Abstract

Reducing sedentary behavior portends massive health benefits. It has been assumed that humans are sedentary by nature, and therefore sedentary behavior can be mitigated through vigilance. Growth in sedentary behavior by all measures suggests that this notion is incorrect. Alternatively, I propose that humans became sedentary only recently, and its cause is environmental. There is support for these notions. Natural selection would have eliminated poorly mobile, sedentary hunter-gatherers. Moreover, the word equivalent to sedentary appears in all European languages during the Renaissance, without earlier words of similar meaning, and chairs were used for the first time in homes and workplaces at this time. The implication is that humans historically preferred weight- bearing over sitting and became sedentary just prior to this time. Footwear use by all social classes also commenced for the first time in human history during the Renaissance. Humans prevent plantar skin ulceration from sustained localized pressure while standing through behavioral re-distribution of plantar loads, but footwear caused instability would have resulted in attenuation of this re-distribution, thereby causing plantar skin discomfort, which has been shown to result in sedentary behavior. The temporal relation between shoe use and sedentary lifestyle, combined with a proven causal mechanism meets contemporary scientific standards for causal inference, and therefore footwear was likely the environmental factor that caused humans to become sedentary. I suggest that averting the sedentary lifestyle may be possible through the introduction of stability enhanced footwear. A more certain solution is increased barefoot weight-bearing both in the workplace and outdoors. This transition may be facilitated through a "quasi-shoe"' - a foot decoration which appears to be a shoe but has no sole therefore lets the plantar surface be in direct contact with the support surface. This will satisfy what may be an innate desire for foot decoration without shoes and fulfill current social norms. Further, social norms encouraging shoe use must be eliminated and interfaces with technology re-designed for upright users.

Introduction

Most individual's spend the majority of their wakeful time sitting, whether when eating, during transport, at work and even in leisure time.[1-3] The definition of sedentary combines both sitting and minimal physical activity (Oxford English Dictionary (OED)). Sitting seems justified since prolonged standing consumes at least twice the energy of sitting, and several hours of standing utilizes the energy of low speed running for one hour.[4] However, the contemporary sedentary lifestyle has been shown to account for numerous medical disorders.[5-7]

It has been assumed that humans voluntarily choose to sit for extended periods, and that therefore excessive sedentary behavior is an expression of an innate desire.[3,5,7] Accordingly, health professionals attempt to reduce sedentary behavior in their patients through suggesting individual vigilance through extolling the benefits of weight-bearing and condemning the sedentary lifestyle.[3,5,7] This means of management has failed. Sedentary behavior continues to increase suggesting that assumptions that humans are inherently sedentary and voluntarily choose sitting over weight-bearing are incorrect .[1-,5,7] If not biologically sedentary, than humans probably prefer weight-bearing but environmental constraints on weight-bearing make this difficult. This report explores this possibility through a multi-disciplinary approach.

Are humans biologically sedentary?

Homo sapiens separated from a common ancestor perhaps 250,000 years ago, probably as hunter-gatherers in sub-Saharan Africa.[8,9] Extensive weight-bearing with and without mobility was required for obtaining food and migrations, therefore sedentary tendencies would have impaired human survival and been eliminated through natural selection. In the early Holocene (15,000 years ago), humans began to live in a fixed location which allowed for the development of an agricultural lifestyle.[10-12] Although animals may have aided transport, this lifestyle likely required similar weight-bearing but less mobility compared to the nomadic existence, hence natural selection likely favored weight-bearing over sitting.

Civilizations first appeared 7,500 years ago, and became dominant in the past 5,000 years. Specialization of task, use of animals to aid human transport and technological innovation may have permitted less weight-bearing and mobility, therefore a sedentary lifestyle was possible for certain groups such as some artisans, entrepreneurs and leaders. There is no direct evidence of a sedentary lifestyle then, but Hippocrates warns about the harmful effects of obesity which is usually associated with a sedentary lifestyle.[13] Few individuals were likely sedentary - in fact the word signifying this type of behavior does not appear in any ancient language.

