## 1 Introduction  
Population ageing is a reality of recent decades, especially in developed countries. By 2033, people over 65 years old will make up 25.2% of the Spanish population [1]. Biological aging and sociodemographic changes lead to an increase in chronic diseases, functional disability and the use of health resources [2]. In this context, the concept of frailty has gained interest, as its early detection and intervention can promote healthy aging. Frailty is a clinical condition characterized by reduced capacities in multiple physiological systems, resulting in a state of increased vulnerability and susceptibility to adverse health outcomes, including disability, falls, hospitalization, institutional care, or even death [3]. Around 10% of people aged over 65 years have frailty, rising to between a quarter and a half of those aged over 85 [4], and it is more frequent in women and nursing home residents [5]. There are two major approaches to defining frailty. The first considers frailty as the result of deficit accumulation [6]. The second, proposed by [PERSON], defines the frailty phenotype according to five criteria [3], with the locomotor system being crucial as three of the criteria depend on it (gait speed, physical activity and muscle strength). Although the frailty concept is shared by both approaches (physical phenotype and cumulative deficit theory), the two definitions have different ways to identify frail people and this appears to influence the prevalence of frailty and biomarkers identification or key aspects to consider during diagnosis. Both approaches have strengths and limitations, but both are associated with adverse health outcomes as has been proven [7; 8], and studies comparing their predictive abilities have shown mixed results [9]. The key point is that both identify frail people allowing an integrated and individual intervention plan to reverse this dynamic process. It seems that they approach frailty from different perspectives, so in clinical practice, it is accepted to use either of them. Regardless of the definition used, there is a growing interest in searching for biomarkers (laboratory, imaging or clinical) that identify those subgroup populations at high risk of complications in order to improve clinical decision making [10]. Falls are one of the main \"geriatric syndromes\", representing a complex geriatric syndrome with multifactorial physiopathology as the walking process is the result of neurological, musculoskeletal, nutritional and cardiovascular integration. The relationship between falls and frailty has been widely described [11; 12; 13; 14], falls being one of the first clinical manifestations of frailty adverse events as was described by [PERSON] in 2001 [3; 15]. Changes in bone and muscles (osteosarcopenia) are present in both and can explain why frail people have more falls. Deficits in physical activity, energy and gait speed are some of the components of the frailty phenotype and are intrinsically linked to falls; on the other hand, people who suffer falls with consequences (such as fractures, contusions or fear of falling syndrome) or those with recurrent falls are especially vulnerable and, therefore, present a greater risk or probability of frailty, leading to a greater risk of new falls [13; 16]. Given this close relationship, the World Health Organization (WHO) recommends implementing public health initiatives focused on the early detection of frailty and falls to increase individual's resilience by improving their health status [17]. The multidimensional assessment of frail patients or those with falls must pay special attention to physical performance, gait analysis and balance [18]. This is usually carried out by performing tests such as gait speed [19], the time up and go test (TUG) [20] or the Short Physical Performance Battery (SPPB) [21] that are useful tools in frailty or falls assessment and screening [22; 23; 24; 25].