HUMAN SPRINTERS HAVE LONGER FOREFEET AND SHORTER PLANTARFLEXOR MOMENT ARMS 1 Josh R. Baxter, 1 Thomas A. Novack, 2 David Pennell, and 1 Stephen J. Piazza 1 The Pennsylvania State University, University Park, PA, USA 2 Vanderbilt University, Nashville, TN, USA email: piazza@psu.edu INTRODUCTION Effective sprinting, whether performed to win a race or to outrun predator or prey, requires rapid acceleration from rest. The forward impulse that determines this acceleration depends on large forward-directed contact force and contact time sufficient for that force to act. The best animal sprinters have limb structures that are favorable for generating large forward impulses. The cheetah, for example, has longer toes and metatarsals and a shorter plantarflexor moment arm (pfMA) than are found in less capable sprinters of similar size [1]. These differences contribute to the cheetah having a large “gear ratio” (ratio of the ground reaction force moment arm to the moment arm of the plantarflexor about the ankle), which enables the plantarflexors to operate at a lower shortening velocity and thus maintain muscle force production near toe-off [2,3]. It is less clear, however, whether variation in foot and ankle structure within species, and specifically among humans, might explain differences in sprinting ability. Lee and Piazza [4] found that human sprinters’ moment arms estimated from Achilles tendon excursion were significantly smaller than those of height-matched non-sprinters, but it is unclear whether those measurements of tendon excursion were affected by differences in tendon compliance between the two groups. Lee and Piazza [4] also found sprinters to have longer toes as indicated by external measurements made from the first metatarsal head to the end of the great toe, and the extent to which these measurements were affected by errors in identifying bony landmarks or by differences in soft tissue thickness is not clear. Scholz et al. [5] and Raichlen et al. [6] did not study sprinters, but did identify a link between human foot structure and function; both groups reported that running economy negatively correlates with calcaneal tuber (heel) length in trained distance runners. The purpose of this study was to use magnetic resonance (MR) imaging of the Achilles tendon and the bones of the foot to determine whether the foot and ankle structure of trained sprinters differs from that of non-sprinters. We hypothesized that sprinters would have longer phalanges and metatarsals but have shorter pfMA than those of non-sprinters. METHODS Achilles tendon moment arms and the lengths of foot bones were measured in seven trained sprinters (mean 100 m time of 10.82 ± 0.20 s, 6.6 ± 3.0 y of sprint training) and seven height-matched non- sprinters (see Table 1 for subject characteristics). Sagittal-plane images of the foot and ankle were made with a 3T Siemens MR scanner (Siemens; Erlangen, Germany) at 15° dorsiflexion, neutral position, and 15° plantarflexion. Subjects were positioned supine on the scanner bed with the knee flexed to 30°. Each scan required 5 s, during which time the subjects isometrically co-contracted their ankle muscles. Moment arm was defined to be the shortest distance between the center of tibiotalar rotation obtained from the 15° dorsiflexion and 15° plantarflexion images using a modified Reuleaux method similar to that implemented by Maganaris et al. [7] (Figure 1). The lengths of the first distal phalanx (L DP1 ), the first proximal phalanx (L PP1 ), and the first metatarsal (L MT1 ) were measured from quasi-sagittal-plane images reconstructed from the three-dimensional MR image data set using OsiriX Viewer (Atlanta, Georgia, USA). The lengths of the phalanges were summed to obtain first toe length and L MT1 was added to first toe length to obtain the length of the first ray (L R1 ).