Chair use by members of the population is likely a valid surrogate measure of sedentary behavior. The number of chairs in use in a population is inferred by their extant numbers as estimated by published auction sales and number of museum pieces. According to these data, chairs made their first appearance in early civilizations within many societies, but were rarely used prior to the European Renaissance since there are few extant examples.[14-15] At first many were highly embellished, and appear to be custom-made symbols of power and prestige. Evidence for this comes from the word preasident, which first appears in Late Latin (c. 200 A.D., OED). It is derived from latin prae (before), and sedere (to sit), suggesting that important figures sat before the presumably standing (perhaps kneeling) less privileged masses. The equivalent word also exists in essentially all Latin and Slavic based languages. English differed in that neologisms (newly created words) chairman and chair have the same meaning and appear in Middle English (OED). This is also supported by pre-Renaissance art portraying the seated as affluent and politically important individuals.[14-15]

Mid-Renaissance chairs continue to be abundant at auctions therefore generally are of lower value than earlier examples. This suggests a sudden rise in the fabrication and usage of chairs.[14-15] Written and visual records indicate their presence in the workplace and home for the first time, and use by all social classes. Most were minimally decorated compared to earlier examples - essentially mass produced as they were often manufactured in sets, with no indication of individual or family assignment.[14,15] Within a short period, Europe became the first society in human history in which the general population used chairs widely.[14,15]

Sedentary, sedentaire and sedentario, appear as neologisms in English (OED), French (Bibliotheque nationale de France) and Spanish (Diccionario de la lengua española), in the 14th century, with no similar words in Old and Middle English, Old French and Old Castillion, respectively. A similar pattern appears in most other Latin and Slavic based European languages. This suggests that prior to the Renaissance few individuals were sedentary since neologisms typically are created for phenomena subject to common discourse.

The notion that humans are inherently sedentary seems improbable from an evolutionary perspective. They must have spent most wakeful time bearing weight and likely continue to be well adapted to it through natural selection. Few humans were probably sedentary prior to the advent of civilization. The abrupt and recent rise of sedentary behavior suggests it was caused by an environmental factor that commenced with the Renaissance. Also, humans probably would prefer to bear weight rather than sit if environmental conditions allowed it.

The introduction of footwear use

Footwear use by a society follows either a functional or aesthetic tradition. Functional is used here to indicate advancement of health, whereas aesthetic refers to body art based on any non-functional considerations, such as an individual's sense of beauty or simply a desire to conform. The only functional tradition of footwear use in large social groupings is with those inhabiting cold climates for thousands of years, where footwear would have been essential to survival, therefore used by all.[16] For example, one such kind of footwear, the kamak (mukluk), has ancient origins. It impairs mobility when compared to the bare foot, but poor mobility would have been a less selective disadvantage than foot necrosis from hypothermia.[16] Aside from this group, all footwear use in early civilizations followed an aesthetic tradition.[17-23] They were body art used by the minority as a symbol of elevated status and probably beauty at a cost to mobility and health.[24-30]

The Domesday Book (c. 1086) is the only source of quantitative data regarding whole population composition for any pre-Renaissance society.[24] It indicates that 70 percent were serfs which were surely unshod as were probably some of slightly higher classes.[24] Estimates of the unshod percentage of other early societies that followed the aesthetic footwear tradition are similar whether the lowest class were slaves as with Egyptian, Roman and Greek civilizations or serfs with respect to feudal states.[17-23]

India provides the clearest example of the transition from body decoration to shoe.[23] Foot decoration through henna painting predates human civilizations and recorded history. Later, the anklet and toe ring were added.[23] The sandal subsequently appeared probably as a consequence of European colonization and cultural imperialism, with the anklet and toe loop modified to attach shoe sole material to the plantar surface.[23] The Egyptian Civilization also had an ancient henna foot aesthetic tradition.[17,22]

Examples of early Egyptian footwear were exclusively aesthetic since they were so fragile they both impaired mobility and offered no protection.[17,22,25] Even caligae - the shoes with thick soles used by Roman soldiers were of an aesthetic tradition since they were poorly functional in terms of protection and impaired mobility. It is thought that they served to impress the enemy through augmenting standing height.[22,25]. Romans introduced many currently used leather footwear construction techniques which resulted in improved footwear functionality only relative to previous shoes through improved foot attachment, and better conformation to foot contour.[22,25] Within this largely aesthetic tradition there were occasional examples of functional footwear, such as the simple foot coverings used by Greek and Roman quarry workers to avoid plantar surface damage from sharp stone fragments.[25]

