Heel-to-toe drop of running shoes: a systematic review of its biomechanical effects

Heel-to-toe drop (HTD) values may modify the running biomechanics. However, more than twenty additional footwear characteristics may contribute to this too. The aim of this review was to identify and systematise the specific effects caused by different HTDs on running biomechanics. A systematic search was carried out in seven online databases and Footwear Science according to PRISMA protocol for studies that included the practice of endurance running in running shoes with different HTD types. A modified Downs and Black checklist was used to assess the risk of bias. Characteristics of the studies and footwear, the equipment used, and biomechanical outcomes were extracted for qualitative synthesis. Twelve studies were included. Only one had a randomised control trial design, which was classified as 'good' (score = 24) quality. The studies reported thirty-nine kinematic and sixteen kinetic variables. HTD ranged between -8 to 16 mm. HTDs did not produce modifications in contact and flight time, stride frequency, and stride length. Some controversial evidence supports that the foot strike pattern changes towards forefoot strike only with HTD 0 compared to HTD 8-10. All evidence indicates that HTD values modified neither ankle, knee, or hip kinematics. Greater evidence supports that the modification of HTD in running shoes does not modify the values of GRF. Despite the controversial nature of the data, the trend is that a lower HTD shows a higher vertical loading rate. The different HTDs modified neither of the joint moments. The negative HTD could be an interesting variable to consider for future research. In addition, it is necessary to study the relationship between the HTD values and different combinations of heel height and forefoot height.