## 1 Introduction  
There has been a large increase in the participation of running events within the past decades. However, it has been reported that 37-79% of the runners in a given year experienced running-related injuries at a lower extremity [1, 2]. Improper running mechanics and footwear contribute to the risk of developing running-related injuries [2, 3, 4]. Thus, effective interventions are required to reduce the risk potential in running. Medial longitudinal arch is the largest and most important arch of the foot to provide rigidity of the mid and forefoot structures for effective push-off. Foot orthoses, which refer to shoe inserts that contour plantar foot surface, are frequently used in footwear in attempts to re-distribute the plantar loading, maintain stability, and minimize the localized plantar pressure, hence reducing injuries [5; 6] and improving comfort for better running performance [7]. The use of arch-support orthoses with arch-support aims to increase the foot-insole contact area and pressure at medial longitudinal arch of a foot, enhancing somatosensory inputs over the plantar foot [8] to provide foot support and shock absorption during contact on the ground [9]. Biomechanically, it was shown that the use of arch-support orthoses increased maximum ankle inversion angle during the landing of the basketball lay-up and the shuttle run, suggesting that this may contribute to cumulative fatigue of the proximal portion of the fifth metatarsal bone [10]. This type of injury could theoretically predispose an individual to stress fracture, acute fracture, or combined stress/acute fracture [10]. Furthermore, the use of arch-support orthoses increased vertical impact force and loading rate in running [11; 12]. The higher ground reaction force (GRF) magnitudes were identified as key biomechanical indicators in running-related injury risk among recreational athletes. Collectively, arch-support orthoses can significantly decrease strain in the plantar aponeurosis and reduce the rearfoot pronation, but the orthoses may lead to a poorer perception of forefoot cushioning, heel cushioning, and overall comfort [11; 13]. Furthermore, the inconsistent findings across studies could be related to the effectiveness of arch-support, such as the stack height and heel to toe drop. For the same medial arch-support height, the decreased heel thickness could biomechanically lead to higher pressure at the medial arch region. Though extensive research has been done on the use of foot orthoses with medial arch support, little attention has been paid to the combined effects of arch-support orthoses with heel lifts on running biomechanics. A heel lift is an insert which placed over the heel region of shoes to elevate the foot into a more plantar flexed position during walking and running [14; 15]. It is usually prescribed to manage lower limb pain and injuries such as plantar heel pain [16], posterior leg compartment muscles strains [17], and inflammation at the Achilles tendon [18; 19; 20; 21] and calcaneus [22]. It can also be used to improve between-leg symmetry for patients suffering from leg length discrepancy [23] and with limited joint range of motion of the affected side [24]. However, heel lifts are rather individualized and specific to symptomatic participants. Furthermore, some heel lift heights can be greater than 15 mm, which limits the usage to a broader healthy population [14]. Akin to the heel lift concept applied in recreational sports, shoe heel-to drop is suggested to alter weight transfer by tilting the foot longitudinally toward the mid and forefoot regions of a shoe, and thus affecting running kinematics and loading [14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24]. In short, lower shoe heel-toe drop would result in smaller foot strike angles [25; 26], which is expected to reduce the impact peak and loading rate [27], thereby reducing the risk potential of running-related injuries [14; 25; 26; 28].