Elevated narrow heels and the tapered toe box have become hallmarks of aesthetic footwear for centuries.[17-20,22,25] They attenuate the support base thereby impairing stability and moreover concentrate forces involved in standing to the digits and metatarsal-phalangeal joints resulting in deformities and arthritis.[25-30] One author ascribes their original popularity to Tamerlane (Timur the lame - 1370-1405), because elevated heels appeared only occasionally before him, but continuously after.[25] If true, than ironically this aesthetic tradition was functional for Tamerlane. He required elevated heels to compensate for a calf muscle contracture from a war injury.[25]

The European Renaissance commenced with few wearing shoes and ended with near universal use - essentially a social norm in the cities burgeoning through international trade and nascent manufacturing.[17-20,22] The Renaissance also commenced with a pandemic form of bubonic plague (Yersenia pestis), the Black Death (1346-53), that de-populated countries bordering the Mediterranean Sea . For example, the population of Venice declined by 70 percent.[31,32] It was less destructive north of the Mediterranean, however even England suffered massive population decline from non-pandemic bubonic plague and other infectious diseases over two centuries.[31] De-population changed societies from having excess labor consisting of unpaid serfs existing in marginal agricultural settings, to labor scarcity with ex-serfs migrating to urban centers for paid employment.[31] Footwear cost plummeted due to decline in leather price following massive culling of herds as a consequence of massive oversupply of animal products resulting from the plague killing humans but not animals. The lowest class majority started wearing shoes for the due to their low cost, money and perhaps pseudo-science attributing bare feet to disease transmission.[17-20,22] Consequently, Renaissance Europe was the first society in humans history where all social wore shoes.

European Renaissance shoe design varied by region, however essentially all were of the aesthetic tradition.[17-20,22] Examination of foot bones from hundreds of skeletons dated to the Renaissance reveals a high frequency of hallus valgus in men, but less in women - the opposite of contemporary observations.[30] Extant footwear examples explain these anatomical abnormalities.[17-20,22] Men's shoes were mainly of leather, with less ornate examples possessing relatively thick, modestly flexible soles that were less wide than the bare weight-bearing foot, and heel height greatly exceeding contemporary fashion and particularly narrow at the toe box.[17-20,22,25] Shoes worn by females were less robust with lower heels and wider toe box.[17-20,22]

Pertinent biomechanics and neurophysiology

A six degree of freedom (x, y, z, pitch, yaw, roll) force platform is used to study mechanisms that humans use to maintain stable equilibrium.[33,34] Sway is typically expressed as area or velocity of movement of center of pressure.[33,34] Sway is both a measure of behavior that re-positions plantar load to prevent plantar skin pressure ulceration as well as postural adjustments that prevent falls.[35]

The most stable cohort of the population is healthy adults less then 40 years of age.[36] When unstressed (e.g., eyes open and standing on two feet and with a firm support base) subjects of this group stand barefoot on a force platform, sway is high and increases further with prolonged standing - often exceeding levels seen in the unstable elderly.[37-39] With similar stable groups and low stress conditions, sway is lower when wearing shoes than when barefoot although falls risk is higher when shod according to valid measures.[37-39] Sway also declines with each advancing trimester of gestation as fall risk increases.[40-43] These examples indicate that sway when healthy humans stand is mainly a measure of plantar redistribution behavior that protects the plantar skin from ischemic necrosis rather than behavior maintaining stable equilibrium.[37-43] Also, as balance stress rises from low to higher levels, humans first choose to lower falls risk through reduction of plantar protective behavior.[37-43] Sway becomes positively related to falls risk once plantar skin protective behavior is extinguished.[37-43]

