at a high speed you burn more calories per minute but you end up running fewer minutes than if you had run more slowly.\u00a0 It all balances out because energy consumption is proportional to speed for running.\u00a0 The same is not true for walking and this is where things become interesting.\u00a0 The adjacent figure shows the amount of energy consumed per meter (per kilogram of body mass) for various walking and running rates.<\/p>\n

\"energy<\/a><\/p>\n

This quantity is referred to as the efficiency.\u00a0 The filled circles are running data that can be found in any textbook on exercise physiology.\u00a0 The solid line, called the Rose-Gamble law, describes the energy expense for walking a fixed distance.\u00a0 What this figure shows is the walking, unlike running, has different efficiencies at different speeds.\u00a0 There is a well defined minimum at about 1.2 meters per second at which point human locomotion achieves its best energy efficiency.\u00a0 Prior to the invention of the wheel, there was no more energy conservative way to move across ground than to walk at about 1.2 m\/s.<\/p>\n

The key difference between the energetics of walking and running is that in running the legs are used as springs while in walking they are used as pendulums; masses suspended at a pivot point \u2013 in this case, the hips.\u00a0 There is very little difference between the kind of pendulum that keeps time in grandfather clocks and the free swinging motion of a leg.\u00a0 The only difference is that mass is distributed throughout the leg and in timekeeping device the mass is located in a bob attached to a virtually massless string.\u00a0 The physics is the same and it is the physics that is going to tell us everything we need to know.\u00a0 The most important feature of the pendulum is that it has a natural period.\u00a0 The time it takes for a pendulum to swing through one cycle is independent (to a very good approximation) of the height from which the pendulum is released.\u00a0 For a simple pendulum made from a bob supported by a string, the natural period is 2p times the square root of (string length divided by the gravitational acceleration at the earth\u2019s surface).\u00a0 The natural periods of real legs have been determined by suspending the legs of cadavers and they differ only marginally from the simple pendulum.\u00a0 The natural period of the legs of early humans, determined from fossilized remains, was about 1.5 seconds.<\/p>\n

When we walk with our legs swinging at their natural period we expend minimal energy.\u00a0 At this period the legs are most responsive to the muscle contractions that drive their motion.\u00a0 All pendulums absorb energy most efficiently when they are driven at their natural period.\u00a0 Because our legs are pendulums we don\u2019t have to know how fast to swing them, they swing at with a period close to 1.5 seconds by themselves.\u00a0 In this way we achieve efficient walking without us having to actually know anything.\u00a0 Our legs know.\u00a0 No selection pressure is needed to develop humans that walk efficiently.\u00a0 The Rose-Gamble law has applied to every person that has ever walked.\u00a0 It also applies to four legged animals and so to all of our primary predators.<\/p>\n

Previous: Is 2 Seconds a Meaningful Time Scale?<\/a>\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0\u00a0Next:\u00a0Gait and Memory<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"

Energy Expense in Gait Produces a Time Scale The final chapter of this story has to do with causation.\u00a0 What is there about people or the environment that creates limits on what can be linked together in the mind?\u00a0 Why can\u2019t we hear music when the notes arrive at intervals longer than 2 seconds?\u00a0 Why […]<\/p>\n","protected":false},"author":107,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"_uag_custom_page_level_css":"","footnotes":""},"class_list":["post-251","page","type-page","status-publish","hentry","entry"],"uagb_featured_image_src":{"full":false,"thumbnail":false,"medium":false,"medium_large":false,"large":false,"1536x1536":false,"2048x2048":false,"site-graphic":false},"uagb_author_info":{"display_name":"ecw255","author_link":"https:\/\/labs.la.utexas.edu\/gilden\/author\/ecw255\/"},"uagb_comment_info":0,"uagb_excerpt":"Energy Expense in Gait Produces a Time Scale The final chapter of this story has to do with causation.\u00a0 What is there about people or the environment that creates limits on what can be linked together in the mind?\u00a0 Why can\u2019t we hear music when the notes arrive at intervals longer than 2 seconds?\u00a0 Why…","_links":{"self":[{"href":"https:\/\/labs.la.utexas.edu\/gilden\/wp-json\/wp\/v2\/pages\/251","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/labs.la.utexas.edu\/gilden\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/labs.la.utexas.edu\/gilden\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/labs.la.utexas.edu\/gilden\/wp-json\/wp\/v2\/users\/107"}],"replies":[{"embeddable":true,"href":"https:\/\/labs.la.utexas.edu\/gilden\/wp-json\/wp\/v2\/comments?post=251"}],"version-history":[{"count":7,"href":"https:\/\/labs.la.utexas.edu\/gilden\/wp-json\/wp\/v2\/pages\/251\/revisions"}],"predecessor-version":[{"id":288,"href":"https:\/\/labs.la.utexas.edu\/gilden\/wp-json\/wp\/v2\/pages\/251\/revisions\/288"}],"wp:attachment":[{"href":"https:\/\/labs.la.utexas.edu\/gilden\/wp-json\/wp\/v2\/media?parent=251"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}