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Abstract

IntroductionDistance running is known to reduce the risks of some health problems in runners [7,27,31,41]; however, distance running also has negative effects that result in musculoskeletal overuse injuries in the lower extremities. The incidence of running-related musculoskeletal overuse injuries in previous studies ranged from 20 to 79 % [39]. The lower leg, foot, and ankle, especially the Achilles tendon and tibia, are commonly affected in running-related injuries [19,24,26,38]. On the basis of material fatigue, large forces applied to injured regions will decrease the number of cycles to failure [3]. Therefore, a decrease in the force applied to the lower leg, foot, and ankle during running may be able to reduce the potential risk of running-related injuries at the Achilles tendon and the tibia for a given running distance and frequency.Foot strike pattern is generally classified into 3 types according to the ground reaction force (GRF) application point, relative to foot length, at the instance of foot contact [5]. Foot strike pattern has been thought to influence the potential risk of running-related injuries [8,21,23,36]. Previous studies reported that 69-75 % runners use the rearfoot strike (RFS) and 25-31 % runners use the midfoot strike (MFS) or rearfoot strike (RFS) [8,15]. It was reported that the vertical component of GRF during the early stance phase, and the loading rate of that force, were smaller for FFS and MFS than for RFS [5,22,23,28]. In these studies, it was suggested that FFS might reduce the potential risk of injury as compared with RFS. However, the GRF does not necessarily coincide with the forces applied to the Achilles tendon and the tibia, and the magnitude of forces applied to the Achilles tendon and the tibia may be much greater than the GRF [9,20,32,34]. Although previous studies suggested that measurements of GRF may not be sufficient to evaluate the potential risk of running-related injuries, it is difficult to directly measure the forces applied to the Achilles tendon and the tibia.An inverse dynamics approach might resolve this problem. The forces applied to the Achilles tendon and the tibia can be calculated as the Achilles tendon force and joint reaction force, respectively. At the instance of foot contact, the application point of the GRF is positioned at the ball of the foot for FFS, the middle of the foot for MFS, and the heel for RFS, respectively [5]. When the orientation of the GRF line of action is similar among the three foot strike patterns, the GRF moment arm, defined as the shortest distance between the ankle joint axis and the GRF line of action, is greater for FFS than the other two foot strike patterns. A difference in the GRF moment arm induces a difference in the moment of the GRF, Abstr Ac tGround reaction force is often used to predict the potential risk of injuries but may not coincide with the forces applied to commonly injured regions of the foot. This study examined the forces applied to the foot, and the associated moment arms made by three foot strik...