Plantar cutaneous deformations and shear are transduced by SA II mechanoreceptors into afferent information that humans use for precise foot position determination which is required for accurate postural adjustments.[45-53] When these mechanical transients are attenuated, they turn to muscle and tendon receptors afferents in making foot position judgements, which are inherently less precise because they exhibit sustained discharge following high amplitude stimulation (after-effects).[54,55]

Footwear attenuate these mechanical transients resulting in reliance on muscle and tendon receptors for foot position judgements.[56] Furthermore, most shoes contain resilient sole materials that amplify frontal plane foot movement thereby further amplifying after-effects.[47-53] These mechanisms help explain attenuated plantar load re-distribution behavior, and therefore greater sustained localized plantar loads when humans wear shoes. Sustained localized plantar loading from reduced re-distribution behavior is an adequate stimulus for plantar high threshold mechanoreceptors, such as nociceptors with c-fiber afferents, which are known to be present in human glabrous skin.[57-61] Discharge from these receptors results in discomfort progressing to poorly localized pain as the stimulus is sustained (allodynia).[57-61] These observations seem to adequately describe the sensation one perceives from sustained high amplitude plantar loads.

The relation between amplitude of plantar load and sedentary behavior has been examined.[62-64] A positive relation between plantar pressure and sedentary behavior was found in a random sample of children.[62] Further, more sedentary behavior was recorded in shod obese children who have greater plantar load in relation to plantar surface area.[63,64] This suggests that sedentary behavior is the preferred means of avoidance of plantar discomfort in shod humans.[62] It follows that when a preponderantly barefoot society adopts the fashion of wearing shoes, this could account for a large increase in sedentary behavior.

Formulation

Scientific disciplines have multiplied in an attempt to increase knowledge through breaking down the totality of unknowns into manageable units. This risks compartmentalization of knowledge whereby scientists are poorly informed and perhaps even disinterested in other disciplines. This may interfere with understanding the cause and therefore management of significant contemporary health problems, because convincing causal arguments that meet current scientific standards often fail to respect the boundaries of a single discipline. The multi-disciplinary approach used here is a process of gaining knowledge through essentially integrating verified published reports from many disciplines to gain insight into poorly understood problems - in this case, the cause and prevention of excessive sedentary behavior in humans.

An hypothesis was formulated based on basic precepts of evolutionary biology and failure of currents treatment methods to control sedentary behavior. It was proposed that environmental factors caused humans to recently become sedentary. It can now be further asserted that footwear was the environmental factor that caused humans to become sedentary.

According to contemporary scientific logic, causal inference requires a temporal relation be drawn between the phenomenon in question and a plausible (ideally proven) causal mechanism.[65] Further support is provided through application of parsimony, or Okham's razor, the empirically validated notion that hypotheses with fewest steps and greatest explainability are usually correct.[65] Footwear use by all social classes and sedentary behavior both commenced for the first time in human history with the European Renaissance. The relation between footwear use and avoidance of plantar discomfort through sedentary behavior is a proven mechanism that is parsimonious. It is consistent also with our current understanding of receptor neurophysiology, our understanding of how plantar tactile receptors influence balance control and locomotory mechanics. The explainability of this hypothesis is vast, relating observations in disciplines thought to be as dissimilar as linguistic anthropology to ergonomics to human behavioral ecology.

Discussion

Current methods of preventing sedentary behavior have failed, and will likely continue to be inadequate because they do not address the environmental constraint that caused humans to become and remain sedentary. Humans inherently prefer weight-bearing over sitting, and therefore would have given up their sedentary ways if the aesthetic tradition of foot decoration through footwear use had become unfashionable. But centuries of footwear use as the means of satisfying aesthetic tradition of foot decoration has introduced additional factors that further encourage sedentary behavior, such as social norms promoting footwear use, interfaces with technology that require the users to be seated and foot pseudo-science that encourages footwear use through false notions.

