The goal of this longitudinal study was to test whether sustained natural endurance running (ER) among urban runners in minimal support footwear increased foot strength reflected by changes within intrinsic plantar musculature and the longitudinal arch. Recent studies indicate that human long distance ER evolved some 2 million years ago beginning with the genus Homo (Bramble and Lieberman, 2004). Humans have the ability to sustain ER at relatively high speeds, which may have allowed human ancestors facing a changing landscape of expanding grassland to hunt effectively before the invention of projectile technology (Carrier, 1984; Bramble and Lieberman, 2004; Pontzer et al., 2010). A number of derived traits in Homo appear to be adaptations for ER, among them long hind limbs, short toes, and musculoskeletal adaptations associated with the foot’s mass-spring mechanism of energy restitution (Bramble and Lieberman, 2004; Pontzer et al., 2010). Although the evolutionary anatomy of the running foot is increasingly well understood, we know less about foot strength, particularly the role of the intrinsic plantar muscles in running.
Today, modern humans run long distances on such disparate substrates as the highlands of Ethiopia where runners are often barefoot and on the urban pavements of the United States where they typically run in stiff, cushioned footwear. In order to better understand the functional demands ER places on the foot’s intrinsic musculature and longitudinal arch, I conducted a 12-week study of foot strength in minimally shod endurance runners. I examined the anatomical cross-sectional area and muscle volume of the plantar intrinsic musculature and the functional conformation of the longitudinal arch. Running kinematics and foot anatomy were analyzed in 33 healthy runners (18 males and 15 females) using 3D motion capture and magnetic resonance imaging. Subjects were randomly partitioned into one of two groups, a control group in standard shoes (n=16), and an experimental group transitioning from standard to minimal support shoes (~12mm to 4mm) (n=17). Participants ran a mean of 30 miles/week over the study period.
The results show that while long distance runners shod in both standard and minimal running footwear increased muscle volume in the flexor digitorum brevis muscle (p= 0.03), minimally shod runners alone significantly increased muscle volume in the abductor digiti minimi (25%, p= 0.009) and reduced stiffness in the longitudinal arch (p= 0.013). The abductor in flexing the fifth digit metatarsal phalangeal joint enhances the tie-rod function (windlass mechanism) of the plantar aponeurosis. Evident in its volumetric increase, greater force production in this muscle suggests an increased reliance on longitudinal arch function among the minimal shoe runners.
Study findings support predictions based on both the biomechanics of endurance running and the functional anatomy of the human foot that minimalist shoe running exploits the deformation capacity and energy-restitution system of the longitudinal arch and increases muscle mass within the intrinsic plantar group. Greater foot strength and enhanced arch flexibility associated with the habitual use of minimal shoes reflect the evolutionary capacity of the modern human foot to respond to natural style running.