Definiciones de la satisfacción del consumidor

The literature reviewed thus far reports the effects high heels have on the joints of the lower extremities. At each joint there are two primary changes, the angle of the joint is altered and the loading of the joint changed. Whilst some of the loading is the result of changes in muscle activation much more is a result of the loading due to body weight and the corresponding ground reaction force. Therefore, understanding how the loading of the foot changes when high heels are donned is imperative in order to understand how the joints of the body are loaded. The key measure for this is plantar pressure since this illustrates how the primary loads applied to the foot are distributed under the foot during gait. The relationship between heel height and plantar pressure is therefore a key part of the understanding of the effects of wearing high heeled shoes. However, whilst studies have shown that heel height has an effect on EMG [100], heart rate and oxygen consumption [27] are non-linear, investigations into the effects of heel height on plantar pressure have never systematically changed height and reported corresponding changes in plantar pressure. Instead researchers have studied shoes that are readily available to them. The effect of this is the differences in height between the shoes are arbitrary except for the fact that one is higher than the other. As such, we do not know at what height the plantar pressure significantly increases.

The two figures (Figure 30 & Figure 31) provide a simplified model of how forces act on the plantar surface of the foot in two different shoe designs. As previously explained by Broch et al [101], in the first (the flat shoe), forces acting on

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the plantar surface are located primarily under the forefoot and the heel. (F) The force under the forefoot has a moment arm of (f) between the centre of force application and the line of action around the ankle centre (where line A intersects it). The force under the heel (H) has a similar moment arm (h). The forces that act upon the heel and forefoot must create an equal moment at A in order to maintain stable standing and no rotation of the ankle.

Figure 31 illustrates the effect of elevating the heel above the forefoot, as occurs when wearing a high heeled shoe. The changed foot position results in a reduced moment arm f and an increase of moment arm h. However, there still needs to be equilibrium of moments acting on A, in order to maintain balance. Therefore there is a reduction in force H and an increase in force F.

When wearing high heels the foot is plantarflexed, which reduces its functional length and makes the foot a more rigid structure. This has the net effect of the forefoot carrying more load, and the heel less than during normal walking. Despite the lack of systematic study of heel height and changes in plantar pressure, this effect has been repeatedly observed in the literature. Several studies confirm the general observation that there is an increase in the pressure at the forefoot [23, 39, 44] and a reduction in pressure under the heel when heel height is increased [23, 25]. Furthermore, the increased pressure on the forefoot is focused on the medial side, with a reduction in pressure observed on the lateral side [23, 25, 39, 42, 53, 65]. The vertical and anteroposterior ground reaction forces also increase [25, 27, 43, 51],

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there is an increased impact force [42], reduced time to maximum pressure [44], and higher force-time and pressure-time integrals at the medial forefoot [23].

Figure 30: An unshod foot or foot in a neutral heel shoe during quiet standing (adapted image from Broch., et. al)

Figure 31: A foot wearing a raised heel shoe during quiet standing (adapted image from Broch., et. al)

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Understanding the effects of different footwear designs on gait can enable their appropriate use, since a design might place one person at risk but offer therapeutic benefit to another. It is clear from past research that an increase in heel height results in an increase in forefoot loading when standing [43] particularly on the medial side of the foot. As a result the force experienced at the first metatarsal phalangeal joint (MPJ) when wearing high heels has been reported to be twice that when barefoot [23, 39]. However, shoe inserts under the forefoot change the material properties in contact with the foot and can alter the ‘fit’ of the shoe, and have been shown to reduce pressure and increase comfort [102, 103]. This suggests that plantar pressure and comfort are related to shoe fit (the matching of the foot and shoe shape, which is explained in much greater detail in chapter 5) in high heeled shoes [42].

Other health related effects of high heel use

Most shoes are designed primarily for fashion but some shoes are designed specifically to help prevent health issues or to protect feet. To protect the foot in the work place some industries require the employees to wear metal toe capped shoes, which will make the shoe very stiff and humid, promoting new foot problems [89]. Despite creating problems for foot health the metal toe cap is still required to reduce risk of significant traumatic injury. Conversely, shoes designed for those with diabetes will be made with a soft very wide fitting upper to reduce the risk of ulceration due to loads from the upper.

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Many people and professional organisations (e.g. Society of Podiatrists and Chiropodists) generally consider high heeled shoes to be ill-designed and poor fitting. If these designs lead to increase plantar pressure, as is often proposed [23, 25, 34, 39, 42, 44], then this ‘damning’ of heel height may be valid given that increased plantar pressure has been associated with clinical problems for diabetes sufferers and conditions such as metatarslagia [104]. Indeed, studies have refered to metatarslagia as a symptom of wearing high heels [104, 105] although it is in fact a generic term for pain in the metatarsal region (assumed to be due to excessive plantar loading). This can occur due to a number of conditions, including: inflammation of the metatarsal heads due to overuse, increased stress on the foot from high heels or the individual being overweight, Morton's neuroma, hammer toe or claw deformity, hallux valgus, stress fracture of the metatarsal, arthritis, gout, or diabetes. One theory relating to a specific cause of metatarsalgia is that the thickening of the nerve tissue is the result of compression of the nerve in the inter-metatarsal space (Morton Neuroma) [80]. This compression is thought to be due to too narrow toe boxes. This assumes the upper material stiffness is able to provide a reaction force sufficient to prevent the forefoot widening as it is loaded, thus compressing the tissues in between the metatarsals. Increased forefoot plantar pressure may worsen the condition [4] as greater load passes through all the forefoot tissues. This example illustrates the importance of understanding how design features commonly used with high heels, such as narrow forefoot area and stiff uppers, might further contribute to changes in plantar pressure and the associated foot problems. Stress fractures are often a result of high repetitive loading of a bone and some have suggested habitual high heel use increases fracture

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risk [80, 105]. However, there is currently no data profiling the causes of metatarsal stress fractures to substantiate this.

The effect of high heels on comfort

Studies have demonstrated that high heel shoes are a risk factor for increased plantar pressure under the forefoot associated with increased risk of poor foot health and reduced footwear ‘comfort’ [9-11]. Indeed, an increase in plantar pressure of 15kPa has been shown to reduce comfort rating by 0.6 points on a 5 point rating scale [106]. However, by contrast, high heel shoes have also been used as a therapeutic technique for symptoms such as plantar fasciitis, as well as tendinitis and partial ruptures of the Achilles tendon [101]. Even relatively small changes in the forefoot dimensions of the high heeled shoe can affect comfort. It has been shown that women with a wider foot are more susceptible to foot pain. de Castro 2010 reported that women without foot pain had a forefoot circumference equivalent to 98.27 (±4.42)% of foot length, whilst those with foot pain had a larger relative circumference, 99.39(±5.55)% of foot length (p =0.048) [107]. Thus it seems that poorly fitting footwear can cause mechanical stresses that are detrimental to foot health [108].

Questionnaires assessing the comfort of high heeled shoes have so far shown that discomfort increases with heel height [109] and the design of the shoe plantar surface (footbed) can be optimised for comfort [66]. In particular the heel wedge angle and heel seat length play an important part in the perceived comfort [110]. It has been shown that mediolateral ground reaction force is increased as a function of heel height

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[27] and it has also been identified that stability is an important factor in the rating of a high heeled shoe’s comfort [111].