Pre-scientific era pseudo-science may have contributed to social norms encouraging footwear use outside of the home. For example, no reports exist indicating that being barefoot outdoors commonly transmits disease, yet it is common knowledge that many believe this myth, and choose to wear shoes outdoors for presumed hygienic reasons. This false notion could be dispelled through education. Further, despite scientific data to the contrary, pseudo-sciences also continue to promote and profit from the false notion that the human foot has an ideal conformation such as arch height that if not maintained by footwear or shoe inserts results in pain and discomfort.[66-68] Again education is required to dispel these ideas. The public seems poorly aware of the reports indicting that most common foot ailments are caused by shoe use, and should be considered a health cost for maintaining this primarily aesthetic tradition.[66-68] Protective footwear use is both legitimate and desirable to prevent, for example, electrocution, crush injuries and hypothermic foot damage, but this is irrelevant to most shoe users because most footwear fail to meet even minimal standards for electrical insulation, foot crush resistance and thermal insulation.

No shoe can provide adequate plantar tactile feedback, but shoes could be designed to moderate frontal plane movement, thereby reducing muscle and tendon receptor after-effects. This would improve stability and amplify plantar pressure re-distribution behavior resulting in greater weight-bearing time before discomfort causes avoidance via sedentary behavior. Shoe features such as, minimal heel rise and soles that are thin, firm, wide made of materials of extremely low resiliency improve stability.[50] But less precise receptors would continue to be used in making foot position judgements, therefore prolonged weight-bearing would remain constrained compared to the barefoot condition.

Once the pseudo-scientific notions regarding health through footwear are dispelled, a novel solution is available in what might be called a “quasi-shoe.” This is defined as a highly decorative covering of only the dorsum of the foot. They are not shoes since they lack the shoe sole which is the essential element that distinguishes footwear from foot decoration. This may satisfy the innate desire of humans to decorate their feet. Further, it may not confront the social norm requiring covered feet outside of the home. The difference between the quiasi-shoe and a shoe might be difficult to notice. Since the quasi-shoe would allow the bare plantar surface to directly contacting the substrate, they retain all of the advantages of the bare foot in terms of plantar tactile information. Humans would be now capable of bearing weight for extended periods with their use thereby moderating sedentary behavior.

Barefoot weight-bearing inside of the home is limited by the traditions of sitting while eating and interfacing media, and some wearing some form of plantar surface covering. These practices could be moderated with the goal of increasing weight-bearing time. Explicit and implicit social norms limit barefoot weight-bearing outside of the home even in environments with low risk of foot injury. Explicit norms could be addressed through informing authorities of the erroneous assumptions on which their regulations were based. Implicit norms that promote foot decoration through, such as ostracism from violating social dress codes and desire for current fashion, may actually prove more difficult to overcome since they historically have resulted in many knowingly sacrificing health for social acceptance. They might be dealt with through a quasi-shoe.

Technology of daily life often forces individuals to sit because the technology was designed to interface with seated users. Without changes to this technology, even motivated barefoot individuals are forced to spend most of their wakeful time sitting. Weight-bearing when working offers the single greatest chance of substantial reduction of sedentary behavior. There is presently interest in desks designed for use when standing. This report suggests that their use will be fleeting unless users are barefoot on firm surfaces.

Many workers spend large portions of their day in transportation vehicles. Passengers on large vehicles could stand, but will soon sit if a seat is available if not barefoot. Driving vehicles when upright and barefoot presents no insurmountable technological barriers, but would require a massive and costly re- design of vehicles. One can envision public health advantages to this change beyond substantial decline in sedentary behavior. Upright driving probably would result in fewer accidents caused by poor alertness considering that considerable anecdotal evidence indicates that humans rarely fall asleep when standing perhaps due to an adaptation to prevent injury from falling.

Conclusion

Humans attenuate behavior that protects the plantar surface from ulceration rather than lose stable equilibrium. Footwear impaired stability results in discomfort that is avoided through sitting. Humans inherently prefer weight-bearing over sitting, but will choose to be sedentary once standing causes pain. Aside from populations inhabiting the far North, shoes were used by the majority of the population for aesthetic reasons the first time in human history commencing with the European Renaissance, which caused them to be the first society to be sedentary.

The ideal solution would incorporate barefoot weight-bearing outside of the homes. Intermediate solutions are available through footwear optimization in terms of stability. The sole-less pseudo-shoe may allow all the benefits of barefoot weight-bearing without confronting social norms and the need for body art through foot decoration.

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