# Influence of Footwear Features on Oxygen Consumption and Running Economy: A Review
## 1 Introduction  
Worldwide, running may be considered one of the most practiced sports activities, particularly when considering it as a natural task that has always been part of human existence [1]. In addition, this sport modality presents a low-cost activity from an economic point of view as it does not require much more specialized equipment than footwear and may be performed outdoors. Numerous health benefits have been reported for those who practice running, as it can reduce the prevalence of vascular and neurological diseases [2] and even different types of cancer like colorectal or lung cancer [3, 4], as well as other diseases related to a sedentary lifestyle such as cardiovascular function or increased adiposity [5, 6]. Nevertheless, running may cause injuries, mainly on the lower limb [7, 8] (i.e., medial tibial stress syndrome, Achilles tendinopathy, or plantar fasciiitis). The incidence of injuries associated with running has risen with the increased popularity of the sport among those middle/long-distance runners who practice it [9]. Nowadays there are studies that set the incidence of injuries in the lower limb at between 19.4 and 79.3% [9]. It is known that the origin of these injuries is multifactorial (i.e., factors such as age, history of previous injury, or perception of risk should be considered) [10] and there are diverse risk factors that may predispose to injury in the lower limb. These risk factors may beclassified into three areas, namely (1) personal factors (i.e., age, sex, or height), (2) running/training factors (i.e., weekly running days, distance, or running shoes), and (3) health and lifestyle-related factors (i.e., smoking, a history of comorbidity and previous injuries). Among the characteristics of training, footwear plays an important role [11]. Coinciding with the increase in popularity of running races from 1970 onward, research on sports shoes started in two different directions, the prevention of injuries and the improvement in performance during the race. Despite the investment that has been made in these two areas, the injury rate has barely decreased and the improvement in sports performance has not undergone a marked change until recent years [12; 13]. The present review aimed to analyze the scientific literature on the influence of footwear on running economy at a medium/long distance. In order to understand how this improvement in running performance is quantified, it is important to know the following concepts. Three physiological parameters are usually studied to predict the speed that an athlete can maintain in competitive events greater than 3000 m, the maximal rate of oxygen consumption (VO\({}_{\text{2 max}}\)), the blood lactate threshold and the energy cost of running [14; 15]. From these data, we can obtain the oxygen cost of running at a given speed, which is known as running economy and is measured in ml O\({}_{2}\)/kg/min [15]. This is the parameter that has traditionally been used to measure the performance of athletes and even today many studies continue to assess improvement in running by measuring oxygen consumption [16; 17]. At the same time, it should be noted that there has recently been a proposal to express running economy in terms of metabolic cost measured in kJ/kg/min since this value would have greater sensitivity than oxygen consumption [18]. So, if an athlete has a greater running economy, that is, a lower oxygen cost/metabolic cost, he or she will be able to run at the same speed with less physiological effort, which will translate into an improvement and greater distance traveled in less time [19]. Therefore, in this review, running economy may be considered as the metabolic cost measured in kJ/kg/min whenever possible because, although this measurement may be more reliable, this fact has only been confirmed in recent years and the vast majority of previous studies [15; 16; 17] use running economy as a reference in relation to oxygen consumption in ml O\({}_{2}\)/kg/min.