**1. Introduction**

Millions of people take part in sports activities that involve running as it is an activity available to all ages, and is low cost, versatile, and brings health benefits [1]. Currently, running is one of the most popular sports in the world, having experienced substantial growth over the past decades [1–3].

At the end of the 20th century, the considerable increase in running practice resulted in a higher prevalence of injuries in the lower limbs [2, 4, 5], with percentages of reported injuries of 5.7 to 39.3% over a period of one year in those who practice sports [1, 2]. Among the most frequent injuries, a retrospective study with 2002 runners revealed that plantar fasciitis is the third most common musculoskeletal injury in runners [6], reaching approximately 10% of them [3, 6, 7]. According to

Tauton et al., [7], of the 267 cases of plantar fasciitis analyzed in various sports, 160 of these cases involved only the running modality. In addition, it is the most common cause of acute and chronic calcaneus pain [8]. These facts have led to growing interest in research regarding the causal factors of plantar fasciitis in runners.

Plantar fasciitis is characterized by a musculoskeletal disorder of inflammatory and degenerative origin of the plantar fascia. The most common clinical symptom of plantar fasciitis is pain in the lower and medial region of the calcaneus, most typically close to its insertion in the medial tubercle of the calcaneus [9, 10].

There are several intrinsic and extrinsic factors related to the disease [11]. However, some specific intrinsic factors related to the development of plantar fasciitis have been further explored in the literature. Among them, obesity [12], decreased ankle dorsiflexion [12, 13], the type of medial plantar longitudinal arch [6, 7, 14–16], rearfoot pronation [11, 7], and increased plantar load stand out [17–20]. However, these factors still remain controversial, especially with regard to their involvement to a greater or lesser extent with the etiology of this injury [11].

The majority of studies report that the factors most strongly associated with the development of plantar fasciitis are biomechanical factors directed to the type of longitudinal plantar arch, excessive pronation of the rearfoot [6, 7, 14, 15], and increased pressure on the surface of the feet [17–20]. Thus, it is evident that changes in the plantar arch and in the rearfoot alignment can directly or indirectly influence the functionality of the plantar fascia and, consequently, in the redistribution of the plantar load.

Generally, the most accepted theoretical and scientific reasoning is that plantar overload, resulting from possible biomechanical imbalances in the structure of the feet, would result in greater pressure and increased force rates on the plantar fascia, inducing microtrauma and inflammation, characterizing the acute phase of the injury [21, 22]. Throughout the period of the disease, the repetitive support of the feet in contact with the ground would evolve to the fragmentation and degeneration of the plantar fascia, characterizing the chronic phase of plantar fasciitis associated with degeneration and fragmentation in the thickness of the plantar fascia [22, 23].

However, even though plantar fasciitis is the third most prevalent injury in runners [6, 7], the majority of studies that investigated calcaneus overload were directed to walking [17–19]. Another fact worth noting is that the presence or absence of the painful stimulus, common in the different phases - acute and chronic plantar fasciitis - has been the subject of studies on plantar loads in walking and running [16–19, 24, 25]. In addition, Ribeiro et al., [24] and Hong et al., [26] emphasize the importance of studying the distribution of plantar load during running in a training and competition environment, since running on a treadmill can influence the foot support pattern of runners affected by the disease.

According to the biomechanical studies carried out to date, it is likely that the plantar loads undergo changes during the acute (with pain) and chronic (without pain) phases of plantar fasciitis, especially in the running population [20, 24]. In addition, this possible increase in plantar load rates in runners with plantar fasciitis may be directly related to changes in the plantar arch, which remains elevated [6, 25, 27] and with the alignment of the rearfoot that maintains greater pronation of the feet [16, 25].

In this chapter, we aim to provide better understanding of scientific studies regarding the static and dynamic biomechanical patterns of the plantar support of the feet of runners with plantar fasciitis as well as their relationship with sports shoes and the prescription of orthoses and insoles, in order to assist in the clinical parameters of health professionals working in the conservative treatment of plantar fasciitis in runner athletes.
